We can correlate between SCFM and ACFM with Equation 2:ĪCFM = SCFM * * (T act / T std) We can size an air compressor that is not at “Standard” conditions, and we can use this value to calculate velocity and pressure drop in a system. Even though this term is hardly used, there are reasons why we will need to know this value. We will talk more about this later in this blog.ĪCFM is the volumetric air flow under actual conditions. We will need to know where the air began in order to reach that reference point. Similarly, we are “moving” the air from its actual condition to a reference or “Standard” condition. It is like having a location of a restaurant as a reference, but if you do not know your current location, you cannot reach it. Since we have a reference point, we still need to know the actual conditions for comparison. In this blog, I will be using the Compressed Air and Gas Institute, CAGI, reference where the “Standard” condition is at 14.5 PSIA, 68 deg. But as long as you use the same reference point, then you can still compare the results. There have been many debates about an appropriate standard temperature and pressure, or STP. So, no matter the pressure, temperature, or relative humidity, the volumetric air flows can be compared to each other at that reference point. The idea of this volumetric air flow is to set a reference point for comparisons. SCFM is the most commonly used term, and it can be the most confusing. The more water vapor, or higher RH value, the less molecules of air is in a given volume. Where is the relative humidity? This would be referenced in the “n” term. The volume of air can change in reference to pressure, temperature, and the number of molecules. We can see this in the Ideal Gas Law: P * V = n * R * T or Equation 1: Volume is measured with three areas: temperature, pressure, and relative humidity. This is important for comparing pneumatic components or for properly sizing pneumatic systems. The first letter in front of CFM above now defines the conditions in which the volumetric air flow is being measured. (For your information, air compressors still use CFM as a unit of air flow, but now this is defined at standard temperature and pressure). To better clarify these conditions, compressor manufacturers decided to add terms with definition. As conditions change like altitude, temperature, and relative humidity, the value of CFM changes. With the volume of the compression chamber and the rotations per minute of the motor, RPM, they could calculate the volumetric air flows. From history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of cylinder. This term is commonly used for rating air compressors. The volumetric component of the flow rate is CFM or Cubic Feet per Minute. In this blog, I will go over the various acronyms and the reasons behind them. The trick begins when volumetric flow rates are used for a compressible gas. Volumetric flow rates are stated in cubic feet per minute or liters per hour. For example, mass flow rates are in units of pounds per minute or kilograms per hour. Generally, the quantity of material can be expressed as a mass or a volume. Flow rate is the quantity of material that is moved per unit of time.
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