844 Passive Air Preheaters

Passive gas-to-gas regenerators are available for applications where cross-contamination cannot be tolerated. One such type of regenerator, the plate-type, is shown in Figure 8.24. A second type, the heat pipe array is shown in Figure 8.25. Passive air preheaters are used in the low- and medium-temperature applications. Those include drying, curing, and baking ovens; air pre-heaters in steam boilers; air dryers; waste heat recovery from exhaust steam; secondary recovery from refractory kilns and reverbatory furnaces; and waste heat recovery from conditioned air.

The plate-type regenerator is constructed of alter nate channels which separate adjacent flows of heated and heating gases by a thin wall of conducting metal. Although their use eliminates cross-contamination, they are bulkier, heavier, and more expensive than a heat wheel of similar heat-recovery and flow capacities. Furthermore, it is difficult to achieve temperature control of the heated gas, while fouling may be a more serious problem.

The heat pipe is a heat-transfer element that is assembled into arrays which are used as compact and efficient passive gas-to-gas heat exchangers. Figure 8.25 shows how the bundle of finned heat pipes extend through the wall separating the inlet and exhaust ducts in a pattern that resembles the conventional finned tube heat exchangers. Each of the separate pipes, however, is a separate sealed element. Each consists of an annular wick on the inside of the full length of the tube, in which an appropriate heat-transfer fluid is absorbed. Figure 8.26 shows how the heat transferred from the hot exhaust gases evaporates the fluid in the wick. This causes the vapor to expand into the center core of the heat pipe. The latent heat of evaporation is carried with the vapor to the cold end of the tube. There it is

Hot waste gas

Hot waste gas

Cooled waste gas

Cold air inlet

Heat pipe bundle

Cooled waste gas

Cold air inlet

Heat pipe bundle

Figure 8.25 Heat pipe.

Heat In

Heat Out

Evaporation

Figure 8.26 Heat pipe operation. Heot In

Figure 8.25 Heat pipe.

Heat In

Heat Out

Condensation

Heat Out

Heat Out

Condensation removed by transferral to the cold gas as the vapor is recondensed. The condensate is then carried back in the wick to the hot end of the tube. This takes place by capillary action and by gravitational forces if the axis of the tube is tilted from the horizontal. At the hot end of the tube the fluid is then recycled.

The heat pipe is compact and efficient for two reasons. The finned-tube bundle is inherently a good configuration for convective heat transfer between the gases and the outside of the tubes in both ducts. The evaporative-condensing cycle within the heat tubes is a highly efficient method of transferring heat internally. This design is also free of cross-contamination. However, the temperature range over which waste heat can be recovered is severely limited by the thermal and physical properties of the fluids used within the heat pipes. Table 8.6 lists some of the transfer fluids and the temperature ranges in which they are applicable.

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