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U.S. ARMY CORPS OF ENGINEERS

NAVAL FACILITIES ENGINEERING COMMAND (Preparing Activity)

AIR FORCE CIVIL ENGINEER SUPPORT AGENCY

Record of Changes (changes are indicated by \1\ ... /1/)

Change No. Date Location

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This UFC supersedes TM 5-81 0-2, dated 31 December 1991. The format of this UFC does not conform to UFC 1-300-01; however, the format will be adjusted to conform at the next revision. The body of this UFC is a document of a different number.

1 UFC 3-430-04FA 15 May 2003 FOREWORD \1\ The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and provides planning, design, construction, sustainment, restoration, and modernization criteria, and applies to the Military Departments, the Defense Agencies, and the DoD Field Activities in accordance with USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and work for other customers where appropriate. All construction outside of the United States is also governed by Status of forces Agreements (SOFA), Host Nation Funded Construction Agreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.) Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, the SOFA, the HNFA, and the BIA, as applicable.

UFC are living documents and will be periodically reviewed, updated, and made available to users as part of the Services’ responsibility for providing technical criteria for military construction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval Facilities Engineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) are responsible for administration of the UFC system. Defense agencies should contact the preparing service for document interpretation and improvements. Technical content of UFC is the responsibility of the cognizant DoD working group. Recommended changes with supporting rationale should be sent to the respective service proponent office by the following electronic form: Criteria Change Request (CCR). The form is also accessible from the Internet sites listed below.

UFC are effective upon issuance and are distributed only in electronic media from the following source:

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AUTHORIZED BY:

______DONALD L. BASHAM, P.E. DR. JAMES W WRIGHT, P.E. Chief, Engineering and Construction Chief Engineer U.S. Army Corps of Engineers Naval Facilities Engineering Command

______KATHLEEN I. FERGUSON, P.E. Dr. GET W. MOY, P.E. The Deputy Civil Engineer Director, Installations Requirements and DCS/Installations & Logistics Management Department of theINACTIVE Air Force Office of the Deputy Under Secretary of Defense (Installations and Environment)

2 ARMY TM 5-810-2 AIR FORCE JOINTMANUAL 32-1057 (FORMERLY AFR 88-28)

31 DECEMBER 1991

HIGH TEMPERATURE WATER HEATING SYSTEMS

THIS COVER PAGE OFFICIALLY CHANGES THE AIR FORCE PUBLICATION NUMBER FROMAFR 88-28 TO AFJMAN 32-1057

(Affix to thefront ofthe publication)

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DEPARTMENTS OF THE ARMYAND THE AIR FORCE

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HIGH TEMPERATURE WATER HEATING SYSTEMS

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED

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This manual has been prepared by or for the Government and, except to the extent indicated below, is public property and not subject to copyright.

Copyrighted material included in the manual has been used with the knowledge and permission of the proprietors and is acknowledged as such at point of use. Anyone wishing to make further use of any copyrighted material, by itself and apart from this text, should seek necessary permission directly from the proprietors.

Reprint or republications of this manual should include a credit substantially as follows: "Joint Departments of the Army and Air Force, TM b-810-2/AFR 88-28".

If the reprint or republication includes copyrighted material, the credit should also state: "Anyone wishing to make further use of copyrighted material, by itself and apart from this text, should seek necessary permission directly from the proprietor .".

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TM 5-810-2/AFR 88-28

TECHNICAL MA,, uAL HEADQUARTERS No. 5-810-2 DEPARTMENTS OF THE ARMY AIR FORCE REGULATION AND THE AIR FORCE No. 88-28 Washington, DC, 31 December 1991 HIGH TEMPERATURE WATER HEATING SYSTEMS I Approved for public release; distribution is unlimited

CHAPTER 1 . DESIGN CONSIDERATIONS A~h Purpose ...... 1-1 1-1 Scope ...... 1-2 1-1 References...... 1-3 1-1 General...... 1-4 1-1 Advantages of HTW systems ...... 1-5 1-1 Properties of high temperature water...... 1-6 1-2 Pressurization ...... 1-7 1-3 Water circulation...... 1-8 1-4 HTW generators ...... 1-9 1-12 Design and selection procedure ...... 1-10 1-12 Economic justification ...... 1-11 1-18 CHAPTER 2. LOAD CHARACTERISTICS AND CALCULATIONS HTW requirements...... 2-1 2-1 Space heating ...... 2-2 2-1 Process heating ...... 2-3 2-1 Diversity factors...... 2-4 2-2 Operating temperatures and pressures ...... 2-5 2-2 Maximum initial load ...... 2-6 2-3 Maximum ultimate load ...... 2-7 2-3 Essential load ...... 2-8 2-8 Matching plant capacity to load ...... 2-9 2-4 System heat loss...... 2-10 2-4 Flywheel factor ...... 2-11 2-4 Calculations ...... 2-12 2-4 CHAPTER 3. DISTRIBUTION PIPING AND EQUIPMENT Design of system ...... 3-1 3-1 Pipe sizing ...... 3-2 3-1 Distribution piping ...... 3-3 3-1 Underground and aboveground systems ...... 3-4 3-4 CHAPTER 4. HEATING PLANT Introduction ...... 4-1 4-1 HTW generators ...... 4-2 4-1 Combustion equipment and controls ...... 4-3 4-3 Pressurization system...... 4-4 4-5 Pumps ...... 4-5 4-6 Makeup water treatment ...... 4-6 4-7 Instrumentation...... 4-7 4-8 Pollution control ...... 4-8 4-9 CHAPTER 5. CONVERSION AND U71LIZATION Potential users of the system ...... 5-1 5-1 Building service...... 5-2 5-1 Location of equipment ...... 5-3 5-1 Design ofheat exchangers ...... 5-4 5-5 Controls ...... 5-5 5-6 APPRNDCx A. REFERENCESINACTIVE...... A-1 APPENDIX B. SAMPLE CALCULATIONS FOR DATA GIVEN IN CHAPTER 2...... B-1 APPENDIX C. EXAMPLE DISTRIBUTION LAYOUTS ...... C-1

'This manual supersedes TM 5-810-2/AFM 88-28, dated 12 September 1984

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*TM 5-810-2/AFR 88-28

LIST OF FIGURES

Ague 7Ytk POO 1-1. Inert Gas-Pressurized Single Circulation Method ...... 1-5 1-2 . Inert Gas-Pressurized Dual Circulation Method ...... 1-7 1-3. Steam-Pressurized Single Circulation Method ...... 1-9 1-4. Steam-Pressurized Dual Circulation Method...... 1-11 1-5. Flow Diagram-Steam-Pressurized Single Circulation System ...... 1-14 1-6. Flow Diagram-Steam-Pressurized Dual Circulation System...... 1-15 1-7. Flow Diagram Inert Gas Pressurized Single Circulation System...... 1-16 1-8. Flow Diagram Inert Gas Pressurized Dual Circulation System ...... 1-17 1-9. Inert Gas Pressurization Using Variable Gas Quantity with Gas Recovery ...... 1-18 2-1. Expansion Tank Volumes ...... 2-6 3-1. Typical Vent and Typical Drain ...... 8-8 4-1. Cascade HTW System in Process Steam System...... 4-2 4-2. Typical Combustion Control Systems...... 4-4 5-1. Various Heat Converters ...... 5-2 5-2. Heat Exchangers and Control Valves ...... 5-4 C-1. Direct Supply Reverse-Return ...... C-1 C-2. Direct Supply Radial ...... C-2 C-3. One-Pipe Loop Main ...... C-8 C-4. Primary and Secondary Systems ...... C-4

LIST OF TABLES

Tabk 1Ttk Pao 2-1. Influence of Temperature Differentials on Selection of Pump Sizes for HTW Systems ...... 2-8

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Information Handling Services, 2000 TM 5-810-2/AFR 88"-28 CHAPTER 1 DESIGN CONSIDERATIONS

1-1. PURPOSE 1 % to 3 percent of the total boiler output because This manual provides guidance for the design of frequent blowdowns are required. Uneven firing in high temperature water (HTW) heating systems steam systems results in excessively high stack classified as operating with supply water tempera- losses because of excessive boiler heat transfer and ture above 240 degrees F. and designed to a pres- frequent maladjustments in the combustion air sure rating of 300 psi. supply . Due to the heat storage capacity of HTW systems, short peak loads may be absorbed from 1-2. SCOPE the accumulated heat in the system and uneven This manual presents the unique features of HTW firing is substantially reduced, keeping these losses systems, factors for comparison with other heat to a minimum. This results in higher generator ef- distribution mediums, and criteria to design or ficiencies than the equivalent steam boiler would modify HTWsystems. have since the pressure in a steam boiler drops di- rectly after a change in load requiring an adjusted 1-3. REFERENCES firing rate. HTW has many characteristics which Appendix A contains a list of references used in make substantial savings possible in the operation this manual. and installation costs of properly designed heat 1-4. GENERAL distribution systems. a. Design. The closed recirculation system re- In district and area heating systems, water is gen- duces transmission and thermal losses to a mini- erally circulated at temperatures from 320 to 440 while eliminating corrosion and degrees F., corresponding to a saturated pressure mum practically range from 75 to 367 psig. The usual practical tern- scaling of generators, heat transfer equipment, . of systems perature limit is 440 degrees F. because of pres- and piping Makeup requirements HTW sure limitations on pipe and fittings, equipment, are almost nonexistent, less than '/z of one percent and accessories. HTW systems are similar to the water loss per day of the total contents of the more familiar low temperature hot water systems system. Operation within closed circuits permits but must be carefully designed because of the reducing the size of water treating systems to a rapid rate of pressure rise occurs in hot water over minimum. Both supply and return high tempera- 440 degrees F. Higher pressures increase system ture water piping can be run up or down and at costs as higher pressure rated components are re- various levels to suit the physical conditions of quired. Heat generation equipment will be de- structures and contours of the ground between signed in accordance with ASME Boiler Codes. buildings without the problems of trapping and Compared to a boiler which will generate steam or pumping condensate . Traps and pressure reducing hot water, the high temperature water generator valves, which require substantial maintenance and is specifically designed to keep water in the liquid which are the causes of substantial losses in steam state at high temperatures. The system must be systems, are eliminated. These features simplify maintained at a positive pressure to do this and a both new design and subsequent extensions to ex- uniform flow through the generator must be main- isting systems. Transmission distances do not offer tained at all operating conditions. unacceptable constraints. Steam suffers rather ex- treme pressure and temperature drops during 1-5. ADVANTAGES OF HTW SYSTEMS transmission; high temperature water is much less HTW systems have numerous advantages over affected by such pressure drops for a given pipe steam heat distribution systems. The inherent size. A circulating pump head takes care of pipe losses of a steam system may be saved resulting in resistances. Since the requirement of several tem- possible fuel savings in a HTW system over the perature levels can be met with HTW systems equivalent capacity steam system. The amount of without reducing the pressure of the heating blowdown requiredINACTIVEby a boiler depends on the medium, pressure reducing valves are not needed. amount and nature of the makeup water supplied. Since the water is circulated at generator pressure HTW systems have closed circuits, require little or slightly higher, single-stage low head circula- makeup, therefore, practically never require blow- tion pumps take the place of the high-pressure down whereas steam systems commonly lose about boiler feed pumps required in steam systems. The

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TM 5-810-2/AFR 88-28

heat storage capacity of HTW systems evens out ings. Leakage in HTW systems is limited to the heating load on the generator which gives amounts lost from pump glands and valve packing. higher generator efficiencies because of the elimi- d. Repairs and Maintenance. Maintenance of nation of overfiring and sudden changes in firing traps and reducing valves, a substantial expense in which result in poor combustion and high stack steam systems, is eliminated with HTW systems gas temperatures. Small generator installations which require only the maintenance of valve and are possible due to the more uniform firing rate pump packing to eliminate leakage. As an exam- and the capability of the heat accumulated in the ple, one HTW system, in operation for 15 years system to take peak loads. Precise modulated tem- without water treatment, was found to be free of perature control may be obtained by hand or auto- scale and corrosion, proving substantial reduction matic means. Because the heat transfer character- in maintenance of this type. Steam condensate istics of high temperature water can be more ex- piping, on the other hand, must commonly be re- actly calculated and predicted than those of steam, newed every five ten if correctly designed, high temperature water pro- to years due to high corro- sion caused by oxygen in the . duces more dependable and uniform surface tem- condensate peratures of the heat transfer equipment than e. Safety of Operation. Breaks or leaks in HTW steam can achieve. ASHRAE "HVAC Systems and lines are not nearly as dangerous as they are in Applications Handbook", chapter 15, "Medium and steam lines. One reason for this is the refrigerat- High Temperature Water Heating Systems" will ing action accompanying release of the water as it be consulted for design guidance as well as for ref- expands, which makes it possible to hold the hand erences for specialized applications . within a foot or two of the rupture without being b. Capital Investment. Smaller pipe sizes are burned. The water is cooled further by evaporation used with HTW systems than with steam systems. in the air. Another reason is that the amount of This, together with the 15 to 20 percent smaller saturated water which can pass through an open- generator requirements because of elimination of ing is about one half the amount of cold water condensate return losses, the reduction in size of which would pass through, and less than the the feedwater treatment plant, and the long life of amount of steam which would pass through. the installation, results in a lower capital invest- Therefore, combining these two effects, the ment. Even though the generator may be smaller amount of heat, in Btu's, which would pass than a corresponding steam boiler, the HTW gen- through an opening is from 5 to 10 times as great erator may typically cost more. The analysis will with saturated steam than with saturated water, be based on overall system costs. The cost of heat depending upon the pressures involved, the size of exchangers to convert the heat to lower tempera- the break, and the length and size of the pipe. Any ture and pressure mediums is usually more than high-pressure system, however, whether steam or justified, based on an overall system cost analysis, water, requires experienced operation as well as by the elimination of traps, return condensate good design. Operational risks such as those due to pumps, pressure reducing stations at the heat water hammer must be avoided in the design and using device, as well as the reduction in fuel cost operation of both types of systems. of HTW systems as compared to steam systems of f. Provision for Future Expansion. Future ex- comparable size. pansion should be considered in the initial design c. Operation and Operating Costs. Savings in of any system so that the system can be expanded operating costs are possible in the closed circula- at any time up to the design capacity of the plant tion high temperature water system which returns and the distribution piping. Heating plant and dis- all heat unused by the users or not lost through tribution system capacity may be equally expanda- pipe radiation to the heating plant. This elimi- ble in either system. nates the losses of condensate and the heat in the condensate due to faulty operation of traps and 1-6. PROPERTIES OF HIGH TEMPERATURE leakage in a steam system. Simple methods can be WATER used to determine the heat produced and delivered The properties of low temperature water are fa- to the various buildings and heat users since only miliar to most engineers. It is a fluid with a high the temperatures and flow rates are required to density, high specific heat, low viscosity, and low INACTIVEthermal conductivity, and requires high pressure compute these quantities. Reduction in distribu- tion temperatures to correspond with heat de- to be maintained at high temperature. Because mands and seasonal variations makes possible ad- water is inexpensive and readily available, the un- ditional operational savings. Steady firing of gen- favorable high pressure requirements are counter- erators results in higher efficiency and fuel sav- balanced economically . It is also known that vari-

Information Handling Services, 2000 TM 5-810- 2/AFR 88-28

ations in density, specific heat, viscosity, and con- temperature water accounts for the large heat ac- ductivity with changes in pressure are negligible. cumulation capacity of HTW systems in relatively It is not well known that the properties of water small pipelines. at high temperatures are even more favorable d. Piping Pressure Drops. Comparisons of than those at low temperatures with the main dis- steam and HTW piping pressure drops cannot be advantage being high pressure . As an example, the made without reference to assumed comparable specific heat of water is as high. as 2.0 Btu/lb/ conditions. Pressure drops in high temperature degree F. at pressures of about 160 atmospheres water circuits have only a very minute effect upon and temperature of 660 degrees F. Of greater im- the water temperature and are important only for portance, however, are the properties of water selecting pumping power and pipe sizing. Steam within the temperature range of 300 to 400 de- suffers, in practice, many times the pressure drops grees F. as applied to process and district heating suffered by water, causing a substantial energy systems. The influence of pressure on the proper- loss as well as a temperature ties of high temperature water within this operat- drop. This is due to the large volume of steam and consequent ve- ing range has negligible effect upon its properties . high locities commonly used to transmit heat with mod- The influence of temperature, however, is consid- erable and deserves closer study. Refer to erate pipe sizes. ASHRAE "HVAC Systems and Applications Hand- 1-7. PRESSURIZATION book", chapter 15 for tables of water properties for The maintenance of the proper temperature in the temperatures up to 400 degrees F. and other pub- lished handbooks for the thermal properties of distribution system is a function of the pressure water from 400 degrees F. to 700 degrees F. Note maintained on the system. There are two basic the rapid rise in pressure as the temperature rises pressurization methods employed and an alternate above 400 degrees F. and the increase in specific (hydraulic pressurization) method for standby serv- heat above 240 degrees F. ice. a. Pressure/Temperature Relations. As temper- a. Steam Pressurized-The steam pressurization ature rises, the pressure rises rapidly causing the method utilizes an expansion vessel separate from economic pressure limit to be reached at 450 de- and downstream of the HTW generators. In this grees F. or below for most applications. Beyond vessel HTW is allowed to flash into steam to pro- this point the cost of equipment and piping is pro- vide a cushion to take care of expansion of water hibitive thus eliminating the savings in using in the piping system. The selected steam pressure HTW. determines the temperature of water in the expan- b. Density. This property is very important sion vessel which is then utilized to supply the dis- since it reflects the expansion and contraction of tribution system pumps. The selected saturation water in a system with temperature changes and pressure is chosen with due consideration of re- thereby determines the size of the expansion ves- quired HTW delivery temperature and with proper sels required in hot water systems. Between 340 allowances for system heat losses . The expansion and 450 degrees F., the volume of water in the vessel must be located above the HTW generator HTW system increases from 10 to 18 percent above outlets. This method should not be used for a new that at 70 degrees F. An expansion vessel able to system or for system upgrades because of the lack store this additional volume is required when the of sufficient extra pressure (above the saturation system is brought up to maximum temperature. pressure of the liquid) needed to prevent flashing Two expansion tanks are recommended, each sized under all operating conditions . This method may for 50 percent of the total capacity of the operat- only be used for a base loaded plant where the ing system plus the additional expansion volume . steam demands are relatively constant. This will facilitate required periodic inspection of b. Inert Gas Pressurization. This method also either tank without causing whole system shut- utilizes an expansion vessel separate from and con- down. nected to the system return water header. An c. Relative Heat ofSteam and High Temperature inert gas cushion is maintained within the expan- Water. A comparison of the heat contained in a sion vessel. Nitrogen is usually utilized as the cubic foot of water going through a 150 degree F. inert gas because it is inexpensive and widely or other valueINACTIVEused in design, with the latent available. Air must not be used as it contributes to drop, heat of steam at utilization temperature shows corrosion of the system. Pressure is usually main- that the HTW contains much more heat per cubic tained at 40 to 60 psi above saturation tempera- foot than does the steam . This property of high ture of the distribution system. In determining the

Information Handling Services, 2000 TM 5-810-2/AFR 88-28

minimum starting pressure, a 10 degrees F. safety 1-8. WATER CIRCULATION factor should be added to the HTW delivery tem- Water is circulated by either a single or dual cy- perature which is the governing factor . Pressure is cling method. The single cycling method uses one maintained by automatic control independently of set of pumps to circulate water through both the the heating load. The expansion vessel can be lo- HTW generator and the distribution system using cated on the operating floor of the heating plant. a bypass control valve to regulate flow through the c. Hydraulic Pressurization Method. The hy- HTW generator. The dual cycling method uses two draulic system consists of a pressurizing pump sets of pumps, one to circulate water through the with a regulator valve which continuously by- HTW generators and a separate set of pumps to passes pump discharge water to a makeup storage circulate water through the distribution system. tank and injects water into the system to maintain Combinations of these methods result in four basic the desired pressure. This system may be utilized types of water circulation: for smaller systems only or included as a standby system to keep the HTW system operational when a. Inert Gas-Pressurized Single Circulation the system expansion tank is out of service for in- Method. A single circulation system, also called a spection and maintenance. one-pump system, is shown in figure 1-1 .

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Information Handling Services, 2000 HTW Supply

By-Pass Control Control Line i INACTIVEValve

HTW Return

Expansion Vessel

t HTW Generator t t

Relief Valve LL--- i Flow Generator Meter Pump rW1 Makeup t Feed Water r Drain

U .S . Army Corps of Engineers

Figure 1-1. Inert Gas-Premurired Single Circulation Method. TM 5-810-2/AFR 88-28

b. Inert Gas-Pressurized, Dual Circulation Method. A dual circulation system, also called two-pump system, is shown in figure 1-2.

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Expansion Vessel

inert Gas Supply w to w

0 w 0 ti ti .n .c

Drain

U .S . Army Corps of Engineers

fun 1-Y Inert Cor"Prasariud Dual Cuculation Method. °°

is *a TM 5-810-2/AFR 88-28

c. Steam-Pressurized, Single Circulation Method. A single circulation system, also called one-pump system, is shown in figure 1-S.

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Information Handling Services, 2000 INACTIVE Drain

m1 HTW Generator

w En r Feed HTW Supply Water 02 Lq 50 Makeup ' 0 Feed Water w a Pump N By--Pass Control w 4 a Flow Control Valve 1-11 -1 Meter ! Line w

HTW Return y

P U .S . Army Corps of Engineers

Figure 1-9. Sfeom-Prmwind Single Circulation Method ao

00N TM 5-810-2/AFR 88-28

d. Steam-Pressurized, Dual Circulation Method. A dual circulation system, also called two-pump system, is shown in figure 1-4.

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1-10

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INACTIVEExpansion Vessel Temperature Controller

HTW Supply w

to m

0 02 w 0 w w

0 w

Makeup Generator Pump Feed Water ,-_r --I -0-D4.. "'--)) i_ 1

U .S . Army Corps of Engineers

6Fgurr 1-1- Steam-PrmurisodDual Ci-ulation .Method r r

TM 5-810-2/AFR 88-28

1-9. HTW GENERATORS (1) Unequal heat distribution with no real There are three basic types of HTW generators. control of water circulation. However, only the water tube controlled forced cir- (2) Tendency to make steam. culation once through type HTW generator is suit- (3) Thermal shock is encountered even with able for all fuels and is recommended for all sys- internal distribution tubes. tems. Generators must be specifically designed for (9) Unequal expansion of tubes results in ex- HTW service and no attempt should be made to cessive maintenance when operated HTW field-adapt a steam boiler for HTW service. at tem- The peratures. natural circulation HTW generator and the fire tube Scotch marine generator should not be used 1-10. DESIGN AND SELECTION PROCEDURE for a new system for the reasons cited in the fol- Planning for a new heating system should be a lowing paragraphs. Use of the water tube natural systematic method that in addition to HTW also circulation HTW generator or the fire tube Scotch includes consideration of alternative heating sys- marine generator must be approved by the using tems such as steam as the heating medium . agency. a. System Design. Loads for various heat using a. Water Tube Controlled Forced Circulation devices at the facility will be determined taking HTW Generator. The water tube controlled forced into consideration future expansion or a possible circulation type is primarily designed for high future change in the mission of the facility. This temperatures and high rate of heat transfer. can be done by various approximating or estimat- Water is strained and metered by orifice/strainers ing procedures. In some instances the actual loads to each tube circuit to accommodate the heat ab- for existing facilities are available. On a copy of sorbing capacity of the circuit. The generator is de- the master plan, indicate demands of various heat signed for low waterside pressure drop and vapor users. The thermal center of demand will be deter- binding will not occur under any operating condi- mined to see if it is compatible with the plan for tion since all tube circuits are vented to the outlet the location of the central heating plant. If not, headers. All circuits are drainable. The counter select a site that is compatible with the master flow of gas and water assures maximum efficiency plan. Zones of distribution from the heating plant (better heat transfer for a set of given conditions). will be developed. Make every attempt to have bal- The undesirable features are: external pumping is anced loads in each zone if possible . Using pipe required and chemical control of the water in the sizing tables suitable for water above 300 degrees system must be closely maintained . However, good F., approximate main and branch sizes. water treatment and proper operation and mainte- b. System Selection. Review chapter 2 of this nance usually avoid tube failure. In small sizes up manual and also TM 5-810-1 and approximate the to 10 mega Btu/hr output, a single-pass continuous equipment and the heating plant configuration for water tube type is available. HTW and for steam. At this point it is not neces- b. Water Tube Natural Circulation HTW Gener- sary to develop highly refined calculations or se- ator. This type is available from most generator lections . Define selections to the extent necessary manufacturers in a wide range of sizes; there are to meet basic requirements for developing the proven designs for steam operation; and a large system. Fuel for generators will be selected in ac- group of operators experienced in their use. The cordance with current DOD policy and agency or undesirable features are: the unequal heat distri- service directives and criteria . bution which is overcome by water mass which (1) Small Systems. HTW central heating sys- permits no real control of circulation; the large tems with an estimated total capacity ranging drums and headers provided for steaming result in from 10,000,000 to 50,000,000 Btu/hr will be de areas in which water is not exposed to high tem- signed as a single circulation system also known perature and, therefore, internal turbulence exists ; as a one-pump system, with combined pumps as the large size of the unit; and the fact that the use shown in figure 1-5 if system is an existing base of steaming boilers is, at best, a compromise loaded, steam pressurized system, or figure 1-7 if whether water is taken from below the water line an inert gas pressurized system. In these systems or obtained from cascades. pumps take suction from the expansion vessel c. Fire Tube and ScotchINACTIVEMarine. This generator (also called expansion drum) and deliver water to is low in cost and the complete packaged unit is the system and through the generators to the ex- available as a shelf item in smaller sizes but pansion drum. These plants are relatively small in should not be used because of the following unde- size and will be designed as simply and as trouble- sirable features: free as possible.

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TM 5-810-2/AFR 88-28 (2) Medium Size Systems. HTW central heat- the system will investigate current trends in ing systems larger than 50,000,000 Btu/hr but not design and equipment before selecting the system. over 120,000,000 Btu/hr will be designed as dual Typical basic flow diagrams for the systems are circulation systems with separate generator recir- shown on figures 1-5, 1-6, 1-7, and 1-8. Figure 1-9 culating and system circulating pumps. This illustrates inert gas-pressurization using variable system is also known as a two-pump system. Pres- gas quantity with gas recovery. The total heating surization will be inert gas pressurized. system plan must be reviewed to determine and (8) Large Size Systems. HTW central heating develop costs for incremental development based systems larger than 120,000,000 Btu/hr will be de- upon proposed development phases of the master signed the same as those over 50,000,000 and will plan. Tabulate the system plan by development be inert gas pressurized. phase and determine costs using current costing (4) Final Selection. The engineer designing procedures.

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Information Handling Services, 2000 TM 5-810-2/AFB 88-28

Reprinted with permission from 1987 ASHRAE Handbook HVAC Systems and Applications

Figure 1-5. Flow Diagram Steam Pnmsurized Single Circulation System. INACTIVE

1-14

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TM 5-810-2/AFR 88-28

Reprinted with permission from 1987 ASHRAE Handbook HVAC Systems and Applications

Figure 1-t". Flow Diagram Steam Pressurized Dual Circulation System . INACTIVE

1-15

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TM 5-810-2/AFR 88-28

BURNER CUT-OFF SWITCH IN HTW GENERATOR HTW GENERATOR SERIES WITH LOW FLAW SWITCH e ORIFICE PLATE

SAFETY VALVE N2 PRESS INDICATING FLOW VENT REGULATOR METER WITH LAW CUT-OUT SWITCH DISCHARGE EXPANSION N TO WASTE TANK 2 STORAGE VENT

IGH LEVEL ALARM

EXPANSION RANGE WATER SUPPLY LOW LEV PUMP START METER LOW LEV CUT OFF MAKE-UP TANK DUMP VALVE ACTIVATED BY HIGH LEVEL SWITCH

DUPLEX MAKE-UP PUMPS SYSTEM PUMPS

SYSTEM EXPANSION LINE

THERMAL ISOLATION LOOP RETURN

SYSTEM FLOW SYSTEM BY-PASS BALANCE OPERATED BY VALVE - rmloNo.. LOW FLOW SIGNAL INACTIVE1SUPPLY U.S. Army Corps of Engineers

Figure 1-7. Flow Diagram Inert Gas Pressurized Single Circulation System.

1-16

Information Handling Services, 2000 TM 5-810-2/AFR 88-28

SYSTEM CIRC PUMPS

U.S. Army Corps of Engineers INACTIVEFigure 1-8. Flow Diagram Inert Gas Pressurized Dual Circulation System.

1-17

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TM 5-810-2/AFR 88-28

SUCTION SIDE Of MAIN CIRCULATING PUMPS

RELIEF VALVE OS -NIGH AND LOW MESSUREGAUGE PRESSURE ALARM PRESSURE CONTROLLER 6O- WATER LEVEL LOW PRESSURE CONTROL

Reprinted with permission from 1987 ASHRAE Handbook HVAC Systems and Applications

Figure I-9. Inert Gas-Pressurization Using Variable Gas quantity with Gas Recovery.

1-11. ECONOMIC JUSTIFICATION b. Central Heating Plants. Central heating Heating systems for all installations will be de- plants are justified when the total life cycle costs signed for lowest overall initial and operating costs of central heating plants with connecting distribu- for the life of the facility. tion systems for groups of two or more independ- a. Economic Analysis. The selection of one par- ent buildings (to be built simultaneously or within ticular type of design for a heating plant, when a period of years) are less than totals for individ- two or more types of design are known to be feasi- ual heating plants which would provide the same ble, must be based on the results of an economic service. However, this comparison does not apply study in accordance with the requirements of ap- when steam-electric power plants are involved and plicable criteria . The results of all studies are to the overall cost of providing heat from extraction be included in the design analysis documentation INACTIVEsteam would be less than either of the above meth- for the project. Clarification of the basic criteria ods. Central plants will have their own enclosures, for a particular design application in the Military Construction Program may be obtained by request but when economically justified, small plants may to HQ USACE (CEMP-ET), Washington, DC be located in one of the buildings of the facilities 20332-1000 . they serve.

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c. Individual Heating Plants. These plants (b) When operating and maintenance costs may be in the buildings they serve or in separate of an entire HTW system are less than those of a buildings when economically justified. Such plants steam system. shall be considered in preference to central heat- ing plants under the following conditions: (c) When costs of excavation, makeup water, and heat losses are appreciably reduced by using a (1) When total life cycle costs of individual HTW distribution system. heating plants are less than the costs of a central heating plant with connecting distribution piping (d) When the pressure and temperature re- to buildings receiving heat. quirements of heat using equipment may be satis- (2) When installation and maintenance costs fied more economically by HTW distribution. for constructing an extension of an existing distri- (e) When needed expansion of existing bution system from a central plant to an isolated steam or low temperature water systems are more building are not economically justified. costly than installing a new HTW system. (3) When dispersal of facilities and continuity (2) A steam central heating plant will be se- of services are so essential that disruption of serv- lected under the following conditions: ice to a central heating plant or its distribution piping cannot be tolerated. (a) When a HTW system cannot be justified on the basis of the analysis above. (4) When only a single building is involved without prospects of adding buildings in the (b) When fluid pressures and temperatures future. required by equipment cannot be provided by a d. HTW Versus Steam Heating Plants. HTW system. (1) A HTW central heating plant will be se- (c) When the engineering design required lected in preference to a steam plant under the fol- for a HTW system and its equipment is not avail- lowing conditions: able. (a) When the thermal storage capacity of an (d) When rehabilitation of an existing steam entire HTW system results in less cost in equip- heating plant and distribution system is more eco- ment such as boilers, pumps, and piping. nomical overall.

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TM 5-810-2/AFR 88-28 CHAPTER 2 LOAD CHARACTERISTICS AND CALCULATIONS

2-1. HTW REQUIREMENTS separated as (a) hot water requirements, (b) low- This chapter deals generally with the load charac- pressure steam requirements, and (c) high-pressure teristics and calculations for load summaries for steam requirements . Laundries are generally con- HTW systems for area or district heating. Area or sidered to require high-pressure steam at 75 to 100 district heating involves the distribution of heat to psig and low-pressure steam at 5 to 15 psig. Before space heaters and various equipment for process the pressure ordinarily prescribed for steam laun- heating. The various types of heat users may be dry equipment is accepted as the basis for designs separated into these two categories: space heating in high temperature water systems, careful consid- loads which are subject to variations in weather eration should be given to the actual equipment conditions and process heating loads which are needs. Frequently, if the steam is generated in usually steady. The general features of these appli- high temperature water converters, somewhat cations are discussed in this chapter. Equipment lower pressures can be used without sacrifice of commonly used is described in a later chapter. performance. It is not economical to produce all of the steam needed in a laundry at the highest pres- 2-2. SPACE HEATING sure in one central steam generating converter Space heating is usually provided by indirect heat- using high temperature water as this will result in ing using secondary heating medium such as unnecessarily high return temperatures in the steam or hot water. High temperature may be high temperature water distribution system. used as a direct heating medium only in limited Therefore, separate steam generating converters situations and only where close temperature con- are used for high and low pressure steam. Low- trol is not a requirement. pressure steam from 5 to 15 psig can be produced 2-3. PROCESS HEATING in a low-pressure steam generating converter. If This term applies to all forms of heating other space heating steam at 5 psig is also required, a than space heating including domestic hot water; combination may be made producing 10 prig steam steam and hot water for kitchens, laundries, and for both purposes . However, such a combination hospitals; and steam cleaning and snow melting with 30 prig steam would certainly be impractical. equipment. High-pressure steam requirements can be met a. Direct Process Heating. Direct use of high either by using high temperature water directly, temperature water is the most economical use of provided that the equipment is designed for it, or heat transmission for air and roller driers and by producing 75 to 100 psig steam in a separate washing equipment in laundries; for washing high-pressure steam generating converter. The equipment in kitchens; and for sterilizers and high-pressure steam requirements are never a other equipment in hospitals. Equipment for direct large fraction of the total heat load for a laundry. application will be of special designs and materials The steam generator must be equipped with an and not standard designs of manufacturers, automatic condensate return system and makeup b. Indirect Process Heating. water system as well as adequate controls to limit the pressure of the steam when the maximum (1) Domestic Hot Water. Domestic hot water reached. Hot water require- is required for showers, lavatories, bathrooms, and steam pressure is be met, ordinarily, by a storage water kitchens. It can be produced by high temperature ments will similar domestic water heater. water coils inserted in the lower part of the do- heater to a mestic water storage tank. Generally domestic hot (3) Kitchens. Kitchens require hot water at water temperatures up to 140 degrees F. are rec- 140 degrees F. for dishwashing which may be ob- ommended . Control of the quantity of high temper- tained from a domestic water heater described ature water flowing through the coils is main- above. Water at 180 degrees F. required for mess tained by an elementINACTIVEsensitive to the temperature and diet kitchen areas for final rinsing will be ob- of the water in the storage tank. tained from booster heaters located in the service (2) Laundries. To keep the high temperature areas. Steam at 40 to 80 prig is required in the water return temperature at a minimum, it is sug- kitchen for cooking. The high temperature water gested that the heat requirements for laundries be is used, in this case, to generate steam at the re-

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TM 5-810-2/AFR 88-28

quired pressures. Steam production will be similar fully diversified to get the most economical instal- to that discussed above. lation. (4) Hospitals. Hospitals require domestic hot a. Heating Loads. A diversity factor of 80 per- water for baths, lavatories, showers, sinks, and for cent is usually used for heating loads. This factor other uses. In addition, steam is needed for steriliz may be lowered to 70 to 75 percent in systems uti- ers and autoclaves at 40 to 80 psig. A high temper- lizing all automatic control for heating. The factor ature water system has the advantage that heat may also be adjusted after a review of the system can be taken to any number of heat exchangers lo- indicates need for both automatically controlled cated adjacent to the sterilizers in different parts heating and nonautomatically controlled heating. of several hospital buildings without needing b. Process Loads. For process loads encoun- steam traps and condensate return. tered for only brief periods of time, such as for hot (5) Steam Cleaning. Steam cleaning equip- water and process steam, a diversity factor of 65 ment, sometimes required for cleaning airplanes percent is usually used. The use point must be by ejecting water using or machinery, operates analyzed for demand characteristics, and the di- In an HTW system this high-pressure steam. versity factor adjusted upward with longer or of high-pressure method would require production more continuous demands. steam by heat exchangers . It is advisable to employ a high-pressure circulation pump which 2-5. OPERATING TEMPERATURES AND can supply water heated to the desired tempera- PRESSURES thereby accomplishing ture in a heat exchanger, Temperatures and pressures used in HTW systems the same purpose as conventional steam cleaning depend upon the nature of the application. The equipment utilizing high-pressure water. factors determining the flow temperatures are the (6) Snow Melting. Snow melting may use highest temperature needed in the system for high temperature water as the primary heat carri- heating or process requirements and the length of er. The secondary heat carrier can receive its heat the distribution system . If the length of the distri- through a converter and may be conventional low bution system exceeds six miles, it is frequently temperature water of 150 to 200 degrees F. or, advisable to maintain a flow temperature substan- preferably, a suitable heat transferring fluid such tially greater than that needed so that the pipe as Glycol or high temperature water. Both low and size may be reduced. Pressures in the system high temperature water should contain antifreeze depend upon the temperatures required and at all such as Glycol when used for this duty. If high times are maintained higher than the saturation temperature water is used as the secondary heat pressure corresponding to the water temperature. carrier, it should be heated to temperatures of 300 a. Supply and Return Temperature. A supply to 350 degrees F. These elevated temperatures temperature limit of 440 degrees F. is generally permit spacing of the lines in snow melting coils found to be the economic limit for space and proc- at 3 to 5 feet on centers, a design especially adapt- ess heating because of the high pressures (400 psig) able for large aprons, runways, and other areas required. Higher supply temperatures require rap- where an expansive surface justifies application of idly increasing pressures throughout the system, high temperatures . and while high pressures result in higher heat car- 2-4. DIVERSITY FACTORS rying capacities in smaller pipe sizes, the saving in offset by the expense On any system serving more than one point of use, pipe size is partially or fully and heat exchang- the possibility of all use points requiring maxi- of generators, fittings, pipelines higher pressures. mum heat input is almost nonexistent. Therefore, ers strong enough to withstand diversity factors are applied to the demand loads. Return water temperatures should be between 250 Each individual use point load is not diversified, degrees F. to 275 degrees F. minimum. Primary content but total system loads are. When a system is being fuels or alternate fuels with high sulfur corrode. The selection designed with plans for large numbers of future can cause generator tubes to buildings, it is advisable to use diversity factors of temperatures in the hot water generator during special only for equipment sizing. Piping in the distribu- all operating conditions should be given content fuels. tion system shouldINACTIVEbe designed for undiversified considerations with high sulfur point, the conditions to allow for unscheduled future addi- When flue gas is cooled below the dew corrosion. tions. This allows greatest flexibility in the piping. gas side of boiler tubes is subjected to flue gas When a system is initially fixed with minimum Corrosion is caused by moisture in the combination of future modifications or additions anticipated, then and acids which result from the the distribution system piping will be designed condensed moisture and sulfur compounds in the

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TM 5-810-2/AFR 88-28

flue gas. The higher the percent by weight of Pressure head to overcome frictional and other sulfur the higher the minimum metal temperature losses must be added to the operating pressure must be to prevent this type of corrosion. When it when selecting pumps, piping, fittings, and other is desired to transfer very large heat quantities system components. The pumps should never be over unusually long distances, an increase in the used as part of the pressurization system. generator pressure to 600 psig corresponding to a supply temperature of about 480 degrees F. might be justified. For distances up to six miles, the high- Table 2-l. Influence ofTemperature Differentials on Selection est supply temperature will be selected below 440 of Pump Sizes for HTW Systems. degrees F. The generator supply temperature is Temperature always maintained somewhat above the tempera- Difference (deg . ture of the water distributed to the system. In F.) ...... 20 50 100 150 200 steam pressurized systems this is necessary to pre- Discharge vent vaporization from taking place if the pumps temperature should (deg . F.) ...... 270 300 350 400 450 suddenly be stopped and where the heat Return fusers are located at elevations higher than the temperature generator. The HTW system supply temperature is (deg . F.) ...... 250 250 250 250 250 maintained by blending the generator output Mean Temperature water with the system return water at the pump (deg. F.)...... 260 275 300 375 350 suction header. The HTW generator Flow rate per 20 water is pro- Mega Btu/Hr (M portioned through an automatic mixing valve. Due lbs/hr) ...... 1,000 400 200 133 100 to heat losses from the distribution system piping, Density of the supply temperature at the use points will be Returning Water reduced by approximately three to five degrees per (lbs/gal) ...... 7.86 7.86 7 .86 7 .86 7 .86 mile, depending upon the Pump Capacity insulation efficiency and (GPM) the ...... 2,091 840 421 283 213 amount of water being circulated. The faster Assumed pump the water is circulated, the less the temperature head (ft) ...... 100 100 100 100 100 drops. Process equipment using either direct high Pump HP required temperature water or steam produced by convert- (HP) ...... 82.7 32.9 16.3 10.8 7.9 ers generally requires temperatures at the use Pump efficiency points ranging from 250 to 400 degrees W...... 60 60 60 60 60 F. For ex- Pump suction size ample, some laundry drying equipment requires (not pipe size)...... 8' 6' 4' 3`/a' 3' steam pressures of 100 psig which necessitates high temperature water of more than 355 degrees F. Therefore a heating installation which includes such laundry equipment will need a supply tem- 2-6. MAXIMUM INITIAL LOAD perature of at least 375 degrees F. to provide for The maximum initial load is the sum of heating, temperature losses and use point requirements process, distribution loss, and plant auxiliary and a generator pressure of no less than 175 to 200 loads, all suitably diversified. Maximum summer psig. It often happens that while temperatures as load is also determined so equipment may be se- high as these are not needed for space heating, lected for ultimate loads which are as compatible they are economical supply temperatures to use with this load as possible. If not compatible, then for lower temperature applications because small- special equipment for summer load only may be er pipe sizes are required. All HTW systems will selected . be designed for a minimum of 150 degrees F. dif- ferential between supply and return water tem- 2-7. MAXIMUM ULTIMATE LOAD peratures. See table 2-1 for influence of various temperature differentials on system components; Maximum ultimate load is the total of estimated compare 100 degrees F. with 150 degrees F. and future loads added to the maximum initial load. note the difference in pump horsepower and pipe sizes as an exampleINACTIVE. 2-8. ESSENTIAL LOAD b. Operating Pressure. This pressure is directly The essential load is the diversified load on all related to the operating temperature which is es- buildings where no cutback can be tolerated plus sentially equal to saturation temperature in the the minimum permissible loads where cutback can expansion drum in a steam-pressurized system and be tolerated plus additional minimum heating slightly higher in an inert gas-pressurized system. loads to avoid freezing.

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TM 5-810-2/AFR 88-28

2-9. MATCHING PLANT CAPACITY TO LOAD matically when the demand increases. Avoid fully a. Maximum Initial Load Generation. Where automatic generator operation of the entire plant. possible, size the initial heating plant for at least Plants over 30,000,000 Btu/hr will have a continu- three generators of equal size. The maximum con- ous watch. tinuous capacity of the plant with one generator (4) Use more but smaller generators to lower down will not be less than the essential plant load, the plant's minimum capacity. and the total maximum continuous capacity of all (5) Provide minimum load generators. Where generators will not be less than the maximum ini- the gap between a plant's minimum load and a tial plant load. Where the ultimate plant load is plant's minimum capacity when using the main known or may be estimated accurately, and where generators is very large, a small, fully automatic, the construction program indicates plant expan- packaged generator unit with its own circulating sion will be required within three years of the pumps may be used to fill the gap. startup of the initial plant, then the ultimate load shall be weighed carefully as the basis for select- (6) Consideration should be given to the fact ing the capacities of the initial generators. Under that the peak operating efficiency of a HTW unit no condition shall the flow through any one gener- is at approximately 80 percent of design capacity. ator be less than 100 percent of its designed capac- At 80 percent load, the operators should be start- ity. ing to think about bringing an additional unit on (1) Avoid installation of initial main genera- line and splitting the load. Typical operating effi- tors of capacities radically smaller than those to ciency curves are fairly flat from 50 percent to full be added when the plant is expanded to the ulti loads. mate size. Also avoid an unnecessarily large (7) The best control philosophy is to employ a number of generators of size equal to the size of fully modulating burner with a turndown capabil- the initial generators. ity of at least 8 to 1. The unit can modulate over (2) Include one small "summer load" genera- this range and be set up for automatic recycle tor where increased efficiency at low loads eco- after load decays below the maximum turndown. nomically justifies its installation . The simplest control utilizes single point position- ing jack for fuel and air. This method pre- (3) The selection of oversize generators for ini- shaft dominates the industry and eliminates many of tial installations will be submitted to HQ USACE the operational problems associated with more so- (CEMP-ET), Washington, DC 20314-1000 or HQ USAF/CECE, Washington, DC 20332-5000, with phisticated systems as well as the requirement'for supporting data, for approval before proceeding more experienced operators. with final design. 2-10. SYSTEM HEAT LOSS (4) For plants over 50,000,000 Btu/hr, size the System heat loss is the heat loss from the distribu- ultimate heating plant for three more genera- or tion system and is dependent upon the length of tors such that down, when one generator is the re lines, ground water conditions, and type of conduit maining generators shall carry the essential load. and insulation. It is recommended that a factor of b. Minimum Load Generation. Choose a means 5 percent be applied to the diversified peak load. of bridging the gap between minimum and maxi- mum loads to suit job conditions from the follow- 2-11. FLYWHEEL FACTOR ing design possibilities. Flywheel factor or storage effect of the system is (1) Establish operating ranges of combustion another consideration applied when selecting control. equipment for system sizing. Because of the (2) Provide manual operation at minimum volume of heated water, there is a considerable load. The turndown range of burners, or one of a heat storage which can be considered available at number of burners on a generator, must include peak design conditions . The accepted factor ap- the minimum load. By changing burner tips in oil plied to the peak load is 85 percent. firing, very low minimum loads may be obtained. 2-12. CALCULATIONS (3) Establish intermittent operations for plants 30,000,000 Btu/hrINACTIVEor smaller, which will For the purpose of illustration of typical calcula- ex- preclude a continuous watch or allow a single op- tions for a high temperature water system, an B. erator to leave a generator room for trouble calls ample is given in appendix on other matters by providing an auxiliary, fully a. Calculations of Temperatures and Flow Quan- automatic packaged generator which will shut off tities. The heat required by the using equipment on satisfaction of heat demand and restart auto- is usually the starting point in calculating the

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TM 5-810-2/AFR 88-28

return temperatures and the required flow quanti- Qt=w. cp (T.-T,.) (eq 2-7) ties in the system. It may be expressed using equa- Combining, we get: tions which give results in pound per hour fol- as T lows: r =(w,T,+w2T2+w2T3)/w, . (eq 2-8) The calculations above will supply an average tem- Qi=w, (h.-h,)=w, cp (T.-T,). (eq 2-1) perature condition for a system with a variety of WHERE: is the heat used by equipment Q, X, ex- loads, i.e., heating, domestic, and process. If the pressed in Btu/hr; undiversified. system is essentially heating (80 percent or great- w, is the flow rate, expressed in lb/hr, passing er), then T.-Tr equal to 150 degrees F. design drop through equipment X. may be used to simplify calculations without intro- h. is the enthalpy of the supply to equipment ducing an unacceptable amount of overdesign. X; expressed in Btu/lb . b. Pump Selection. For a single circulation h, is the enthalpy of the return from equip- system, (or a one-pump system), minimum flow ca- ment X; expressed in Btu/lb . pacity of any pump is based on the essential load, T, is the supply temperature; expressed in de- and pump head requirement is the resistance grees F. through the HTW circulation system added to the flow T, is the return temperature from equipment resistance through the HTW piping system. X. Total pumping capacity is determined by combin- ing the maximum initial load with the HTW circu- cp is the specific heat at constant pressure in lation load. For a dual circulation system, (or a Btu/1b.F two-pump system), minimum pump capacity is T, is determined by using equipment require- based on the future anticipated load added to the ments, i.e., assume heating with 220 degrees F. low maximum initial load. Multiple pumps are used to temperature water, then with 20 degrees F. ap- provide the flow needed for the essential load on proach on convertor 220+20=240 degrees each boiler. If there is no substantial anticipated F.=220+20=240 degrees F.=T,. Considering a load, the essential load can be utilized for sizing system with three heat using devices, use the fol- the generator circulation circuit. In any event, the lowing equations: (the subscripts identify the con- minimum flow in this circuit must always be at sumer under consideration). least equal to the essential requirements. Q=w, cp (T.-T,) (eq-2-2) c. Expansion Vessel Design . Graphical repre- Q,=w2 cp (T,-T2) (eq-2-3) sentation of volumes required for calculations are Q,=wa cp (T.-Ta) (eq-2-4) illustrated in figure 2-1. Figure 2-1 illustrates a Qt=cp [(w,+w2+waXTd-w1T,-w2T2-wsT3] circular tank section as for a horizontal tank, how- ever, the expansion tank may be horizontal or ver- Qt=cP [(w.XTd-w,T,-w2T2-w2T2] (eq 2-5) tical. For small systems between 1,000,000 and Since w.=(w,+w2+w2), the sum of the flows 10,000,000 Btu/hr, it is practical to size the expan- through all three , heat users equals the total flow sion vessel for the total water expansion from the through the system. Qt is the total of amount undi- initial fill temperature. For larger systems water versified heat supplied to the system by the gener- expansion is based upon operating conditions and ator and is equal to the sum of the heat supplied not on startup condition. When placing the system to all consumers, or: in operation, it is necessary to bleed off the Qt=Q,+Q+Q9 (eq 2-6) volume of water because of expansion from the ini- The return temperature, Tr, may be found from tial starting temperature to the lowest operating the above equations and the expression: temperature. INACTIVE

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U .S . Army Corps of Engineers

Figure 2-1. Expansion Tank Volumes.

(1) System Expansion Volume. Water temper- VIr,=specific volume at maximum return tem- ature is assumed to vary approximately 10 percent perature . under normal operating conditions. The tempera Vfr2=specific volume at minimum return tem- ture changes in supply and return lines are gener- perature . ally computed separately and the results combined (This temperature is Tr less the 10 percent AT to obtain the total system expansion volume. found for Vf1 and Vf2). Where: Vf,=specific volume of supply piping Ar=percent change in return system piping. water at supplyINACTIVEtemperature. =(Vfrt-Ir2) / VIr2Xl00 (eq 2-10) Vf2 =specific volume of supply piping water at Then: Total expansion volume= V, temperature of 10 percent less. =A.+AVr/zooxsystem volume . (eq 2-11) AVs=percent volume change in supply piping (2) Steam Expansion Vessel Design Data. The =(Vf, =Vf2) / Vf2X 100 (eq. 2-9) minimum volume to be provided in a steam-pres-

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TM 5-810-2/AFB 88-28

surized expansion vessel is the sum of the system sorption by water. Gas wastage will affect operat- liquid expansion volume, a reserve volume for 1/z- ing costs. The gas recovery system as shown in minute pumping capacity, and a pressurization al- figure 1-9 should be analyzed on the economics of lowance of 20 percent of the sum of the two vol- each application. It is generally more applicable to umes above, or as shown on figure 2-1 for a hori- larger systems, as shown in figure 1-8. The sim- zontal tank: plest gas pressurization system has a fixed quanti- V, =system expansion. ty of gas in the expansion tank to accommodate V2= 1/2-minute reserve for pumps based on the the change in water volume within the tank. As pump capacities outlined previously. the water temperature increases the expansion of water into the tank raises the pressure of the V9=pressurization space=0.2 (VI +Vg). system and the system pressure (ASHRAE states that 20 percent of the sum of decreases as the water V, +V2 is a reasonable allowance.) temperature of the system drops. The pres- sure is allowed to vary a minimum value above Then: Minimum expansion tank volume saturation pressure to a maximum determined to = V, +V,+Vs+allowance for level controls + be within pressure range of the piping equipment sludge allowance. (eq 2-12) and HTW generators. Referring to figure 2-1 and The allowance for level controls should = (tank di- assuming a vertical tank, the minimum total ameter) (length) (1-ft. depth), or 150 cubic feet volume required in the expansion tank is nominal allowance should be used to determine V, + Vs + V,9 + sludge allowance (eq 2-13) tank volume and diameter. Then this volume should be checked against tank diameter selected . Where: V, = a volume required for water expan- The sludge allowance should be at least equal to sion. the bottom 6 inches of the tank, but the require- Vs= a volume for a 1/2-minute pumping re- ment can vary depending on the size of the system serve. and HTW generator capacities. ASHRAE states V9 = the pressurization space. Select an oper- that 40 percent of V, is a reasonable value. Tank ating range to keep system pressures within diameters over 8 feet are uneconomical because of pressure rating of valves and fittings and com- the shell thickness required to withstand the oper- pute V,. The nominal operation pressure ating pressures and they are difficult and costly to should be a minimum of the saturation pres- fabricate. Bottom of tanks must be located above sure for the HTW generator plus a differential the high point of the HTW generators. System will pressure (AP,). This differential pressure is be designed that a single expansion vessel will usually taken as 40 psi. Since the return meet the operational requirements of the system. water temperature will vary, some pressures An exception may be a system which includes a will be lower and some higher than this as- large process load requiring continuous operation. sumed nominal operating pressure. Assume a In such a situation two tanks might be needed to differential operating pressure range (AP2). allow isolation of one tank to periodically inspect Add 0.75 APQ to the nominal operation pres- for development of stress cracks without total sure to obtain the maximum operating pres- system shutdown . Steam-pressurized systems usu- sure P,9 and subtract 0.25 AP, from the nomi- ally require horizontal tanks since it is necessary nal operation pressure to obtain a minimum that the tank bottom be higher than the hot water operating pressure, P,. generator outlet. To size the expansion tank, select a convenient (3) Inert Gas Pressurized Expansion Vessel diameter and compute the required tank A single Design. expansion tank is preferred as height as follows: the most economical installation . A system with large process heating, which requires some contin- Let: Vu = volume in cubic feet per foot of tank ous operation, might require two tanks each sized height = wD'/4 for 50 percent of total system expansion to allow where D = diameter of tank in feet. for one tank to be periodically inspected without Vertical displacement for V, - V,/Vu total system . A arrangement shutdown two-tank is Expansion displacement: V., can be deter- shown in figure 1-8. When multiple tanks are mined from the relationship V, minimum = used, nitrogen andINACTIVEwater equalization lines are re- P,V,/(Pp-P,) quired between the two tanks. The tank will con- Displacement = V,/Vy tain a gas pressurization space and should be a . vertical tank to reduce the contact area between Total tank height is the sum of the above comput- the gas and water and thereby minimize gas ab- ed vertical displacements plus an allowance for

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sludge equivalent to 10 percent of V,+Vs. lion vessel . The makeup water should be handled For the smaller systems it is possible to accommo- in a separate tank and not be part of the expan- date total system expansion volume in the expan- sion vessel circuit.

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TM 5-810-2/AFR 88-28 CHAPTER 3 DISTRIBUTION PIPING AND EQUIPMENT

3-1 . DESIGN OF SYSTEM locity of 7 fps may be used for long delivery lines The distribution system for a HTW heating system with pipe sizes 6 inches and larger which have few includes the supply and return piping, conduit (if branch takeoffs, provided protection is incorporat- buried), and related equipment extending from the ed in the design to take care of surges caused by heating plant to the buildings to be heated. The power failure at the system circulating pumps. master or site plan of the facility must be studied Lower velocities result in lower transient surges in to plan the distribution lines. The heat using long lines. equipment (consumers) of a facility may be served b. Calculation of Pressure Drop. The pressure by a number of different arrangements, so various drop due to the friction of water flowing through distribution plans should be studied before choos- pipe and fittings may be calculated by formulae as ing sizes and the number of distribution zones. covered in ASHRAE "Fundamentals Handbook", Ideally, zones should be drawn to segregate those or may be selected from charts which have been consumers having high return temperatures from developed and published in many handbooks. those having low return temperatures ; those Since selection of pipe sizes is limited to sizes com- which require constant high supply temperatures, mercially available, using extensively refined such as those serving processes, from those which for- mulae for extreme accuracy prove might have viable supply temperatures; and, final- time-consuming and impractical for the average system. ly, separating consumers which must be operated throughout the year from those which operate 3-3. DISTRIBUTION PIPING intermittently . These separations of loads help a General. Standard weight steel pipe, Sched- make the system as flexible and economical as pos- ule 40, is generally satisfactory for most HTW sys- sible. One factor which tempers these ideal ap- tems. Seamless is the preferred type fabrication; proaches is the selection of distribution lines based however, it is more expensive than welded piping on maximum capacity of lines of practical dimen- and this should be considered in designing the sions. A layout will be prepared following the pro- cedures outlined above showing the distribution system. Extra strong weight (Schedule 80) pipe will be 2 system and the flow required in each zone, main used on sizes inches and smaller. All joints will be welded and designed in supply, and return line and branch line. In con- accordance with ASME 1331.1, however, 300 trast to steam systems where buildings can be con- Class insulating flanges will be used for dielectric connections nected to the steam mains which form an open at loop around the site, high temperature water will every pipe connection from a trench system or have a circulating system with supply and return aboveground system to an underground system mains. However, it is possible to connect designat- and at dissimilar metals. Gaskets for flanged con- ed buildings, such as hospitals, to two different dis- nections will be of material designed for dielectric tribution zones so that either of the zones may service for pressure/temperature at each point of serve these buildings at any time. Switching over application in the system . Flanged connections from one zone to the other must be done very care- may be allowed at connections to converters and fully, however, to avoid shocks . For this reason, all equipment. Great care must be exercised in the valves used for changing zones must have by- design and installation of piping and vessels to passes. Example distribution system layouts are ensure enough flexibility to permit thermal expan- shown in appendix C. sion to take place without creating stresses greater than those allowable for the pipe, fittings, or ves- 3-2. PIPE SIZING sels. Distribution lines are installed with properly a. General. In a HTW district heating system, designed U-bends, L-bends, or z-bends to permit ex- pipe sizing is determined largely by the allowable pansion and with anchor points and guides where velocities used for design and resultant pressure needed. Provision must be made for venting and drops. Minimum allowableINACTIVEvelocity should be 2 draining all lines. Branch takeoffs must be de- feet per second (fps) to avoid stratification except signed properly to prevent interference with flow that minimum pipe size shall be lV2 inches, maxi- through both the main distribution lines and the mum allowable velocity should be 7 fps. Five fps is branch circuits. Serious problems have occurred a good nominal design velocity . The maximum vo- where specifications depend on ASME B31.1 for

Information Handling Services, 2000

TM 5-810-2/AFR 88-28

nondestructive testing. ASME does not require pansion of distribution piping located either above such unless the temperature is above 350 degrees or below the ground. Loops and elbows are prefer- F. at 1025 psi. Radiographic inspection of all welds able to expansion joints because they are not sub- in the distribution systems may be highly benefi- ject to the hazards of misalignment which can cial at little increase in cost. Proper pipe and cause line breaks . Expansion U-bends are general- vessel design stress limits require a working ly located at the midpoint between two anchor knowledge of the provisions of ASME B31.1, para- points with guides at the loop and a vertical re- graph 102.3 .2 relating to thermal stress. straint at the midpoint of the loop, if aboveground. b. Expansion Loops and Anchors. Steel pipe ex- It is preferable to keep the axes of the long legs of pands approximately 3 inches per 100 feet when these bends in a common plane. Anchors, solidly subjected to a temperature change of 400 degrees connected to a concrete base, must be sufficiently F. Refer to table Cl in ASME B31 .3 for unit ex- strong to withstand the full unbalanced pressure pansions for steel pipe. Expansion of steel pipe of the water and the stresses of expansion as well with temperatures as follows: as the weight of the line filled with water. An- chors are commonly located between expansion Expansion, bends both in underground and aboveground in- in./100 ft, stallations. Pipe guides of the type used in steam from 0 line Pipe Temperature degrees F. to construction must also be used. In general, pipe the procedures used in the design of steam lines temperature should be followed taking into account the addi- tional weight of the water carrying conduit. 100...... 0.6 150 ...... 0.9 c. Expansion Joints. Slip, bellows, ball, and 200 ...... 1.3 corrugated type expansion joints are not practical 250 ...... 1.7 with HTW systems and will not be used. 800 ...... 2.2 d Air Bottles, Drains and Vents. Air bottles of 850 ...... 2.6 400 ...... 3.0 adequate size must be provided at all the high 450 ...... 3.5 points of HTW systems. Automatic drain valves 500 ...... 4.0 are not practical. The vents on air bottles very seldom have to be used since air enters the system only when filling the system with water or when The flexibility of piping systems must be adequate starting up after a long stoppage . Drain connec- to prevent thermal expansion from causing unsafe tions will be installed at the low points of the dis- stresses in the pipe and fittings, excessive bending tribution piping. When drains cannot be run to moments at the joints, or excessive thrusts on sewers, draining may be accomplished by portable equipment or at the anchorage points. Credit for self-priming pumps. Figure 3-1 shows a typical cold springing will not be used in calculations for drain and a typical vent with air bottle . When dis- determining amount of expansion to be incorporat- tribution piping is run underground, it is neces- ed. Methods for calculating expansion bends are sary to locate drains and vents in suitably located available in handbooks and in pipe manufacturers' manholes with connections to the outside. Drains literature. Expansion loops or elbows will be used and vents are required on both sides of sectioning as the most practical means of accommodating ex- valves. INACTIVE

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H M 0 n N3 nN. 0

x m m P w oo . A. a H ~ 0

(D og rr m n A e 0 INACTIVE n W m ~<5. Sbo 00 "'Vm N 0 0 AIR BOTTLE 0 a ONE PIPE SIZE o W ~' c GLOBE VALVE (TYPICAL) LARGER THAN LINE SIZE BUT NOT LESS THAN TROUGH DRAIN MOUNTED ON WALL w 3" NOR MORE COLUMN OR HAND RAIL co a '140M tg THAN 6" ---'"' a .n PITCH DOWN ---~ a w DISCHARGE OVER "----ahj ru w FLOOR DRAIN i" SIZE --441 PIPE CAP HIGH TEMP. , mo NA -1 WATER SUPPLY 0 $. a OR RETURN ---1 FLOOR LINE GLOBE VALVE'

~ ~ rr

~ a r~ ' ~ TYPICAL VENT TYPICAL DRAIN b

Dar. ArKJ Corps of Engineers

TM 5-810-2/AFR 88-28

branch valves at junction with main piping on ac- that the lines be located below the frost line, high count of inaccessibility in the case of fire or other temperature water lines have not been found to hazards which might make the building inaccessi- freeze when located only 2 feet underground even ble. Valves such as sectioning valves in the mains when they have been out of operation for many which are used only in the fully open or fully hours. One of the greatest concerns in the design closed positions can be gate valves. All other of underground piping is the elimination of mois- valves which must be opened gradually or which ture from conduits and manholes. Sumps will be are used to make adjustments preferably should be provided in the manholes with facilities for pump- globe types. Branch valving is commonly located ing out the water. Proper sealing of conduit en- in manholes when the piping is run underground trances and exists in manholes is of very great im- or, when lines are not too far below grade, in valve portance. The arrangement of manholes for high boxes. In either case branch valves must be acces- temperature water applications is similar to that sible for maintenance. used for underground steam lines except that no f. Sectioning Valves. Sectioning valves on the provision need be made for the disposal of drip supply and return lines must be provided at a line condensate. number of locations to isolate sections of the b. Basic Criteria for All Types of Conduit Sys- mains for repairs and emergencies. Sectioning tems. Since all underground systems eventually valves are operated only infrequent intervals at become wet and water is the major adverse factor and, therefore, should be valves especially de- encountered, all systems will be of a type that can signed for easy operation, tight seating, and resist- be drained and dried. The insulation will be of a ance to . corrosion Lever-operated rising stem type that can be drained and dried. The insulation valves the would be suitable sectioning valves for will be of a type that can withstand repeated or smaller pipe sizes. Gate valves designed for the extended boiling and drying without physical pressures and temperatures expected are generally damage and/or loss of insulating characteristics. used for section valves. The project designer will include in contract docu- g. Bypass Values. Bypass valves may be in- ments the following information regarding the site stalled at the end of zones which are planned for and where conditions vary along the proposed extension to assure circulation of hot water and path of the system and will define separately the prevent stagnation. Normally globe a 1 1/2-inch conditions for the various segments of the system. valve is satisfactory for this service . If at all practicable, a geotechnical engineer famil- h. Insulation. All parts of the plant supply and iar with underground water conditions at the site return lines operating above 140 degrees F. will be shall be employed to establish the following classi- insulated with insulation suitable for the operat- fications. If the system to be installed is expected ing temperature. Flanges, valves, and pumps shall to be used for less than 10 years, consideration also be insulated. should be given to classifying the site one class i. Relief Valves. Relief valves should be high lower than it would ordinarily be classified (e.g., quality carbon steel with stainless steel disc and bad rather than severe). nozzle rated at 750 psi and 800 degrees F. Valves (1) Severe. The water table is expected to rise should be of the type which can be repacked with- frequently above the bottom of the system; or the out removing them from the line. water table is expected to occasionally rise above 3-4. UNDERGROUND AND ABOVEGROUND the bottom of the system and surface water is ex- SYSTEMS. pected to accumulate and remain for long periods in the soil surrounding the system. a. General. High temperature water distribu- tion lines may be run either above or below the (2) Bad. The water table is expected to rise ground. Underground lines may follow the con- occasionally above the bottom of the system and tours of the ground. Lines aboveground may be surface water is expected to accumulate and run on short concrete supports, or 10 to 12 feet remain for short periods (or not at all) in the soil is above the ground on wooden, concrete, or steel surrounding the system; or the water table the supports . They may be run over or under obstruc- never expected to rise above the bottom of above tions without difficulties. While concrete supports system but surface water is expected to rise INACTIVEthe bottom of the system but surface water is ex- may be best suited to many applications, they are not necessarily the least expensive. It is essential pected to accumulate and remain for long periods that the lines be protected by metal covers called in the soil surrounding the system. jackets. Underground piping may be located 2 feet (3) Moderate. The water table is never ex- below the ground level. Although it is not essential pected to rise above the bottom of the system but

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TM 5-810-2/AFR 88-28

surface water is expected to accumulate and should be summarized in a report and submitted remain for short periods in the soil surrounding by the design organization to the Contracting Offi- the system. cer with the contract documents. (4) Mild. The water table is never expected d Soil pH. If there is any reason to suspect to rise above the bottom of the system and surface that the soil pH will be less than 5.0 anywhere water is not expected to accumulate in the soil along the proposed path of the system, pH meas- surrounding the system. urements should be made at close intervals along c. Corrosive Classification. The soil of each site the proposed route, and all locations in which the should be classified as corrosive or noncorrosive on pH is less than 5.0 should be indicated in the con- the basis of the following criteria: tract documents. Soil pH should be determined by (1) Corrosive. The soil resistivity is less than an experienced geotechnical engineer, preferably 30,000 ohms per centimeter (ohm/cm) or stray the same engineer responsible for other soils engi- direct currents can be detected underground; all neering work. The load bearing qualities of the sites classified as having severe water conditions soil in which the system will be installed should be should be classified as corrosive. investigated by an experienced geotechnical engi- (2) Noncorrosive. The soil resistivity is 30,000 neer, again preferably the same engineer responsi- ohm/cm or greater and no stray direct currents ble for other soils engineering work, and the loca- can be detected underground. The classification tion and nature of potential soils problems should shall be made by an experienced corrosion engi- be identified. neer after a field survey of the site carried out in e. Cathodic Protection. Cathodic protection will accordance with recognized guidelines for conduct- be designed in accordance with TM 5-811-7 when ing such surveys. The results of the field survey required for metallic casing systems.

INACTIVE

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information Handling Services, 2000

TM 5-810-2/AFB 88-28

CHAPTER 4 HEATING PLANT

4-1. INTRODUCTION for generating HTW. There may also be instances This chapter describes the elements that go into of process steam systems being available to the central heating plant of a high temperature produce HTW. water system. The information is of a general a. Steam-Activated HTW Generators. nature because of the many equipment manufac- (1) Heat-Exchanger HTW Generator. A basic turers involved in the special specification require- (shell-and-tube) heat exchanger could be utilized to ments of each system. generate HTW, and the HTW system from the a. Capacity. The capacity of the central heat- outlet of the HTW generating heat exchanger ing plant must be large enough to handle the would be a standard system requiring expansion design loads of the connected system. The capac- tank, pressurization, and circulating pumps with ities of installed generators must be able to pro- the return water directed to the heat exchanger. vide the essentfal plant load with one generator (2) Cascade HTW Generator. There is another out of operation. Both winter and summer loads type of HTW source which utilizes a cascade and day and night loads will be considered when heater. This is a direct contact vertical vessel these loads are highly variable. In addition, plan- where system return water is cascaded over trays ning plant size must phase heating loads for the in the upper part of the vessel and makes direct initial installation and for the future extensions so contact with the steam supply. The lower part of that the heating plant will operate efficiently the cascade heater serves as the system expansion during each stage of expansion. The individual tank and the upper part serves as the steam-pres- generators selected should not be too small and surization chamber. Surplus water generated by the larger the number of generators the more eco- the water absorption of steam is usually returned nomical will it be to adapt to the variations in the directly to the boiler through a pipe from the cir- heating load and to operate the plant continuously culating pump discharge header. This unit re- at maximum efficiency . Usually three or more quires little equipment and lends itself to being lo- generators are required to permit installation of cated in any location convenient to the steam dis- the plant in increments. tribution system. Different zones can be handled 6. Savings. If the heating plant can be de- by providing multiple units matching zone require- signed initially for the final maximum size, careful ments. A typical cascade HTW system is shown on analysis will be undertaken to see whether subdi- figure 4-1. Cascade heaters are especially applica- vision of generator units greater than three or ble where high-pressure process steam is available. four is justified. The savings obtainable by reduc- An ideal situation would be a boiler plant supply- ing the size of the spare unit may be overbalanced ing cogeneration steam turbines for local power by the additional costs created by the subdivision. and also supplying high-pressure process steam. In this evaluation the possible increase in operat- Since the cascade heater raises the water tempera- ing efficiency should not be overlooked. tures by direct absorption of steam by the HTW circuit, the practically continuous excess liquid 4-2. HTW GENERATORS content is bled off and becomes part of the boiler Most HTW generators are the fuel-fired type gen- condensate return system. This is an ideal condi- erating HTW directly. There are other installa- tion both for heating HTW and obtaining good tions where HTW can be generated from steam. quality condensate return. If high-pressure steam One such condition would be obtaining HTW from is distributed to distant buildings, then a cascade a steam-powered turbine generating electric type heater fits into the remote location and sim- power. Boiler operating temperatures and pressure plifies zoning and running of extra water lines would fall withinINACTIVEthe range of heat requirements from a central plant.

Information Handling Services, 2000 TM 5-810-2/AFR 88-28

dw d Ma 00 q W w 0 a w

Figure 4-J. Cascade HTW System in Process Steam System.

b. Direct-Fined HTW Generators. Most manu- lation generators for high temperature water ap- facturers offer water tube generators specifically plications use the recirculation principle. Evapora- designed for high temperature water application tion is limited to the small amount necessary to using controlledINACTIVEforced circulation. The majority of raise the return water from the system to the generators installed are of this type. Natural cir- saturation temperature. The forced circulation se- culation generators using larger-sized water tubes cures positive flow in one direction through each ..~ without orificing should not be used. Forced circu- tube circuit at all times regardless of the rate of

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TM 5-810-2/AFR 88-28

heat transfer. Orifices with strainers placed at the pieces: the furnace section couples with the con- entrance of the tube circuits, proportion, equalize, vector section on site. Above the 150 Mega Btu/hr and direct the required amount of water flow in size, the units are generally "knocked-down" and each circuit. The degree and location of the restric- built up in panels: four walls, base, and top for tion may be varied to suit the length, arrange- field assembly . Through all the size ranges, burn- ment, and heat absorption of the circuit. Forced ers and accessories may be shipped separately if circulation generators are usually designed with mounted dimensions exceed route clearances . Stok- smaller overall heating surface than natural circu- ers for coal firing are generally installed at the lation generators of the same output . This does not site. mean that the critically loaded parts of the gener- ator surface such as the water walls are more 4-3. COMBUSTION EQUIPMENT AND CON- highly loaded than they would be with natural cir- TROLS culation generators. The higher overall heat trans- Fuel for generators will be selected in accordance fer per unit surface area results largely from the with the current DOD policy and agency or service fact that a larger portion of the generator surface directives and criteria. Select fuels which produce is operating under higher loading than would be the required plant performance at lowest overall possible with natural circulation generators. In production cost for an entire plant, including am- natural circulation generators, the heat absorbed ortization and operating and maintenance costs for by the water produces the buoyancy which starts all elements. and maintains the circulation. The circulation a. Combustion Equipment. Oil and gas burners therefore varies with the rate of firing or the local should be UL 733 and UL 795 approved units. rate of heat transfer. In order not to restrict this Stokers for coal will be selected on the basis of circulation, comparatively large tubes are favored. generator size, the type of coal used, and the char- The effectiveness of the heating surface of the gen- acteristics of the load. In general, however, spread- erator is reduced by the fact that the necessary er stokers are recommended as most suitable for downcomers must not be heated as intensely as HTW generators. They will be of the continuous the risers so as to assist circulation. This increases ash discharge type. Oil storage and handling the size of natural circulation generators in com- storage and handling parison with forced circulation generators. Great equipment and coal and ash and sized based upon care must be exercised in the application of natu- equipment should be selected stokers selected and agency ral circulation generators to HTW systems so that the size of burners or the forced circulation in the external circuit does or service directives and criteria. not interfere with the natural circulation inside b. Control Systems. Combustion controls usual- the generator. ly are set to regulate the firing rate to maintain a c. Generator Configuration. Two basic outlines preset water temperature at the discharge nozzle have evolved and are in general use today: the of the generator. For closer control and more horizontal unit, in which gases travel horizontally stable operation, anticipating compensation is fre- out of the furnace and through the convector sec- quently used. Since flow through the generator is tion, has a large base area and a relative low very rapid, the sensing device must respond rapid- height; and the vertical unit, in which the convec- ly to temperature changes. One arrangement, de- tor section sets above the furnace and which has a veloped specifically for high temperature water ap- relatively small base area and a high profile. This plication and successfully applied on several in- variance in shape affects the space design in the stallations, make use of two small and extremely heating plant so that early selection of the genera- sensitive and highly reliable solid-state sensors tor is desirable to obtain an economical structure. called thermistors. One is located in the generator If early selection is not possible, the building discharge water and the other in the system should be designed to house either configuration. return water flowing to the generator. These sen- d. Package Unit. The term "package unit" is sors are electronically balanced in the bridge cir- used rather loosely, and manufacturers ship pack- cuit which adjusts the firing rate of each generator aged units in more than one piece. However, the in direct proportion to the temperature spread be- tendency is to prefabricate as much as is economi- tween inlet and outlet water and maintains the cally feasible andINACTIVEto field-assemble as little as pos- outlet water temperature at the controllers set- sible. Railway and road clearances are the factors ting. Manual potentiometers are provided to adjust limiting size. Most manufacturers supply units up settings and throttling range. The two common to 75 Mega Btu/hr in a single factory-assembled typical control systems are shown schematically package, and sizes up to 160 Mega Btu/hr in two on figure 4-2.

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TM 5-810- 2/AFR 88-28

Fuel/Air Ratio Controller

Furnace Draft Signal METERING TYPE

Water Temperature Signal

Actuator For Generator Outlet Damper

Master Controller Actuator For Fuel Control Valve POSITIONING TYPE

U .S . Army Corps of Engineers

Rgure 4-2. Typical Combustion Control Systems . (1) For systems having generators of 20 Mega (d) For systems having generators of more Btu/hr capacity each, or less, the following is rec- than 20 Mega Btu/hr capacity, the following ommended : INACTIVEshould be used: (a) Positioning controls. (e) Metering controls. (b) Separate actuators on fuel and air. (f) Fuel and air actuators linked for tandem permits. (c) Manual fuel-to-air ratio controllers. operation, where physical location

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TM 5-810-2/AFR 88-28

(g) Automatic fuel-to-air ratio controllers. lowing the inert gas to relieve to a low-pressure re- Safety devices and limiting switches will be inter- ceiver. A compressor picks up the gas in the low- locked for low water flow, high temperature, and pressure receiver and compresses it into a high- high pressure at each generator. Good practice dic- pressure receiver for storage. As water contracts, tates a redundancy of sensing elements; that is, the control valve closes and the gas supply valve even though a sensor is signalling a generator's opens to permit the required increase in gas quan- discharge water temperature to a controller, a dif- tity. ferent sensor will be used to indicate high limiting (2) Separate expansion vessel, fixed gas quan- temperature. tity, fixed water quantity, variable pressure. As (2) To aid pollution control and energy conser- water expands, inert gas is compressed, increasing vation, closer control of fuel-air ratio is evolving. system pressure. As water contracts, system pres- Research on current methods will be made prior to sure is decreased. Although system pressure is al- final selection of controls. lowed to fluctuate, the pressure is never allowed to drop below saturation pressure. The need for keep- SYSTEM 4-4. PRESSURIZATION ing the expansion vessel within reasonable size a. Criteria for Selection of Pressurization and for avoiding pressures in excess of the rating Method. The following criteria for selection will of fittings, piping, and heat exchangers usually be used: limits the size of such systems. (1) Collapse of pressurization must be avoided d. Sizing the Expansion System. The expansion when system is in operation. system is sized to take the volume fluctuations at (2) Water must be kept free of oxygen. operating conditions. It is impractical to try to size (3) The control of pressurization must be an expansion system for HTW based upon cold simple . conditions, as too large a volume change takes (4) Waste of compressed inert gas must be place. The expansion system must, however, in- avoided. clude reserve capacity for some sludge buildup and supply of 30 seconds for each circulating (5) System should minimize fluctuations in pump boiler and system on separate pump HTW system pressure and outgoing temperature pump (both of water from generator. systems). large, (6) System should provide for a modulating (1) Steam-pressurization makes use of a steam chest in control of firing rates. horizontal expansion drum with a void to impose saturation pressure on (7) Maintenance of pressurization should be the upper the system. Generator discharge water is taken to easy and minimal. the drum supply water to the distribution system operating costs should be (8) Installation and is taken from the drum so that drum water is the low. hottest in the system with the highest vapor pres- (9) Proper utilization of safety devices should sure. The saturation pressure corresponding to the be assured. highest drum water temperature is the system b. Steam-Pressurization. One type of steam pressure. Consequently, system operating pres- pressurization is acceptable, that is a separate ex- sures are the very lowest possible. Inert gas pres- pansion vessel. This system has the forced circula- surization of the void space of the expansion drum tion hot water generators connected to a separate at about 40 prig above saturation pressure will expansion vessel. System water is drawn from the assure stable operation and prevent flashing with expansion vessel for supply to the pumps. Water changes in system volume demand. With inert gas in the expansion vessel is allowed to flash into pressurization the expansion drum can be smaller steam to maintain sufficient pressure on the entire and set vertically at grade level. In some cases the system. Sufficient net positive suction head must use of inert gas pressurization will result in in- be maintained to prevent flashing at the eye of the creasing the system pressure so that valves and pump. fittings would be heavier and costlier and require c. Inert Gas-Pressurization . Two methods of added surveillance and maintenance of the pres- inert pressurization are acceptable. sure control system by the operating personnel. (1) SeparateINACTIVEexpansion vessel, variable gas (2) Draining and filling connections must be quantity, constant water quantity. This system in- provided so that the expansion vessel and the con- cludes a high and low pressure gas receiver, com necting lines can be completely drained and com pressor, and necessary control valves. As water ex- pletely filled with water after a shutdown. Vents pands, the control valves open at a preset point al- are necessary at all the high points of both the

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Information Handling Services, 2000 TM 5-810-2/AFR 88-28

tank and piping to permit purging of any trapped gpm, with heads up to 250 feet of water. The head air. A single tank is preferred on the basis that it characteristic of circulating pumps should be flat is the most economical installation . If the volume in order to deliver nearly constant head through- variation cannot be practically handled in one out the range of operating capacities. At the same tank, two, or a maximum of three tanks may be time the maximum head should occur at shutoff used. These tanks must be equipped with equaliza- and should fall off gradually up to maximum gal- tion nitrogen and water lines to permit adjustment lonage, with a decrease in pressure no more than of water level and pressure differences between about 15 percent below the shutoff pressure at the tanks. Provision must be made for filling and maximum operating gallonage. Pumps operating draining the expansion drum. Suitable water level in parallel will carry more or less their proper por- controls may be installed to provide for overflow if tion of the capacity when the pump characteristic the water level gets too high and for manual or has a continuous drop of 10 to 15 percent. Circula- automatic supply if it should get too low. The over- tion pumps are located in the supply line of the flow connections must be equipped with a single or system to maintain the highest possible positive double manual shutoff valve and may be equipped suction head. To further improve efficient oper- with a cooler or a cold water mixer to eliminate ation of these pumps, a mixing connection is pro- flashing and increase the capacity of the line. vided so that a portion of the system return water Large quantities of water should be drained only mixes with the water from the generator thus low- through double sets of blowoff valves in case one ering its temperature to avoid the danger of flash- should fail to seat. Makeup and emergency feed ing at the pump suction. The suction intake of connections to the system will have nonreturn high temperature water pumps must be carefully type valves such as check valves to prevent system designed to avoid sudden changes in velocity or di- pressures from being imposed on equipment when rection which might contribute to flashing and in- not in service. Safety valves must be installed in efficient operation. With nitrogen cushioned sys- the steam space for the full generator capacity tems any pump suction rumbling (flashing) can be connected to the expansion drum. These should be quieted simply by raising the pressure slightly (in- set according to the practice of setting boiler suring that the expansion line connection is as safety valves. Purge or vent valves need not be close to the pump suction as possible also helps). larger than 1 1/z inches. Saddles and supports for This cannot be done with a steam cushioned horizontal expansion tank must be designed to system as no means to increase the pressure other permit movement due to thermal expansion of the than raising the water temperature exists . Pumps tank and to support a weight of 1 1/a to 2 times the which have split casing with an axial suction and weight of the tank and its contents. Pressure upward discharge are preferred. Adequate water gauges are required at the top of the tank, and cooling must be provided for all pumps. Drip water thermometers should be located at several levels and cooling water should have drains near the of the expansion tank, usually at 1/a points of pump and the drains should be visible. Mechanical vessel. A long gage glass is needed to follow the seals are recommended after the pump has had a water level. Leaving a nitrogen cushioned expan- "run-in" period of a few months. This gives the sion tank uninsulated should be considered so the system water time to become stabilized and most tank will radiate and operate at a lower tempera- dirt will be out of the system. Mechanical seals ture. The inclusion of a heat trap loop in the ex- will be selected to suit each pump manufacturer's pansion line is always desirable if space permits as recommendation and installed accordingly. Me- this will aid in preventing heat migration from the chanical seals provide for less maintenance and return piping into the water in the tank. elimination of packing leakage. Each pump should be direct-connected to its motor on a common cast 4-5. PUMPS iron or structural steel base. When individual gen- Single-stage centrifugal pumps are used to circu- erator circulation pumps are used, one pump must late high temperature water through the distribu- be provided for each generator. The head under tion system and through the generator or through which these pumps must operate, as well as the both the distribution system and the generator, de- capacity, is determined by the generator require- pending upon the type of circuit selected for the ments. These pumps take their suction from the HTW system. PumpsINACTIVEselected for this service must return water line. Systems having a number of be designed especially for high temperature water zones should have the total capacity of the zones to secure efficient, reliable operation with a mini- subdivided for several pumps of equal capacity to mum of maintenance. Standard pumps for this ap- avoid operation of pumps at low efficiencies rather plication have capacities ranging from 100 to 2000 than having one pump for each zone. A common

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pump discharge header then supplies the zones. struments and other equipment are needed to con- The operating head of the system circulation trol the flow through the generator because the pumps must be sufficient to handle the calculated circulation pumps have a constant capacity and pressure drop through the complete circuit which are entirely independent of the system circulation. the pump serves. This includes the pressure drops The only safety devices required to protect the through valves and piping in the heating plant, generators of this system are the generator pres- the distribution system, and the heat-consuming sure differential control and a thermocouple con- equipment. In case the system circulation pumps trol which safeguards the tubes against excessive also serve to circulate the water through the gen- temperatures . erators, the pump head must also be sufficient for b. Single-Pump Systems. Single-pump systems the generator circuit. Provide multiple pumps in have one set of pumps which circulate water parallel so that when one pump is inoperative the through both the system and the generators. The remaining pumps will have capacity to provide 100 circulation pumps must have sufficient head to cir- percent flow. Yoke mounted pumps should be culate water first through the system and subse- avoided. Use of centerline mounted pumps exclu- quently through the generators. The water volume sively will avoid associated expansion problems. of either the generators or the system will deter- Variable speed pumps should be considered for mine the capacity of the pumps, depending upon system pumps in a dual pumping arrangement. which is greater. The water volume of the system Constant speed (volume) pumps will be used with will vary with the head load and the temperature forced circulation HTW generators as they re- drop between the supply and return temperatures, quired a constant flow, minimum 90 percent of since the heat users throttle the flow of high tem- design or 100 percent of the flow required by the perature water to adjust to changing heat require- manufacturer of the boiler installed, whichever is ments. The volume of water circulated through greater, (only return water temperature will vary the generators may be allowed to vary, but to pro- with load). The water flow element must be locat- tect the generator tubes from overheating it must ed in the return line to the generator and in never be allowed to fall below the minimum piping straight runs to insure an accurate reading. amount required to guarantee proper distribution Flow switches will be included to prevent startup of flow through all the water circuits of the gener- if flow has not been established and will take the ator. A generator flow meter or other device which unit off line should the flow drop below the mini- indirectly indicates the flow is required to operate mum rate. The pressure loss of the bypass valve and automatic bypass valve which assures ade- and associated piping should be as close to that of quate volume of water to the generators at all the HTW generator loop to insure that the circu- times. This bypass valve is sized for the minimum lating pump operates on its curve under any condi- flow required at the generators. tion. a. Dual-Pump Systems. Dual-pump systems 4-6 . MAKEUP WATER TREATMENT have generator circulating pumps and system cir- HTW systems are closed systems and, therefore, culating pumps. The generator circulation pump makeup is limited to the extremely small amount draws water from the return system and delivers of leakage which occurs at pump glands and valve it directly to the generator inlet header. These stems. Additional makeup is required for filling pumps must be designed to circulate the quantity lines or equipment which are drained. When of water specified by the manufacturer against the makeup is small, accumulation of salts and other head required to overcome the resistance of the impurities in the generator is so slow that genera- generator plus the connecting piping and fittings. tor blowdown, another cause of losses, is hardly The quantity of water circulated through each ever needed. The makeup water requirements generator is therefore kept more or less constant even in the largest systems should not exceed 200 regardless of the generator firing rate. The system to 1000 gal. per day. circulation pumps are designed to circulate the a. External Treatment. All makeup water in- quantity of water determined by the heat load of troduced into the HTW system must be filtered to the heat users and the design temperature drop remove suspended matter and treated to remove between the supply and return lines against the hard elements of calcium and magnesium, and total resistanceINACTIVEof this circuit. There need be no must be oxygen free. A demineralized unit is usu- relationship during operation between the quanti- ally not required. The calcium and magnesium ties of water circulated by the system pumps and will be removed by a water softener. The softening the generator circulation pumps in this arrange- operation is performed by filtering makeup water ment. With this dual pump arrangement no in- through a bed of ion-exchanger material common-

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ly called "zeolite." At intervals determined by the on a generator panel and, where applicable, should amount of water used for makeup, the zeolite ma- include the following: terial must be regenerated by backwashing with (1) Multipoint draft gage, 4 points with me- concentrated salt brine solution. Capacity of the chanical ash collector, 3 points without mechanical system is normally in the range of 15 to 20 gal. per ash collector (varies with fuel). min. The water softening system should consist of (2) Water flowmeter and recorder with 3-pen dual zeolite tanks, a brine solution tank, manually circular chart: operated multiport control valve, bell alarm water (a) Water flow. meter, and water distribution manifold . This water softener system may need to be supplemented for (b) Outlet water temperature gage. the initial fill requirements when raw water condi- (c) Return water temperature gage. tions are poor. The initial fill should be relatively (3) Recorder with 2-pen circular chart: free of oxygen. Any remaining oxygen should be (a) Combustion air flow. scavenged with sodium sulfite or similar treat- (b) Flue gas temperature. ment. (4) COs meter and recorder with 1-pen circular b. Analysis of Water. Analysis for control of chart. water is essential in HTW systems to prevent the formation of insoluble deposits of scale within the (5) Smoke density indicator and alarm. generator tubes and other parts of the system and (6) Individual inlet and outlet water pressure to prevent corrosion and deterioration. To provide gages. a system with the greatest resistance to corrosion (7) Stoker grate temperature multipoint indi- and chemical attack economically possible, a rela- cator (solid fuel-fired generators only). tively high level of alkalinity is maintained in the (8) Opacity monitor. system water. Tests have shown that iron is least b. Master Control Board. A master control soluble at a pH of 9.3 and that corrosion of iron in- board with annunciating alarm panel should be creases rapidly as the pH falls below 9.3. To assure provided to include the following: maximum protection it is recommended that a pH pressure gage. of 9.3 to 9.9 with zero hardness and zero oxygen be (1) Expansion drum maintained. The services of a competent water (2) Zone distribution meters and recorders specialist should be used for the primary and sec- with 3- or 4-pen circular chart: ondary water circuits on a regular schedule to (a) Water flow rate. sample, analyze system water, and make recom- (b) Water temperature in. mendations for corrections, if necessary. (c) Water temperature out. c. Storage Tank. A treated water storage tank (d) Expansion drum pressure (on first zone will be provided for emergency pumping require- only). ments. This tank is usually sized for approximate- indicator. ly 20 minutes pump demand. It is advisable also to (3) Expansion drum water level provide a means of heating the treated water to at (4) Annunciator with illuminated windows least 210 degrees F. to avoid cold shock to the and horn to indicate the following: system. (a) Overflow . (b) Low level. 4-7. INSTRUMENTATION (c) Low level cutoff. Instrumentation serves the following necessary unit). functions: records and indicates vital factors such (d) Generator low flow (one for each as water flow, temperatures, and pressures which (e) Burner low pressure (each oil burner are essential to the operators; provides supervisory when used). check readings and information for determining (5) Indoor and outdoor thermometer. efficiency of operation; assures maximum utiliza- (6) Electric clock. tion of total plant; and provides monitoring of (7) Circulating pumps status lights . emission controls. The minimum instrumentation annunciator requirements will be reviewed with current emis- (8) Excess water temperature INACTIVEpoint. sion monitoring requirements of EPA or state or local codes. (9) how expansion tank pressure (inert gas a. Generator Panel. The minimum instrumen- pressurized systems). tation for each HTW generator should be mounted (10) Makeup pump status lights.

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c. Additional Features. Fault finders and an- this installation should be analyzed before includ- nunciators are available which will aid the opera- ing in the design. tor in determining which limit has caused a "fail 4-8 . POLLUTION CONTROL to operate on demand" condition or an inadvertent Pollution control will conform to the latest re- shutdown . Because of the high costs of installation quirements of EPA or the state or local codes, and maintenance during operation, the benefit to whichever is more stringent.

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CHAPTER 5 CONVERSION AND UTILIZATION

5-1. POTENTIAL USERS OF THE SYSTEM enter the building. The lines beyond the shutoff The design of building distribution circuits in- valves must be drained if there is danger of freez- cludes the arrangements for bringing high temper- ing due to long periods of shutdown . A minimum ature water supply and return lines into each circulation must continue when none of the heat building, locating equipment in the equipment exchangers require heat. For this reason, a bypass line equipped valve should be roams, and designing suitable heat exchangers, with a %-inch gate installed at the entrance to each equipment room control systems, and auxiliary piping such as cir- ahead of the shutoff valves. culation systems, drains, vents, and bypasses. In general, branches are taken off the high tempera- 5-3. LOCATION OF EQUIPMENT ture distribution lines to serve one or more equip- Heat exchangers or converters may be located space heating ment rooms in each building. For either in an equipment room or in an open base- and process applications, heat exchangers or con- ment since a special room is not essential. If the verters are required to transfer the heat from the individual buildings are very large or extensive, high thermal potential of the high temperature such as hospitals and research laboratories, it may supply water lines to the lower temperature levels cost less to distribute the heat exchangers of the spaces and equipment requiring heat. Heat throughout the building serving them from main will be transferred directly by radiation and con- HTW headers. In laying out the equipment spaces, vection only with special approval otherwise may sufficient space must be provided for the removal be indirectly by radiation and convection, or indi- of HTW coils, especially if the low temperature rectly by the circulation of secondary heating heat carrier is expected to form scale or other de- media such as air, water, or steam. Suitable con- posits . Since the HTW will not foul, scale, or cor- trol equipment is required so that the desired tem- rode the piping through which it passes, no provi- perature levels can be maintained under varying sions are needed for cleaning this circuit. heat loads. a. Heat Exchangers. Figure 5-1 shows a simpli- fied subcircuit including the various types of con- 5-2. BUILDING SERVICE verters commonly used in district heating applica- At the point where the HTW supply and return tions. Steam generator, domestic water converter, lines enter a building, shutoff valves are required. and hot water converter use steam or water as sec- For maximum accessibility and convenience, ondary heat carriers. All of the converters are ar- locate them inside the building in the mechanical ranged in parallel between the supply and return equipment room nearest to where the service lines lines of the HTW system.

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Domestic Water Converter

Hot Water Converter

U .S . Army Corps of Engineers

Figure 5-1. Various Heat Converters. b. Piping. It is most convenient to locate most pair of flanges in the HTW lines must permit re- of the piping connecting converters and heat ex- moval of the tube bundle . If walls or windows are changers above rather than below the equipment located close to the tube bundle, removable sec- with risers and downcomers serving the various tions must be provided to permit removal of the pieces of apparatus. This arrangement keeps the bundle. Piping distributing the secondary heat car- piping up and out of the way making it easier to riers should be designed in the conventional fulfill the venting and draining requirements . manner as recommended by ASHRAE Handbook. Vents and air bottles must be located at all the c. Valves. Each heat exchanger using HTW re- high points in theINACTIVEpiping, and drains and filling quires at least one supply and one return shutoff connections must be provided at the low points to valve to permit maintenance. In addition, an auto- facilitate maintenance of the equipment. No matic control valve in the return is usually re- piping should be located in the space reserved for quired with provision for its removal or replace- removing the tube bundles. The disconnection of a ment. Figure 5-2 shows three types of heat ex-

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changers commonly used. HTW connections and verter should not be used. They can easily deprive controls are basically the same for all three types. the other circuits of flow and produce high return Shutoff valves may be either gate or plug valves. temperatures and excessive pump loads thereby A balancing valve, either a globe or plug, also is lowering the overall efficiency of the system. installed ahead of the return shutoff valve so the Bypass globe valves of the same size as the control subsystem may be balanced to avoid short circuit- valves are provided around the control valves. The ing or excessive flow of supply water when control shutoff valves must be located so as to permit valve drives to full open position . The control maintenance of the control valves as well as of the valves are operated by thermostats or pressure- heat exchanger. To assure no short circuiting and stats which actuate the valves through a position- loss of heating capacity, great care should be er using electric, electronic, or pneumatic modula- taken that flow is assured to all heat exchangers tion. Control devices sensing secondary media tem- at all times: for this reason, piping and valuing peratures must be selected to provide flow and must be sized carefully and branch lines leading to positive shutoff when required under all conditions individual heat exchangers should be taken off of operation. Controls will be located in return common headers. In the case of air heaters used to since this reduces or eliminates flashing of the water flowing through the valve, provides better heat outside air, if the control valve is tight-clos- control characteristics, and prevents plug erosion ing a bypass with a V2-inch globe valve must be caused by high temperature water flashing to provided to maintain enough circulation at all steam at lower discharge pressure. Control valves times to prevent freezing of the water in the coils. in HTW supply are not recommended. Control The construction of valve seats and plug contours valves of the three-way type or three-way pressure will be carefully selected to minimize erosion in controls which bypass hot water around the con- HTW piping.

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Pressure Control Instrument Compressed Air Supply \1E l_.-....~ High Water Line Flow Control m~ Balancing Valve_ i ..l team Generator

High Temp. Hot Water Supply D And Return Feedwater STEAM GENERATOR Temperature Control Instrument Compressed Air Supply Globe Valve Flow Control Hot Water Supply Balancing Valy e~7 I ~------

High Temp. Hot Water Supply And Return

HOT WATER CONVERTER

Temperature Control Compressed Air Supply Instrument ------\E Domestic Hot Water

High Temp. Hot Water 'Cold Water Supply Supply And Return DOMESTIC HOT WATER CONVERTER

U .S . Army Corps of Engineers

Figure 5-2. Heat Exchangers and Control Valves.

d. Flow Control. Provisions shall be made in be controlled by the plant operators and must be . the HTW subsystem so accurate balancing can be kept in their jurisdiction will be provided INACTIVEsystems to e. Temperature Gages. Gages achieved. Control of flow to individual on the supply and return lines to each piece of is very avoid excessive flow and short circuiting HTW equipment. Gages allow a quick visual check important. The continual checking of the flows of the temperature differential and an indication cannot be overstressed to the designer. Flow must of unbalance in the system if differential is below

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design. Design differential will be posted near the shell and tube heat exchangers, the high tempera- equipment or kept on file at plant operations. ture water will be in the tubes and lower tempera- tures and pressure medium in the shell. The thick- 5-4. DESIGN OF HEAT EXCHANGERS ness of the shell and the gage of the tubes must be a. Standards. Heat exchangers using HTW are sufficient to carry the required pressures; extra best classified according to their secondary mode thickness is not required to allow for corrosion. of heat transfer since they all circulate high tem- The tubes or coils will always be arranged as mul- perature water in the primary circuit. Heat may tipass so that nearly equal velocity occurs through be transferred in the secondary circuit by direct all the tubes of all passes. The overall pressure radiation, by radiation and convection to air, by drop on the shell or coil side should generally not convection to water or to steam. Each heat ex- exceed 7 feet of water. Coils must be removable changer must be designed to deliver a specified through a flanged opening and be accessible for amount of heat per hour which represents the rerolling of the tubes in the tubesheets. The coils maximum design heating load to which it is con- should be capable of being cleaned on the outside nected. It must have sufficient area to deliver this mechanically or chemically and on the inside by amount of heat with specified velocity and enter- chemical means only since accumulations are ing and leaving temperatures of the primary HTW hardly ever found on the surfaces contacted by to the secondary heat carrier under specified con- high temperature water. It is very important to re- ditions and in the case of convection, with speci- alize that the economy of the entire installation fied circulation. The film coefficients of heat trans- depends to a great extent upon the design of the fer by convection or radiation may be determined heat exchangers . It is generally cheaper to in- by suitable formulae available in the standards of crease the heat exchanger surface area than to in- equipment manufacturers. Fouling factors may be crease pipe sizes and pump capacity. It is impor- included when fouling or scaling of the secondary tant that a serious effort be made to reduce return heat transfer surface will occur over a period of temperatures of the HTW as much as feasible time. No fouling factor need be applied to the since this determines the pump capacity and pipe HTW side since this closed circuit is not subject to sizing as well as transmission losses . High pres- fouling, scaling, or corrosion. The specified supply sure drops through the exchangers and their valu- temperature of the HTW circuit is determined ing and piping are also uneconomical although a from the design supply temperature leaving the certain minimum must be allowed for good balanc- generator plant less a proper allowance for heat ing and operation. A 40-foot total drop is consid- losses in transmission, usually about 4 to 8 degrees ered a good design parameter. Water coil velocities per mile. The return HTW temperature for each should be at least 4 feet per second (fps) and not piece of heat transfer equipment may generally be exceed 8 fps. specified as between 10 to 20 degrees higher than b. Radiant Coils and Panels. HTW may be the outlet temperature of the secondary heat carri- used directly with special approval in radiant coils er for peak load. For very large heat exchangers, and panels when they can be mounted more than temperature crossing is practical. The flow rate of 15 feet above the floor. Radiant coils and panels HTW through the heat exchangers is determined are especially desirable since comfort conditions by the heat load and the design supply and return can be obtained at lower room temperatures than temperatures of the primary circuit. The physical those required for convection heating. This is a arrangement of HTW heat exchangers must be particularly advantageous method for heating carefully considered to be certain that it complies warehouses, factories, hangars, and other areas with the special requirements of this heat carrier. which have high ceilings or roofs. The disadvan- In general, the coils must be horizontal to permit tage is that close temperature control is difficult. draining and venting, and must not permit bypass- The panels or coils must be carefully distributed to ing or short circuiting of the water from inlet to produce uniform radiation throughout the spaces outlet or permit stratification to occur. The coils to be heated. The return water temperatures from or tubes of HTW heat exchangers should be made radiant coils and panels depend upon the heat from cupro-nickel, stainless steel, or Admiralty load, the effective radiant area, the room tempera- metal. Most available material is 90-10 cupro- tures, and the mean coil temperature; they should nickel which isINACTIVEgood for normally expected pres- be kept as low as possible, as pointed out above, sure and temperatures. Other cupro-nickel such as without requiring excessive surface area. When 70-30 or 80-20 is available on special order. Stain- coils are located in floors and walls, lower surface less steels are used for higher pressures. Bronze temperatures are required and HTW cannot be and brass are not suitable for this service. In all used directly. In this case, heat exchangers may be

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*rm 5-8102/AFR 88-28

used to transform the heat to lower temperature is used, but may be eliminated, of course, in single- levels. The heat radiated by HTW coils may be pipe systems of the gravity type or in systems regulated to suit demand by throttling the flow which waste the steam. The control valve throttles through the coil which reduces the mean tempera- the flow of HTW in the primary circuit and is op- tures of the coils. erated by a pressurestat connected to the steam c. Forced Circulation Hot Water Converters. chamber of the converter. A safety valve is re- Forced circulation hot water converters are re- quired to relieve pressure in the shell should the quired to produce lower temperature hot water for controls fail to operate properly. space and process heating. The use of circulating hot water for building heating is preferable to the 5-5. CONTROLS use of steam when HTW is used as the primary a. General. Control of temperature within close heat carrier. Not only is greater comfort and limits is an important factor in all heating instal- easier control achieved, but also a more economi- lations, comfort and process, both for proper func- cal arrangement results since lower return HTW tioning of the equipment and for best economy. temperatures are produced . The distribution of the Considerable savings in fuel can be achieved by low temperature water may be designed in the controls which adjust the consumption of the heat- same way as conventional forced circulation hot ing medium closely to the heat demand. For this water systems. Such a design would require a cir- reason, the cost of proper controls is usually easily culating pump, a mixing valve controlled by inside justified from the fuel savings alone. Automatic or outside temperatures, zone control valves, con- controls must be selected carefully to suit the ap- ventional convectors or radiant panels, and an ex- plication. This is particularly true in the case of pansion tank to take care of variations in the HTW controls due to the great heat potential of volume of water in the system with temperature. this heat medium . Controls for HTW equipment The heat exchanger required for this application generally receive their control impulses from a should be the multipass type on the HTW side and thermostat or pressurestat located where they cor- should be well baffled on the secondary circuit to rectly indicate the temperature or pressure of the prevent short circuiting of the lower temperature secondary heating medium . Great care must be water. Relief valve or valves installed on the shell taken not to locate thermostats in stagnant re- to relieve pressure should be sized with added ca- gions where they could give false readings. Control pacity should the control valves on the HTW side valves should be the two-way single seated type. fail. There are two types of controls which can be oper- d. Large Domestic Hot Water Converters. Stor- ated either electrically or pneumatically; on-off age type domestic hot water converters are used controls and modulating controls. for heating cold water to about 140 to 180 degrees b. On-Off Controls, Quick-Opening Valves. F. Converters for this application are similar to When the load is fairly constant, and when wide those used with forced hot water circulation. Baf- temperature differentials may be permitted, on-off fles may be installed in the shell to prevent by- controls are often satisfactory . On-off controls may passing of the entering cold water to the outlet cause serious water hammer, so their use should connection and stratification of water in the tank. be restricted to small units and to short runs of Throttle controls are required to vary the flow of pipe. Quick-opening valves are not suitable for the HTW to maintain a constant temperature of the close temperature control required for hot water domestic water in the upper portion of the tank. heaters or domestic hot water converters or for the e. Steam Generators. Steam generators can be close pressure control required for most steam pro- used to raise steam at any desired pressure, limit- ducers. Regulating valves suitable for use with this ed only by the temperature of the HTW available type of equipment must be of the modulating type. to the converter. Slightly wet steam is produced c. Modulating Controls. Modulating controls in unless provision is made to eliminate the en- general are far more satisfactory than on-off con- trained water. A steam separator built into the trols, but they cost considerably more. Their use is steam space of the converter and suitable water always justified, however, by the better control level control are necessary for these converters. and higher economy which they produce. A modu- Automatic condensate return to the steam genera- lating control assembly will consist of a thermo- INACTIVEtaken that the stat or a pressurestat, a control instrument, and a tor is practical, but care must be control should not deliver more than the necessary control valve. The thermostat or pressurestat may amount of fresh makeup water to the generator. A transmit its impulse to the instrument by gas, condensate pump and condensate tank are re- vapor, or liquid pressure or by electrical impulse. quired when a two-pipe steam distribution system The impulse from the instrument to the control

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valve also may be transmitted by gas, liquid, or stant may use controls with narrow ranges of only electricity. The control valve itself in general 10 percent band widths and without any reset. should be designed with an equal percentage flow Based on present experience applications where characteristic . Valve positioners are preferable for the inlet pressure of the valve varies widely due to all valves 2 inches and larger if the distribution the fluctuations in load, instruments with band pump pressure head is in excess of 100 psig. The widths up to 150 percent and with automatic reset valves must always be arranged to be closed will provide a safe control. Exact sizing of the against the impulse of the instrument so that the valves in accordance with available pressure drop controls will close automatically should the im- is essential and it is recommended that a pressure pulse from the instrument fail. Control applica- differential of 20 feet of water should not be ex- tions where the pump pressure is more or less con- ceeded .

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APPENDIX A REFERENCES

Government Publications Departments of the Army and the Air Force TM 5-810-1 Mechanical Design-Heating, Ventilating and Air Conditioning TM 5-811-7 Electrical Design, Cathodic Protection Non-Government Publications American Society ofHeating, Refrigerating, and Air Conditioning Engineers (ASHRAE), 1791 Tullie Circle NE, Atlanta, GA 30329 Handbook, Equipment, 1988 Handbook, Fundamentals, 1989 Handbook, HVAC Systems and Applications, Chapter 15, 1987 Handbook, Refrigeration Systems and Applications, 1990 American Society ofMechanical Engineers (ASME), 22 Law Drive, Box 2300, Fairl"ield, NJ 07007-2300 Boiler Pressure Vessel Code (1989) B31.1-1989 Power Piping B31.3-1990 Chemical Plant and Petroleum Refinery Piping Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook, IL 60062 773-75 Oil-Fired Air Heaters and Direct-Fired Heaters, Third Edition, 14 August 1985 795-73 Commercial-Industrial Gas Heating Equipment, Third Edition, 13 July 1989

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TM 5-810-2/AFB 88-28 APPENDIX B SAMPLE CALCULATIONS FOR DATA GIVEN IN CHAPTER 2

B-1. Heating load and distribution system requirements are based upon average system tem- First the heat requirements throughout the distri- perature differential. bution system are tabulated; then system pres- 120,000,000/159.59 =751,927 lb/hr, based on maxi- sures and temperatures are determined. In this ex- mum initial load. ample system the heating 80 load is percent of the 40,000,000/159.59 = 250,642 lb/hr, based on future total load. T,-Tr is then selected at 150 degrees F. anticipated load. Tr averaged 240 degrees F. following the procedure set forth in subparagraph 2.12.a. The supply tem- If the distribution system design is for a two-pump perature T, is system the best selection would be three pumps Tr = 240 F. plus one spare based upon the 250,000 lb/hr. If the 150 design is for a single pump system, then the best + F. selection would be two pumps plus one spare based T, = 390 F. + allowance for loss (40 degrees F.) _ upon the 344,630 lb/hr for the boiler. Pipe sizes for 430 degrees F. The system supply is selected at 440 the mains in the circulating loop and at the mani- degrees F. at the generators. folds for the pumps and generators should be sized Tr = 440 - 150 = 290 degrees F. for maximum future load. This type of analysis should be made for every system to assure that B-2. HTW generator selection adequate pumping capacity is provided. For this Using the following requirements the basic calcu- example a single pump system is selected. lations are performed. Maximum Initial Load (Qd=120,000,000 Btu/hr B-4. Expansion vessel design Future Anticipated Load=40,000,000 Btu/hr The first step is to determine the system expan- Maximum Ultimate Load =160,000,000 Btu/hr sion volume as outlined in subparagraph 2.12.c(1): Essential Load =55,000,000 Btu/hr T, = 440 F. Flywheel Factor =0.85 Tr = 290 F. Temperature Supply (Ts)=440 F. Vt, = specific volume at 440 F. = 0.01926 Temperature Return (Tr)=290 F. Vt 2 = specific volume at Average Temperature Drop= 150 F. T, - 10 percent = 400 F. = 0.0186.¢ Volume in Ultimate System=4,000 cubic feet VfI -Vf, = 0.00062 Use two 55,000,000 Btu/hr units initially; one Utilizing Equation (2-9): down Vf will provide essential load. Add one AV, = (Vt i - z)/Vf , ] 100 = (0.00062X100)/ 55,000,000 Btu/hr unit in future. Generator circu- 0.01864 = 3.3 percent change in supply system lation requirements are based upon average water. system temperature differential. This establishes the operating range between 400 T, = 440 F. = 418.9 Btu/lb (enthalpy) degrees F. and 440 degrees F. This 40-degree differ- Tr = 290 F. = ,259.31 Btu/lb ential is applied to Tr to obtain the minimum Ah = 159.59 Btu/lb return temperature. 55,000,000/159.59 = 344,630 lb/hr, based on essen- Vt,, = specific volume at Tr = 290 F. --- 0.01733 tial load. Vni = specific volume at minimum return = Tr - B-3. Circulating pump selection 40 F. = 250 F. = 0.01700 Two generator circulation pumps are selected for Vf, - Vr ar = 0.00033 this flow requirement with head based upon resist- Utilizing Equation (2-10): ance through generator and associated piping for AVr = [Vtrl - Vha]100/Vt2 = 0 .00033/0.01700 two-pump systemsINACTIVE. For single-pump systems, 1.9 change in return system water. (100) = percent pumps must provide this flow as a minimum con- dition; pump head requirements would be the From Equation (2-11): above resistance added to system resistance. Distri- V, =total system expansion volume = AV, + bution system circulation, pumps, and pipe size, AV,/100 (system volume)

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V, = 3.3 + 1.9/100 (4000) = (0.052x4000) = 208 From Equation (2-12): cubic feet. Required nominal volume = V, + V2 + Va + Steam expansion vessel design follows data in sub- control allowance = 208 + 167.2 + 75.04 + 150 = paragraph 2.12.c.(2): 600.24 cubic feet. V, = 208 cubic feet per calculations above. Length required = 600.24/7r Dz/4 = 600.24/ V2 = '/z-minute reserve for pumps based on the (0.7854X64) = 1.94 tank length in feet . maximum initial load plus the essential load which is the circulating load through the These proportions are not good, therefore assume boiler in this single-pump system: tank length of 25 feet. Then 600.24 = 0.7854 = 344,630 + 751,927 = 1,096,557 lb/hr or: D2(25): D2 = 30.57, D = 5.53 feet. Use tank with 6- V2 = (1,096,557X0.0183 ft3/lbXV2 minute)/60 min- foot inside diameter. Volume for controls = utes = 167.2 cubic feet where specific volume (6)(25X1) = 150 cubic feet, which checks with the is selected for average of T, and Tr minimum recommended. =440 + 290/2 = 365 F. Total volume of tank = (6)2 yr (25)/4 = 706 .86 V, = 0.2(V, + V2) Required computed volume = 600.24 = 0.2(208 + 167.2) = 75.04 cubic feet. Allowance for level controls = (tank Net volume available for sludge = 106.62 cubic diameter)(lengthXl ft). Assume largest practical di- feet. ASHRAE recommendation = 40 percent (V,) ameter = 8 ft and use 150 cubic feet nominal al- = 0.4(208) = .83.2 cubic feet. Therefore, tank lowance. sludge capacity is adequate.

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Information Handling Services, 2000 TM 5-810-2/AFR 88-28 APPENDIX C EXAMPLE DISTRIBUTION LAYOUTS

Selection of a distribution layout will depend upon the particular terrain and the need, if any, for separa- tion of consumer loads. Several example layouts are illustrated. Figure C-1. In this arrangement balancing of the system is easily accomplished and pressure differentials at all building connections are nearly equal.

BUILDING SERVICE CONNECTIONS (TYPICAL)

Figure C-1. Direct Supply, Reverse-Return. INACTIVE

Information Handling Services, 2000

TM 5-810-2/AFR 88-28

Figure C-2. In this arrangement a number of individual circuits are used. Control valve sizing is not as difficult as with direct supply, single circuit (not shown) where pressure at each connection is different.

BUILDING SERVICE CONNECTION (TYPICAL)

-yo- -~1 CENTRAL I I HEATING i PLANT -L-fl4-- I s

0 0

Figure C-2. Direct Supply, Radial.

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Information Handling Services, 2000 TM 5-810-1/AFR 88-28

Figure C-3. Return from each connection is fed back into the loop main. The effect of lower temperatures at connections farthest from the central plant must be considered.

CENTRAL HEATING PLANT

INDIVIDUAL BUILDING (EXAMPLE)

t-j14

BUILDING SERVICE CONNECTION (TYPICAL)

Figure C-9. One-Pipe Loop Main. INACTIVE

C-3

Information Handling Services, 2000 TM 5-810-2/AFR 88-28

Figure C-4. This is a type of distribution that can be used with any layout, where high temperature water is converted for distribution of lower temperature water.

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WN w z N O_ t9 1- z W - W O 2 J Z O m V

3 W

J 9 a N

I ~W K INACTIVEf

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Information Handling Services, 2000

TM 5-810-2/AFR 88-28

The proponent agency of this publication is the Office of the Chief of Engineers, United States Army. Users are invited to send com- ments and suggested improvements on DA Form 2028 (Recom- mended Changes to Publications and Blank Forms) to HQUSACE (CEMP-ET), WASH DC 20314-1000.

By Order of the Secretaries of the Army and the Air Force:

GORDON R. SULLIVAN General, iefof States Army Official: 4 , Chief Staff MILTONwH. HAMILTON Administrative Assistant to the Secretary of the Army

MERRILL A. McPEAK General, USAF Official : ChiefofStaff EDWARD A. PARDINI Colonel, USAF Director of Information Management

Distribution: Army: To be distributed in accordance with DA Form 12-34B, requirements for TM 5-810-2. Air Force: F

*U .S . GOVERNMENT PRINTING OFFICE : 1992 - 309-808

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Information Handling Services, 2000