Why do we use return fan in AHU? Even though it is return duct for exhaust?
What kind of system are you talking about? MORE Info please.
Yes, too many applications to adequately answer your question properly, please give us some more details.
Originally Posted by safwan
In basic terms based on the limited info.
A return fan is sometimes used as an aid in controlling building static pressure.
On larger high volume systems (air) it is difficult to naturally remove enough air from the conditioned space without the aid of a return fan to "pull" the air from the building fast enough to keep the building from over pressurizing while bringing in outside air.
If you were to build a large air handler system that did not require a return fan, the return duct would have to be enormous and have few or no restrictions for flow...Otherwise, you would be pressurizing the building much like a balloon.
Hope this answers your question....
If a day goes by and you have learned nothing, I hope you got a lot of sleep.
GT's explanation good, I would like to add..
That when the return duct system is extensive And you have an economizer Then a single fan that provides the ESP for both the supply and return duct will not allow the Economizer to work properly because there is no positive Relief.
PS And a power exhaust IS NOT A RETURN FAN.
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!