Units For Evaluating an Air Compressor

When I was first introduced to the world of air compressors, it felt quite overwhelming. However, as with learning anything new, starting with the basics can make the process more manageable. Given the wide variety of air compressors available, I decided to begin by understanding the technical units of airflow measurement. 

Just when I thought I had grasped the fundamentals, the variety of numbers and ratings associated with different compressor features added to the complexity. If you're new to air compressors, recently changed roles and are looking to dive in, or are an enthusiast wanting to test your knowledge, here's what you need to know to evaluate the right air compressor for your needs.

Cubic Feet per Minute (CFM)

CFM, or Cubic Feet per Minute, is the most common measurement of airflow. It indicates the volume of air an air compressor can deliver over time (volume/time) under atmospheric conditions. Each compressor has specifications for the cubic feet of compressed air per minute (CFM) it can deliver from its discharge port. Knowing the CFM is crucial for choosing the right compressor. If you know the power requirement for the tool you’re using, you can determine your CFM needs. Higher CFM ratings mean the compressor can handle bigger jobs and more demanding tools. CFM measures airflow in systems where the air’s temperature and pressure are unknown.

Standard Cubic Feet per Minute (SCFM)

SCFM stands for Standard Cubic Feet per Minute. It measures the flow rate of gas or air through a compressor at standard atmospheric conditions: 14.7 PSI (atmospheric pressure), 68° Fahrenheit, and 36% relative humidity, as established by the Compressed Air and Gas Institute (CAGI) and the American Society of Mechanical Engineers (ASME). SCFM accounts for both air volume and density, making it useful for comparing different tools and compressors. For example, a 90lb pavement breaker might require 68-85 SCFM at 90 PSI. Remember, you may sometimes work outside of these standard conditions.

Inlet Cubic Feet per Minute (ICFM)

ICFM, or Inlet Cubic Feet per Minute, measures the airflow entering the compressor at the atmospheric pressure and temperature of the operating environment, before any compression components like inlet filters. This measurement is taken before any pressure drop occurs, which is important for comparing compressor ratings. The operating environment can affect the compressor’s output; for instance, if the intake is in a high-heat area, the air output will be reduced.

Recapping the Differences Between CFM, SCFM, and ICFM

CFM, SCFM, and ICFM are the most common air compressor ratings. The difference between CFM and SCFM is that SCFM is measured under controlled ambient conditions, whereas CFM is measured in variable ambient conditions. Compressors are not rated in SCFM because they are displacement devices, meaning their output volume changes with operating conditions. However, SCFM and ICFM can be used interchangeably if standard and inlet conditions are identical.

Air Tool Consumption

Air compressors can power a variety of tools, from jitterbugs to impact wrenches. When sourcing an air compressor, consider both your current and future needs. SCFM measures the compressor’s performance at specific temperatures and pressures. Tools requiring constant air, like sandblasters, need compressors with larger tanks. Once you know the highest SCFM required by your tools, scale that number for safety.

Additional Units to Evaluate

There are other units to consider when evaluating air compressors:

  • PSI (Pounds per Square Inch): Measures the air pressure delivered by the compressor. Most air tools require either 40 PSI or 90 PSI. 
  • Duty Cycle: Rotary screw compressors are suitable for continuous use, while reciprocating compressors are designed for cyclical operation. Duty cycle refers to the time between on and off periods. 
  • Gallons: A reciprocating compressor’s tank size, measured in gallons, indicates how much air can be pressurized at a given time. Larger tanks are preferred for continuous airflow jobs. 
  • Horsepower (HP): Measures the power output of the compressor’s engine or motor. Generally, 1 HP equals 4 CFM at 90 PSI.
  • Understanding these units and measurements will help you choose the right air compressor for your needs and ensure safe and efficient operation.

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