Flow rate through large air duct

[mattibo]

I am trying to find the flow rate in a duct that is quite large (around 4'x3') but is very short. This duct leads to a fan which leads to a separate unit through another short duct section. Both these duct sections have different sizes (the fan outlet is much bigger than its inlet). I will take static pressure readings before and after the fan to calculate the flow rate. How will the change in duct sizing effect my flow rate estimation? I guess that the dynamic pressure head variation will have a significnt effect.

Thank you for reading.

 

[anorlunda]

ping @russ_watters . Can you help with this spring cleaning thread?

 

[russ_watters]

I am trying to find the flow rate in a duct that is quite large (around 4'x3') but is very short. This duct leads to a fan which leads to a separate unit through another short duct section. Both these duct sections have different sizes (the fan outlet is much bigger than its inlet). I will take static pressure readings before and after the fan to calculate the flow rate. How will the change in duct sizing effect my flow rate estimation? I guess that the dynamic pressure head variation will have a significnt effect.

You can't calculate the flow rate from a static (or differential) pressure measurement across the fan alone. But if you have the fan curve, you can plot the static pressure on it and find the flow. Either way, if you have the equipment to measure the pressure it would be better to use a pito-tube and measure the velocity -- then calculate the flow rate with the duct area.