Flow rate is the quantity of material that moves over a period of time. Generally, the quantity of material can be expressed as a mass or a volume. For example, mass flow rates are generally in units of pounds per minute (lbs./min) or kilograms per hour (Kg/hr). Volumetric flow rates are stated in cubic feet per minute (CFM) or liters per hour (LPH). The trick begins when volumetric flow rates are used for a compressible gas. In this blog, I will go over the various acronyms and the reasons behind them.

What acronyms will be covered?

CFM – Cubic Feet per Minute

SCFM – Standard Cubic Feet per Minute

ACFM – Actual Cubic Feet per Minute

ICFM – Inlet Cubic Feet per Minute

The volumetric component of the flow rate above is CFM or Cubic Feet per Minute. This term is commonly used in rating air compressors and pneumatic equipment. From their history, they would calculate the volume of air being drawn into the air compressor by the size of cylinder. With the rotations per minute of the motor, RPM, they could calculate the volumetric flow rate. As conditions change like altitude, temperature, and relative humidity, the value of CFM changes. To better clarify these conditions, compressor manufacturers decided to add terms with definition. (For your information, air compressors still use CFM as a unit of air flow, but now this is defined at standard temperature and pressure).

The first letter in front of CFM above now defines the conditions in which the volumetric air flow is being measured. This is important for comparing pneumatic components or for properly sizing pneumatic systems. Volume is measured by three areas: temperature, pressure, and relative humidity as seen in the Ideal Gas Law.

Equation 1:

V = n * R * T / P

V – Volume

n – Number of molecules of gas

R – Universal Gas Constant

T – Absolute Temperature

P – Absolute Pressure

The volume of air can change in reference to pressure, temperature, and the number of molecules. Where is the relative humidity? This would be referenced in the “n” term. The more water vapor, or higher RH values, the less molecules of air is in a given volume.

SCFM is the most commonly used term, and it can be the most confusing. The idea of this volumetric air flow is to set a reference point for comparisons. So, no matter the pressure, temperature, or relative humidity; the volumetric air flows can be compared to each other at one reference point. There have been many debates about an appropriate standard temperature and pressure, or STP. But as long as you use the same reference point, then you can still compare the results. 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^{o}F, and 0% RH. Since we have the reference point, we still need to know the actual conditions. As an example, it is like having a location for a restaurant as a reference, but if you do not know your current location, you cannot reach it. Similarly, we are “moving” the air from one condition to a reference or “Standard” condition. We will need to know where the air began in order to reach that reference point. We will talk more about this later in this blog.

ACFM is the volumetric air flow under actual conditions. This is actually the “true” flow rate. Even though this term is hardly used, there are reasons why we will need to know this value. We can resize an air compressor that is not at “Standard” conditions, and we can use this value to calculate velocities and pressure drop in a system. We can correlate between SCFM and ACFM:

Equation 2:

ACFM = SCFM * [P_{std} / (P_{act} – P_{sat} Φ)] * (T_{act} / T_{std})

Where:

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 (^{o}R) or (^{o}F + 460)

T_{std} = Standard temperature (^{o}R) or (^{o}F + 460)

ICFM, or Inlet Cubic Feet per Minute, is one of the newest terms in the history of air compressors. This is where devices are added to the inlet of an air compressor, affecting the flow conditions. If you have a blower on the inlet of an air compressor, the volumetric flow rate changes as the pressure and temperature rises at the “Inlet”. If an intake filter is used, then the pressure drop will decrease the incoming pressure at the “Inlet”. These devices that affect the volumetric flow rate for an air compressor should be considered. Equation 3 shows the relationship to ACFM and ICFM:

Equation 3:

ICFM = ACFM * (P_{act }/ P_{f}) * (T_{f} / T_{act})

Where:

ICFM = Inlet Cubic Feet Per Minute

ACFM = Actual Cubic Feet per Minute

P_{f } = Pressure after filter or inlet equipment (PSIA)

T_{f} = Temperature after filter or inlet equipment (°R)

To expand on my explanation above about SCFM and ACFM, a technical question comes up often about the pressure when using SCFM. The reference point of 14.5 PSIA is in the definition of SCFM. Remember, this is only a reference point. The starting location is actually needed. This would be the ACFM value where the air values are true and actual. As an example, two air nozzles are rated for 60 SCFM. An EXAIR Super Air Nozzle, model 1106, is cataloged at 80 PSIG, and a competitor is cataloged at 60 PSIG. By comparison, they look like they use the same amount of compressed air, but do they actually? To simplify Equation 2 above, we can compare the two nozzles at the same temperature, 68^{o}F, and 0% RH. This equation can be reduced to:

Equation 4:

ACFM = SCFM * 14.5 / (P + 14.5)

@60 PSIG Competitor:

ACFM = 60 SCFM * 14.5 PSIA/ (60 PSIG + 14.5 PSIA)

=** 11.7 ACFM**

@80 PSIG EXAIR Super Air Nozzle:

ACFM = 60 SCFM * 14.5 PSIA / (80 PSIG + 14.5PSIA)

= **9.2 ACFM**

Even though the SCFM rating is the same but at two different pressures, the actual flow shows that you are using 21% more compressed air with the competitive nozzle.

Another example would be for sizing an air compressor. Since air compressors are rated at sea level (14.5 PSIA), 68^{o}F and 0% RH, what happens if you are in Denver? A manufacturing company was needing a 500 SCFM air compressor to run their plant. They were located at 1,000 feet above sea level with a site temperature of 85^{o}F and a relative humidity of 60%. Since they were not at the standard conditions, we can calculate the ACFM to properly size the air compressor. The atmospheric pressure at 1,000 feet was 14.2 PSIG. The saturation pressure at 85^{o}F is 0.595 PSIA. From Equation 2, we can calculate the ACFM.

ACFM = SCFM * [P_{std} / (P_{act} – P_{sat} Φ)] * (T_{act} / T_{std})

ACFM = 500 SCFM * [14.5 / (14.2 – 0.595 * 60%)] * [(85^{o}F + 460) / (68^{o}F + 460)]

ACFM = 500 SCFM * 1.0474 * 1.0322

ACFM = 540

For this manufacturing plant, they will need to increase the capacity to 540 SCFM to run their 500 SCFM pneumatic system at their location.

When it comes to rating compressed air products or air compressors, always ask the conditions of the pressure, temperature and RH. The more you know about volumetric flow rates, the better decision that you can make in selecting the correct product. If you need any help in selecting point-of-use blow-off devices, you can contact an Application Engineer at EXAIR. We will be happy to help you.

John Ball

Application Engineer

Email: johnball@exair.com

Twitter: @EXAIR_jb