How Do You Measure Static Pressure for Axial Flow Fan Efficiency?

[mawais15]

have a manometer. i want to measure the static pressure for calculation of axial flow fan efficiency. please tell me how would i do it.

Regards.

 

[Skrambles]

http://en.wikipedia.org/wiki/Pressure_measurement

Scroll down to the section on hydrostatic pressure and take a look at the manometer diagram/formula. You'll need to know the density of the fluid in your manometer. Are you sure you're trying to measure the static pressure and not the stagnation pressure?

 

[mawais15]

http://en.wikipedia.org/wiki/Pressure_measurement

Scroll down to the section on hydrostatic pressure and take a look at the manometer diagram/formula. You'll need to know the density of the fluid in your manometer. Are you sure you're trying to measure the static pressure and not the stagnation pressure?

of course i am measuring the static pressure in the supply air duct!

thanks for the guide!

 

[mender]

To increase the resolution, mount the manometer on a slant. For example, a 30 degree slant will double the reading.

 

[mawais15]

To increase the resolution, mount the manometer on a slant. For example, a 30 degree slant will double the reading.

i don't understand. .

what i thought is to place the manometer in the duct at delivery side and the reading will be the static pressure.

 

[xxChrisxx]

For static pressure don't you place a pressure probe 90 degrees to the flow? (so just of of direct flow.

Or use a pitot-static probe in the flow.

 

[mawais15]

i think for static pressure measurement, the tube should be perpendicular to the air flow

 

[boneh3ad]

That is what he just said.

Is there any reason you are using a manometer and not something a little nicer like a pressure transducer?

 

[mawais15]

i don't have any other instrument or transducer available. dats why

 

[mender]

http://upload.wikimedia.org/wikipedia/commons/4/4b/Flow_bench_schematic.GIF

Look at the right side of the drawing; you'll see that the inclined manometer is mounted on a 30 degree slant in order to get more precise readings. It's a very reasonable alternative to a transducer.

The pressure probe remains perpendicular to the air flow.

 

[boneh3ad]

I think the issue, mender, is that you used the term "double the reading", which could imply to some people that you are going to get double the real pressure.

 

[mender]

Okay, let me clarify using an example.

If the readings are in inches of water, having the manometer at a slant of 30 degrees will result in the water moving 2 inches along the tube instead of just one. The reading is doubled for the same pressure differential, allowing more precise readings.

 

[mawais15]

thanks mender for explanation. i got your point that when the liquid column is inclined. it will be more sensitive to the pressure response. . . THANKS1

 

[Herr-Kuhn]

Nothing wrong with using a manometer...very accurate device. The height of the water in the tube will end up at a head pressure height to equalize with the measured pressure. Remember, the volume of water has nothing to do with the pressure...it's all about the height. Thus the comments above on the angle on the tube.

 

[mawais15]

yeah that's right.

but according to mender, if you get a greater rise in the water height for the same pressure differential, that will increase your measurement resolution; this is what i am agree with.

the formula for calculation of the pressure is

P = density*height*g

where g = gravitational acc.

 

[russ_watters]

You're also measuring the static pressure at the fan intake plenum, right...?

 

[mawais15]

You're also measuring the static pressure at the fan intake plenum, right...?

yeah i will measure the pressure at inlet and outlet of the fan and then the difference of these will give me the required value.

 

[Herr-Kuhn]

The angle would seem to be a good idea, though you are still measuring the pressure head held in equilibrium. So long as you know the fluid and the height, the pressure is easy to calculate.

I just recently installed a new furnace which was re-jetted to propane. We first set the gas pressure valve by eye (watching the flame) and it was close, but a manometer showed the LPG (gas) pressure was 1 inch of water too low. A simple turn of the screw and all was in-line. I just learned that most furnace guys don't like working with LPG...it tends to hang around a bit longer than Natural Gas...they claim it will hang out in a sink bowl and stay there unrecognized...then poof!

Unfortunately furnaces don't have closed loop measurement of the flames...you would wonder if there are any savings to be had by being able to make some adjustments on the fly as conditions vary.