Economizers and air preheaters are heat exchangers used by some boilers as auxiliaries to recover more heat from the flue gases, heat that otherwise would be lost up the stack. Air preheaters can be classified as either indirect exchange types, external heat source types, or direct exchange types. An economizer normally consists of a bank of tubes located in the path of the flue gases downstream of the steam generating surfaces in the boiler. The low-temperature boiler feedwater is pumped through the tubes in this tube bank and is heated before passing into the boiler.
Air preheaters raise the combustion air temperature. There are two basic types of air preheaters: recuperative and regenerative. The recuperative type is similar in principle to a conventional heat exchanger with the hot flue gases on one side of the heat transfer surface and the cool air on the other side. The most common recuperative type is the tubular air preheater, which consists of a tube bank with the tubes rolled into a stationary tube sheet at the top of the unit and a floating tube sheet at the bottom. Flue gas flows on the outside of the tubes and air flows on the inside of the tubes. The use of a floating tubesheet accommodates the difference in expansion caused by temperature differences between the tubes and the casing. In this type, the hot gases flow through the tubes, and the air passes around the tubes. Another recuperative type is made up of plates arranged with passages for the flue gas on one side of the plates and passages for air on the other side.
The most common regenerative type is called a rotating heat transfer wheel and is made up of many closely spaced sheets of metal. This metal absorbs heat as it rotates through the flue-gas compartment of its housing and gives up heat as it rotates through the air compartment. The heat transfer wheel is rotated at approximately 3 rpm by a driving motor through a reduction gear. Diaphragms and seals divide the unit lengthwise to separate the hot flue gases from the air, which flow through the preheater in opposite directions.
The preheating of combustion air has high economic value. In the conventional air preheater, cold air from the forced draft fan flows through the air preheater and extracts heat from the flue gases as they flow to the stack. Economizers or air preheaters are used when fuel savings justify them.