Page 278 - Mechanical Engineers' Handbook (Volume 4)

11 Waste Heat Recovery Systems 267

potential. Smaller units are cleaned periodically by removal and washing. Large in-
dustrial units are cleaned in place. A possible objection to their use is a slight increase
in the ozone content of treated air.
3. Wet scrubbers are particularly effective for removing water-soluble contaminants
such as sulfur and chlorine compounds. They can be used in place of filters for
handling heavy loads of solid particulates such as from foundry cupola furnaces,
metal-refining processes, and lime kilns. Waste material is collected as a mud or
slurry, requiring proper disposal to avoid solid-waste problems.
4. Combustible wastes, such as the solvent vapors from organic coating ovens, may be
burned in incinerator units by adding combustion air and additional fuel as required.
Fuel economy may be improved by using waste heat from combustion to preheat
incoming gases through a recuperator. The same system may be used for combustible
solid particulates suspended in flue gases.
5. Radioactive wastes from nuclear power plants will usually be in the form of sus-
pended solids that can be treated accordingly if suitable facilities for disposal of
collected material are available, or as radioactive cooling water for which a suitable
dumping area will be needed.

11 WASTE HEAT RECOVERY SYSTEMS
In fuel-fired furnaces, a fraction of the energy from combustion leaves the combustion cham-
ber as sensible heat in waste gases, and the latent heat of evaporation for any water vapor
content resulting from the combustion of hydrogen. Losses increase with flue gas temperature
and excess air, and can reach 100% of input when furnace temperatures equal theoretical
flame temperatures.
Waste heat can be recovered in several ways:

1. Preheating incoming loads in a separate enclosure ahead of the furnace.
2. Generating process steam, or steam for electric power generation. Standby facilities
will be needed for continuous demand, to cover interruptions of furnace operation.
3. Preheating combustion air, or low-Btu fuels, with regenerative or recuperative firing
systems.

11.1 Regenerative Air Preheating
For the high flue gas temperatures associated with glass- and metal-melting processes, for
which metallic recuperators are impractical, air may be preheated by periodical reversal of
the direction of firing, with air passing consecutively through a hot refractory bed or checker
chamber, the furnace combustion chamber, and another heat-storage chamber in the waste-
gas flue. The necessary use of the furnace firing port as an exhaust port after reversal limits
the degree of control of flame patterns and the accuracy of fuel/air control in multiple port
furnaces. Regenerative firing is still preferred, however, for open hearth furnaces used to
convert blast furnace iron to steel, for large glass-melting furnaces, and for some forging
operations.
A functional diagram of a regenerative furnace is shown in Fig. 44. The direction of
flow of combustion air and flue gas is reversed by a valve arrangement, connecting the low-
temperature end of the regenerator chamber to either the combustion air supply or the exhaust