Classification. Air heaters are classified as either recuperative
or regenerative. Recuperative air heaters pass heat through a heat exchanger
directly from one side of the surface to the other. Recuperative air heaters
are generally of the tubular or plate type. Regenerative air heaters use heat
from the hot gasses to heat a heat storage medium then use the heated medium
Corrosion. The tube metal temperatures must stay above the
temperatures indicated in Figure 37. The tube metal temperatures may be
controlled by one of the following methods.
a) Recirculate a portion of the preheated air from the air heater
outlet to the inlet of the forced draft fan.
b) Bypass a portion of the air around the air heater which will
reduce flow through the air heater and allow the metal temperature to
c) Use a steam coil located in the air duct prior to the inlet of
the air heater.
Use corrosion resistant materials or material coating.
e) The temperature of combustion air discharge from the air heater
should not exceed the limits listed below:
1) Stoker fired units for coal or weed chips can generally
accommodate temperatures up to 350 degrees F (177 degrees C).
2) Pulverized coal fired units may utilize temperatures up to
600 degrees F (315 degrees C) for drying the fuel as well as additional
Some of the factors to consider in stack design are:
a) Flue gas conditions; i.e., erosive and corrosive constituents,
dew point temperature, maximum temperatures if bypassing economizer or air
b) Temperature restrictions (related to stack lining materials and
c) Corrosion Resistance. Stack and/or lining material must be
selected to withstand corrosive gases and condensed acids (related to sulfur
in the fuel).