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SCFM vs ACFM, How This Relates to Velocity

Is there a distinction similar to that between ACFM and SCFM when interpreting air velocity data (in feet per minute)?

Hi,

I have been doing some reading to understand the relationship between SCFM (Standard Cubic Feet Per Minute) and ACFM (Actual Cubic Feet Per Minute). I understand that for a given SCFM, as you raise your temperature, the ACFM will be higher because gasses expand at higher temperatures.

Both of these values (SCFM and ACFM) are indications of the volume of air that is being moved.

Sometimes, I am presented with air flow data in terms of velocity (in feet per minute). When I am presented with velocity data like this, is there a distinction between whether it is "actual" feet per minute or "standard" feet per minute? Or does velocity data like this have the same meaning regardless of whether it is measured at actual conditions or converted to standard conditions?

Thanks in advance,

Paul

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"SCFM" is really a mass flow measurement. A Standard cubic foot of gas is the amount of that gas at 14.696 psia and 60 degF conditions.

"ACFM" is the volume of gas flowing at the actual pressure and temperature conditions inside the pipe containing the gas. A gas velocity measurement (or calculation) would be done at actual conditions.

By Feucht on 14 March, 2017 - 1:09 pm

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SCFM is Standard Cubic Feet per Minute when air is at STP (Standard Temperature and Pressure) at 60 deg F and 14.696 psia. ACFM is Actual Cubic Feet per minute at what ever the actual operating temperature and pressure is. It is often corrected to SCFM.

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ACFM is simple current temp and pressure gas flow rate.

SCFM is an adjusted SCFM value for standard conditions. It can be thought of the flow for the pounds of gas passing at STP (Standard temperature and pressure). SCFM then can be thought of as pounds at STP or mass flow rate.

The same ACFM may not represent the number of pounds of flow if conditions change. SCFM will represent the same number of pounds at different conditions.

Hope this helps.

Mike W

Hi Otised, Feucht, and Mike W,

Thank you for your help. Now I understand that if someone gives me air flow data listed in CFM, it is ambiguous - I have to ask whether to interpret it as SCFM or ACFM.

But, what if someone gives only gives me velocity data in ft/min (with no mention of volume)? Is there more than one possible interpretation for this? Or is velocity listed in ft/min a non-ambiguous measurement?

For example, I see quite a few instruments that advertise their capability in terms of air velocity measurement, and they list their range in ft/min. If I had one of these instruments, I assume I would have to calculate my volumetric flow (in ACFM) using my duct size and the density of air I am moving (which will vary with temperature and pressure). I can totally understand how for a given air velocity, volumetric flow (in ACFM) will vary depending on temperature and pressure. But...for an instrument that measures velocity in terms of ft/min, can I expect this measurement to be accurate (and non-ambiguous)regardless of temperature or pressure. In other words, does 300 ft/min always mean 300 ft/min? (even though at this velocity my volumetric flow will be different at different temperatures?)

Thanks again for your help.

Paul

By d- on 15 March, 2017 - 7:05 pm

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CFM is commonly used in HVAC systems, in part since the pressure os near atmospheric. If precise measurements/calculations are required, then more specifics are required.

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Paul,

Velocity is what it says - the speed of the gas stream in the pipe or duct that it is flowing in. Think of it like wind speed. To convert velocity to ACFM all you need to know is the area of the pipe or duct. The density of the gas doesn't matter. If the velocity is 100 ft/min and the area of the pipe or duct is, say, 0.83 square feet, then the ACFM = 83.

To convert that to SCFM you will need to know the gas density at the flowing condition and the gas density at standard conditions.