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Static pressure measurement vs venturi effect

mingmongmang (Mechanical) (OP)
4 Jun 10 16:09
I'm using a differential pressure gage to measure static pressure inside a forced air cooled enclosure. My point of measurement is quite close to the blower and I'm just using plain 4mm ID tubing from the gage.

My concern is venturi effect changing my readings - I can blow across the end of the tube and pull 1" of water. Should I have some kind of baffle on the end of the tube to eliminate any venturi effect?
willard3 (Mechanical)
6 Jun 10 9:49
Static pressure in the control volume (cooling enclosure) will always be positive  (above atmospheric) if you are adding flow energy.

You may want to find total pressure and subtract velocity pressure to find static pressure. I don't know what you have for instruments.
AbbyNormal (Mechanical)
6 Jun 10 16:27
It is not uncommon for air to be so turbulent at a fan discharge that you would measure negative static pressures.

It is not uncommon in my experience that you have to measure the static some distance from the discharge in order to get a stable,positive, repeatable reading.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

mingmongmang (Mechanical) (OP)
7 Jun 10 16:49
My problem is there are a lot of obstructions to airflow close to the blower (which actually pulls air from the unit) and I need to know the pressure drop across the fan to use it's CFM/pressure drop curve. I'm concerned that measuring from the  far side of the obstructions would show a falsely small pressure drop and I'm pretty sure there'll be all kinds of turbulence between the blower and obstructions.
Is there any way to get a reliable static pressure measurement behind the blower? (assuming I have the instrumentation)
AbbyNormal (Mechanical)
7 Jun 10 18:23
Can you not traverse the inlet duct with a pitot tube?

Or get the face velocity into the fan inlet with an aneometer or a velocity grid?

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

willard3 (Mechanical)
8 Jun 10 10:46
You will also find that the fan performance curve is rated with clean inlet and outlet conditions. Check AMCA for rating parameters.
mingmongmang (Mechanical) (OP)
8 Jun 10 17:35
OK so I may as well forget pressure/CFM curve and just measure CFM entering or leaving the unit. Does that sound like the best approach and would a velocity grid be the best tool for that?

I guess I should say I'm trying to improve the airflow (cooling)through some complex equipment and I'm just looking for a repeatable measurement so I can see improvements and be sure I'm within the capability of the blower.
I should also say I've never worked with forced air cooled equipment or airflow measurement before.
IRstuff (Aerospace)
8 Jun 10 18:02
As I posted in your other thread, you should also make temperature measurements as well.  Don't make assumptions about what to fix if you have no data to base that assumption on.  These things are a SYSTEM, and the weakest link might not be where you think it is.


FAQ731-376: Forum Policies

mingmongmang (Mechanical) (OP)
9 Jun 10 11:57
Well the temp of each component is monitored so I know where the additional cooling is needed. Problem is the enclosure is absolutely packed with components and the air already has to take a very tortuous route. I want to remove unnecessary restrictions and maximize flow and then have a way to monitor total flow as I add/tweak diverters etc.

I'm leaning towards just monitoring the flow entering or leaving the unit from outside rather than trying to measure pressure inside, but I still need to find the best way to do this.  
IRstuff (Aerospace)
9 Jun 10 17:54
Then you have part of your answer already.  A tortuous path may be much worse than the port restrictions you're looking at.   


FAQ731-376: Forum Policies

willard3 (Mechanical)
10 Jun 10 9:05
Even measuring the cfm at inlet and outlet is not simple as flow will vary over the area of the inlet/outlet.

SMACNA or the Air Balance Council have good guides for flow measurement.

AbbyNormal (Mechanical)
10 Jun 10 19:47
it is simplest at the inlet to a grille, no 'vena contracta'

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

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