In induced arrangement plenum chambers are mounted on top of the bundle and protect the fin tubes against wind, rain, snow, etc.
In most cases the advantages of induced draft design outweigh the disadvantages.
Advantages of Induced Draft Better distribution of air across the bundle.
Less possibility of hot effluent air recirculating into the intake.
Better process control and stability because the plenum covers 60% of the bundle face area, reducing the effects of sun, rain, and hall.
Increased capacity in the fan-off or fan failure condition, since the natural draft stack effect is much greater.
Disadvantages and limitations Possibly higher horsepower requirements if the effluent air is very hot.
Effluent air temperature should be limited to 100°C to prevent damage to fan blades, bearings, or other mechanical equipment in the hot air stream. When the process inlet temperature exceeds 180°C, forced draft design should be considered because high effluent air temperatures may occur during fan-off or low air flow operation.
Fans are less accessible for maintenance, and maintenance may have to be done in the hot air generated by natural convection.
Plenums must be removed to replace bundles.