Page 256 - Boiler plant and distribution system optimization manual

Waste Heat Recovery 241

Figure 12.14—Three popular designs of direct contact flue gas condensing heat recovery tower.

(Figure 12.16). It is possible to preheat combus- hot section where the matrix or plates are heat-
tion air, process fluids, building make up air, etc. ed by the flue gasses, then on the other side of a
by this type of heat exchanger. It is a relatively sealing section it rotates through a cold air section
simple system, but the gas streams must pass in giving up its heat.
close proximity. Heat wheels can exceed 50 feet in diameter.
One manufacturer has overcome several Matrix materials can include aluminum, stainless
problems with this type of exchanger. A unit has steel, and ceramics for higher temperatures.
been designed to be torqued together using resil- Disadvantages include cross contamination
ient mountings, eliminating thermal stress and of the two streams due to sealing and purging
cracked welds associated with expansion and problems, clogging of passages, gas and air flow
thermal shocks. Also, these plate units can be con- restriction and drive motor horsepower. The large
structed of corrosion resistant materials or coat- size of the units and supporting ducting is anoth-
ed with Teflon to insure a long useful life. These er disadvantage. These units can be used to pre-
units are also designed to benefit from latent heat heat combustion air, but cannot be used to heat
recovery and to cope with acid corrosion and con- building air because of cross contamination of
densation problems. combustion products.

The Heat Wheel Tubular Heat Exchangers
The heat wheel is a regenerative type of heat Tubular heat exchangers are also used for
exchanger which extracts heat from one source, combustion air preheating. The heater is arranged
briefly stores it and then releases it to a cooler for vertical gas flow through a nest of tubes. The
stream {Figure 12.17). It consists of a large rotat- air passes horizontally across the tubes in sections
ing wheel frame, packed with a heat absorbing of the air heater defined by built in baffles.
matrix. Its primary application is for combustion In contrast to regenerative designs, tubular
air preheating. As it rotates it passes through the or recuperative air heaters have more severe cold