Ducts and Dampers Design Considerations
Air preheater systems should have bypass ducts and dampers to allow the preheater to be isolated for maintenance. There should be air bypass doors that open automatically on fan failure. This permits the heater to operate at reduced rate on natural draft. (The vendor should state what that reduced rate will be.)
Dampers in air preheater systems have developed major leaks due to warpage and deformation which result from temperature and pressure differences. Dampers should be designed and reinforced to minimize these problems.
In the event of a change in ambient air temperature or a reduction in the temperature of the flue gas entering the preheater, it may be necessary to raise the cold end temperature to protect the cold end heating elements. This is done by bypassing a portion of the inlet air around the preheater.
An observation port and light should be provided in the air inlet duct to allow onstream inspection of the cold end of the preheater.