Air Recirculation Recirculation of air which has been heated as
it crosses the tube bundle provides the best means of preventing operating
problems due to low-temperature inlet air. Internal recirculation
is the movement of air within a bay so that the heated air which has
crossed the bundle is directed by a fan with reverse flow across another
part of the bundle. Wind skirts and louvers are generally provided to
minimize the entry of low-temperature air from the surroundings.
Contained internal recirculation uses louvers within the bay to control
the flow of warm air in the bay as illustrated in Fig. 11-47. Note that
low-temperature inlet air has access to the tube bundle.
External recirculation is the movement of the heated air within the
bay to an external duct, where this air mixes with inlet air, and the mixture
serves as the cooling fluid within the bay. Inlet air does not have
direct access to the tube bundle; an adequate mixing chamber is
essential. Recirculation over the end of the exchanger is illustrated in
Fig. 11-48. Over-the-side recirculation also is used. External recirculation
systems maintain the desired low temperature of the air crossing
the tube bundle.
