Measuring air velocity and relating to volumetric flow at different temperatures 2

[PaulKraemer]

Hi,

I am trying to evaluate some instruments for measuring the velocity of air in a duct. Most of the instruments I am looking at list their ranges in ft/min, with a note that volumetric flow can be calculated.

I have been reading up on the difference between ACFM (Actual CFM) and SCFM (Standard CFM). Please correct me if I am wrong, but my understanding is that if you are moving the same mass of air at the same rate, the ACFM will be higher at high temperatures and lower at low temperatures, where as the SCFM should be the same because SCFM is "adjusted" to standard temperature and pressure.

What is confusing to me is, if I buy an instrument that measures air velocity in ft/min, and I multiply this velocity by my cross-sectional duct area, will this give me ACFM? Or would my calculation have to do something more complex (like taking temperature into account)?

Also, for instruments that measure ft/min, is this an all-telling, unambiguous measurement that has the same meaning at any temperature? Or does even a velocity measurement typically have to be compensated/adjusted somehow for temperature at the measurement location.

Any input will be greatly appreciated.

Thanks in advance,
Paul

 

[3DDave]

For a given velocity (neglecting the lack of uniformity of velocity in a duct) the ACFM is constant. Because density changes with temperature and pressure the corrected SCFM will change.

It depends on the type of instrument as to corrections for temperature.

 

[PaulKraemer]

Thanks 3D Dave - I appreciate your help.

I just want to make sure I understand your comment that it depends on the type of instrument whether corrections for temperature are required. The three types of instruments for measuring air velocity that I have come across are:

(1) instruments that calculate air velocity by measuring a pressure drop
(2) instruments that have one unheated sensor that measures ambient temperature of the moving air and another sensor that is heated to a fixed offset above ambient. I believe that by monitoring how much power is required to keep this heated sensor at the fixed offset, these instruments are able to calculate air velocity.
(3) instruments that have a rotating vane.

I would guess that instruments of type (1) must do internal calculations based on temperature because air at different temperatures will have different densities, and air at a given velocity will create a different pressure drop at different densities. I would say that instruments of type (2) must also do internal calculations based on temperature due to density differences at different temperatures. It would seem that more dense air would have a greater cooling effect on the heated sensor than less dense air, therefore requiring more power to keep it at the fixed offset. I would say that instruments of type (3) would not have to do internal calculations because regardless of temperature and/or density, it would seem air of a given velocity would make a vane rotate at a repeatable/predictable RPM.

Regardless of whether the instrument I choose has to do internal temperature calculations or not, as long as it is calibrated correctly to give me air velocity in ft/min, am I correct in thinking that velocity in ft/min is an unambiguous measurement...in other words...can I assume that 300 ft/min always means that the air stream is moving at 300 ft/min, and that this can be interpreted in no other way regardless of temperature and/or pressure.

Assuming this is correct, then it totally makes sense to me that ACFM would simply be the velocity (in ft/min) multiplied by duct cross-sectional area, regardless of temperature and/or pressure. Temperature and pressure only come into play if I want to convert ACFM to SCFM.

Do I have this right?

Thank you again for your help - I really appreciate it.

Best regards,
Paul

 

[Lnewqban]

The air/gas velocity in a system is independent from the mass flow rate through it.
Nevertheless, we don't have a direct way to measure that air/gas velocity.

Let's see what happens to a rotating turbine within the moving fluid:
Its angular velocity (what we can see) depends on the amount of energy that the molecules of the fluid transfer to the shaft via impact.

Using some unreal/non-proportional numbers:
- Three molecules occupying the volume of a 1 cubic foot and traveling at V1 induce 10 rpm.
- Three molecules occupying the volume of a 1 cubic foot and traveling at V2 induce 20 rpm.
- Nine molecules occupying the volume of a 1 cubic foot and traveling at V1 induce 30 rpm.
- Nine molecules occupying the volume of a 1 cubic foot and traveling at V2 induce 60 rpm.

We then have two rpm's readings for the same actual speed of the fluid, for the same actual cubic feet per minute.
Therefore, we must know the temperature and pressure of that air/gas (how many molecules are traveling within the volume of 1 cubic foot) in order to combine those parameters with the read rpm's of the turbine.

Either the instruments or ourselves can do that calculation.

"Engineering is achieving function while avoiding failure." - Henry Petroski

 

[IRstuff]

You don't seem to have specified the desired speed range nor accuracy.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[PaulKraemer]

Thank you Lnewqban,

That totally makes sense. So even with instruments with a rotating vane, we still have to take temperature and pressure into account to accurately relate RPM to air velocity.

I guess my question now is, let's say a customer presents me with air velocity data listed in units of ft/min. As long as the instrument they used to collect this data was properly calibrated and any necessary temperature/pressure adjustments/calculations were done correctly for this particular instrument, can I assume that this air velocity data (in ft/min) is unambiguous.

In other words, if someone tells me they measured air to be moving at a velocity of 300 ft/min, can I assume this means that regardless of the instrument used or the temperature or pressure at the time the measurement was made (assuming that they used their instrument and did their calculations correctly), that this unequivocally means that the average velocity of the air molecules in the air stream (regardless of density) was 300 ft/min.

Thanks again for your help.

Best regards,
Paul

 

[3DDave]

The turbine example doesn't work. As a thought experiment, a few molecules is a rarified gas and is unlikely to be seen in ordinary use. Secondly, if the rotating velocities are different then that would indicate different amounts of power were being pulled from the air stream; anemometers don't absorb energy so if that was true the rotors would continue to accelerate.

Rotating cup anemometers are drag-balances and operate at the speed where the drag of the receding cup equals the drag on the the advancing cups. Since drag is proportional to density and the density is equal for all cups there should be no affect on operating speeds for reasonable densities and constant air speeds. Temperature does not affect drag.

For vane anemometers they tend to operate at zero angle of attack. This is also not dependent on density or temperature for reasonable densities and constant air speeds.

Pitot static and other direct drag meters do depend on the density but not temperature.

 

[Lnewqban]

Paul,
I would respond yes to both of your previous questions.
In case that I am inadvertently misleading you in any way, please refer to this more detailed explanation:

http://www.omega.com/literature/transactions/volume4/T9904-06-FLOW.html#flow_1

"Engineering is achieving function while avoiding failure." - Henry Petroski

 

[PaulKraemer]

Thank you 3DDave and Lneqban again for your help.

I read the omega article sent by Lnewqban and I am feeling confident that regardless of the instrument used or the temperature or pressure at the time an air velocity measurement is made (assuming that the instrument is used correctly and any necessary temperature/pressure calculations are done correctly), that air velocity data (in ft/min) can only have a single meaning. If air velocity is reported as 300 ft/min, this unequivocally means that the average velocity of the air molecules in the air stream (regardless of density) is 300 ft/min.

As far as how these instruments actually work, for example the turbine drag-balance explanation by 3DDave, I have to admit this is over my head at the current time. I am ok with that for now. I feel I am ready to start talking to suppliers about my particular application and requirements with a much better understanding of the units of measure commonly used. I am sure I will have other questions, but I will most likely start another thread when the time comes.

I really appreciate your help.

Best regards,
Paul