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Show Fourth - Lower the ambient temperature in the burner area. At times, this temperature reached 60°C (140F). Consequently, maintenance of the burners and roll bearings was both agonizing and expensive. II - THE APPROACH It was clear that some type of waste heat recovery device would be required to attain the improvements desired. To better understand how we decided what type of heat recovery system to use, let's briefly review the types that are available. There are two (2) basic types of heat exchangers for waste heat recovery: Recuperators and Regenerators. Recuperators have metallic or ceramic walls that separate the flue gas stream from the combustion air stream. Heat is transferred from the hot flue gas, through the walls, to the cold combustion air. Regenerators consist of two (2) chambers, each containing a permeable storage bed of ceramic or metal. Flue gas flows through one chamber giving up heat to the bed, while combustion air flows through the other, absorbing heat. After a period of time, flows are switched, and air flows through the heated bedwhi1e flue gas heats the other bed in preparation for the next cycle. These flow reversals occur at fixed time intervals. Here is a purely schematic diagram of a combustion system with a recuperator. Note cold combustion air from a blower passes through a recuperator, receiving heat, through a wall, from the hot flue gas. The pre-heated air then goes to the burner where it is mixed with the fuel and burned in the furnace. The flue gas then exits through the recuperator. Fig. 3 - Schematic Drawing of Recuperator 66 ~ Fig. 4 - Cross-Section of Radiation Recuperator There are, of course, many types of recuperators. Here is a radiation type. Note, in the cross-section, the flue gas travels vertically upward in the center while the combustion air travels vertically upward in the annulus. This is called parallel-flow. If the air traveled vertically downward, it would be counter-flow. If the air traveled horizontally past the flue gas tube, it would be cross-flow. Counter-flow is the most efficient method but it also subjects the heat transfer walls to the highest temperatures. Fi g . 5 - Cross-Section of Regenerative Furnace This is a picture of a regenerator on a stee l open hearth furnace. You will appreciate the size of the regenerator when I tell you that the furnace proper is about 15.25 m (50 feet) long. Cold air flows up through the brick checkerwork on the left end of the furnace becoming heated to about 13l5°C (2400F). F~e1 is added as the air enters the furnace chamber generating a very hot flame which maintains ' |
