## CFM, ICFM, ACFM, SCFM: Volumetric Flow Rates Explained

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, 68oF, 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 * [Pstd / (Pact – Psat Φ)] * (Tact / Tstd)

Where:

ACFM = Actual Cubic Feet per Minute
SCFM = Standard Cubic Feet per Minute
Pstd = standard absolute air pressure (psia)
Pact = absolute pressure at the actual level (psia)
Psat = saturation pressure at the actual temperature (psi)
Φ = Actual relative humidity
Tact = Actual ambient air temperature (oR) or (oF + 460)
Tstd = Standard temperature (oR) or (oF + 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 * (Pact / Pf) * (Tf / Tact)

Where:

ICFM = Inlet Cubic Feet Per Minute

ACFM = Actual Cubic Feet per Minute

Pf  = Pressure after filter or inlet equipment (PSIA)

Tf = 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, 68oF, 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), 68oF 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 85oF 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 85oF is 0.595 PSIA.  From Equation 2, we can calculate the ACFM.

ACFM = SCFM * [Pstd / (Pact – Psat Φ)] * (Tact / Tstd)

ACFM = 500 SCFM * [14.5 / (14.2 – 0.595 * 60%)] * [(85oF + 460) / (68oF + 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

## People of Interest: Robert Boyle – 1627 to 1691

Being in the compressed air industry for over 35 years, you come across many interesting people from the past that have created laws that we are still using today.   Robert Boyle is one of those people.  He was born on January 25, 1627 in Lismore Castle in Ireland.  He published the book “The Sceptical Chymist” in 1661, and many considered his work to be the foundation of modern chemistry.  He dabbled in many areas of study, but with a young university student, Robert Hooke, they found Boyle’s Law.

The experiment was performed using a ‘J’ shaped glass tube sealed on the shorter leg, and open to atmosphere on the longer leg.  Mercury was poured into the tube, such that the level was equal on each side. The volume of the trapped air was noted. Additional mercury was poured into the tube, and it was observed that the mercury did not stay level, and measurements of the heights of each tube leg were recorded.  The height difference of the mercury is effectively a measure of the pressure of the trapped air.  Through the experiment and the data, Boyle discovered a relationship between the volume and the pressure of air.  The data as published, is shown below.

Boyle noticed the pressure times the volume of air for the initial condition equaled the pressure times the volume at any other mercury height.  So, the pressure is proportional to the inverse of the volume, Equation 1.

Equation 1: P ∝ 1/V

Or P * V = k (a constant)

For comparing the same substance under two different sets of conditions, Boyle’s law can be expressed as Equation 2.

Equation 2:  P1 * V1 = P2 * V2

Equation 2 looks very familiar.  One of Boyle’s most famous discoveries was to become the first of the gas laws, relating the pressure of a gas to its volume. Combining Boyle’s Law with Charles’s Law, Gay-Lussac’s Law, and Avogadro’s Law; you will have the basis and creation of the ideal gas law;

Equation 3:   P * V = n * R * T

which includes the major factors that affect a gas; temperature, pressure, volume, the amount of the gas, and the ideal gas constant.

Robert Boyle passed away on December 31st, 1691, and from his work, EXAIR uses the pressure and volume of compressed air for our Intelligent Compressed Air® Products to make them efficient, safe, and effective.  If you would like to speak more about how EXAIR can benefit your pneumatic system, one of our Application Engineers can help you determine the best solution.

John Ball
Application Engineer
Email: johnball@exair.com

Robert Boyle image courtesy of Skara KommunCreative Commons License

## Compressor Intake – Air Flows

Flow rate is the quantity of material that is moved per unit of time.  Generally, the quantity of material can be expressed as a mass or a volume.  For example, mass flow rates are in units of pounds per minute or kilograms per hour.  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 with compressible gases.  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 is CFM or Cubic Feet per Minute.  This term is commonly used for rating air compressors.  From the history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of the cylinder.  With the volume of the compression chamber and the rotations per minute of the motor, RPM, they could calculate the volumetric air flows.  As conditions change like altitude, temperature, and relative humidity, the volumetric value of CFM changes.  To better clarify these conditions, compressor manufacturers have decided to add terms with a 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 volumetric air flow is being measured.  This is important for comparing pneumatic components or for properly sizing pneumatic systems.  Volume is measured within three areas; temperature, pressure, and relative humidity.  We can see this in the Ideal Gas Law, reference Equation 1.

Equation 1:

P * V = n * R * T

Where:

P – Absolute Pressure

V – Volume

n – Number of molecules of gas

R – Universal Gas Constant

T – Absolute Temperature

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

SCFM is the most commonly used term, and it can be the most confusing.  The idea behind 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 that 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 a reference point, we still need to know the actual conditions for comparison.  It is like having the location of a restaurant as a reference, but if you do not know your current location, you cannot move toward it.   Similarly, we are “moving” the air from its actual condition to a reference or “Standard” condition.  If we do not know the actual state where the air began, then we cannot “move” toward 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 size an air compressor that is not at “Standard” conditions, and we can use this value to calculate velocity and pressure drop in a pneumatic system.  We can correlate between SCFM and ACFM with Equation 2.

Equation 2:

ACFM = SCFM * [Pstd / (Pact – Psat * Φ)] * (Tact / Tstd)

Where:

ACFM – Actual Cubic Feet per Minute

SCFM – Standard Cubic Feet per Minute

Pstd – standard absolute air pressure (PSIA)

Pact – absolute pressure at the actual level (PSIA)

Psat – saturation pressure at the actual temperature (PSI)

Φ – Actual relative humidity (%)

Tact – Actual ambient air temperature (oR)

Tstd – Standard temperature (oR)

ICFM 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 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 a 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.  The equation to relate ACFM to ICFM is Equation 3.

Equation 3:

ICFM = ACFM * (Pact / Pf) * (Tf / Tact)

Where:

ICFM – Inlet Cubic Feet Per Minute

ACFM – Actual Cubic Feet per Minute

Pact – absolute pressure at the actual level (PSIA)

Pf – Pressure after filter or inlet equipment (PSIA)

Tact – Actual ambient air temperature (oR)

Tf – Temperature after filter or inlet equipment (°R)

To expand on my explanation above about SCFM and ACFM, a technical question is asked often about the pressure when using SCFM.  The reference point of 14.5 PSIA is in the definition of the term for SCFM.  Remember, this is only a reference point.  The starting location is also needed as it gives us the ACFM value where the air values are true and actual.  Then we can make a comparison of actual air usage.

As an example, let’s look at two air nozzles that are rated at the same air flow; 60 SCFM.  The EXAIR Super Air Nozzle, model 1106, is cataloged at 60 SCFM at 80 PSIG, and a competitor is cataloged at 60 SCFM at 72 PSIG.  By comparison, they look like they use the same amount of compressed air, but actually they do not.  To simplify Equation 2, we can compare the two nozzles at the same temperature and RH at 68 oF and 0% RH respectively.  This equation can be reduced to form Equation 4.

Equation 4:

ACFM = SCFM * 14.5 / (P + 14.5)

@72 PSIG Competitor:

ACFM = 60 SCFM * 14.5 PSIA/ (72 PSIG + 14.5 PSIA)

= 10.1 ACFM

@80 PSIG EXAIR Super Air Nozzle:

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

= 9.2 ACFM

Even though the SCFM is the same amount, you are actually using 10% more air with the competitive nozzle that was reported at 60 PSIG.  So, when it comes to rating pneumatic products, improving efficiency, and saving money; always determine the pressure that you are at.  The more you know about volumetric flow rates, the better decision that you can make.  If you need more information, you can always contact our Application Engineers at EXAIR.  We will be happy to assist.

John Ball
Application Engineer
Email: johnball@exair.com

## Robert Boyle the Father of Chemistry and Boyles Law

Robert Boyle, one of the founding fathers of modern chemistry and a man who changed the very way we look at scientific research. From the Scientific Method to the very laws that govern gasses, Robert Boyle was able to change the very way we look at life and solve our problems. One could say that Robert Boyle didn’t really have what you would call a humble beginning; he was born in January 1627 to the 1st Earl of Cork Richard Boyle and his wife Catherine Fenton at Lismore Castle in Ireland. When he was only 8 years of age, he was sent off to Eton College in order to study under a private tutor. In 1641 Robert would spend the winter in Florence Italy studying the “paradoxes of the great star-gazer” Galileo Galilei.

Starting in mid-1644 Robert would make his residence in Dorset England were he conducted many experiments and from then devote his life to research. In 1654, Boyle would move to Oxford from Ireland in order to further pursue his studies in chemistry. It was here in 1657 that he would read about Otto von Guericke’s air pump, and would set out to improve the system along with Robert Hooke. In 1659 the “Pneumatic Engine” would be completed and he began a series of experiments on the properties of air. He would further go on to coin the term factitious airs which is a term used to describe synthetic gases after isolating what is now understood to be hydrogen.

Though he was primarily interested in chemistry, one of Boyle’s most famous discovery was what is now known as the first of the gas laws, rightfully named Boyles’s Law.  Boyle’s Law defines the relationship between pressure and volume in a closed area given the mass of an ideal gas. Boyle and his assistant Robert Hooke used a closed J-Shaped tube and poured mercury in from the open side, forcing the air on the other side to contract under the pressure. After repeating this using several different amounts of mercury Boyle deducted that the pressure of a gas is inversely proportional to the volume occupied by it.

In 1669 his health, although which was never very good, began to fail seriously and he withdrew from the public. In his later days he would propose some important chemical investigations which he wanted to leave as a sort of legacy for those who would were also “Disciples of the Art”, essentially future chemists. On the winters day on December 31, 1691 Robert Boyle took his final breath. In his will Robert Boyle left a series of lectures known as the Boyle Lectures the talked about the relationship between Christianity and today’s science.

Here at EXAIR we use Boyle’s Law everyday as nitrogen, oxygen, and hydrogen (the three main elements that make up air) are all considered ideal gas. This means that all of our products are governed by the relationship between pressure and volume.

If you have questions about any of our quiet EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR or any Application Engineer.

Cody Biehle
Application Engineer
EXAIR Corporation
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