Types of Air Preheaters
There are two types to consider: regenerative and recuperative.
There is no cost advantage for either type, although domestically the regenerative type is the most frequently used, with the recuperative type most popular in Europe.
Regenerative. This type has a record of successful operation, and is referred to as a Ljungstrom design. It has a compartmented rotor contained in a rotor housing supported by bearings, each filled with metallic heating elements. The rotor slowly rotates, alternatively through the gas and air streams. Hot flue gases flow through one side of the rotor and heat the elements. Air flows through the other side where the stored heat is released to the air stream. The air and gas flows are separated by diaphragms in the rotor and seals between the rotor and the rotor housing. While this design has little fouling and no heat transfer losses due to soot deposits, it does require a motor driver and there is some leakage around seals between the incoming air and the exhaust gases.
Recuperative. This is a fixed air-to-air flue gas, with no moving parts. It is a bundle of tubes expanded into a tube sheet and enclosed in a casing. Flue gases flow through the tubes, and air to be heated flows over the tubes. Alternatively, extended surface tubes may be used with flue gas flowing over the tubes while the air to be heated flows through the tubes. While soot deposits can lower the heat transfer and increase draft losses, these units are stationary and there is no leakage between the incoming air and the exhaust gases.