Contents
CFM, Free Air
CFM, free air, is a critical measurement of a compressor’s performance. It represents the volume of air that a compressor can take in, compress to a specific pressure and then deliver. This output is calculated back to the free air conditions at the compressor’s intake — the specific temperature, atmospheric pressure and humidity of the surrounding environment.
FAQs
Free air delivery (FAD), also known as CFM, free air, is the actual, usable air output of a compressor measured at the discharge point and converted back to the inlet conditions.
While often used interchangeably, free air delivery is a specific performance metric, while cubic feet per minute (CFM) is a unit of measurement for air flow rate. Free air delivery is expressed in CFM.
The most reliable way to identify a compressor’s FAD is to review the manufacturer’s data sheet. Alternatively, use an online free air delivery to CFM calculator that uses the tank pump-up time method. This technique times how long it takes for the compressor to fill its air receiver tank from a starting pressure to an ending pressure.
The formula is:
CFM = (Tank in volume cu. ft. x (end pressure PSI – start pressure PSI)) / (time in minutes x 14.7)
For positive displacement compressors, multiply the compressor’s displacement by its volumetric efficiency to calculate FAD.
Understanding the difference between CFM, free air and other CFM ratings is essential for selecting the correct compressor for your application.
CFM, free air tells you the real-world, usable airflow you can expect from the compressor to power your tools. In contrast, displacement CFM is a calculation of the compressor’s pump theoretical capacity. It overlooks real-world inefficiencies, such as pressure drops and heat loss.
The standard CFM (SCFM) measures the airflow rate under an idealized, fixed set of conditions. This standardized benchmark makes SCFM useful for comparing compressors in a controlled, laboratory-like setting.
An air compressor CFM chart is a reference table that helps you match your required CFM with the right compressor. It shows the CFM output of a specific air compressor model at various pressure levels. As the pressure increases, the CFM output decreases.
The chart also lists various pneumatic tools and their average CFM consumption at a specific pounds per square inch (PSI), enabling you to determine your tools’ total CFM demand.
FAD is crucial in helping you select the correct air compressor for your specific needs. It tells you the actual, real-world volume of air the compressor can deliver to power your tools.
It also allows for an honest comparison between compressors. Since the volume of air changes with temperature and pressure, a compressor tested in a cold, high-pressure setting will show a higher output than one tested in a hot, low-pressure environment.
By defining FAD as the output volume calculated back to the inlet conditions, this creates a standardized baseline, enabling you to compare the performance of two different compressors regardless of the manufacturer.
In a reciprocating piston compressor, FAD is the volume of fresh air drawn into the cylinder during the intake stroke, compressed, and then delivered. This process is not completely efficient, as not all the air in the cylinder is expelled due to the clearance volume — a small space left at the end of the piston’s stroke.
The FAD of a reciprocating compressor is determined by its displacement, speed and volumetric efficiency. The volumetric efficiency is affected by factors like the compression ratio, operating conditions and valve design.