Specifying Steam Surface Condensers cooling water entering the condenser by Elliot Spencer and the quantity of water available. Graham Mfg.Co., Inc., Great Neck, Long Island, N.Y. The steam condensing temperature can get down to as low as 5F above Factors affecting condenser design and selection. the entering cooling water temperature, A typical air conditioning surface condenser specification is included. but in most air conditioning installations the differential is 20F, sometimes 10F, but seldom smaller, as from the turbine manufacturer. the closer approach would require a costly amount of condenser surface. THE FORMULA for determining the Generally speaking, on systems where In practice, cooling tower water is first amount of surface required in a surface the motive steam pressure is used in the refrigerant condenser, condenser is as follows: approximately 150 lb and the turbine is where it rises about 10F and is then Q = UA (LMTD) (1) discharging to a vacuum of 26.inches available for steam condensing. The where of mercury (4 in. Hg absolute) , a steam great bulk of steam-driven cen-trifugal Q = Heat to be absorbed, Btuh = rate of 14-16 lb per hr per bhp is refrigeration systems operate with a pounds per hour of steam times 950 economically feasible. Although steam condenser vacuum of Btu per pound; greater efficiency is available at approximately 4 in. Hg abs., which is U = Overall heat transfer rate, Btu per increased cost, utility costs for this equivalent to a condensing (hr) (sq ft) (F); type of installation seldom justify temperature of 125.4F. Thus, referring A = Surface required, sq ft; reducing the steam consumption by to Fig 1 and presuming that 85F cooling water is supplied to the system LMTD = Loge mean temperature means of higher ef-ficiency turbines or difference between condensing lower condensing pressures. When from the cooling towers, water temperature and cooling water. special conditions are present, the temperature into the steam condenser No attempt will be made to go through specifying engineer may consider is approximately 95F. Assuming a a formal condenser selection. The either of the above modifications to turbine steam rate of 14-16 lb per hr per procedure for condenser rating is improve performance. bhp (bhp per ton of refrigeration is amply covered in the publication of approximately 1) and a condensing Heat Exchange Institute entitled, Condenser Vacuum water circulation rate of 3 gpm per ton "Standards for Steam Surface The vacuum produced in a steam of refrigeration, water rise on the steam Condensers."* Rather we shall attempt condenser is that pressure condenser is also about 10F. Thus, the to discuss qualitatively the factors corresponding to the temperature at leaving water temperature approach is affecting selections and their effects which the steam condenses. This 20F(125F minus 105F). on size and price. steam condensing temperature, in turn, is dependent upon the temperature of Turbine Steam Rate Turbine steam rate is usually expressed in pounds per hour of steam per brake horsepower and is commonly called the water rate. This water rate is a function of motive steam pressure and temperature, condensing temperature, or equivalent vacuum, and turbine efficiency. Specific information for a given turbine application can be obtained ______This Standard is currently out of print, but a new edition is in preparation and will be available in the future from Heat Exchange Institute, 122 E. 42nd St., 'New York, N.Y. 10017. If, on the other hand, we designed the be specified. This temperature is accessory items have built-in excess steam condenser for 3 in. Hg. abs., determined by the water source and capacity), no statement of excess which is equivalent to a condensing varies with the location of the capacity need be made. temperature of 115F for the same installation. Water flow rate to the approximate water temperature rise, the condenser is usually determined by Water Pressure Drop approach would be 10F (115F minus the refrigeration manufacturer and is Water side pressure drop is a function 105F). Thus from Eq (1), because the generally set at approximately 3 gpm of tube velocity, condenser length and log mean temperature difference is per ton of refrigeration when cooling the number of passes the water has to greater, we see that the condenser towers are used. However, when river make in a given condenser. Air designed for 4 in. Hg abs. requires less water or water at relatively low conditioning practice permits surface than the one designed for 3 in temperature is available, water flow velocities as high as 10 fps in the tube Hg abs. rate varies and specifications should side of refrigerant condensers and, in The foregoing example is an take these variations into account. recent years, this has been extended to oversimplification. In practice, the A recent trend in large multi-story apply to steam surface condensers for steam rate of a turbine discharging at 4 office buildings has been to use 2 gpm air conditioning installations. in. Hg. abs. is somewhat higher than per ton of refrigeration. Although this It has been common practice to limit one discharging at 3 in. Hg. abs. affects condenser, and turbine water pressure drop to 10 ft. However, Choice of a lower absolute pressure adversely due to higher condensing if this limitation is rigidly enforced, might permit using a turbine with one temperatures, there is a saving in velocities must be reduced to comply less wheel, which would result in condenser water piping. with this pressure drop requirement, reduced turbine cost. This reduced Too often, specifications state that the resulting in lower heat transfer rates turbine cost might or might not be surface condenser shall be capable of and higher condenser costs. Generally, offset by the increased condenser handling turbine exhaust while water pressure drops of approximately cost. (See "Is There a Steam Turbine in maintaining the desired vacuum with a 15 ft can be attained in most single- Your Future," ACHV, April, 1968, pp 5% increase in normal steam load. pass condensers operating at 4 in. Hg 43-47.) Similarly, condenser cost could Compliance with this requirement is abs. Most two-pass condensers, be reduced if the condenser were almost impossible. Specifications usually those running at 3 in. Hg abs., operated at 5 in. Hg. abs. (133.8F seldom provide the 5% greater water require pressure drops as high as 20-30 condensing). However, turbine cost quantity needed to balance this 5% ft. might increase more rapidly if the loading factor. Further, usually no Specification of water velocity and steam rate had to be held to a provision is made for the extra load on materials of construction for tubes and previously set value, which, as the cooling tower and the change in tube sheets where river water or indicated, is generally in the range of steam rate of the turbine when brackish water is involved must be 14-16 lb per hr per bhp. overload hand valves are used. carefully evaluated. Information Thus, the specification should simply Furthermore, the refrigeration machine should be obtained on the experience state the number of pounds per hour is operating at a different point on its of other users in the area having of steam to be used and the maximum curve and may be rejecting more heat similar design conditions before a temperature and quantity of available which, in turn, means hotter water to specific material is selected. Generally cooling water, omitting the precise the steam condenser. The net result speaking, for brackish water operating vacuum to be maintained by under the 5% excess load condition is applications, velocities in excess of 6.5 the surface condenser. This permits that the steam condenser receives fps can seriously reduce tube life, but the manufacturer to offer the best cooling water that is more than 5% for cooling tower applications a 10 fps combination of condenser, turbine and above the initial design temperature velocity is acceptable. refrigeration unit to comply with the and the resulting excess heat imparted limitations on steam consumption to the water overloads the cooling Fouling Factor specified. tower, further aggravating the Most surface condensers for air Other considerations, such as length situation. conditioning and power plant limitations and turbine vapor exhaust If it has been decided that the surface applications are designed with a size, very often have a determining condenser should have additional cleanliness factor of 85%. This means affect on condenser cost. surface in it, the most practical that the heat transfer rate used in approach would be to state that the designing the condenser is 85% of the unit shall have 5% excess surface or Water Temperature clean heat transfer coefficient. For 10% or whatever is needed. In general, As cooling water temperature those installations where cooling water a surface condenser will handle the determines the vacuum that can be sources produce rapid fouling of produced, it is imperative that the entire excess load at a slight increase maximum anticipated water temperature in back pressure. If this is permissible (and it must happen unless all the tubes, a higher factor must be field for proper vapor velocities within leakage quantities based on various used. This holds true for both the the shell. team quantities entering the refrigerant condenser and the steam Calculation of condenser heat load is condenser. Specification that the condenser. based on the fact that steam entering ejector shall be designed for the For most applications using clean the condenser has a value of quantities of air leakage as designated cooling tower water, refrigeration approximately 950 Btu per lb. More in HEI Standards is sufficient. condensers should be specified with a precise calculations can be made if the fouling factor of 0.0005 and steam specific turbine characteristics are Codes surface condensers designed for an available, but this figure is sufficiently 85% clean tube coefficient. If the accurate for the sizing of the cooling Steam surface condensers operate refrigerant fouling factor is increased, tower and condenser. under a vacuum and are, therefore, not the surface condenser cleanliness considered pressure vessels. The factor should be increased by the same Space Limitations ASME Code is a pressure vessel code percentage. Thus, if the refrigerant and is not, strictly speaking, applicable condenser is specified as 0.001 vs. Space considerations often influence to surface condensers operating under 0.0005, the corresponding cleanliness the cost of a condenser. For example, if a vacuum. However, the tube side of a factor for the surface condenser is 70% the surface condenser is limited in tube surface condenser is considered a clean vs. 85 % clean. length, a multipass design may be pressure vessel, as it is subjected to required in place of a single-pass the full water pressure. When Steam Inlet Velocity design. This results in a short, stubby necessary, this side of the condenser unit, which is generally more expensive can be designed and constructed to Velocity at the exhaust of the steam than a long, thin unit. ASME Code requirements. Most turbine is limited by National Electrical Specifications of head room limitations surface condensers are designed and Manufacturers Association to 450 fps. and permissible condenser tube length constructed in accordance with HEI Should the exhaust line from turbine to should take into consideration the Standards. They can be built to ASME condenser be long, there can be distance beyond the tube sheets of the Code requirements and so stamped by excessive pressure drop in the vacuum condenser required to allow removing a qualified inspector. This type of line, which means that pressure at the tubes for replacement. Condensers construction, which requires specific condenser inlet is penalized by the must be mounted so that liquid level methods of welding, specially qualified pressure drop between turbine and on the hotwell is sufficiently high to welders and keeping of material condenser. Condenser manufacturers permit removal by condensate pumps. records with certified copies of these usually guarantee to maintain the For air conditioning installations, the records to be supplied, results in a specified vacuum at the inlet to the liquid level should be at least 5 ft more expensive surface condenser. steam condenser. However, pressure above the inlet to the condensate However, if the unit is to be built to drop between turbine exhaust and pump and preferably 1 or 2 ft higher. ASME Code requirements, it should be steam condenser must be calculated This permits an economical pump stated that both the shell and tube by the design engineer and selection. sides, or simply the tube side, shall be compensated for by specifying that designed, constructed and stamped in the condenser shall maintain a vacuum Air Leakage accordance with ASME Code and shall slightly below the vacuum at the be accompanied by certifications exhaust of the turbine. A specification As both the exhaust of the steam signed by a qualified insurance that calls upon the supplier to supply a turbine and the condenser are inspector. Do not be mislead by the specified steam consumption obviates operating under a substantial vacuum, statement, "Condenser is constructed this problem. Where a given velocity is air is bound to leak into the system. in accordance with applicable ASME used to determine the pressure drop, This leakage occurs through the gland Codes." It is common for some this should be stated in the seals on the steam turbine and through manufacturers to indicate that, as specification so that it is not exceeded. minute holes in the piping connections ASME is a pressure vessel code, it is In any event, velocities in excess of associated with the surface condenser not applicable to vacuum condensers 450 fps should not be permitted. itself. Over a period of years, Heat and therefore their equipment complies Impingement plates should be Exchange Institute has determined the with applicable ASME Codes. provided below the condenser steam normal quantity of air that should leak inlet to protect tubes from the effect of through properly designed turbines locally high inlet velocities and and piping systems, and these are impingement of condensate. Steam specified in their "Standards for inlet velocities should not exceed 400 Surface Condensers." Manufacturers fps and provision should be made have similarly standardized their based upon design experience in this ejector sets so that several standard sizes are available for specific air Materials of Construction which are the only moving parts in a surface condenser system should be Materials used in construction of supplied in duplicate. Power plant steam surface condensers are given in practice usually requires twin air Table 1. ejector sets, one a standby, but for air Use of copper alloy tube sheets in conditioning installations, a single set steam surface condensers with the is sufficient. accompanying requirement that tube sheets be bolted to the shell by means Ejectors of collar bolts, is a carryover from marine practice. There is no reason For condensing pressures of 5 in. Hg why a steel tube sheet cannot be used, abs. (25 in. of vacuum) and lower, a in view of the fact that the refrigeration two-stage ejector system is condenser just upstream of the steam recommended because, as the condenser uses this type of condenser water temperature drops, construction. When a steel tube sheet the two-stage ejector permits operating is used, the specification should at reduced condenser pressure that, in indicate that it may be welded to the turn, reduces overall steam shell. consumption of the turbine. Also, as these systems are designed on the Specification Considerations basis of maximum summer temperatures, a two-stage ejector When tube alloys that differ from the provides operating economies during standard admiralty material are used, those days when maximum water ratings must conform to HEI temperature levels are not reached. Standards. These standards reduce the It is important in specifying steam heat transfer rate to conform with the ejectors that the minimum anticipated characteristic performance of various steam pressure be stated. Allowance alloys. Similarly, when tube gages should be made for pressure drop must be increased above the normal 18 between the source of steam and the BWG, heat transfer rates should be point at which steam enters the corrected in accordance with HEI ejectors. If an ejector is designed for Standards to compensate for increased 100 lb steam and lower pressure steam tube wall resistance. is delivered, the ejector may not function properly; if the pressure is It is therefore recommended that It is not uncommon in power plant condensate be used as a cooling practice to use a divided water box more than 10% below design level it will fail to operate altogether. medium for the inter and construction on surface condensers, aftercondensers, as condensate is but this type of construction is Specifications should state that design air leakage for the ejector system be in practically pure water and contains unusual in air conditioning practice, as little or no solids. A typical schematic air conditioning equipment is seldom accordance with HEI Standards. When more than one turbine discharges into control diagram using condensate over critical. However, where cooling water the inter and aftercondensers is shown is particularly dirty, fouling of the a common condenser, the ejector must be designed for 1 1/2 times the normal in Fig. 2. tubes may be rapid and, under these Condensate from the intercondenser circumstances, a divided water box air leakage for the total steam quantity. For more than two turbines, other must be drained back to the hotwell, as condenser might have some the intercondenser is under a vacuum. advantage. correction factors are applicable. The cooling medium for the inter and This can be done by means of a Corrosive effects on water boxes condensate trap or, where headroom should be compensated for by use of aftercondensers of the two-stage ejector system can be cooling water permits, by means of a loop seal, which alloy cladding, fiber glass reinforced must be at least 8 ft high. Condensate polyester lining or similar procedure. from the cooling tower or condensate from the hotwell of the surface from the aftercondenser can be drained back to the hotwell through a trap or Auxiliary Equipment condenser. As intercondenser condensing temperature is about 145F may simply be drained to an open and aftercondenser condensing sight drain. The writer prefers the Selection of auxiliary equipment to be temperature is about 212F, fouling of second method used in conjunction with the surface the inter and aftercondensers will be condenser can be very important. more rapid than that of the main Failure of the ejector set or condensate condenser. pump is critical to the operation of the entire system. The condensate pumps, possible. The greater the available overflow to drop into an open sight NPSH, the easier job the pump has and drain. Thus, as long as there is an because it eliminates the possibility of the less danger there is of cavitation. overflow, we know that the valve is a malfunction of this trap, which could Condensate pumps are generally of the supplied with water. bleed air back to the hotwell. horizontal close coupled type, 3450 rph and with open drip-proof motors. Liquid Level Control Pumps The method of control used depends Condensate pumps for air conditioning Relief Valves upon whether or not condensate is are customarily of the horizontal type, Surface condensers most commonly used as the cooling medium for the designed for approximately 4 1/2 - 5 ft use atmospheric relief valves of the inter and aftercondensers of the net positive suction head (NPSH) and water-sealed or O-ring type, although ejectors. When condensate is used as total dynamic heads of from 80-150 ft. bursting discs are occasionally the cooling medium, a scheme such as Actual NPSH available for these applied. Size and rating characteristics that shown in Fig. 2 should be used. If pumps depends upon the physical for these valves are given in HEI condensate is simply dumped location of the condenser. The Standards. In operation, a small overboard, then a simple mechanically discharge head is affected by where amount of water is kept above the operated level control on the discharge the condensate is pumped after it relief disc to prevent the inleakage air. of the condensate pump is sufficient. leaves the condensate pump; that is, Water in the valve must be The arrangement shown in Fig. 2 to an open condensate tank or back to supplemented periodically, as a certain requires that all of the condensate be the cooling tower, etc. The importance amount will evaporate. A sight glass is pumped over the inter and of providing sufficient NPSH for provided, but because relief valves are aftercondensers before diverting a proper operation of the condensate usually located at a considerable portion back to the hotwell, or pumps cannot be overemphasized. The height and the sight glasses are overboard, depending upon load condenser should be located frequently obscured by sediment, they conditions. sufficiently high off the floor to put as are difficult to read. This author prefers much head on the inlet of the to supply a small constant drip of condensate pumps as is physically water to the valve, permitting the Minimum Simplified Specification for Steam Surface Condensers in Air Conditioning Practice

Vacuum at inlet to steam condenser______Cleanliness factor______Steam flow to condenser, lb per hr______(85% clean for cooling tower applications) Condenser water inlet temperature, F______Waterbox design pressure______Condenser water gpm______Water test pressure______Permissible condenser water pressure drop, ft______(1.5 times design pressure except for cast iron Number of water passes______which is 5 Lb above design pressure)______(Single-pass for 26 in. of vacuum and two-pass Shell side design pressure ______for .27 in. Should be left up to supplier) . (Full vacuum test to 20 Lb) Maximum water velocity, fps______Steam pressure and temperature at ejector inlet______10 fps for cooling tower applications) Condensate pump capacity, gpm ______Tube diameter and gage______Condensate pump net positive suction head______(3/4 in. X 18 BWG) Condensate pump discharge head______(Add pressure drop through inter- and aftercondensers and overboard valve to total system loss.) Electrical characteristics of pump motors ______

1. The steam condenser shall be of the shell and tube type, single or multipass, designed and constructed in accordance with Heat Exchange Institute (HEI) Standards, except as noted herein, and shall be suitable for the design conditions noted above. Condenser manufacturer shall demonstrate successful installations of similar equipment that has been in operation for at least______(5 or 10) years.

2. Condenser shall have cast iron or steel waterboxes, steel tube sheets, steel tube supports, admiralty tubes and a steel hotwell complete with screen, strainer and hand-hole clean-out. Hotwell shall be of the vertical tank type, suitable for one-minute storage capacity.

3. Provision for tube expansion shall be made either by means of bowed tubes or shell side expansion joint. Covers of the waterboxes shall be removable without disturbing water piping. Connections shall be provided in the hotwell for ejector drain, condensate outlet, liquid level control bypass, fresh water startup, gage glass and high level water alarm. Connections shall also be supplied in the condenser shell for pump vent, vacuum gage, pressure gages to be located on waterboxes, an atmospheric relief valve and steam inlet from the turbine drive. Waterboxes shall be supplied with a plugged vent and drain and shall have flanged water connections. All vapor side flanges shall be 125 lb FF. Water side flanges shall be______lb (raised or flat face, depending upon pressure).

4. A single-element, two-stage ejector assembly, complete with internal interconnecting water and steam piping with surface inter- and after-condenser shall be provided. Ejector shall be cast iron. Ejector motive steam nozzles shall be stainless steel. The two-stage ejector shall be a self-contained package, complete with steam valves, interconnecting steam piping, steam strainer and interconnecting vapor piping, and shall be provided with mounting bracket so that ejector can be mounted directly on the surface condenser.

5. Condensate pumps shall be supplied in accordance with design conditions and shall be close coupled, single suction, enclosed impeller type with cast iron casings, bronze or cast iron impellers, stainless steel shafts, shaft seals suitable for the service, grease lubricated bearings and open drip-proof motors. Pump motor starters will be provided by others. Electrical characteristics will be as specified under ______herein.

6. A hotwell gage glass shall be provided.

7. A steam inlet expansion joint shall be provided, sized to conform with the diameter of the turbine exhaust with corrugations. Expansion joint shall have copper or stainless steel bellows and shall be provided with a liner if steam velocity exceeds 300 fps.

8. An atmospheric relief valve shall be furnished, sized for protection in accordance with HEI Standards. head beyond the overboard control valve. Expansion Joint Gages and Thermometers A single or multi-corrugation A combination pressure and expansion joint should be placed vacuum gage of the dial type should between the exhaust of the turbine be supplied to give a rough and the inlet to the surface indication of condenser vacuum. condenser. These expansion joints Exact vacuum can be properly usually have copper or stainless determined only by use of a mercury steel bellows and a steel flange. manometer, which can be specified. However, as the cost of copper Pressure gages should be installed increases, stainless steel joints are on the inlet and outlet water boxes. replacing them. The number of It is also common practice to install corrugations is determined by the pressure gages at the condensate amount of expansion that must be pump discharge, as well as at the accommodated by the joint and inlet to the condensate pump, so the should be determined by the When condensate quantity exceeds operating engineer can easily check engineer prior to completing the that required by the inter and the performance of pump and specification. When the number of aftercondensers, a suitable bypass condenser. In addition, corrugations is not stated, a single- should be supplied. thermometers should be placed so corrugation joint is supplied, which Selection of air-operated level that inlet and outlet water may or may not be sufficient for the control valves is dependent upon temperatures, as well as temperature job. information supplied by the of the condensate in the hotwell of When steam velocities through the consulting engineer; namely, the the surface condenser, may be read. expansion joint exceed 300 fps, it is discharge head beyond the Air Leakage Meter advisable to install a stainless steel overboard level control valve and Most operating engineers of air liner to prevent noise and excess the total dynamic head the pump conditioning installations judge vibration of the joint. must develop. Specifications should excessive air leakage by the amount It is possible to obtain reduced indicate whether the total dynamic of steam and droplets of condensate steam consumption simply by head stated for the pump has taken issuing from the aftercondenser designing the surface condenser for into account the condensate vent. However, an air leakage meter a higher vacuum. Relative costs of pressure drop through the inter and placed on the vent of the condensers designed for 26 and 27 aftercondensers and the overboard aftercondenser is more reliable and, in. of vacuum are given in Fig. 3. valve. Generally, pressure drop as it represents an investment of through the inter and approximately $150, is well worth the aftercondensers is less than 15 ft expense. and that on the overboard valve is 10-30% of the available discharge