ACFM vs SCFM 3D

CFM represents cubic feet per minute, which can represent a 1’ x 1’ x 1’ box of air. The blower moves hundreds of these boxes per minute. That air may have varying temperature or humidity levels. The standard CFM (SCFM) conditions consist of air at 68.3 degrees Fahrenheit and 0% humidity at atmospheric pressure. A cubic foot of air will weigh 0.075 pounds under these conditions, but this weight will change when there are changes in temperature, pressure, and humidity. HVAC systems also move several boxes of air, with each one exerting force on the others.

The air we breathe primarily consists of nitrogen (78%) and oxygen (21%), plus some trace substances. These molecules move faster when we apply heat, making them more spaced out and less dense, which reduces the weight of a cubic foot of air. Water vapor (humidity) weighs less than diatomic nitrogen and oxygen in the atmosphere, so higher humidity levels will reduce the weight of a cubic foot of air. Lower pressures, such as at high elevations, will make the air less dense, which will also decrease the weight of a cubic foot of air.

For those reasons, the 400 CFM per ton rule of thumb is not applicable to all climates. We want to focus on maintaining a fixed mass flow rate, which accounts for the weight of each cubic foot of air. ACFM accounts for the weight of the air we’re moving to account for variations in humidity, altitude (pressure), and temperature. Arid climates are likely to have higher target ACFM rates, and humid climates are likely to have lower target ACFM rates. SCFM is a common reference point during the commissioning and testing of equipment, and ACFM is more likely to give us a nuanced idea of the pounds of air our systems are moving.