Pressure drop calculation for exhaust header / parallel fan?

[m2e]

What is the pressure drop calculation method for an exhaust ductwork system where multiple exhaust fans discharge into a common header, and the combined E/A duct run out the building? Can you assume the friction is the same as a single fan system? Also, how do you size the fan with the back pressure from the other fans?

Thanks.

 

[willard3]

There are lots and lots of variables in parallel fan configurations, control can be very difficult.

 

[MTES93]

This was fairly common in older buildings that I have worked on. As far as the back pressure thing I am not sure how much comes into affect when/if the system is designed as a low pressure system. On systems I have designed or modified I have done the following:
Design common header w/ fairly low pressure drop and w/ sum of the connected exhaust fan air flow rates.
Install back draft dampers on all the fans to prevent reverse flow incase they are off and others are on.

From an engineering perspective, remember that the building exhaust outlet is at zero gauge pressure so the air from these exhaust fans are moving from a slightly positive air pressure at fan outlet to this neutral pressure at this outlet, so the air will go that way.

I think higher pressure/velocity systems you could see back pressure issues, not sure how much this comes into play when it is a normal, low velocity exhaust duct system.