Air heaters in a steam boiler system are classified according to their principle of operation as:
§ Tubular Air Heater
|Fig. 1: Tubular Air Heater|
In a tubular air heater, energy is transferred from the hot flue gas flowing inside many thin walled tubes to the cold combustion air flowing outside the tubes. The unit consists of a nest of straight tubes that are roll expanded or welded into tubesheet and enclosed in a steel casing. The casing serves as the enclosure for the air passing outside of the tubes and has both air inlet and outlet opening. In the vertical type tubes are supported from either the upper or lower tubesheet.
The tubular air heater is simple to fabricate and does not increase the existing auxiliary power or erosion problems (in case of coal-fired boilers), because there are no leakages from air to gas side such as in the case of an Rotary Air Heater and no maintenance issues because the assembly is static.
§ Rotary Air Heater
|Fig.2: Rotary Air Heater|
Rotary air heater can be known as Ljungstrom type which features a cylindrical shell plus a rotor which is packed with bundles of heating surface elements and is rotated through counter flowing air and gas streams. The rotor is enclosed by a stationary housing which has duct at both ends. Air flows through one half of the rotor and gas flows through the other half. The most prevalent flow arrangement has the hot gas entering the top of the rotor as cold air enters the bottom in counter flow.
The rotary air heater needs regular adjustment to the seals and is also maintenance prone but the real advantage lies in its compactness and simplified duct layout. With several shop fabricated subassemblies, the site erection takes much less time. Also, with enameled baskets, lower gas-exit temperatures can affect boiler efficiency favorably.