When It’s Not So Simple…

I’m a big fan of things being simple.  – If you are facing the right direction, all you have to do is keep on walking.  – Simple.  Compressed air isn’t always so simple.

Many of the specifications we give for our products are in SCFM (Standard Cubic Feet per Minute), a standardized volumetric flow rate.  Recently, I’ve been assisting several EXAIR customers who are using our product in non-standardized environments.  These setups can include increased temperature or humidity, or significant altitude differences.  The effects that these changes have on the ambient air can lead to deviation from the supplied values in our catalog.

One such application was evacuating a large heat load from an oven at a high temperature using an Air Amplifier.  The customer needed to know the ACFM (Actual Cubic Feet per Minute) value of the air flowing through the Air Amplifier.  ACFM and SCFM are not the same animal.  An SCFM rating can change drastically when converted to ACFM, especially when dealing with a large temperature value.  So, I went to the engineering toolbox and pulled out the following equation:

ACFM = SCFM [P(std) / (P(act) – P(sat) Φ)](T(act) / T(std))


ACFM = Actual Cubic Feet per Minute

SCFM = Standard Cubic Feet per Minute

P(std) = Standard absolute air pressure (psia)

P(act) = absolute pressure at the actual level (psia)

P(sat) = Saturation pressure at the actual temperature (psi)

Φ = Actual relative humidity

T(act) = Actual ambient air temperature (Degrees R)

T(std) = Standard temperature (Degrees R)

In the application I dealt with, the following values were used:

ACFM = ?

SCFM = 89

P(std) = 14.7 psia

P(act) = 14.4 psia

P(sat) = .3631 (Absolute Vapor Pressure determined as a function of temperature)

Φ = 55%

T(act) = 1410R

T(std) = 530R

Using the values from our customer, including the supplied atmospheric pressure reading based on elevation, I calculated the ACFM to be 245 ACFM.  Knowing this value was extremely helpful to our customer, because it provided a quantitative value he could use to examine the cooling effects of using our Air Amplifier.

When the customer contacted me with this issue, he said he wasn’t sure if EXAIR could help him, he wasn’t sure if we offered this kind of product support.  With a staff of engineers dedicated to our product line, we are more that capable of providing this kind of support.  If you have a compressed air application that is outside of the norm, email me, I’ll be glad to help.

Lee Evans
Application Engineer

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