How do you calculate Static Pressure in ventilation ducts?

Question

I am trying to find the simplest way to calculate the Static Pressure. I will be using this on a regular basis at work. If anyone knows of a website that can help with this please let me know.

Answer 1

Calculation of static duct pressure is done with the knowledge of the fan or blower outlet pressure, the material (friction factor) of the duct work, the shape of the duct (round or rectangular) the volume flow rate (and temperature) of the air through the duct and the and type of flow disturbances with their respective equivalent lengths.

At that point you can calculate the anticipated pressure drop across the various sections of duct and subtract that from the fan outlet pressure.

But that in reality gets you in the ball park. I would get my hands on a u-tube manometer, and a standard pitot tube, (you can get them fairly cheep from www.dwyer-inst.com), stick one end of the rubber hose into the manometer (series 1221) the other into the static end (the end perpendicular to the length) of the tube and simply measure the pressure. You can get a 6" long telescoping pitot tube(series 166T) that can easily be stuck into duct vents. The actual static pressure is much more valuable than the calculated static pressure in an existing HVAC system. The measurement is significantly more accurate.

Answer 2

1/2 * density * velocity squared