8. WASTE HEAT RECOVERY - Bureau of Energy Efficiency

1 8. WASTE heat RECOVERY173 Bureau of Energy EfficiencySyllabusWaste heat RECOVERY : Classification, Advantages and applications, Commercially viablewaste heat RECOVERY devices, Saving IntroductionWaste heat is heat , which is generated in a process by way of fuel combustion or chemical reaction, and then dumped into the environment even though it could still be reused for someuseful and economic purpose. The essential quality of heat is not the amount but rather its value . The strategy of how to recover this heat depends in part on the temperature of the wasteheat gases and the economics quantity of hot flue gases is generated from Boilers, Kilns, Ovens and some of this WASTE heat could be recovered, a considerable amount of primary fuel couldbe saved. The Energy lost in WASTE gases cannot be fully recovered.

2 However, much of theheat could be recovered and loss minimized by adopting following measures as outlined inthis Losses QualityDepending upon the type of process, WASTE heat can be rejected at virtually any temperaturefrom that of chilled cooling water to high temperature WASTE gases from an industrial fur-nace or kiln. Usually higher the temperature, higher the quality and more cost effective isthe heat RECOVERY . In any study of WASTE heat RECOVERY , it is absolutely necessary that thereshould be some use for the recovered heat . Typical examples of use would be preheating ofcombustion air, space heating, or pre-heating boiler feed water or process water. With hightemperature heat RECOVERY , a cascade system of WASTE heat RECOVERY may be practiced toensure that the maximum amount of heat is recovered at the highest potential.

3 An exampleof this technique of WASTE heat RECOVERY would be where the high temperature stage wasused for air pre-heating and the low temperature stage used for process feed water heatingor steam Losses QuantityIn any heat RECOVERY situation it is essential to know the amount of heat recoverable and alsohow it can be used. An example of the availability of WASTE heat is given below: heat RECOVERY from heat treatment furnaceIn a heat treatment furnace, the exhaust gases are leaving the furnace at 900 C at the rate of2100 m3/hour. The total heat recoverable at 180oC final exhaust can be calculated asQ = V Cp TQ is the heat content in kCalV is the flowrate of the substance in m3 2/23/2005 11:25 AM Page 1738. WASTE heat Recovery174 Bureau of Energy EfficiencyTABLE WASTE SOURCE AND in flue higher the temperature, the greater the potential value for in vapour above but when condensed, latent heat also and radiant heatLow grade if collected may be used for space heating or airlost from exterior losses in cooling grade useful gains if heat is exchanged with incoming losses in providinga) High grade if it can be utilized to reduce demand for water or in theb) Low grade if refrigeration unit used as a form of heat of chilled stored in productsQuality depends upon the in gaseous and liquidPoor if heavily contaminated and thus requiring alloy heat leaving process.

4 Is density of the flue gas in kg/m3 Cpis the specific heat of the substance in kCal/kg C T is the temperature difference in CCp (Specific heat of flue gas) = kCal/kg/ CHeat available (Q) = 2100 ((900-180) = 4,31,827 kCal/hrBy installing a recuperator, this heat can be recovered to pre- heat the combustion air. The fuel savings would be 33% (@ 1% fuel reduction for every 22 C reduction in temper-ature of flue Classification and ApplicationIn considering the potential for heat RECOVERY , it is useful to note all the possibilities, andgrade the WASTE heat in terms of potential value as shown in the following Table :High Temperature heat RecoveryThe following Table gives temperatures of WASTE gases from industrial process equipmentin the high temperature range.))

5 All of these results from direct fuel fired Temperature heat RecoveryThe following Table gives the temperatures of WASTE gases from process equipment in themedium temperature range. Most of the WASTE heat in this temperature range comes from theexhaust of directly fired process 2/23/2005 11:25 AM Page 1748. WASTE heat Recovery175 Bureau of Energy EfficiencyLow Temperature heat RecoveryThe following Table lists some heat sources in the low temperature range. In this range itis usually not practical to extract work from the source, though steam production may not becompletely excluded if there is a need for low-pressure steam. Low temperature WASTE heat maybe useful in a supplementary way for preheating of DeviceTemperature, CNickel refining furnace1370 1650 Aluminium refining furnace650 760 Zinc refining furnace760 1100 Copper refining furnace760 815 Steel heating furnaces925 1050 Copper reverberatory furnace900 1100 Open hearth furnace650 700 Cement kiln (Dry process)620 730 Glass melting furnace1000 1550 Hydrogen plants650 1000 Solid WASTE incinerators650 1000 Fume incinerators650 1450 TABLE TYPICAL WASTE heat TEMPERATURE AT HIGHTEMPERATURE RANGE FROM VARIOUS SOURCESType of DeviceTemperature, CSteam boiler exhausts230 480 Gas turbine exhausts370 540 Reciprocating engine exhausts315 600 Reciprocating engine exhausts (turbo charged)

6 230 370 heat treating furnaces425 650 Drying and baking ovens230 600 Catalytic crackers425 650 Annealing furnace cooling systems425 650 TABLE TYPICAL WASTE heat TEMPERATURE AT MEDIUMTEMPERATURE RANGE FROM VARIOUS 2/23/2005 11:25 AM Page Benefits of WASTE heat RecoveryBenefits of ' WASTE heat RECOVERY ' can be broadly classified in two categories:Direct Benefits: RECOVERY of WASTE heat has a direct effect on the Efficiency of the process. This is reflected byreduction in the utility consumption & costs, and process Benefits:a)Reduction in pollution:A number of toxic combustible wastes such as carbon monoxidegas, sour gas, carbon black off gases, oil sludge, Acrylonitrile and other plastic chemicalsetc, releasing to atmosphere if/when burnt in the incinerators serves dual purpose recovers heat and reduces the environmental pollution ) Reduction in equipment sizes: WASTE heat RECOVERY reduces the fuel consumption, whichleads to reduction in the flue gas produced.

7 This results in reduction in equipment sizes ofall flue gas handling equipments such as fans, stacks, ducts, burners, ) Reduction in auxiliary Energy consumption:Reduction in equipment sizes gives additional benefits in the form of reduction in auxiliary Energy consumption like electricityfor fans, pumps WASTE heat Recovery176 Bureau of Energy EfficiencySourceTemperature, CProcess steam condensate55 88 Cooling water from:Furnace doors32 55 Bearings32 88 Welding machines32 88 Injection molding machines32 88 Annealing furnaces66 230 Forming dies27 88 Air compressors27 50 Pumps27 88 Internal combustion engines66 120 Air conditioning and refrigeration condensers32 43 Liquid still condensers32 88 Drying, baking and curing ovens93 230 Hot processed liquids32 232 Hot processed solids93 232 TABLE TYPICAL WASTE heat TEMPERATURE AT LOWTEMPERATURE RANGE FROM VARIOUS 2/23/2005 11:25 AM Page 1768.

8 WASTE heat Recovery177 Bureau of Energy Development of a WASTE heat RECOVERY SystemUnderstanding the processUnderstanding the process is essential for development of WASTE heat RECOVERY system. This can beaccomplished by reviewing the process flow sheets, layout diagrams, piping isometrics, electricaland instrumentation cable ducting etc. Detail review of these documents will help in identifying:a) Sources and uses of WASTE heatb) Upset conditions occurring in the plant due to heat recoveryc) Availability of spaced) Any other constraint, such as dew point occurring in an equipments identifying source of WASTE heat and the possible use of it, the next step is to selectsuitable heat RECOVERY system and equipments to recover and utilise the Evaluation of WASTE heat RECOVERY System It is necessary to evaluate the selected WASTE heat RECOVERY system on the basis of financialanalysis such as investment, depreciation, payback period, rate of return etc.

9 In addition theadvice of experienced consultants and suppliers must be obtained for rational section gives a brief description of common heat RECOVERY devices available commer-cially and its typical industrial Commercial WASTE heat RECOVERY DevicesRecuperatorsIn a recuperator, heat exchange takesplace between the flue gases and theair through metallic or ceramicwalls. duct or tubes carry the air forcombustion to be pre-heated, theother side contains the WASTE heatstream. A recuperator for recoveringwaste heat from flue gases is shownin Figure simplest configuration for arecuperator is the metallic radiationrecuperator, which consists of twoconcentric lengths of metal tubing asshown in Figure inner tube carries the hotexhaust gases while the external annulus carries the combustion air from the atmosphere tothe air inlets of the furnace burners.

10 The hot gases are cooled by the incoming combustionair which now carries additional Energy into the combustion chamber. This is Energy whichdoes not have to be supplied by the fuel; consequently, less fuel is burned for a given furnace loading. The saving in fuel also means a decrease in combustion air and thereforeFigure WASTE heat RECOVERY using 2/23/2005 11:25 AM Page 1778. WASTE heat Recovery178 Bureau of Energy Efficiencystack losses are decreased not only by loweringthe stack gas temperatures but also by discharg-ing smaller quantities of exhaust gas. The radi-ation recuperator gets its name from the fact thata substantial portion of the heat transfer from thehot gases to the surface of the inner tube takesplace by radiative heat transfer. The cold air inthe annuals, however, is almost transparent toinfrared radiation so that only convection heattransfer takes place to the incoming air.