AHU – Return Air Fan

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A return air fan is optional on small systems but is essential for the proper operation of air economizer systems for free cooling from outside air if the return path has a significant pressure drop (greater than about 75 Pa) It provides a positive return and exhaust from the conditioned area, particularly when mixing dampers permit cooling with outdoor air in intermediate seasons and winter. The return air fan ensures that the proper volume of air returns from the conditioned space. It prevents excess pressure when economizer cycles introduce more than the minimum quantity of outside air. It also reduces the static pressure the supply fan has to work against. (The use of a return fan can increase system energy use if the system is so arranged that the return fan works against more static pressure than the incremental static pressure the supply fan would have to provide to overcome the return path pressure drop) The supply fan(s) must be carefully matched with the return fans, particularly in variable air volume (VAV) systems. The return air fan should handle a slightly smaller amount of air to account for fixed exhaust systems, such as the toilet exhaust, and to ensure a slight positive pressure in the conditioned space. Chapter 45 of the 1999 ASHRAE Handbook—Applications has design details.

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