Kilns used in the brick industry are a good example of this. These continuous furnaces operate at temperatures of 2,000oF (1,100oC) and higher, but their exhaust gases don’t leave the stack until the temperature has dropped to about 600oF (320oC). The result is high thermal efficiency despite high operating temperatures. Combustion gases from the high temperature areas are forced to pass over cooler incoming product. Thus the heat is transferred to the product, bringing it part-way up to full processing temperature. As the exhaust gases cool on their way out, they continue to encounter cooler and cooler incoming product, so heat transfer continues. If you have a furnace or oven that operates at moderate to high temperatures, it is worth investigating design modifications that would route some or all of the exhaust over the cold incoming product. The energy efficiency benefits can be substantial.

Never Waste Heat: Even if it’s not practical to use exhaust gases to preheat product, it may still be possible to salvage some of the energy in the exhaust by sending it to a lower temperature process. Obviously the donor and recipient heating equipment must be situated fairly close to each other so large amounts of heat are not lost during transfer, and the two pieces of equipment have to operate on compatible cycles. It doesn’t make sense to run Furnace 1 just to keep Furnace 2 going, and Furnace 2 should be operating frequently enough to make good use of Furnace 1’s exhaust. If you can’t get a perfect match-up of operating cycles, an auxiliary heater on Furnace 2 can be used to fill the gaps.