1、11FTM26AGMA Technical PaperAtmosphere FurnaceHeating SystemsBy J.W. Gottschalk, SurfaceCombustion Inc.Atmosphere Furnace Heating SystemsJohn W. Gottschalk, Surface Combustion Inc.The statements and opinions contained herein are those of the author and should not be construed as anofficial action or
2、opinion of the American Gear Manufacturers Association.AbstractA detailed evaluation of furnace heating systems will be performed. Topics of discussion will includeapplication guidelines for both gas fired and electrically heated furnaces. Heating system selection willconsideroperatingtemperature,pr
3、ocessingatmosphereandheatingmethod(radiantorconvectiveheating)along with heating system orientation within the furnace chamber.The evaluation will consist of a comparison of operating costs, environmental considerations and lifetimemaintenance costs of the various systems. Systems to be evaluated co
4、nsist of alloy radiant tubes (singleended, U-tube, etc.), ceramic radiant tubes (single ended and U-tubes) and a variety of electrical elementdesigns. Actual case studies of the various heating systems will be presented with respect to maintenanceand operating costs.Copyright 2011American Gear Manuf
5、acturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-026-13 11FTM27Atmosphere Furnace Heating SystemsJohn W. Gottschalk, Surface Combustion Inc.Oneoftheprimaryprocessesassociatedwiththemanufacturingofgearsisheattreatment. Theseprocessesinclude s
6、tress relieving, normalizing, hardening and tempering. Simply stated, heat treatment is therequirementtoheatpartstoaprescribedtemperaturenecessarytoachievespecific metallurgicalproperties.Heating methods used include fuel fired, electric and induction heating systems. This article will discussprimar
7、ily fuel fired and electrically heated systems as applied to conventional atmosphere heat treatingfurnaces.Some of the key criteria used to evaluate the proper heating system for a given application are listed below:S Installed cost of the heating systemS Operating cost of the heating systemS Life t
8、ime maintenance cost of the heating systemS Contribution of heating system to part qualityS Environmental concernsForthepurposesofthisarticle,thecarburizingprocesswillbeusedtoevaluatethecriteriadefinedabove. Themost common type of carburizing furnace used in industry today is the batch integral quen
9、ch (BIQ) furnace(Figure 1). This furnacedesignwillserveas thebasis of our evaluation, whichcanthen beused for compar-isontootherbatchandcontinuousheattreatmentsystems. Thespecificationsandparametersof acommonBIQ furnace and processing cycle are listed below (Table 1).Figure 1. Allcase batch integral
10、 quench (BIQ) furnaceTable 1. Common batch integral quench (BIQ) process cycleLoad size 36” wide X 48” long X 36” highLoad weight 2500 pounds including parts and fixturesProcessing temperature 1700FProcess CarburizingCycle time 6 HoursProcessing atmosphere Endothermic gas with natural gas enrichment
11、Net heat release 150 kw/hour (512,000 BTU/hour)4 11FTM27Inordertosimplifythecomparisonofthevariousheatingsystems,heatingsystemoutputswillbeconvertedtokilowatts to allow for thermal efficiency calculations required for fuel fired systems. Their net heat release isdependent upon the degree of heat rec
12、overy from fuel fired heating system exhaust gasses.The following heating systems will be compared:S Gas fired alloy radiant U tubes with external recuperator (Figure 2)S Gas fired single ended alloy radiant (SER) tubes with internal recuperator (Figure 3)S Gas fired single ended silicon carbide rad
13、iant tubes with internal recuperatorS Electric heating elements in single ended radiant tube (Figure 4)S Prolectricelectric heating elements directly in furnace atmosphere (Figure 5)For the purposes of this evaluation, a net heat release of 50 BTU/inch2/hr and 90 BTU/inch2/hr have beenused for the a
14、lloy and silicon carbide materials respectively. The parameters of each system are listed inTable 2.Figure 2. Gas fired alloy radiant U tubes with external recuperatorFigure 3. Gas fired single ended alloy radiant (SER) tubes with internal recuperator5 11FTM27Figure 4. Electric heating elements in s
15、ingle ended radiant tubFigure 5. Prolectricelectric heating elements directly in furnace atmosphereTable 2. Heating system operating parametersAlloyU-tubesAlloy SERSiliconcarbide SERElectricelement inSERProlectricelement infurnaceatmosphereQuantity of tubes 4 8 6 8 6Tube diameter 8” 8” 6” 8” 1.5”Tub
16、e material HT cast alloy HT cast alloySiliconcarbideHT cast alloy 330 SSTotal alloy area 10240 in210240 in25690 in210240 in210240 in2Total pounds alloy provided 280 lbs 280 lbs N/A 280 lbs 320 lbsThermal efficiency 60% 70% 70% 100% 100%Heating system operatingcost per 2500 pound load1)$21.30 $18.24
17、$18.24 $37.70 $37.70Installed cost of system2)$72,000 $90,000 $115,000 $55,000 $82,000Annual operating cost3)$21,920 $18,240 $18,240 $37,700 $37,70020 year utility cost $425,000 $365,000 $365,000 $754,000 $754,000NOTES:1)For the purposes of operating cost evaluation, a natural gas cost of $10.00 per
18、 1000 cubic feet of 1000 BTU/CFnaturalgaswasused. Anelectricrateof$0.10perKWHwasusedforcomparison. Thechartshownbelow(Figure 6)shows equivalent gas and electric rates.2)The installed costof thesystems reflects all ofthe necessary components requiredto makean operationalsystem,i.e., burners, radiant
19、tubes, heat recovery devices, air blowers, exhaust hoods, piping and safety components,electrical transformers and installation labor.3)Annual operating costs based on a 6,000 hour per year schedule.6 11FTM27Figure 6. Equivalent natural gas vs. electric ratesThe lifetime operating costs of the vario
20、us heating systems are evaluated in a different way than the initialinstalled cost of the equipment. As all of the systems havesignificantly different maintenance practices, thedifferent maintenance functions of the furnace heating systems will be evaluated along with typical failuremodes of the sys
21、tems. This evaluation will be carried out through the first 20 years of the system operation(seeTable 3). Theprimarycauseoftube/elementfailureinthisevaluationisconsideredtobethroughcarbur-izationofthealloycomponentexposedtothefurnaceatmosphere. Inthecaseofthesiliconcarbidetubesthatare not subject to
22、 carburization, failure is generally caused by contact of the load with thetubes or breakageduring other maintenance activities, i.e., re work of furnace refractory systems.Table 3. Lifetime maintenance costsMaintenance requirementsAlloyU-tubesAlloy SERSiliconcarbideSERElectricelement inSERProlectri
23、celement infurnaceatmosphereOuter tube replacement 3 years 3 years 10 years 3 years N/ARecuperator replacement 10 years 10 years 10 years N/A N/AHeating element replacement N/A N/A N/A 3 years 5 yearsAir blower replacement 20 years 20 years 20 years N/A N/ATransformer/SER replacement N/A N/A N/A 20
24、years 20 yearsAnnual maintenance labor 40 hours 40 hours 40 hours 40 hours 20 hoursLife time maintenance costsover 20 years1)$285,000 $175,000 $125,000 $250,000 $135,00020 year utility cost $425,000 $365,000 $365,000 $754,000 $754,000Total 20 year cost of system $710,000 $540,000 $490,000 $1,004,000
25、 $889,000NOTE:1)Includes labor cost to change elements and/or tubes and burners.7 11FTM27There are several areas where the heating system design affects the quality of parts produced within thesystem. The primary two areas are temperature uniformity and atmosphere contamination of the system.As the
26、heating system is the primary source of energy within the heating chamber, its ability to uniformlydeliver heat to the parts is critical to insure that uniformities are within acceptable limits. The uniformity of asystemistypicallymeasuredasarangeaboveandbelowthefurnacesetpoint. Optimizeduniformitie
27、softhevarious systems are shown in Table 4.Asseenbythedata,allsystemsarewithintheacceptableranges withelectric heatingsystems deliveringthebest over all uniformity.Themost commoncauseof atmospherecontaminationwithintheheatingchamber is abreachin theradianttube/element. Throughcarburizationorcreepfai
28、lurearethemostcommonreasonandresultsfromextendedexposuretoelevatedtemperatures andcarbonpotentials. Other failuremodes areimproper combustionsetup leading to localized overheating of the radiant tubes or short circuits within the electric heating system.Typicalcausesofshortcircuitsarescalebuildupwit
29、hintheradianttubeintheSERsystemorcarbonbuildupin power feed through areas in the case of the Prolectric electric heating system.Evidence of the tube/element failure is exhibited by a decrease in furnace carbon potential caused byincreased CO2and water vapor in the furnace atmosphere.Risk of atmosphe
30、re contamination from the various systems is ranked in Table 5.The silicon carbide SER offers the best resistance to furnace contamination in that it is both resistant tocarburization and creep based on the physical properties of silicon carbide. The prolectric electric elementdirectly in the furnac
31、e atmosphere offers benefits over the other alloy systems in that the element does notcarburize due to special consideration within the design.Anareaof growingconcernwithallheatingsystem applications istheenvironmentalfoot printof thesystem.Of major concernaregreenhousegasses associatedwithglobalwar
32、mingandacidrain. Expectedemissionvalues for the various systems are listed in Table 6.Table 4. Heating system temperature uniformityTemperatureuniformityAlloyU-tubesAlloy SERSiliconcarbideSERElectricelement inSERProlectric elementin furnaceatmosphereUniformity at 1700F10 F10 F10 F5 F5 FTable 5. Atmo
33、sphere contamination riskAtmospherecontaminationAlloyU-tubesAlloy SERSiliconcarbideSERElectricelement inSERProlectric elementin furnaceatmosphereRisk factor Highest Highest Lowest Highest MediumTable 6. Heating system emissions comparisonEnvironmentalAlloyU-tubesAlloy SERSiliconcarbide SERElectricel
34、ement inSERProlectricelement infurnaceatmosphereLifetime pounds CO25,700,000 lbs 4,885,000 lbs 4,885,000 lbs 10,100,000 lbs 10,100,000 lbsLifetime pounds NOX4,500 lbs 3,000 lbs 3,000 lbs 5,750 lbs 5,750 lbsLifetime pounds SO240 lbs 40 lbs 40 lbs 100,000 lbs 100,000 lbs8 11FTM27The values presented a
35、re typical totals based on published burner ratings operating on natural gas. Valuesgenerated for the electric systems are based on a U.S. Department of Energy (DOE) and American WindEquipment Association (AWEA) reports for average power plant emissions within the United States. Thesignificantly hig
36、her emissions from the electrically heated systems are attributed to the high percentage ofelectricity that is generated from coal combustion. In areas where the electrical generation is more heavilyshiftedtonaturalgas,nuclear,ornoncombustionsources,certainemissionvaluescouldbereducedbyupto80%. Futu
37、re generations of coalfired power generation technology suggest improvedefficiencies whichwillmake natural gas and electrically heated furnaces equivalent with regard to CO2emissions with futurereductions in SO2expected from improving gas scrubbing technologies.ConclusionsAll of the furnace heating
38、system technologies presented arecapable of producing quality parts withvaryingdegrees of maintenance and risk of atmosphere contamination from system failures. Selection based onperformanceof thesystem wouldsuggest that electric systems offer somebenefits insystem uniformity andreduced maintenance
39、costs.However, these benefits will be over shadowed by the operating cost of the system over the lifetime of theequipment. Only in areas where natural gas or other fuels are not available or where electric rates aresubstantially lowerthannationalaverages, doelectrically heatedsystemsofferthepossibilityof financialpayback over gas fired systems.And finally, the environmental footprint of the gas fired systems will in almost all cases be substantially lessthan an electrically heated system due to the significant shift in emissions associated with coal fired powergeneration.
