Volume Flow Rate vs Mass Flow Rate w/ Jim Bergmann

One of the most fundamental equations that drive the work we do is Q = mass x specific heat x ΔT. This is the equation we use to find BTUs. We can use a similar equation to figure out how many pounds of air we’re moving if we already know how many CFM we’re moving. Air density will affect the mass of the air and CFM, so that requires us to differentiate between standard CFM (SCFM) and the actual CFM (ACFM). ACFM differs from SCFM in that it factors in the effects of relative humidity, temperature, and barometric pressure on the mass of the air. Depending on the type of fan you have, you may really have a variable mass flow rate with a constant volume.

SCFM measurements are based on air at sea level, 68.3 degrees Fahrenheit, and 0% relative humidity (0.075 lbs/ft cubed). Higher relative humidity levels will reduce the density of the air, as water (H2O) is lighter than nitrogen (N2) and oxygen (O2), which make up most of our air. MeasureQuick’s ACFM accounts for those humidity changes as well as pressure and temperature deviations from the sea-level, 68.3-degree standard.

Fans move a constant volume of air (SCFM), but the mass flow rate (ACFM) is much more variable based on location and climate. You can also use a psychrometric chart to help you with ACFM. The line of specific volume for 68.3 degrees and 0% relative humidity on a psychrometric chart also happens to be the inverse of the 0.075 lbs per ft cubed, which is the SCFM. (The ACCA manuals use SCFM for equipment design.)

ECMs work off RPM and torque; changes in air density affect the torque and affect ECM outputs. Since we’re cooling the mass of the air, not the volume, it also helps to think about ACFM beyond ECM outputs.