847Waste Heat Boilers

Waste-heat boilers are water tube boilers in which hot exhaust gases are used to generate steam. The exhaust gases may be from a gas turbine, an incinerator, a diesel engine, or any other source of medium- to high-temperature waste heat. Figure 8.29 shows a conventional, two-pass waste-heat boiler. When the heat source is in the medium-temperature range, the boiler tends to become bulky. The use of finned tubes extends the heat transfer areas and allows a more compact size. If the quantity of waste heat is insufficient for generating a needed quantity of steam, it is possible to add auxiliary burners to the boiler or an afterburner to the ducting upstream of the boiler. The conventional waste-heat boiler cannot generate super-heated steam so that an external superheater is required if superheat is needed.

A more recently designed waste-heat boiler utilizes a finned-tube bundle for the evaporator, an external drum, and forced recirculation of the feedwater. The design, which is modular, makes for a compact unit with high boiler efficiency. Additional tube bundles can be added for superheating the steam and for preheating the feedwater. The degree of superheat which can be achieved is limited by the waste-heat temperature. The salient features of the boiler are shown on the schematic diagram in Figure 8.30.

Waste-heat boilers are commercially available in capacities from less than 1000 up to 1 million cfm of exhaust gas intake.

0 0

Post a comment