Boiler Types

4/19/2012

BOILERS AND THERMAL SYSTEMS; BOILER EFFICIENCY IMPROVEMENT SECTION P

BOILER TYPES

 FIRE TUBE As the name implies, the fire is in the tubes and the water outside. Most of the small “packaged” boilers in buildings and industry today are fire tube boilers and thus are likely the ones we will encounter.

Source: Illustration recreated for web by Technologists Inc. using graphic supplied by The Boiler Efficiency Institute, Auburn, Alabama to PNL, for use with FEMP O&M Best Practices, as a model. Section P - 2

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 WATER TUBE Again, as the name implies, now the water is in the tube and the fire outside. Most of the larger boiler systems today are constructed water tube boilers. There are smaller ppgackaged water tube boilers also. We will see water tube boilers but most of the larger ones are well designed and controlled. Thus, we will not spend much time on water tube boilers.

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PROPERTIES AND USE OF STEAM

 Steam is water in a vapor state  Steam temperature is in °C or K K = C + 273  Steam pressure is in kPa, MPa or bar 1 bar = 100 KPa = 0.1 Mpa  Steam conditions are either:  saturated - temperature or pressure specifies its properties  superheated - must know temperature and pressure to find its properties

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HEAT CONTENT OF STEAM

 The heat content of steam is called its enthalpy (h or H) measured in kJ/Kg

 By definition h = H = 0 at 0°C

 Enthalpy is the sum of the sensible plus latent heat.  Sensible heat - heat absorbed or removed during a change in temperature without a change in state of phase  Latent heat - heat absorbed or removed during a change of state or phase at constant temperature

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STEAM DISTRIBUTION SYSTEM

 The steam distribution system on the next page shows a “typical” distribution system.  The major components include  The steam manifold and piping,  heat exchangers,  steam traps at each use point and at various locations throughout the system,  pressure reducing valves,  condensate return piping,  condensate return tank(s), and  various pumps.

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STEAM SYSTEM

170 C 8 BAR (800 KPA) 150 C & 450 KPA

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Not shown in the previous diagram are:  make up water,  water treatment for the make up water, and  a feed wa ter tan k whi ch is typi call y bet ween the condensate return tank and the boiler.

The feedwater tank will also normally have feedwater preheating and removal of oxygen (deaeration).

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STEAM TRAPS Purposes 1. Reject (Return) Condensate 2. Reject Air 3. Hold Back Steam

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MONITORING In a poorly maintained or non-maintained system, 20% to 30% of the steam traps are likely stuck open wasting significant amounts of expensive steam. To locate failed steam traps, use:

1. Sight (Watch the Discharge) 2. Sound (Listen to the Operation Possibly Ultrasonically) 3. Temperature (Watch the Delta T)

Note: Real Time MMS Available

Other indications that a steam trap may be malfunctioning:  the system has trouble holding pressure,  the condensate return tank is unpressurized and significant flash steam is formed

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COMBUSTION EFFICIENCY  Combustion is an exothermic (heat producing) chemical reaction.  The chemistry for the combustion of methane (natural gas is about 96% methane) is shown below.

CH4 + 2O2  CO2 + 2H2O

+ _O2 + _ O2

+ _N2 + _ N2

+ _ NOx

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CONTROLLING COMBUSTION

 In any closed combustion system such as a boiler or a furnace without secondary air, we can measure precisely what occurred at the burner by carefully measuring the exhaust.  The goal is to be able to carefully control the fuel and airflow to ensure complete and efficient combustion.  We will see why excess air is important and why too much excess air is expensive.

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NATURAL GAS

FLUE GAS ANALYSIS VS. % COMBUSTION AIR

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ALL TEMPERATURES ARE STR IN °C (STACK TEMPERATURE RISE)

New efficiency- Old efficiency Percent savings  New efficiency

Savings  Percent savings  Fuel consumption

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SCALE AND SOOT Whether fire tube or water tube, it is important that the tubes remain clean. On the waterside, scaling can lead to a dramatic loss in efficiency. Scale is a good insulator, thus as scale forms the temperature of the fireside must increase to do the work on the waterside. Water treatment is essential to maintain this condensate return never have a scaling problem. Most of us will fin d it necessary to per io dica lly s hut t he bo iler down an d remove sca le. The Figure on the next page demonstrates the expected amount of loss due to scaling. Reverse osmosis water treatment will almost eliminate scaling problems.

Soot on the fireside has a similar effect. Running good combustion systems slightly on the excess airside ensures complete combustion and minimizes soot. However, soot blowers may be required, especially for certain fuels, and periodic brushing gas fired system, this brushing is likely required about once a year.

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LOAD BALANCING The boiler room is an interesting place. If the boiler room operator keeps the boiler working with adequate pressure everywhere in the plant, no one complains. Normally, there is an extra boiler or two for back up and load variation reasons. Therefore, there is a natural tendency for the operator to fire all the available boilers and run them at part load. If one goes down, the others quickly move up and the pressure is maintained.

Boilers don't run well at low loads and each boiler has about 3-5% “skin losses”, so there is a significant energy penalty for this philosophy. Instead, profiling the efficiencyyygy of each boiler for varying loads will yield data that can be used to determi ne th e opti mum fir ing pro file for any load.

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BOILER BLOWDOWN

Additional details on Blowdown in Appendix Section P - 19

FLASH STEAM When a hot pressurized liquid is placed in a tank with lower pressure, some “flash steam” will form as the enthalpy of the saturated liquid is reduced.

This phenomenon is sometimes a major loss to the steam system (700 kPa blowdown going to an unpressurized vessel will produce significant atmospheric pressure steam whhhich is worthless and thus is a loss). It is easy to calculate how much as shown in the example following.

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FLASH STEAM Alternatively, the high-pressure liquid can be taken to a low pressure tank. The flash steam now has enoug h pressure to use, an d inexpensive low-pressure steam is the result.

The next example shows how to calculate that.

Continuous top blowdown is an excellent source for this purpose and sometimes enough pressure is left in the condensate return to accomplish the same thing.

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BOILER LOG SHEETS As with any other operating equipment, log sheets are extremely important.

They will tell you wh at is happeni ng, wh at is goi ng wrong, and usually what is causing that. It is vitally important to keep good log sheets for all boiler systems. The numbers are usually taken or observed anyway. The log sheet is simply an organized method of keeping those numbers. Log sheets should be developed for individual sites, boilers, and operators.

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BOILER AND STEAM PLANT ECM’S INCREASE BOILER EFFICIENCY 1. Reduce Excess Air to Boiler(s) 2. Provide Sufficient Air to Boiler(s) for Complete Combustion 3. Install Low Excess Air Burner (()s) 4. Repair/Replace Faulty Burner (s) 5. Repair Natural Draft Burner(s) with Forced Draft Burner(s) 6. Install Turbulators in Firetube Boiler(s) 7. Replace Existing Boiler(s) with New More Efficient Boiler(s) 8. Install a Condensing Boiler/Water Heater 9. Install a Pulse Combustion Boiler/Water Heater 10. Install a Small Boiler for Summer Operation 11. Clean Boiler(s) to Eliminate Fouling and Scale 12. Improve Feedwater Chemical Treatment to Reduce Scaling 13. Optimize Boiler Loading When Using Multiple Boilers

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INSULATION 14. Install Insulation on Steam Line(s) 15. Install Insulation Jacket on Steam Fitting(s) 16. Install Insulation on Feedwater Line(s) 17. Install Insulation on Condensate Return Line(s) 18. Install Insulation on Condensate/Feedwater/Deaerator Tank(s) 19. Install Insulation on (Domestic) Hot Water Line(s) 20. Install Insulation on (Domestic) Hot Water Tank 21. Install Insulation Jacket on Boiler Shell 22. Install Insulation to Reduce Heat Loss

REDUCE BOILER LOAD 23. Repair Steam Leak(s) 24. RiRepair FildFailed Steam TT()rap(s) 25. Reduce Boiler Blowdown 26. Return Condensate to Boiler(s)

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27. Shut Off Steam Tracer(s) During Summer 28. Shut Off or Turn Back Boiler During Long Periods of No Use 29. Change Boiler Steam Pressure 30. Replace Continuous Gas Pilot(s) with Electronic Pilot(s) 31. Install Stack Damper(s) to Reduce Natural Draft Heat Loss 32. Pressurize Condensate Return System 33. Reduce or Utilize Flash Steam 34. Reduce Boiler Load and/or Steam Requirement

WASTE HEAT RECOVERY 35. Install an Economizer to Preheat Boiler Feedwater 36. Install Heat Exchanger to Preheat Boiler Makeup or Feedwater 37. Install Heat Exchanger to Recover Blowdown Heat 38. Install Recuperator to Preheat Combustion Air 39. Recover Waste Heat to Supplement (Domestic) Hot Water Demand 40. Recover Heat from Boiler Flue Gas to Supplement Bldg Heat 41. Install Heat Recovery Steam Generator on Incinerator 42. Direct Contact Condensation Heat Recovery

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OTHER 43. Vary (Domestic or Heating) Hot Water Temp. Based on Demand 44. Eliminate Air Conditioning in Boiler Room 45. Install Back Pressure Steam Turbine for Cogeneration 46. Switch to an interruptible Natural Gas Source 47. Request Change to a Different Utility Rate Schedule 48. Switch to a More Economical Fuel Source 49. Install Heat Pump to Supplement (Domestic) Hot Water Demand 50. Replace Electric Boiler(s) with Natural Gas Fired Boiler(s) 51. Ins ta ll a StSate llite BilBoiler 52. Install a Variable Frequency Drive on Pump(s) and Fan(s) 53. Replace On/Off Control System with Variable Firing Rate

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ADDITIONAL PRACTICE EXAMPLES

 1. A large facility assumed that it was too costlyyppg in distribution piping to return condensate to the boiler. Find the savings for utilizing this source of heat for condensate unit heaters to provide space heating for 4,000 hours per year. Assume the temperature leaving the heater is 40°C, the temperature of condensate is125°C, there is 9000 kg/hr of condensate available, and cost of present fuel to heat the building is $8/GJ. Boiler efficiency is 85%.

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SOLUTION FOR EXAMPLE 1 Analysis: qqp = m x Cp x deltaT = (9000 kg/hr)(4.2 kJ/kg°C)(125-40)°C = 3.2 x 106 kJ/hr

Savings = (3.2 x 106 kJ/hr) x (1 GJ/1,000,000 kJ) x ($8.00/GJ) x (4000 hrs/ yr)(1/0 . 85) = $120,471/year

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EXAMPLE 2

 In operating a boiler with dual fuel capability, comment on the lowest cost of fuel given the following.

Natural Gas $5.80/GJ efficiency = 0.92 Residual Fuel Oil $310/ton (40,000 kJ/kg) efficiency = 0.88

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SOLUTION FOR EXAMPLE 2 Analysis: Convert each fuel source to $/GJ delivered

Natural Gas = ($5.80/GJ)/0.92 = $6.30/GJ delivered Residual = ($310/ton)(1/1000 ton/kg) x ((g)1/40000 kg/kJ) x (106 kJ/GJ)(1/0.88) =$8.81/GJ delivered Best Choice: Natural Gas

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EXAMPLE PROBLEM

3. Last year a 20 GJ/hr boiler consumed 19,000 GJ of natural gas at

$8/GJ. The boiler operates at 6% O2 and 350 °C STR. What is the savings for correcting that to 3% O2?

Eff1=75% and Eff2=77% 77 - 75 % savings   0.026 or 2.6% 77 19,000GJ $8.00 $3,948 Cost savings  0.026   Year GJ yr

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AEE/ Certified Energy Manager CEM ® Section P - 32

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4. Now, you can install an economizer that will reduce the stack temperature rise to 200°C. What is the % fuel savings for that change? Eff 1 = 77% Eff 2 = 83% 83- 77 % savings   7.2% 83

3) What is the % excess air for 3% O2 –Natural Gas?

Approximately 15%

What would the CO2 reading be if we measured it?

Approximately 10%

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STEAM EXAMPLES

1a. Find the enthalpy of 800 kPa (8 bar) saturated steam. Using Table H = 2768.7 kJ/kg for 800 kPa steam

1b. Find the enthalpy of 165 °C steam. Using Table H = 2762 kJ/kg for 165°C steam

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2. How much heat is required to raise the temperature of 3000 kg of water from 20°C to 80°C? For 20°C water from Table H = 83.9 For 80°C water from Table H = 334.9 3000 kg (334.9 83.9) kJ x kJ = kg x = 753,000 kJ x ≈0.753 GJ

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3. Find the amount of heat required to convert 3000 kg of boiler feedwater at 90°C to saturated steam at 800 kPa (8 bar). For 90°C water H = 376.9 (Table) For 800 kPa steam H = 2768.7 (Table)

3000kg ()2768.7 376.9 kJ x kJ = kg x = 7,175,400 kJ x ≈7.18 GJ

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APPENDIX

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TYPES OF STEAM TRAPS

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INVERTED BUCKET STEAM TRAP 1) Steam enters trap under bottom of bucket, where it rises and collects at top, imparting buoyancy. Bucket then rises and lifts valve towards its seat until valve is snapped tightly shut. Air and carbon dioxide continually pass through bucket vent and collect at top of trap. Any steam passing through vent is condensed by radiation from trap.

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INVERTED BUCKET STEAM TRAP 2) As the entering condensate starts to fill the bucket, the bucket begins to exert a pull on the lever. As the condensate continues to rise, more force is exerted until there is enough to open the valve against the differential pressure.

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F&T STEAM TRAP

1) When steam reaches the trap, the thermostatic air vent closes in response to higher temperature. Condensate continues to flow through the main valve which is positioned by the float to discharge condensa te at the same rate that it flows to the trap.

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F&T STEAM TRAP

2) As air accumulates in the trap, the temperature drops below that of saturated steam. The balanced pressure thermostatic air vent opens and discharges air.

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THERMOSTATIC STEAM TRAP As the temperature inside the trap increases, it quickly heats the charged bellows element, increasing the vapor pressure inside. When pressure inside the element becomes balanced with system pressure in the trap body, the spring effect of the bellows causes the element to expand, closing the valve. When temperature in the trap drops a few degrees below saturated steam temperature, imbalanced pressure contracts the bellows, opening the valve.

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BOILER BLOWDOWN

Condensate is distilled water and thus is very clean. If it is not all returned, make up water is needed and this water is not as clean. Thus, solids (mud) will build up in the boiler.

Some of that mud will float at the top and some will sink to the bottom. The mud on the bottom would eventually plug the boiler. As the boiler is firing, these residual solids are formed at the steam water interface where some sink and some float.

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To cure these problems, boilers will have to be “blown down” in some fashion. There is usually a top blowdown (skimming) that is often continuous or could easily be automated by blowing down when the conductivity of the water reaches a certain level. This top blowdown is relatively clean and usually a small volume flow. If the boiler is producing 700 kPa steam, then this blowdown is saturated water at 165 C.

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Thus, TOP BLOWDOWN IS AN EXCELLENT OPPORTUNITY FOR WASTE HEAT RECOVERY.

The bottom or mud blowdown is usually done manually, by timers, or aut omati call y. To be sure the mud is adequately removed, this blowdown is usually drawn off in a large line with agitation. Mud blowdown is usually not a good candidate for heat recovery.

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An excellent system takes the high pressure blowdown, develops low pressure steam in the tank described and then runs the liquid left in the low pressure tank through a shell and tube heat exchanger to preheat the make up water. These systems are available commercially.

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BOILER BLOWDOWN (SEE FIGURE)

Purpose: Avoid “mud” build up in boiler Problem: Very hot liquid at boiler drum pressure is rejected

Management:  Maintain proper BD rate  Recover heat from blowdown with a shell & tbtube heat exch anger (conti nuous or top blowdown)  Recover flash steam (see heat recovery Q-8)

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CALCULATING BOILER BLOWDOWN

% BD = A x 100 (B – A)

A = ppm impurities in feedwater

B = ppm allowed in boiler

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BOILER BLOWDOWN EXAMPLE Calculate the percentage of blowdown for a boiler that has an allowable limit of 500 ppm of impurities and uses feedwater with 10 ppm of impurities.

A = 10 ppm B = 500 ppm

%BD = 10 x 100 = 2% 500 - 10

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BLOWDOWN CALCULATION EXAMPLE Calculate the % of flash steam generated by expanding saturated liquid from 10 bar to 2 bar. From steam tables: Hf1 = 762.5 kJ/kg ( 10 bar liquid) Hf2 = 504.7 kJ/kg ( 2 bar liquid) Hfg2 = 2201.5 kJ/kg (2 bar liquid to vapor)

% FLASH = 762.5 – 504.7 x100 = 11.7% 2201.5

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SI STEAM TABLES

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END OF SECTION P

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