Where to place a fan along a duct?

[TSN79]

Picture a circular horizontal piece of duct, open at each end. If a fan is to be placed somewhere along the duct and move a certain amount of air, does it make a difference for the fan and it's operating point where it is positioned? Or does it only come down to the total length of duct and the friction it creates?

 

[Bystander]

What are the ΔP values?

 

[TSN79]

What are the ΔP values?

It's just a pictured scenario, but typically 1-2 Pa/m in the situations I deal with.

 

[russ_watters]

For what you described, no, it does not matter. If there are other factors you aren't telling us about though, it might.

 

[rbelli1]

If there are any leaks in the duct you will want to arrange the pressure inside so that the leakage is in an acceptable direction. For example in the duct-work for an airplane shore cooling system the fan must be at the inlet (positive pressure) even if rigid ducting were used as the leaked ambient air is polluted by engine exhaust and is also the wrong temperature.

If the duct is used for transporting particulates then the fan must be at the outlet (negative pressure) otherwise the transported materiel will be sprayed out the leak.

BoB