ACFM (Actual Cubic Feet per Minute) vs. SCFM (Standard Cubic Feet per Minute)

The air compressor industry loves to use acronyms, especially when measuring airflow and air consumption. Two common acronyms, representing measurements, are ACFM and SCFM.

CFM—or Cubic Feet per Minute—is an important metrics when choosing an air compressor and related pneumatic equipment. Pneumatic tools and equipment require a specific minimum air mass, or CFM, to perform properly on the job site.

But what exactly does it mean when manufacturers use ACFM or SCFM to perform properly on the job site?

What is ACFM?

ACFM (Actual Cubic Feet per Minute) may have different definitions depending on the industry.  VMAC defines ACFM as the true air mass flow given a certain set of real-life conditions. We will simulate a real life environment and then measure the CFM based on the actual output of air, resulting in ACFM.

But ACFM is impacted by atmospheric conditions and the surrounding environments. For example, an air compressor on a mountain top is likely to have lower output than that same air compressor will have at ocean level.

What is SCFM?

SCFM (Standard Cubic Feet per Minute) measures air output, like ACFM, but uses a standard that takes atmospheric conditions into account.

SCFM is a set of specific parameters determined by the American Society of Mechanical Engineers (ASME) that are recognized across many industries. These SCFM calculations are based on atmospheric conditions (“standard conditions”) to measure air mass flow from an air compressor.

SCFM “standard conditions” include:

  • atmospheric pressure at sea-level of 14.7 PSIA (760 mmHg),
  • relative humidity of 36%, and
  • ambient temperature of 68°F (19°C).

After completing the calculations using these conditions, the maximum SCFM output of the air compressor is revealed.

SCFM is the only way to compare “apples to apples”, or air compressor to air compressor, as operating conditions may otherwise vary depending on where the air compressor air mass flow is being measured. VMAC defines SCFM as the air mass flow generated at “standard conditions”.

Differences Between ACFM & SCFM

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At “standard conditions”, with no efficiency losses, SCFM equals ACFM.  Where inlet conditions vary from “standard conditions”, consideration may be necessary to ensure the specified air compressor has enough power to generate adequate air mass flow for the tool to perform properly on the job site.

ACFM demand by a tool may be higher if any one of the following conditions is different from standard conditions:

  • atmospheric pressure is lower (elevation)
  • humidity is higher
  • temperature is higher

Another way of thinking about this is the air compressor needs to work harder as the job site drifts away from “standard conditions”.  A more powerful air compressor (with higher SCFM rating) may be required to generate adequate ACFM for the pneumatic tool to perform properly on the job site.

Quick Calculations:

  • For every 1000 ft (305 m) of elevation increase over “standard conditions”, ACFM demand increases by approximately 5%.
  • For every 20oF (11.1oC) of ambient temperature increase over “standard conditions”, ACFM demand increases by approximately 5%.
  • For every 20% of humidity increase over “standard conditions”, ACFM demand increases by approximately .5%.

Example Calculation:

A 1” impact gun may have a minimum air mass flow requirement of 45 CFM.  On the job site in Ft McMurray, Alberta at an elevation of 1,211 ft (369 m) on June 22, 2011 at 2:00PM, atmospheric pressure is 14.2 PSIA (732 mmHg), humidity is 24%, and temperature is 84oF (27oC).   For an air compressor to generate 45 ACFM in these conditions, it actually needs to have an SCFM rating of approximately 49 SCFM.

Why Does ACFM vs. SCFM Matter?

You need to ensure your air compressor system produces enough air to properly power your tools. If the environment you work in impacts air production, SCFM ratings may not perfectly reflect your real-world needs. Unless you are only using your air compressor on the beach on a cool spring day, add a buffer when specifying your equipment.

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