What’s the difference between SCFM, ACFM, and ICFM?

CFM is cubic feet per minute. In simple terms, this term references an air compressor’s flow rate, or the amount of air that a compressor can produce at a given pressure level. Typically, a compressor with a higher CFM rating can supply more air, making it ideal for applications where larger flows are required. Where it gets a bit more complicated is defining exactly which condition is being referenced when we say “CFM.” Because all applications and environments are different, there isn’t only one way to describe compressed air flow – there are many! This comes about because CFM is dependent on a wide variety of variables, including altitude, temperature, and elevation. Three of the most popular of the CFM-related terms are SCFM, ACFM, and ICFM. Let’s learn more about each of these below.

What is SCFM?

SCFM, or standard cubic feet per minute, is the compressor’s flow rate at standard conditions. This is a very common measurement to use when comparing compressors, as it allows a more “apples to apples” assessment to be made. Keep in mind that standard conditions are difficult to meet in real life, but this is nevertheless the more practical measurement to publish because it allows the fair comparison between compressors.

What is ACFM?

ACFM is the actual cubic feet per minute, meaning it describes the compressor’s actual flow rate when taking into consideration the environmental conditions. While this measurement may more precisely describe the compressors’ flow rate, it’s simply not logical to specify compressors based on ACFM because of the extremely wide range of environmental conditions that are possible.

To convert SCFM to ACFM, you can use the following formula:

ACFM = SCFM [P_std / (P_act – Psat 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 (^oR)
T_std = Standard temperature (^oR)

What is the difference between SCFM and ACFM?

The core difference between SCFM and ACFM comes down to the conditions under which the flow rate is measured. SCFM is measured at a fixed, standardized set of conditions, typically a set temperature, pressure, and humidity level, making it a reliable baseline for comparing one compressor against another. ACFM, on the other hand, reflects what's actually happening in your specific environment, accounting for real-world variables like your facility's altitude, ambient temperature, and humidity. 

Think of it this way: SCFM is the number on the label, and ACFM is the number in the field. In most cases, the two will not be identical. For instance, if your facility is located at a higher altitude where air is less dense, your ACFM will typically be higher than the SCFM rating, because the compressor needs to move a greater volume of air to deliver the same mass flow. Conversely, in cooler or higher-pressure environments, ACFM may be lower than SCFM. 

This is why understanding the difference between SCFM and ACFM is so important when sizing a compressor for your application. Relying solely on the SCFM rating without accounting for your actual operating conditions could result in a compressor that underperforms in the field, even if it looked perfect on paper. 

What is ICFM?

ICFM is inlet cubic feet per minute, which identifies the flow rate at the inlet of the compressor. Don’t forget that a pressure drop occurs as the air passes through the inlet, which is something that you must account for because ICFM is measure before the air has passed through the inlet. To convert ACFM to ICFM, use the following formula:

ICFM = ACFM (P_act / P_f) (T_f / T_act)

ICFM = Inlet Cubic Feet Per Minute

Pf  = Pressure after filter or inlet equipment (psia)

Tf = Temperature after filter or inlet equipment (°R)

When sizing a compressor system, it’s essential that everyone be on the same page regarding the specific CFM that is being used in the specifications. Knowing this upfront will help greatly in the compressor selection process and will help prevent any potential issues down the road.

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