Fan in series

[remp]

Fans in series

I have a fan coil unit with about 210pa exteranl static pressure on it (pretty low). I am low in available pressure and the fan is to do both supply and return duct (100% recirc)

To help the situation I am putting a return air fan in the return air duct from the returan air grilles to a box at the back of the fan coil unit.
Is it ok to run two fans in series even if they are different fan curves. The return air fan will have a variable speed drive on it to match the suplly air fan.

 

[quark]

No problems as long as you maintain the return fan flowrate equal to that of supply fan. Ideally, the return fan should be selected at the supply fan cfm and at a static pressure equal to the total SP required minus the external static of supply fan.

 

[willard3]

If the fans are either axial or forward-curved blades and the fan curve has a bump, you will get some interesting aerodynamic effects unless the fans are exactly the same and start simultaneously.