Static Pressure in air blowers 2

[marc400317]

Hi all, it could be a simple question for some, why blowers are specified by static pressure not total pressure or dynamic pressure? I am new in this area!
Thanks!

 

[willard3]

First things first:

Total pressure = velocity pressure + static pressure

 

[imok2]

In air conditioning, heating and ventilating work, it is helpful to understand the techniques used to determine air velocity. In this field, air velocity (distance traveled per unit of time) is usually expressed in feet per minute (FPM). By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in cubic feet per minute (CFM
You can go here to read all about it:

http://www.dwyer-inst.com/htdocs/airvelocity/AirVelocityIntroduction.cfm

 

[marc400317]

Imok thanks for that link, I've read and I understood completly the diffrence between static pressure and velocity pressure, however still confused why air blowers are specified by static pressure not total pressure. I have a sugestion, maybe because the fan static pressure is the diffrence between the static pressure at the fan outlet and the total pressure at the fan inlet, and in my case where the fan draws directly from the atmosphere then, the total pressure at the inlet is zero which leads to saying, the fan static pressure is equal to the static pressure at the fan outlet. in Math form: Ps=Ps2-Pt1, Pt1=0 therefore Ps=Ps2.
I will apreciate your comment on that
Thanks

 

[marcoh]

It is because the system pressure loss (ie duct losses etc) are calculated as a static pressure loss which the fan needs to overcome.

Velocity pressure at the fan will affect the fan motor size (ie system efficiency), hence fans should be selected/specified with low velocity discharges to decrease absorbed power and increase fan efficiency.

 

[fredt]

It is a convention which is probably related to "useful" pressure as somewhere along the line you are likely to lose velocity pressure when the air is discharged. However, I do understand the confusion and it is incorrect to specify "Fan Static Pressure" (which is a defined term in the Fan Standards as Fan Total Pressure less the velocity pressure at a defined evase area) without specifying the evase area. It is necessary to know the total pressure rise when designing a system where the final discharge is different from the stated fan evase area.

Hope that this helps.

 

[marc400317]

Thanks guys

 

[imok2]

When a mfg makes a supply fan he determines what his fan is capable of performing at various pressure drops from whatever will be attached to his fan therefore he takes his readings, in static pressure and determines how much CFM the fan will produce.

 

[newmep]

I have a question along these same ideas. In pumps, there is an NPSH, required for pump to operate. Is there such a term and requirement for fans? Is there a minimum total head required on the fan inlet.
Thanks,

 

[RossABQ]

NewMEP, not exactly. There are similar inlet conditions that will ruin pump or fan performance, much of which has been documented by AMCA. Unlike a pump, if a fan is choked on the inlet side there are (usually) no destructive consequences for the fan itself. In fact, inlet vane control used to be popular for some types of fans.

 

[imok2]

Any air conditioning supply fan that has return air supply ducts will be operating in a negative pressure as the supply ducts will be in a positive pressure and when you are measuring external static pressure you add both the negative and the positive pressures for total static pressure

 

[SAK9]

Hi Marc,

Thanks for asking a very good question!Fan selection should be done based on total pressure only.

Fan static pressure is equal to fan total pressure only when fan outlet area is equal to the discharge duct area.This is not the case always and selection should be based on total pressure.Please read ASHRAE (Systems and Application ) chapter on Fans and your doubts will be cleared.

Cheers

 

[SAK9]

Sorry ASHRAE(Systems and Equipment)

 

[willard3]

I always specify fan systems by total pressure so that you don't get a nasty blast gate in the scroll to regain static pressure.

Total pressure performance is what you are after in a fan.