Exhaust fan pressure requirement 1

[davidyu2404]

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!

 

[willard3]

On most centrifugal fans, Vp inlet is not equal to Vp outlet.

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.

 

[EnergyProfessional]

homework?

 

[davidyu2404]

NO this NOT homework. Don't you think I would be given more information if this is a homework problem?

 

[EnergyProfessional]

So what are you trying to accomplish? Looks like the fan manufacturer could you give yous specific information.

 

[317069]

what is the cfm of your fan

 

[davidyu2404]

I am a designer at a manufacture company. I am new to the field and I am trying to select a fan with the right static pressure requirement for the duct work. The fan should around 5500 cfm. I was reading ACGIH to get more information and I found the equation Fan Static Pressure=FTP-VP(inlet)=SP(outlet)-SP(inlet)-VP(inlet). I think SP(outlet) is the pressure loss of the duct work and I can figure out VP(inlet) from the duct velocity but I don't know how to calculate SP(outlet).

 

[willard3]

I would suggest, from your questions and responses, that you could use some help from an Engineer with ventilation experience.

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.

 

[junagco]

davidyu2404, as your fan is exhausting air directly to outside, I assume that the ducting is only connected to fan inlet.
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.