Radiant Heat Loss: Do You Really Know What It’s Costing You?
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 summarizes 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 summarized in Table 2.
One often forgets these holding furnaces run 24/7. Thus the estimated 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.
The heat loss is costly, and it results in a lengthened melt cycle.
Thus, it is vital the decision of when to replace the lining must take into consideration the total costs (energy 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 different lining options. Various forms of insulation including newer thin insulation are available. This insulation can reduce the shell temperature and thus the radiant heat loss.
In one example, the addition of 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 upper sidewalls roof was added. This cost $5,000 but saved $14,000 per year.
An often overlooked part of radiative 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 (emittance = 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 insulation, 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.
Click here to see this story as it appears in the December 2017 issue of Modern Casting