INDUSTRIAL FURNACES
[FIRST PA [-7], (1) LINES ——— NORMAL PGENDS: [-7], (1) CONTENTS INDUSTRIAL HEATING PROCESSES INDUSTRIAL PROCESS HEATING FURNACES / CLASSI?CATIONS OF FURNACES / FURNACE CLASSI?CATION BY HEAT SOURCE / FURNACE CLASSI?CATION BY BATCH OR CONTINUOUS, AND BY METHOD OF HANDLING MATERIAL INTO, THROUGH, AND OUT OF THE FURNACE / FURNACE CLASSI?CATION BY FUEL / FURNACE CLASSI?CATION BY RECIRCULATION / FURNACE CLASSI?CATION BY DIRECT-FIRED OR INDIRECT-FIRED / CLASSI?CATION BY FURNACE USE / CLASSI?CATION BY TYPE OF HEAT RECOVERY / OTHER FURNACE TYPE CLASSI?CATIONS / ELEMENTS OF FURNACE CONSTRUCTION / REVIEW QUESTIONS AND PROJECTS / HEAT TRANSFER IN INDUSTRIAL FURNACES HEAT REQUIRED FOR LOAD AND FURNACE / HEAT REQUIRED FOR HEATING AND MELTING METALS / [-8], (2) LINES HEAT REQUIRED FOR FUSION (VITRI?CATION) AND CHEMICAL REACTION / FLOW OF HEAT WITHIN THE CHARGED LOAD / THERMAL CONDUCTIVITY AND DIFFUSION / LAG TIME / HEAT TRANSFER TO THE CHARGED LOAD SURFACE / CONDUCTION HEAT TRANSFER / CONVECTION HEAT TRANSFER / RADIATION BETWEEN SOLIDS / RADIATION FROM CLEAR FLAMES AND GASES / RADIATION FROM LUMINOUS FLAMES / DETERMINING FURNACE GAS EXIT TEMPERATURE / ENHANCED HEATING / PIER DESIGN / THERMAL INTERACTION IN FURNACES / INTERACTING HEAT TRANSFER MODES / EVALUATING HYDROGEN ATMOSPHERES FOR BETTER HEAT TRANSFER / TEMPERATURE UNIFORMITY / EFFECTIVE AREA FOR HEAT TRANSFER / GAS RADIATION INTENSITY / SOLID RADIATION INTENSITY / MOVEMENT OF GASEOUS PRODUCTS OF COMBUSTION / TEMPERATURE DIFFERENCE / TURNDOWN / REVIEW QUESTIONS AND PROJECT / HEATING CAPACITY OF BATCH FURNACES DE?NITION OF HEATING CAPACITY / EFFECT OF RATE OF HEAT LIBERATION / EFFECT OF RATE OF HEAT ABSORPTION BY THE LOAD / MAJOR FACTORS AFFECTING FURNACE CAPACITY / EFFECT OF LOAD ARRANGEMENT / AVOID DEEP LAYERS / EFFECT OF LOAD THICKNESS / [-9], (3) LINES VERTICAL HEATING / BATCH INDIRECT-FIRED FURNACES / BATCH FURNACE HEATING CAPACITY PRACTICE / BATCH OVENS AND LOW-TEMPERATURE BATCH FURNACES / DRYING AND PREHEATING MOLTEN METAL CONTAINERS / LOW TEMPERATURE MELTING PROCESSES / STACK ANNEALING FURNACES / MIDRANGE HEAT TREAT FURNACES / COPPER AND ITS ALLOYS / HIGH-TEMPERATURE BATCH FURNACES, 1990 F TO 2500 F / BATCH FURNACES WITH LIQUID BATHS / CONTROLLED COOLING IN OR AFTER BATCH FURNACES / HEATING CAPACITY OF CONTINUOUS FURNACES CONTINUOUS FURNACES COMPARED TO BATCH FURNACES / PRESCRIPTIONS FOR OPERATING FLEXIBILITY / CONTINUOUS DRYERS, OVENS, AND FURNACES FOR <1400 F (<760 C) / EXPLOSION HAZARDS / MASS TRANSFER / ROTARY DRUM DRYERS, INCINERATORS / TOWER DRYERS AND SPRAY DRYERS / TUNNEL OVENS / AIR HEATERS / CONTINUOUS MIDRANGE FURNACES, 1200 TO 1800 F (650 TO 980 C) / CONVEYORIZED TUNNEL FURNACES OR KILNS / ROLLER-HEARTH OVENS, FURNACES, AND KILNS / SHUTTLE CAR-HEARTH FURNACES AND KILNS / SAWTOOTH WALKING BEAMS / CATENARY FURNACE SIZE / SINTERING AND PELLETIZING FURNACES / PELLETIZING / AXIAL CONTINUOUS FURNACES FOR ABOVE 2000 F (1260 C) / BARREL FURNACES / SHAFT FURNACES / LIME KILNS / LINES FLUIDIZED BEDS / HIGH-TEMPERATURE ROTARY DRUM LIME AND CEMENT KILNS / CONTINUOUS FURNACES FOR 1900 TO 2500 F (1038 TO 1370 C) / FACTORS LIMITING HEATING CAPACITY / FRONT-END-FIRED CONTINUOUS FURNACES / FRONT-END-FIRING, TOP AND BOTTOM / SIDE-FIRING REHEAT FURNACES / PUSHER HEARTHS ARE LIMITED BY BUCKLING/PILING / WALKING CONVEYING FURNACES / CONTINUOUS FURNACE HEATING CAPACITY PRACTICE / EIGHT WAYS TO RAISE CAPACITY IN HIGH-TEMPERATURE CONTINUOUS FURNACES / SLOT HEAT LOSSES FROM ROTARY AND WALKING HEARTH FURNACES / SOAK ZONE AND DISCHARGE (DROPOUT) LOSSES / CONTINUOUS LIQUID HEATING FURNACES / CONTINUOUS LIQUID BATH FURNACES / CONTINUOUS LIQUID FLOW FURNACES / SAVING ENERGY IN INDUSTRIAL FURNACE SYSTEMS FURNACE EF?CIENCY, METHODS FOR SAVING HEAT / FLUE GAS EXIT TEMPERATURE / HEAT DISTRIBUTION IN A FURNACE / CONCURRENT HEAT RELEASE AND HEAT TRANSFER / POC GAS TEMPERATURE HISTORY THROUGH A FURNACE / FURNACE, KILN, AND OVEN HEAT LOSSES / LOSSES WITH EXITING FURNACE GASES / PARTIAL-LOAD HEATING / LOSSES FROM WATER COOLING / LOSSES TO CONTAINERS, CONVEYORS, TRAYS, ROLLERS, KILN FURNITURE, PIERS, SUPPORTS, SPACERS, BOXES, PACKING FOR ATMOSPHERE PROTECTION, AND CHARGING EQUIPMENT, INCLUDING HAND TONGS AND CHARGING MACHINE TONGS / LOSSES THROUGH OPEN DOORS, CRACKS, SLOTS, AND DROPOUTS, PLUS GAP LOSSES FROM WALKING HEARTH, WALKING BEAM, ROTARY, AND CAR-HEARTH FURNACES / WALL LOSSES DURING STEADY OPERATION / WALL LOSSES DURING INTERMITTENT OPERATION / HEAT SAVING IN DIRECT-FIRED LOW-TEMPERATURE OVENS / SAVING FUEL IN BATCH FURNACES / SAVING FUEL IN CONTINUOUS FURNACES / FACTORS AFFECTING FLUE GAS EXIT TEMPERATURE / EFFECT OF LOAD THICKNESS ON FUEL ECONOMY / SAVING FUEL IN REHEAT FURNACES / SIDE-FIRED REHEAT FURNACES / ROTARY HEARTH REHEAT FURNACES / FUEL CONSUMPTION CALCULATION / FUEL CONSUMPTION DATA FOR VARIOUS FURNACE TYPES / ENERGY CONSERVATION BY HEAT RECOVERY FROM FLUE GASES / PREHEATING COLD LOADS / STEAM GENERATION IN WASTE HEAT BOILERS / SAVING FUEL BY PREHEATING COMBUSTION AIR / OXY-FUEL FIRING SAVES FUEL, IMPROVES HEAT TRANSFER, AND LOWERS NOX / ENERGY COSTS OF POLLUTION CONTROL / REVIEW QUESTIONS, PROBLEMS, PROJECT / OPERATION AND CONTROL OF INDUSTRIAL FURNACES BURNER AND FLAME TYPES, LOCATION / SIDE-FIRED BOX AND CAR-BOTTOM FURNACES / SIDE FIRING IN-AND-OUT FURNACES / SIDE FIRING REHEAT FURNACES / LONGITUDINAL FIRING OF STEEL REHEAT FURNACES / ROOF FIRING / FLAME FITTING / LUMINOUS FLAMES VERSUS NONLUMINOUS FLAMES / FLAME TYPES / FLAME PRO?LES / UNWANTED NOX FORMATION / CONTROLS AND SENSORS: CARE, LOCATION, ZONES / ROTARY HEARTH FURNACES / ZONE TEMPERATURE IN CAR FURNACES / MELTING FURNACE CONTROL / AIR/FUEL RATIO CONTROL / AIR/FUEL RATIO CONTROL MUST BE UNDERSTOOD / AIR/FUEL RATIO IS CRUCIAL TO SAFETY / AIR/FUEL RATIO AFFECTS PRODUCT QUALITY / MINIMIZING SCALE / FURNACE PRESSURE CONTROL / VISUALIZING FURNACE PRESSURE / CONTROL AND COMPENSATING PRESSURE TAP LOCATIONS / DAMPERS FOR FURNACE PRESSURE CONTROL / TURNDOWN RATIO / TURNDOWN DEVICES / TURNDOWN RANGES / FURNACE CONTROL DATA NEEDS / SOAKING PIT HEATING CONTROL / HEAT-SOAKING INGOTS—EVOLUTION OF ONE-WAY- FIRED PITS / PROBLEMS WITH ONE-WAY, TOP-FIRED SOAK PITS / HEAT-SOAKING SLABS / UNIFORMITY CONTROL IN FORGE FURNACES / TEMPERATURE CONTROL ABOVE THE LOAD(S) / TEMPERATURE CONTROL BELOW THE LOAD(S) / CONTINUOUS REHEAT FURNACE CONTROL / USE MORE ZONES, SHORTER ZONES / SUGGESTED CONTROL ARRANGEMENTS / EFFECTS OF (AND STRATEGIES FOR HANDLING) DELAYS / REVIEW QUESTIONS / GAS MOVEMENT IN INDUSTRIAL FURNACES LAWS OF GAS MOVEMENT / BUOYANCY / FLUID FRICTION, VELOCITY HEAD, FLOW INDUCTION / FURNACE PRESSURE; FLUE PORT SIZE AND LOCATION / FLUE AND STACK SIZING, LOCATION / THE LONG AND SHORT OF STACKS / MULTIPLE FLUES / GAS CIRCULATION IN FURNACES / MECHANICAL CIRCULATION / CONTROLLED BURNER JET DIRECTION, TIMING, AND REACH / BAF?ES AND BRIDGEWALLS / IMPINGEMENT HEATING / LOAD POSITIONING RELATIVE TO BURNERS, WALLS, HEARTH, ROOFS, AND FLUES / OXY-FUEL FIRING REDUCES CIRCULATION / CIRCULATION CAN CURE COLD BOTTOMS / CALCULATIONS/MAINTENANCE/QUALITY/SPECIFYING A FURNACE CALCULATING LOAD HEATING CURVES / SAMPLE PROBLEM: SHANNON METHOD FOR TEMPERATURE-VERSUS-TIME CURVES / PLOTTING THE FURNACE TEMPERATURE PRO?LE, ZONE BY ZONE ON FIGS 8 6, 8 7, AND 8 8 / PLOTTING THE LOAD TEMPERATURE PRO?LE / HEAT BALANCE—TO FIND NEEDED FUEL INPUTS / MAINTENANCE / FURNACE MAINTENANCE / AIR SUPPLY EQUIPMENT MAINTENANCE / RECUPERATORS AND DILUTION AIR SUPPLY MAINTENANCE / EXHORTATIONS / PRODUCT QUALITY PROBLEMS / OXIDATION, SCALE, SLAG, DROSS / DECARBURIZTION / BURNED STEEL / MELTING METALS / SPECIFYING A FURNACE / FURNACE FUEL REQUIREMENT / APPLYING BURNERS / FURNACE SPECI?CATION PROCEDURES / MATERIALS IN INDUSTRIAL FURNACE CONSTRUCTION BASIC ELEMENTS OF A FURNACE / INFORMATION A FURNACE DESIGNER NEEDS TO KNOW / REFRACTORY COMPONENTS FOR WALLS, ROOF, HEARTH / THERMAL AND PHYSICAL PROPERTIES / MONOLITHIC REFRACTORIES / FURNACE CONSTRUCTION WITH MONOLITHIC REFRACTORIES / FIBER REFRACTORIES / WAYS IN WHICH REFRACTORIES FAIL / INSULATIONS / INSTALLATION, DRYING, WARM-UP, REPAIRS / COATINGS, MORTARS, CEMENTS / HEARTHS, SKID PIPES, HANGERS, ANCHORS / HEARTHS / SKID PIPE PROTECTION / HANGERS AND ANCHORS / WATER-COOLED SUPPORT SYSTEMS / METALS FOR FURNACE COMPONENTS / CAST IRONS / CARBON STEELS / ALLOY STEELS / REVIEW QUESTIONS, PROBLEM, PROJECT / [FIRST PA [1], (1) LINES ——— NORMAL * PGENDS: [1], (1) INDUSTRIAL HEATING PROCESSES 1 1 INDUSTRIAL PROCESS HEATING FURNACES INDUSTRIAL PROCESS HEATING FURNACES ARE INSULATED ENCLOSURES DESIGNED TO DELIVER HEAT TO LOADS FOR MANY FORMS OF HEAT PROCESSING MELTING FERROUS METALS AND GLASSES RE- QUIRES VERY HIGH TEMPERATURES,* AND MAY INVOLVE EROSIVE AND CORROSIVE CONDITIONS SHAPING OPERATIONS USE HIGH TEMPERATURES* TO SOFTEN MANY MATERIALS FOR PROCESSES SUCH AS FORGING, SWEDGING, ROLLING, PRESSING, BENDING, AND EXTRUDING TREATING MAY USE MIDRANGE TEMPERATURES* TO PHYSICALLY CHANGE CRYSTALLINE STRUCTURES OR CHEMICALLY (METALLURGICALLY) ALTER SURFACE COMPOUNDS, INCLUDING HARDENING OR RELIEVING STRAINS IN METALS, OR MODIFYING THEIR DUCTILITY THESE INCLUDE AGING, ANNEALING, AUSTENITIZING, CARBURIZING, HARDENING, MALLEABLIZING, MARTINIZING, NITRIDING, SINTERING, SPHEROIDIZ- ING, STRESS-RELIEVING, AND TEMPERING INDUSTRIAL PROCESSES THAT USE LOW TEMPERATURES* INCLUDE DRYING, POLYMERIZING, AND OTHER CHEMICAL CHANGES ALTHOUGH PROFESSOR TRINKS’ EARLY EDITIONS RELATED MOSTLY TO METAL HEATING, PARTIC- ULARLY STEEL HEATING, HIS LATER EDITIONS (AND ESPECIALLY THIS SIXTH EDITION) BROADEN THE SCOPE TO HEATING OTHER MATERIALS THOUGH THE TEXT MAY NOT SPECI?CALLY MENTION OTHER MATERIALS, READERS WILL ?ND MUCH OF THE CONTENT OF THIS EDITION APPLICABLE TO A VARIETY OF INDUSTRIAL PROCESSES INDUSTRIAL FURNACES THAT DO NOT “SHOW COLOR,” THAT IS, IN WHICH THE TEMPERATURE IS BELOW 1200 F (650 C), ARE COMMONLY CALLED “OVENS” IN NORTH AMERICA HOWEVER, THE DIVIDING LINE BETWEEN OVENS AND FURNACES IS NOT SHARP, FOR EXAMPLE, COKE OVENS OPER- ATE AT TEMPERATURES ABOVE 2200 F (1478 C) IN EUROPE, MANY “FURNACES” ARE TERMED “OVENS ” IN THE CERAMIC INDUSTRY, FURNACES ARE CALLED “KILNS ” IN THE PETROCHEM AND CPI (CHEMICAL PROCESS INDUSTRIES), FURNACES MAY BE TERMED “HEATERS,” “KILNS,” “AFTER- BURNERS,” “INCINERATORS,” OR “DESTRUCTORS ” THE “FURNACE” OF A BOILER IS ITS ‘?REBOX’ OR ‘COMBUSTION CHAMBER,’ OR A ?RE-TUBE BOILER’S ‘MORRISON TUBE ’ *IN THIS BOOK, “VERY HIGH TEMPERATURES” USUALLY MEAN >2300 F (>1260 C), “HIGH TEMPERATURES” = 1900– 2300 F (1038–1260 C), “MIDRANGE TEMPERATURES” = 1100–1900 F (593–1038 C), AND “LOW TEMPERATURES” = < 1100 F (<593 C) [2], (2) LINES INDUSTRIAL PROCESS HEATING FURNACES [3], (3) LINES EVERDUR (CONTINUED) [4], (4) LINES [5], (5) LINES [6], (6) LINES 1REFER TO APPENDIX FOR TYPICAL POURING TEMPERATURES 2INCLUDES GRAY AND DUCTILE IRON 3AUSTENITIC STAINLESS STEELS ONLY (AISI 200 AND 300 SERIES) 4FERRITIC STAINLESS STEELS ONLY (AISI 400 SERIES) 5MARTENSITIC STAINLESS STEELS ONLY (AISI 400 SERIES) 6AUSTENITIC AND MARTENSITIC STAINLESS STEELS ONLY ALL RJR 5-26-03 ARE BY PERMISSION FROM REFERENCE 52 CLASSIFICATIONS OF FURNACES [7], (7) LINES ——— 5 67PT ——— NORMAL PGENDS: [7], (7) INDUSTRIAL HEATING OPERATIONS ENCOMPASS A WIDE RANGE OF TEMPERATURES, WHICH DEPEND PARTLY ON THE MATERIAL BEING HEATED AND PARTLY ON THE PURPOSE OF THE HEATING PROCESS AND SUBSEQUENT OPERATIONS TABLE 1 1 LISTS RANGES OF TEMPERATURES FOR A LARGE NUMBER OF MATERIALS AND OPERATIONS VARIATIONS MAY BE DUE TO DIFFERENCES IN THE MATERIAL BEING HEATED (SUCH AS CARBON CONTENTS OF STEELS) AND DIFFERENCES IN PRACTICE OR IN MEASURING TEMPERATURES ROLLING TEMPERATURES OF HIGH QUALITY STEEL BARS HAVE FALLEN FROM ABOUT 2200 F (1200 C) TO ABOUT 1850 F (1283 C) IN THE PROCESS OF IMPROVING ?NE-GRAIN STRUCTURE THE LIMITING OF DECARBURIZATION BY ROLLING AS COLD AS POSSIBLE ALSO HAS REDUCED ROLLING TEMPERATURES IN ANY HEATING PROCESS, THE MAXIMUM FURNACE TEMPERATURE ALWAYS EXCEEDS THE TEMPERATURE TO WHICH THE LOAD OR CHARGE (SEE GLOSSARY) IS TO BE HEATED 1 2 CLASSIFICATIONS OF FURNACES 1 2 1 FURNACE CLASSI?CATION BY HEAT SOURCE HEAT IS GENERATED IN FURNACES TO RAISE THEIR TEMPERATURE TO A LEVEL SOMEWHAT ABOVE THE TEMPERATURE REQUIRED FOR THE PROCESS, EITHER BY (1) COMBUSTION OF FUEL OR BY (2) CONVERSION OF ELECTRIC ENERGY TO HEAT FUEL-?RED (COMBUSTION TYPE) FURNACES ARE MOST WIDELY USED, BUT ELECTRICALLY HEATED FURNACES ARE USED WHERE THEY OFFER ADVANTAGES THAT CANNOT ALWAYS BE MEASURED IN TERMS OF FUEL COST IN FUEL-?RED FURNACES, THE NATURE OF THE FUEL MAY MAKE A DIFFERENCE IN THE FURNACE DESIGN, BUT THAT IS NOT MUCH OF A PROBLEM WITH MODERN INDUSTRIAL FURNACES AND COMBUSTION EQUIPMENT ADDITIONAL BASES FOR CLASSI?CATION MAY RELATE TO THE PLACE WHERE COMBUSTION BEGINS AND THE MEANS FOR DIRECTING THE PRODUCTS OF COMBUSTION 1 2 2 FURNACE CLASSI?CATION BY BATCH (CHAP 3) OR CONTINUOUS (CHAP 4), AND BY METHOD OF HANDLING MATERIAL INTO,THROUGH, AND OUT OF THE FURNACE BATCH-TYPE FURNACES AND KILNS, TERMED “IN-AND-OUT FURNACES” OR “PERIODIC KILNS” (?GS 1 1 AND 1 2), HAVE ONE TEMPERATURE SETPOINT, BUT VIA THREE ZONES OF CONTROL—TO MAIN- TAIN UNIFORM TEMPERATURE THROUGHOUT, BECAUSE OF A NEED FOR MORE HEAT AT A DOOR OR THE ENDS THEY MAY BE LOADED MANUALLY OR BY A MANIPULATOR OR A ROBOT LOADS ARE PLACED IN THE FURNACE; THE FURNACE AND IT LOADS ARE BROUGHT UP TO TEMPER- ATURE TOGETHER, AND DEPENDING ON THE PROCESS, THE FURNACE MAY OR MAY NOT BE COOLED BEFORE IT IS OPENED AND THE LOAD REMOVED—GENERALLY THROUGH A SINGLE CHARGING AND DISCHARGING DOOR BATCH FURNACE CON?GURATIONS INCLUDE BOX, SLOT, CAR-HEARTH, SHUTTLE (SEC 4 3), BELL, ELEVATOR, AND BATH (INCLUDING IMMERSION) FOR LONG SOLID LOADS, CROSS- WISE PIERS AND TOP-LEFT/BOTTOM-RIGHT BURNER LOCATIONS CIRCULATE FOR BETTER UNIFORMITY BELL AND ELEVATOR KILNS ARE OFTEN CYLINDRICAL FURNACES FOR POT, KETTLE, AND DIP-TANK CONTAINERS MAY BE ?RED TANGENTIALLY WITH TYPE H ?AMES INSTEAD OF TYPE E SHOWN [8], (8) LINES ——— LONG PA PGENDS: [8], (8) FIG 1 1 SEVEN (OF MANY KINDS OF) BATCH-TYPE FURNACES (SEE ALSO SHUTTLE KILNS AND FURNACES, ?G 4 8; AND LIQUID BATHS IN ?G 1 12 AND SEC 4 7 ) (FOR ?AME TYPES, SEE ?G 6 2 ) UNLIKE CRUCIBLE, POT, KETTLE, AND DIP-TANK FURNACES, THE REFRACTORY FURNACE LINING ITSELF IS THE ‘CONTAINER’ FOR GLASS “TANKS” AND ALUMINUM MELTING FURNACES, ?GURE 1 2 CAR-HEARTH (CAR TYPE, CAR BOTTOM, LORRY HEARTH) FURNACES, SKETCHED IN ?GURE 1 1, HAVE A MOVABLE HEARTH WITH STEEL WHEELS ON RAILS THE LOAD IS PLACED ON THE CAR-HEARTH, MOVED INTO THE FURNACE ON THE CAR-HEARTH, HEATED ON THE CAR-HEARTH, AND REMOVED FROM THE FURNACE ON THE CAR-HEARTH; THEN THE CAR IS UNLOADED COOLING IS DONE ON THE CAR- HEARTH EITHER IN THE FURNACE OR OUTSIDE BEFORE UNLOADING THIS TYPE OF FURNACE IS USED MAINLY FOR HEATING HEAVY OR BULKY LOADS, OR SHORT RUNS OF ASSORTED SIZES AND SHAPES THE FURNACE DOOR MAY BE AF?XED TO THE CAR HOWEVER, A GUILLOTINE DOOR (PERHAPS ANGLED SLIGHTLY FROM VERTICAL TO LET GRAVITY HELP SEAL LEAKS ALL AROUND THE DOOR JAMB) USUALLY KEEPS TIGHTER FURNACE SEALS AT BOTH DOOR-END AND BACK END * *SEE SUGGESTED PROBLEM/PROJECT AT THE END OF THIS CHAPTER CLASSIFICATIONS OF FURNACES [9], (9) LINES ——— 0 394P ——— LONG PA * PGENDS: [9], (9) FIG 1 2 BATCH-TYPE FURNACE FOR MELTING ANGLED GUILLOTINE DOOR MINIMIZES GAS AND AIR LEAKS IN OR OUT COURTESY OF REMI CLAEYS ALUMINUM SEALING THE SIDES OF A CAR HEARTH OR OF DISC OR DONUT HEARTHS OF ROTARY HEARTH FURNACES IS USUALLY ACCOMPLISHED WITH SAND-SEALS OR WATER-TROUGH SEALS CONTINUOUS FURNACES MOVE THE CHARGED MATERIAL, STOCK, OR LOAD WHILE IT IS BEING HEATED MATERIAL PASSES OVER A STATIONARY HEARTH, OR THE HEARTH ITSELF MOVES IF THE HEARTH IS STATIONARY, THE MATERIAL IS PUSHED OR PULLED OVER SKIDS OR ROLLS, OR IS MOVED THROUGH THE FURNACE BY WOVEN WIRE BELTS OR MECHANICAL PUSHERS EXCEPT FOR DELAYS, A CONTINUOUS FURNACE OPERATES AT A CONSTANT HEAT INPUT RATE, BURNERS BEING RARELY SHUT OFF A CONSTANTLY MOVING (OR FREQUENTLY MOVING) CONVEYOR OR HEARTH ELIMINATES THE NEED TO COOL AND REHEAT THE FURNACE (AS IS THE CASE WITH A BATCH FURNACE), THUS SAVING ENERGY (SEE CHAP 4 ) HORIZONTAL STRAIGHT-LINE CONTINUOUS FURNACES ARE MORE COMMON THAN ROTARY HEARTH FURNACES, ROTARY DRUM FURNACES, VERTICAL SHAFT FURNACES, OR ?UIDIZED BED FURNACES FIG 1 3 FIVE-ZONE STEEL REHEAT FURNACE MANY SHORT ZONES ARE BETTER FOR RECOVERY FROM EFFECTS OF MILL DELAYS USING END-?RED BURNERS UPSTREAM (GAS-?OW-WISE), AS SHOWN HERE, MIGHT DISRUPT ?AME COVERAGE OF SIDE OR ROOF BURNERS END ?RING, OR LONGITUDINAL ?RING, IS MOST COMMON IN ONE-ZONE (SMALLER) FURNACES, BUT CAN BE ACCOMPLISHED WITH SAWTOOTH ROOF AND BOTTOM ZONES, AS SHOWN FIG 1 4 EIGHT-ZONE STEEL REHEAT FURNACE AN UN?RED PREHEAT ZONE WAS ONCE USED TO LOWER ?UE GAS EXIT TEMPERATURE (USING LESS FUEL) LATER, PREHEAT ZONE ROOF BURNERS WERE ADDED TO GET MORE CAPACITY, BUT FUEL RATE WENT UP REGENERATIVE BURNERS NOW HAVE THE SAME LOW ?UE TEMPERATURES AS THE ORIGINAL UN?RED PREHEAT ZONE, REDUCING FUEL AND INCREASING CAPACITY FIG 1 5 CONTINUOUS BELT-CONVEYOR TYPE HEAT TREAT FURNACE (1800 F, 982 C MAXIMUM) EXCEPT FOR VERY SHORT LENGTHS WITH VERY LIGHTWEIGHT LOADS, A BELT NEEDS UNDERSIDE SUPPORTS THAT ARE NONABRASIVE AND HEAT RESISTANT—IN THIS CASE, THIRTEEN ROWS, ?VE WIDE OF VERTICAL 4 IN (100 MM) SERIES 304 STAINLESS-STEEL CAPPED PIPES, BETWEEN THE BURNERS OF ZONES 2 AND 4 AN UN?RED COOLING ONE IS TO THE RIGHT OF ZONE 3 FIGURES 1 3 AND 1 4 ILLUSTRATE SOME VARIATIONS OF STEEL REHEAT FURNACES SIDE DISCHARGE (?G 1 4) USING A PEEL BAR (SEE GLOSSARY) PUSHING MECHANISM PERMITS A SMALLER OPENING THAN THE END (GRAVITY DROPOUT) DISCHARGE OF ?GURE 1 3 THE SMALL OPENING OF THE SIDE DISCHARGE REDUCES HEAT LOSS AND MINIMIZES UNEVEN COOLING OF THE NEXT LOAD PIECE TO BE DISCHARGED OTHER FORMS OF STRAIGHT-LINE CONTINUOUS FURNACES ARE WOVEN ALLOY WIRE BELT CON- VEYOR FURNACES USED FOR HEAT TREATING METALS OR GLASS “LEHRS” (?G 1 5), PLUS ALLOY OR CERAMIC ROLLER HEARTH FURNACES (?G 1 6) AND TUNNEL FURNACES/TUNNEL KILNS (?G 1 7) ALTERNATIVES TO STRAIGHT-LINE HORIZONTAL CONTINUOUS FURNACES ARE ROTARY HEARTH (DISC OR DONUT) FURNACES (?G 1 8 AND SECS 4 6 AND 6 4), INCLINED ROTARY DRUM FURNACES (?G 1 10), TOWER FURNACES, SHAFT FURNACES (?G 1 11), AND ?UIDIZED BED FURNACES (?G 1 12), AND LIQUID HEATERS AND BOILERS (SEC 4 7 1 AND 4 7 2) ROTARY HEARTH OR ROTATING TABLE FURNACES (?G 1 8) ARE VERY USEFUL FOR MANY PUR- POSES LOADS ARE PLACED ON THE MERRY-GO-ROUND-LIKE HEARTH, AND LATER REMOVED AFTER THEY HAVE COMPLETED ALMOST A WHOLE REVOLUTION THE ROTARY HEARTH, DISC OR DONUT (WITH A HOLE IN THE MIDDLE), TRAVELS ON A CIRCULAR TRACK THE ROTARY HEARTH OR ROTATING TABLE FIG 1 6 ROLLER HEARTH FURNACE, TOP- AND BOTTOM-?RED, MULTIZONE ROLLER HEARTH FURNACES ?T IN WELL WITH ASSEMBLY LINES, BUT A Y IN THE ROLLER LINE AT EXIT AND ENTRANCE IS ADVISED FOR ?EXIBILITY, AND TO ACCOMMODATE “PARKING” THE LOADS OUTSIDE THE FURNACE IN CASE OF A PRODUCTION LINE DELAY FOR LOWER TEMPERATURE HEAT TREATING PROCESSES, AND WITH INDIRECT (RADIANT TUBE) HEATING, “PLUG FANS” THROUGH THE FURNACE CEILING CAN PROVIDE ADDED CIRCULATION FOR FASTER, MORE EVEN HEAT TRANSFER COURTESY OF HAL ROACH CONSTRUCTION, INC CLASSIFICATIONS OF FURNACES FIG 1 7 TUNNEL KILN TOP ROW, END- AND SIDE-SECTIONAL VIEWS SHOWING SIDE BURNERS ?RING INTO ?RE LANES BETWEEN CARS; CENTER, ?OW DIAGRAM; BOTTOM, TEMPERATURE VS TIME (DISTANCE) CERAMIC TUNNEL KILNS ARE USED TO “?RE” LARGE-VOLUME PRODUCTS FROM BRICKS AND TILES TO SANITARY WARE, POTTERY, ?NE DINNERWARE, AND TINY ELECTRONIC CHIPS ADAPTED FROM AND WITH THANKS TO REFERENCE 72 FURNACE IS ESPECIALLY USEFUL FOR CYLINDRICAL LOADS, WHICH CANNOT BE PUSHED THROUGH A FURNACE, AND FOR SHORTER PIECES THAT CAN BE STOOD ON END OR LAID END TO END THE CENTRAL COLUMN OF THE DONUT TYPE HELPS TO SEPARATE THE CONTROL ZONES SEE THOROUGH DISCUSSIONS OF ROTARY HEARTH STEEL REHEAT FURNACES IN SECTIONS 4 6 AND 6 4 MULTIHEARTH FURNACES (?G 1 9) ARE A VARIATION OF THE ROTARY HEARTH FURNACE WITH MANY LEVELS OF ROUND STATIONARY HEARTHS WITH ROTATING RABBLE ARMS THAT GRADUALLY PLOW GRANULAR OR SMALL LUMP MATERIALS RADIALLY ACROSS THE HEARTHS, CAUSING THEM TO EVENTUALLY DROP THROUGH PORTS TO THE NEXT LEVEL INCLINED ROTARY DRUM FURNACES, KILNS, INCINERATORS, AND DRYERS OFTEN USE LONG TYPE F OR TYPE G ?AMES (?G 6 2) IF DRYING IS INVOLVED, SUBSTANTIALLY MORE EXCESS AIR THAN NORMAL MAY BE JUSTI?ED TO PROVIDE GREATER MOISTURE PICKUP ABILITY (SEE ?G 1 10 ) TOWER FURNACES CONSERVE ?OOR SPACE BY RUNNING LONG STRIP OR STRAND MATERIALS VERTICALLY ON TALL FURNACES FOR DRYING, COATING, CURING, OR HEAT TREATING (ESPECIALLY ANNEALING) IN SOME CASES, THE LOAD MAY BE PROTECTED BY A SPECIAL ATMOSPHERE, AND HEATED WITH RADIANT TUBES OR ELECTRICAL MEANS SHAFT FURNACES ARE USUALLY REFRACTORY-LINED VERTICAL CYLINDERS, IN WHICH GRAVITY CONVEYS SOLIDS AND LIQUIDS TO THE BOTTOM AND BY-PRODUCT GASES TO THE TOP EXAMPLES ARE CUPOLAS, BLAST FURNACES, AND LIME KILNS FIG 1 8 ROTARY HEARTH FURNACE, DONUT TYPE, SECTIONED PLAN VIEW (DISK TYPE HAS NO HOLE IN THE MIDDLE ) SHORT-?AME BURNERS ?RE FROM ITS OUTER PERIPHERY BURNERS ALSO ARE SOMETIMES ?RED FROM THE INNER WALL OUTWARD LONG-?AME BURNERS ARE SOMETIMES ?RED THROUGH A SAWTOOTH ROOF, BUT NOT THROUGH THE SIDEWALLS BECAUSE THEY TEND TO OVERHEAT THE OPPOSITE WALL AND ENDS OF LOAD PIECES R, REGENERATIVE BURNER; E, ENHANCED HEATING HIGH-VELOCITY BURNER (SEE ALSO ?G 6 7 ) FLUIDIZED BED FURNACES UTILIZE INTENSE GAS CONVECTION HEAT TRANSFER AND PHYSICAL BOMBARDMENT OF SOLID HEAT RECEIVER SURFACES WITH MILLIONS OF RAPIDLY VIBRATING HOT SOLID PARTICLES THE FURNACES TAKE SEVERAL FORMS 1 A REFRACTORY-LINED CONTAINER, WITH A ?NE GRATE BOTTOM, ?LLED WITH INERT (USUALLY REFRACTORY) BALLS, PELLETS, OR GRANULES THAT ARE HEATED BY PRODUCTS OF COMBUSTION FROM A COMBUSTION CHAMBER BELOW THE GRATE LOADS OR BOILER TUBES ARE IM- MERSED IN THE ?UIDIZED BED ABOVE THE GRATE FOR HEAT PROCESSING OR TO GENERATE STEAM CLASSIFICATIONS OF FURNACES FIG 1 9 HERRESHOFF MULTILEVEL FURNACE FOR ROASTING ORES, CALCINING KAOLIN, REGENERATING CARBON, AND INCINERATING SEWAGE SLUDGE COURTESY OF REFERENCE 50 2 SIMILAR TO ABOVE, BUT THE GRANULES ARE FUEL PARTICLES OR SEWAGE SLUDGE TO BE INCINERATED THE SPACE BELOW THE GRATE IS A PRESSURIZED AIR SUPPLY PLENUM THE FUEL PARTICLES ARE IGNITED ABOVE THE GRATE AND BURN IN ?UIDIZED SUSPENSION WHILE PHYSICALLY BOMBARDING THE WATER WALLS OF THE UPPER CHAMBER AND WATER TUBES IMMERSED IN ITS ?UIDIZED BED 3 THE ?UIDIZED BED IS ?LLED WITH COLD GRANULES OF A COATING MATERIAL (E G , POLY- MER), AND LOADS TO BE COATED ARE HEATED IN A SEPARATE OVEN TO A TEMPERATURE ABOVE THE MELTING POINT OF THE GRANULES THE HOT LOADS (E G , DISHWASHER RACKS) ARE THEN DIPPED (BY A CONVEYOR) INTO THE OPEN-TOPPED ?UIDIZED BED FOR COATING FIG 1 10 ROTARY DRUM DRYER/KILN/FURNACE FOR DRYING, CALCINING, RE?NING, INCINERATING GRANULAR MATERIALS SUCH AS ORES, MINERALS, CEMENTS, AGGREGATES, AND WASTES GRAVITY MOVES MATERIAL CO- CURRENT WITH GASES (SEE ?G 4 3 FOR COUNTER?OW ) FIG 1 11 LIME SHAFT KILN COURTESY OF REFERENCE 26, BY HARBISON- WALKER REFRACTORIES CO LIQUID HEATERS SEE LIQUID BATHS AND HEATERS, SEC 4 7 1, AND BOILERS AND LIQUID FLOW HEATERS, SEC 4 7 2 1 2 3 FURNACE CLASSI?CATION BY FUEL IN FUEL-?RED FURNACES, THE NATURE OF THE FUEL MAY MAKE A DIFFERENCE IN THE FURNACE DESIGN, BUT THAT IS NOT MUCH OF A PROBLEM WITH MODERN INDUSTRIAL FURNACES AND BURNERS, EXCEPT IF SOLID FUELS ARE INVOLVED SIMILAR BASES FOR CLASSI?CATION ARE AIR FURNACES, OXYGEN FURNACES, AND ATMOSPHERE FURNACES RELATED BASES FOR CLASSI?CATION MIGHT BE THE POSITION IN THE FURNACE WHERE COMBUSTION BEGINS, AND THE MEANS FOR DIRECTING THE PRODUCTS OF COMBUSTION, E G , INTERNAL FAN FURNACES, HIGH VELOCITY FURNACES, AND BAF?ED FURNACES (SEE SEC 1 2 4 AND THE ROTARY HEARTH FURNACE DISCUSSION ON BAF?ES IN CHAP 6 ) ELECTRIC FURNACES FOR INDUSTRIAL PROCESS HEATING MAY USE RESISTANCE OR INDUCTION HEATING THEORETICALLY, IF THERE IS NO GAS OR AIR EXHAUST, ELECTRIC HEATING HAS NO ?UE GAS LOSS, BUT THE USER MUST RECOGNIZE THAT THE HIGHER COST OF ELECTRICITY AS A FUEL IS THE RESULT OF THE ?UE GAS LOSS FROM THE BOILER FURNACE AT THE POWER PLANT THAT GENERATED THE ELECTRICITY RESISTANCE HEATING USUALLY INVOLVES THE HIGHEST ELECTRICITY COSTS, AND MAY REQUIRE CIRCULATING FANS TO ASSURE THE TEMPERATURE UNIFORMITY ACHIEVABLE BY THE ?OW MOTION OF THE PRODUCTS OF COMBUSTION (POC) IN A FUEL-?RED FURNACE SILICON CONTROL RECTI?ERS HAVE MADE INPUT MODULATION MORE ECONOMICAL WITH RESISTANCE HEATING VARIOUS MATERIALS ARE USED FOR ELECTRIC FURNACE RESISTORS MOST ARE OF A NICKEL–CHROMIUM ALLOY, IN THE FORM OF ROLLED STRIP OR WIRE, OR OF CAST ZIG-ZAG GRIDS (MOSTLY FOR CONVECTION) OTHER CLASSIFICATIONS OF FURNACES FIG 1 12 CIRCULATING ?UIDIZED BED COMBUSTOR SYSTEM (TYPE 2 IN EARLIER LIST) COURTESY OF REFER- ENCE 26, BY HARBISON-WALKER REFRACTORIES CO RESISTOR MATERIALS ARE MOLTEN GLASS, GRANULAR CARBON, SOLID CARBON, GRAPHITE, OR SILICON CARBIDE (GLOW BARS, MOSTLY FOR RADIATION) IT IS SOMETIMES POSSIBLE TO USE THE LOAD THAT IS BEING HEATED AS A RESISTOR IN INDUCTION HEATING, A CURRENT PASSES THROUGH A COIL THAT SURROUNDS THE PIECE TO BE HEATED THE ELECTRIC CURRENT FREQUENCY TO BE USED DEPENDS ON THE MASS OF THE PIECE BEING HEATED THE INDUCTION COIL (OR INDUCTION HEADS FOR SPECI?C LOAD SHAPES) MUST BE WATER COOLED TO PROTECT THEM FROM OVERHEATING THEMSELVES ALTHOUGH INDUCTION HEATING USUALLY USES LESS ELECTRICITY THAN RESISTANCE HEATING, SOME OF THAT GAIN MAY BE LOST DUE TO THE COST OF THE COOLING WATER AND THE HEAT THAT IT CARRIES DOWN THE DRAIN INDUCTION HEATING IS EASILY ADAPTED TO HEATING ONLY LOCALIZED AREAS OF EACH PIECE AND TO MASS-PRODUCTION METHODS SIMILAR APPLICATION OF MODERN PRODUCTION DESIGN TECHNIQUES WITH RAPID IMPINGEMENT HEATING USING GAS ?AMES HAS BEEN VERY SUCCESSFUL IN HARDENING OF GEAR TEETH, HEATING OF ?AT SPRINGS FOR VEHICLES, AND A FEW OTHER HIGH PRODUCTION APPLICATIONS MANY RECENT DEVELOPMENTS AND SUGGESTED NEW METHODS OF ELECTRIC OR ELECTRONIC HEATING OFFER WAYS TO ACCOMPLISH INDUSTRIAL HEAT PROCESSING, USING PLASMA ARCS, LASERS, RADIO FREQUENCY, MICROWAVE, AND ELECTROMAGNETIC HEATING, AND COMBINATIONS OF THESE WITH FUEL ?RING ILLUSTRATES THE PRINCIPLE OF A CONTINUOUS BELT DIRECT-?RED RECIRCULATING OVEN ALL REQUIRE THOUGHTFUL CIRCULATION DESIGN AND CAREFUL POSITIONING RELATIVE TO THE LOADS 1 2 5 FURNACE CLASSI?CATION BY DIRECT-FIRED OR INDIRECT-FIRED IF THE ?AMES ARE DEVELOPED IN THE HEATING CHAMBER PROPER, AS IN ?GURE 1 1, OR IF THE PRODUCTS OF COMBUSTION (POC) ARE CIRCULATED OVER THE SURFACE OF THE WORKLOAD AS IN ?GURE 3 17, THE FURNACE IS SAID TO BE DIRECT-?RED IN MOST OF THE FURNACES, OVENS, AND DRYERS SHOWN EARLIER IN THIS CHAPTER, THE LOADS WERE NOT HARMED BY CONTACT WITH THE PRODUCTS OF COMBUSTION INDIRECT-?RED FURNACES ARE FOR HEATING MATERIALS AND PRODUCTS FOR WHICH THE QUALITY OF THE ?NISHED PRODUCTS MAY BE INFERIOR IF THEY HAVE COME IN CONTACT WITH ?AME OR PRODUCTS OF COMBUSTION (POC) IN SUCH CASES, THE STOCK OR CHARGE MAY BE (A) HEATED IN AN ENCLOSING MUF?E (CONDUCTING CONTAINER) THAT IS HEATED FROM THE OUTSIDE BY PRODUCTS OF COMBUSTION FROM BURNERS OR (B) HEATED BY RADIANT TUBES THAT ENCLOSE THE ?AME AND POC 1 2 5 1 MUF?ES THE PRINCIPLE OF A MUF?E FURNACE IS SKETCHED IN ?GURE 1 14 A POT FURNACE OR CRUCIBLE FURNACE (?G 1 15) IS A FORM OF MUF?E FURNACE IN WHICH THE CONTAINER PREVENTS POC CONTACT WITH THE LOAD A DOUBLE MUF?E ARRANGEMENT IS SHOWN IN ?GURE 1 16 NOT ONLY IS THE CHARGE ENCLOSED IN A MUF?E BUT THE PRODUCTS OF COMBUSTION ARE CON?NED INSIDE MUF?ES CALLED RADIANT TUBES THIS USE OF RADIANT TUBES TO PROTECT THE INNER COVER FROM UNEVEN HEATING CLASSIFICATIONS OF FURNACES FIG 1 14 MUF?E FURNACE THE MUF?E (HEAVY BLACK LINE) MAY BE OF HIGH TEM- PERATURE ALLOY OR CERAMIC IT IS USUALLY PUMPED FULL OF AN INERT GAS FIG 1 15 CRUCIBLE OR POT FURNACE TANGENTIALLY ?RED INTEGRAL REGENERATOR-BURNERS SAVE FUEL, AND THEIR ALTERNATE ?RING FROM POSITIONS 180 DEGREES APART PROVIDES EVEN HEATING AROUND THE POT OR CRUCIBLE PERIPHERY (SEE ALSO ?G 3 20 ) IS BEING REPLACED BY DIRECT-?RED TYPE E OR TYPE H ?AMES (?G 6 2) TO HEAT THE INNER COVER, THEREBY IMPROVING THERMAL CONVERSION EF?CIENCY AND REDUCING HEATING TIME 1 2 5 2 RADIANT TUBES FOR CHARGES THAT REQUIRE A SPECIAL ATMOSPHERE FOR PRO- TECTION OF THE STOCK FROM OXIDATION, DECARBURIZATION, OR FOR OTHER PURPOSES, MOD- ERN INDIRECT-?RED FURNACES ARE BUILT WITH A GAS-TIGHT OUTER CASING SURROUNDING THE FIG 1 16 INDIRECT-?RED FURNACE WITH MUF?ES FOR BOTH LOAD AND ?AME COVER ANNEALING FURNACES FOR COILS OF STRIP OR WIRE ARE BUILT IN SIMILAR FASHION, BUT HAVE A FAN IN THE BASE TO CIRCULATE A PREPARED ATMOSPHERE WITHIN THE INNER COVER REFRACTORY LINING SO THAT THE WHOLE FURNACE CAN BE ?LLED WITH A PREPARED ATMOSPHERE HEAT IS SUPPLIED BY FUEL-?RED RADIANT TUBES OR ELECTRIC RESISTANCE ELEMENTS 1 2 6 CLASSI?CATION BY FURNACE USE (INCLUDING THE SHAPE OF THE MATERIAL TO BE HEATED) THERE ARE SOAKING PITS OR INGOT-HEATING FURNACES, FOR HEATING OR REHEATING LARGE INGOTS, BLOOMS, OR SLABS, USUALLY IN A VERTICAL POSITION THERE ARE FORGE FURNACES FOR HEATING WHOLE PIECES OR FOR HEATING ENDS OF BARS FOR FORGING OR WELDING SLOT FORGE FURNACES (?G 1 1) HAVE A HORIZONTAL SLOT INSTEAD OF A DOOR FOR INSERTING THE MANY BARS THAT ARE TO BE HEATED AT ONE TIME THE SLOT OFTEN ALSO SERVES AS THE ?UE FURNACES NAMED FOR THE MATERIAL BEING HEATED INCLUDE BOLT HEADING FURNACES, PLATE FURNACES, WIRE FURNACES, RIVET FURNACES, AND SHEET FURNACES SOME FURNACES ALSO ARE CLASSI?ED BY THE PROCESS OF WHICH THEY ARE A PART, SUCH AS HARDENING, TEMPER- ING, ANNEALING, MELTING, AND POLYMERIZING IN CARBURIZING FURNACES, THE LOAD TO BE CASE-HARDENED IS PACKED IN A CARBON-RICH POWDER AND HEATED IN POTS/BOXES, OR HEATED IN ROTATING DRUMS IN A CARBURIZING ATMOSPHERE 1 2 7 CLASSI?CATION BY TYPE OF HEAT RECOVERY (IF ANY) MOST HEAT RECOVERY EFFORTS ARE AIMED AT UTILIZING THE “WASTE HEAT” EXITING THROUGH THE ?UES SOME FORMS OF HEAT RECOVERY ARE AIR PREHEATING, FUEL PREHEATING, LOAD PREHEAT- ING (FIG 1 17), RECUPERATIVE, REGENERATIVE, AND WASTE HEAT BOILERS—ALL DISCUSSED IN CHAPTER 5 PREHEATING COM