Air heaters like economisers, recover low grade energy from the flue gas stream. While economisers recover the heat by heating the boiler feed water, air heater recover the heat by preheating the combustion air.
Combustion air preheating enhances the combustion of many fuels and is critical for pulverised coal firing. Pre heated combustion air is required for the transportation of pulverised fuel and to maintain stable ignition of the fuel in the boiler.
Because of the lower gas temperatures and air side heat transfer coefficients, air heaters require far greater surface area per unit of heat recovered. Because the heat transfer coefficient is similar on both sides of the air heater heat transfer surface , extended surfaces are rarely used in air heaters. Air heaters are typically located between the economiser and stack, receiving the hot flue gas from the economiser and the cold combustion air from a forced draft fan.
Economisers are classified into 2 types – recuperative or regenerative.
In a recuperative air heater, heat is transferred continuously and directly from stationary, solid heat transfer surfaces, which separate the hot and cold streams. Typical are tubular air heaters with the flue gas passing through the tubes and cold air flowing outside of the tubes. These units have the tubes expanded or welded into tube sheets. The shell or casing of the heater often incorporates expansion joints or have floating tube sheets. Baffles are use on the air side to direct flow and eliminate flow induced vibration. Besides tubular air heaters, plate type air heaters and steam coil air heaters are used.
Recuperative air heaters transfer heat indirectly by convection as a heat storage material is exposed to the hot and cold streams periodically. A variety of material such as tightly packed corrugated steel plates are used as the heat storage medium. These rotate through the hot and cold streams, periodically receiving and rejecting the heat. The main issue with regenerative heaters is air leakage into the gas stream as sealing cannot be 100%.
As most air heaters are designed to operate at minimum metal temperatures slightly below the acid dew point, there are several operational issues with most air heaters. Most common are cold end corrosion, plugging and cleaning, air leakage and performance degradation and erosion.