Fan selection is not only dependent on the air quantity (cfm) and the static pressure it has to overcome, it also depends on what is in the air that is being moved or what cannot be there. Air handlers that supply air for comfort applications typically utilize a forward curve or backward incline fan arrangement with the motor belts, base and pulleys in the air stream.

	A cleanroom application may require there be no belts or motors in the air stream as the entire duct and air handler interiors require periodic cleaning or disinfecting. The solution here would be to use a direct drive or belt driven plug fan arrangement.

An exhaust fan may be exposed to corrosive chemicals in the air stream. Depending on the quantity and corrosive nature of air stream the use a non-metallic fan wheel and housing may be required.