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SMART ENERGY


Radiant Heat Loss: Do You Really Know What It’s Costing You?


ROBERT EPPICH (LEAD AUTHOR) EPPICH TECHNOLOGIES (SYRACUSE, INDIANA), AND CINDY BELT, METALS ENERGY MANAGEMENT (CALLAHAN, FLORIDA), DR. JAMES WICZER, SENSOR SYNERGY (VERNON HILLS, ILLINOIS), BRIAN REINKE, TDI CONSULTING (LEMONT, ILLINOIS) (CONTRIBUTING AUTHORS)


A


lot has been written about radiant heat loss associated with uncovered furnaces. But


another major area of radiant heat loss is from furnace shells, such as holding furnaces, reverberatory furnaces and rotary melting furnaces. Table 1 sum- marizes the heat loss in Btu/ft2/hr. vs. shell temperature. Keep in mind, radiation heat loss goes up as the 4th power of absolute temperature. Heat loss almost doubles when the shell temperature rises from 150F to 200F (65.5C to 93.3C). To put this in perspective, the


results of a carefully documented study of an aluminum holding furnace with an old lining vs. a new lining showed the following as sum- marized in Table 2. One often forgets these holding


furnaces run 24/7. Tus the esti- mated annual cost of operating with a furnace shell temperature of 160F (71.1C) is about $9,000-$11,000 per year greater than operating with a shell temperature of 130F (54.4C). Another study involved a rotary


furnace gas-fired non-ferrous melting furnace, 8 ft (2.4m) diameter by 20 ft. (6.1m) long. Normal practice was to start with a lining that resulted in a shell temperature of 400F (204.4C) and reline when the shell temperature reached 900F (482.2C). Just to put this in perspective, the energy use at the two different shell temperatures is shown in Table 3. Te heat loss is costly, and it results in a lengthened melt cycle. Tus, it is vital the decision of


when to replace the lining must take into consideration the total costs (en- ergy and throughput) associated with that lining—not just the obvious cost of the relining itself. What type of lining you use is


also a critical decision. Talk to your refractory company to run numbers on


40 | MODERN CASTING December 2017


Radiant heat loss can be reduced on your furnaces. Relining, added insulation, and even aluminum paint can reduce heat loss.


different lining options. Various forms of insulation including newer thin insulation are available. Tis insulation can reduce the shell temperature and thus the radiant heat loss. In one example, the addition of


Table 1. Radiation Heat Loss vs. Temperature


Furnace Shell


Temperature (0F) 100


150 200 250 300 350 400 450 500 550 600 650 700 750 800


Btu Heat Loss/hr/ft2


50


155 285 450 630 865


1,115 1,400 1,740 2,130 2,600 3,125 3,660 4,240 4,850


Condition New Old


Shell Surface Temperature


130F 160F


an insulating castable and board to a rotary furnace cost $2,000 but saved $8,000 per year. In a reverberatory furnace, extra


castable insulation in the roof and block and board insulation in the up- per sidewalls roof was added. Tis cost $5,000 but saved $14,000 per year. An often overlooked part of radia-


tive heat loss is emittance. Emittance is a measure of the ability of a surface to radiate energy. A perfect black body emitter (1.0) would radiate the most energy from a hot surface. If you have a dark, dirty, and rough surface (emittance = 0.9) on your furnace, more heat is radiated. A smooth shiny surface such as aluminum paint (emit- tance = 0.3) would radiate much less. Sometimes repainting your furnace can save more than adding insulation. Radiant heat loss can be reduced on


your furnaces. Relining, added insula- tion, and even aluminum paint can reduce heat loss. Reducing radiation heat loss saves energy, increases melt rate, and even makes the area cooler for everyone.


Table 2. Energy Use of Aluminum Holding Furnace Lining


KW/Hr


13.2 25.0


Table 3. Radiant Heat Loss From the Shell of a Rotary Melting Furnace Shell


Temperature 400F


900F


Heat Loss Btu/ft2/hr


935 5,826


Heat loss from the shell of the 20ft long furnace


470,000 Btu/hr 2,930,000 Btu/hr


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