Exhaust fan pressure requirement
Exhaust fan pressure requirement
(OP)
So I found the following equation from ACGIH: Principle of Ventilation
Fan Total Pressure=TP(outlet)-TP(inlet)=SP(outlet)+VP(outlet)-SP(inlet)-VP(inlet)
Fan Static Pressure=FTP-VP(inlet)=SP(outlet)-SP(inlet)-VP(inlet)
The fan I am using is a centrifugal fan that exhaust directly outside the building
Questions:
Is VP(inlet) approximately equal to VP(outlet) since the velocity is about the same?
I calculate all the pressure losses up to the inlet of the fan, but how do I calculate SP(outlet) and SP(inlet)? If the fan exhaust directly to the outside air, is SP(outlet) atmospheric pressure?
Thanks!
Fan Total Pressure=TP(outlet)-TP(inlet)=SP(outlet)+VP(outlet)-SP(inlet)-VP(inlet)
Fan Static Pressure=FTP-VP(inlet)=SP(outlet)-SP(inlet)-VP(inlet)
The fan I am using is a centrifugal fan that exhaust directly outside the building
Questions:
Is VP(inlet) approximately equal to VP(outlet) since the velocity is about the same?
I calculate all the pressure losses up to the inlet of the fan, but how do I calculate SP(outlet) and SP(inlet)? If the fan exhaust directly to the outside air, is SP(outlet) atmospheric pressure?
Thanks!





RE: Exhaust fan pressure requirement
Look at the fan and calculate the area of the inlet and outlet; are they the same? If they are not, the velocities for equal volume are not the same.
Area x velocity = flow.
RE: Exhaust fan pressure requirement
RE: Exhaust fan pressure requirement
RE: Exhaust fan pressure requirement
RE: Exhaust fan pressure requirement
RE: Exhaust fan pressure requirement
RE: Exhaust fan pressure requirement
Tell your boss that you are not comfortable designing air systems without some help.
This is a web forum and not an air systems class.
RE: Exhaust fan pressure requirement
Fan is usually sized or selected based on airflow (units may be cfm or lps) and external static pressure or ESP (unit is in in-wg or pa).
For the static pressure, i will not use what is in ACGIH as it is complicated. The usual fan pressure formula is TSP = ISP + ESP. TSP is total static pressure, and ISP is intenal static pressure.
I only consider ISP when air handling unit is involved. But for selecting fan or blower alone, we need only the ESP. For your given fan, ESP is the summation of pressure drops along its suction ductworks.
To determine ESP, calculate the: equivalent pressure drop caused by straight duct (usually based on friction rate used - normally it is 0.1 in/100 ft); equivalent pressure drop of all fittings (wye, elbow, collar, plenum, etc.); pressure drops of all accessories (grilles, dampers, etc.).
The ESP calcs process is somehow tedious and long. But you may learn this by reading CARRIER Manual, Part 2, Air Distribution. Or thru HVAC System Duct Design by SMACNA.