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 1 4/19/2012 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. Section P - 3 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 Section P - 4 2 4/19/2012 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 Section P - 5 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. Section P - 6 3 4/19/2012 STEAM SYSTEM 170 C 8 BAR (800 KPA) 150 C & 450 KPA Section P - 7 Not shown in the previous diagram are: make up water, water treatment for the make up water, and a fee dwa ter tan k whic h is typ ica lly be tween the condensate return tank and the boiler. The feedwater tank will also normally have feedwater preheating and removal of oxygen (deaeration). Section P - 8 4 4/19/2012 STEAM TRAPS Purposes 1. Reject (Return) Condensate 2. Reject Air 3. Hold Back Steam Section P - 9 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 Section P - 10 5 4/19/2012 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 Section P - 11 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. Section P - 12 6 4/19/2012 NATURAL GAS FLUE GAS ANALYSIS VS. % COMBUSTION AIR Section P - 13 Section P - 14 7 4/19/2012 ALL TEMPERATURES ARE STR IN °C (STACK TEMPERATURE RISE) New efficiency- Old efficiency Percent savings New efficiency Savings Percent savings Fuel consumption Section P - 15 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 perio dica lly shut the bo iler down and remove scale. 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. Section P - 16 8 4/19/2012 Section P - 17 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 determ ine the optimum fir ing profile for any load. Section P - 18 9 4/19/2012 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. Section P - 20 10 4/19/2012 FLASH STEAM Alternatively, the high-pressure liquid can be taken to a low pressure tank. The flash steam now has enough pressure to use, and 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. Section P - 21 BOILER LOG SHEETS As with any other operating equipment, log sheets are extremely important. They will tell you what is happeni ng, what is going 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. Section P - 22 11 4/19/2012 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 Section P - 23 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 T()Trap(s) 25. Reduce Boiler Blowdown 26. Return Condensate to Boiler(s) Section P - 24 12 4/19/2012 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.