A weird question about airflow measurement

[Vincent Yu]

Hello all,

I have not been here for several months. Currently I have a weird question about airflow rate measurement. I am struggling very hard to solve it. Now I am working on designing dehumidifiers. It is very important for me to measure the airflow rate accurately. I found an interesting phenomenon:

When I measure the airflow, the reading at system inlet is always different from the reading at system outlet. For example: when I measure the airflow at system inlet, the reading is 250 cube feet per minutes; if I use the same device to measure the airflow at system outlet, the reading is about 300 cube feet per minutes. I test several of our products. The airflow readings at outlet are always about 20% higher than the readings at inlet.

I use both pitot-static gauge and Venturi meter in my tests. The results are consistent. When I run the test, the compressor is shut down. The power of the fan is about 120 to 180 watts. So the influence caused by the temperature difference is not so significant.

Could anyone give me a reasonable explanation? What is the best way to measure airflow? Any comments are welcome.

Thanks
Vincent

[Josip]

Hi, Vincent Yu :)

Can you make a sketch with some more info;)

Not use Anemometer :confused:

Best regards, Josip :)

[Brian_UK]

If you have the same size ductwork at inlet and outlet you should be getting the same reading.

I'm assuming that you have the correct duct layout to accurately measure the airflow.

If you are not using ductwork then please provide details of the inlet and outlet configurations and the method of taking measurements.

However is there any leakage on the inlet side to the fan after your duct measurement point.

[Dan]

My guess is the directional velocity of the air is causing your readings to differ. Inlet air is not as focused a stream as outlet air is. Measurements based on velocity will more likely show a higher general velocity on the outlet for this reason. I think that if you passed all inlet and outlet air through a propeller velometer, for example, you would get identical readings both in and out.

Just a wild hunch.

[Peter_1]

According to the law of Bernoulli states that as the speed of a moving fluid increases, the pressure within the fluid decreases and the ideal gas law, derived fram Boyles' and Charles law states that P.V= n.R.T

And then there's also the equation of continuity.

So, there must be something wrong with your measurement.

[MadApprentice]

Umm i am only an apprentice but would referring to the three fan laws be of any help?

[Peter_1]

:confused: what are these 3 fan laws?

[MadApprentice]

I will write them up tomorrow its 2am in the morning here and my books are in my car!

[Josip]

Hi,


:confused: what are these 3 fan laws?


Maybe 3 fun laws ;)

Best regards, Josip :)

[Brian_UK]

Basic Fan Laws

For any individual fan, various rules (http://www.comairrotron.com/engineering_notes_02.asp) have been proved, including one that air flow is proportional to motor speed.

Variable - When Speed Changes

Air Flow Varies directly with speed ratio CFM1(cu.m/s1/cu.m/s2) /CFM2 = RPM1 /RPM2

Air Pressure Varies with square of speed ratio P1 /P2 = (RPM1 /RPM2)2

Power Varies with cube of speed ratio W1 /W2 = (RPM1 /RPM2)3

Data above copied from:- http://www.cpemma.co.uk/fanlaws.html

[Josip]

Hi, Brian_UK :)

good link, (confirmation: we can learn something new every day:) )

Best regards, Josip :)

[MadApprentice]

Yep what brian said apparently you can use this calculation to predict what effect adjustments will have ?

[jeelansharieff]

hi this is jeelan

it means air expands
thats your problem

[Abby Normal]

Air expands is a possible answer but will not account for 20%.

Look at the entering and leaving conditons on a pyschrometric chart.

Perhaps you have air entering at 80F db and 60% RH and leaving at 120F dry bulb and 10.5% RH. Specific volume would have expanded about 6.5%.

If you plotted the entering air condition you would be able to come up with a mass flow rate of dry air and moisture entering the dehumifier.

You could compare this to the leaving condition, you should have the same mass flow rate of dry air coming out but less moisture. The difference in mositure would be the condensate.

So 20% seems high for air expanding, could be a measurement error as well.

[Abby Normal]

The air balancers constant of 4005 is based on dry air at 70F.

You apply it to humid air coming into the dehumidifier than you get an error.

You apply it the hot dry air leaving the dehumidifier and you get an error.

A way of checking the accuracy of how your are measuring would be to just run the fan and not the compressor, with the coil dry.

The entering and leaving airflows should measure pretty close, be minimal fan heat expanding the air. Would point out any kind of downstream turbulance etc putting your measurements off.

[Abby Normal]

make sure the cabinet is not leaking, that the fan draws in air other than what your are measuring in a duct

I see that the original poster tried it with just the fan running. Dan mentioned velocity, so I can just guess then that the velocity profile on the supply side is not fully developed where the measurements are being taken.

[winfred.dela]

I use both pitot-static gauge and Venturi meter in my tests. The results are consistent. When I run the test, the compressor is shut down. The power of the fan is about 120 to 180 watts. So the influence caused by the temperature difference is not so significant.


Hi Vincent Yu,

Are you using a method that is recommended in Testing & Balancing (TAB) manuals or handbook?

Am just trying to figure out the way/location you measured and computed those flow rates :confused:

[Lc_shi]

Hi Vencent
It's not easy to measure air flow volume exactly. If you used air velocity and times duct area which is not very correct dut to the air flow is not even in the duct. you should use throttling nozzle or metering plate and the pressure drop along the duct then you calculate the air flow volume. In usual r&d test, we measure at inlet and plus a mixer at outlet and measure the velocity ,it's around 10% error. only for your ref

regards
LC

[ernestlin]

The air balancers constant of 4005 is based on dry air at 70F.

You apply it to humid air coming into the dehumidifier than you get an error.

You apply it the hot dry air leaving the dehumidifier and you get an error.

A way of checking the accuracy of how your are measuring would be to just run the fan and not the compressor, with the coil dry.

The entering and leaving airflows should measure pretty close, be minimal fan heat expanding the air. Would point out any kind of downstream turbulance etc putting your measurements off.


I use both pitot-static gauge and Venturi meter in my tests. The results are consistent. When I run the test, the compressor is shut down. The power of the fan is about 120 to 180 watts. So the influence caused by the temperature difference is not so significant.

Hi, Abby, This test had been run without the work of comprosser. So, the problem may be at the measure method.