How to Measure Static Pressure

By "efficiency", you mean how high the static pressure of the axial fan can be?

If you have a manometer (which is nothing but a 'U' shaped glass or plastic tube filled with coloured water), connect one end of it at the discharge duct of the fan with suitable tapping point and read/measure the difference in water level between both the arms of 'U' tube. It will give static pressure in mm OR inches of water column. What is the pressure unit required in your formulae for calculation of efficiency? It may be either inches WC or psi. Convert accordingly.

If SP is higher than 1000 mm, you may not be able to measure with standard manometer. In such case make it yourself longer one.

Static pressuree is simply measured from a tapping in the side of the duct.

I'd need to check your formula to be sure, but I think SP should be static pressure differential across the fan. Strictly speaking, formula is only correct if the up and downstream ducts are the same area, otherwise need to take account of velocity pressure.

OK if actual pressure is close to atmospheric, otherwise need to be careful whether CFM is at some standasrd conditions or actual conditions.

Cheers.....Codey

i took this formula from a book. i am not sure whether its right or wrong.

the pressure at the inlet of the fan is nearly atmospheric. that's why i will measure the static pressure in the supply side only.

I'll try to check the formula, but I'm on holiday in Australia at the moment and don't have usual facilities.

Somebody referred to measuring the CFM. Your post implies you know the CFM, but don't forget CFM is not constant, CFM and pressure differential affect each other via the fan performance curve.

Cheers.....Codey

what you mean by CFM is not constant. . . !

i will measure all the parameters at one after another!

for a given speed and pressure differential, the CFM would be constant as far as i know :-D

That's right, but if the system characteristic changes the CFM and SP willl change.

Codey

Also meant to ask - what are units of SP and CFM in the formula?

The formulae given by you "Efficiency = SP*CFM/6345*B.H.P" appears to be grossly wrong. However, please give the unit of static pressure before jumping in to conclusion.

According to my calculation, the formula is about right if SP is in inch w.g. and Efficiency is a fraction.

Cheers.......Codey

of course the units of SP are in inches of W.C.

however, if we use other units, then the formula becomes like

efficiency = (SP*Q)/(102*P)

where

SP = Static pressure in mm W.C

Q = Flow rate in m3/sec

P = Power consumption in kW

and efficiency would be in fractions :-)

Thanks for clarifying. Formulae is right. However, the simpler form of it can be

efficiency = (SP*Q)/P , if a SP is expressed in Pa

http://en.wikipedia.org/wiki/File:Pitot_tube_types.svg

The middle one is for static pressure measurment

How to Use a Manometer | eHow.com

thanks

You Can go to the Dyer Instrument web site and down load a page from one of their manuals that shows you how it is done. They also have some nice graphs to help with this process.

Flow of air or gases is measured easily. Just put two pipes into the duct. One perpendicular to the flow and other with opening facing the flow. Both should be place in the centerline of the duct. Then you connect a glass U-tube, with a scale that is half filled with colored alcohol or colored water, so you can measure difference between kinetic pressure and static pressure of the flow. Yet the dynamic pressure is difference between both. It is the dynamic pressure that counts.

A ready to use instrument is in every plane that is used to measure speed of the plane. Buy one that is in Cessna from a plane junkyard.

To get more accurate measure of slow flows just place the U-tube with scale under angle! It would show pressure difference that is dynamic pressure

Kazakh

Dear friend,

The static pr. is the potential energy put into the system by fan,it is given up to friction in the ducts,it will be measured using U-tube manometer.One end of the U-tube manometer is connected to duct and other is left open to atmosphere.The diff.in the height of the fluid gives static pressure of fan system.

STAIC PRESSURE - height diff. in U-tube manometer X sp.gravity of fluid(filled in manometer)

ELLURU RAMAIAH