Page 254 - Boiler plant and distribution system optimization manual

Waste Heat Recovery 239

Figure 12.12—Flue gas enthalpy as a function of temperature for various fuels.

The direct contact heat exchanger is very ef- peratures.
ficient because there are no heat exchange surfac- Figure 12.13 shows a direct contact flue gas
es exposed to clogging, corrosion or fouling. The condensing heat recovery system in operation.
elimination of an interfering wall greatly increas- The primary water circuit includes the piping
es the heat transfer rate. from the reservoir, a pump and the spray nozzles
The direct contact heat exchanger is an ide- at the top of the column. A heat exchanger is used
al candidate for transferring latent heat from flue to transfer heat from the primary system to a sec-
gas because the spraying of fine “mist like” water ondary system which could be any low tempera-
droplets can produce a large heat transfer surface ture water use such as domestic hot water, boiler
in the presence of relatively small temperature make up feed water or heating applications.
differences between heating and cooling streams. Figure 12.14 shows the three types of col-
The packed tower design also enhances the umns used for direct contact heat recovery. The
available heat exchange surface, providing a open spray tower presents the least obstruction
highly effective scrubbing action which improves to exhaust gas flow, less than one tenth inch-of
heat transfer efficiency and raises reservoir tem- water column; but has the lowest reservoir tem-