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
Twitter: @EXAIR_jb

Photo: Compressor equipment by terimakasih0Pixabay license

About Air Compressors: Air Intake Best Practices

Take a second and think about where the air compressor is located within your facility.  It is more than likely not a major focal point displayed prominently in the floor layout. There is a better chance it is tucked away in a corner of the facility where operators seldom travel.  No matter the type of air compressor, it still has an intake where it pulls in the ambient air from around the compressor then sends it through some process and on the demand side of your compressed air system.  These intakes can easily be placed out of sight and out of mind especially in older facilities that were designed when compressors were loud and the piping layout kept them away from operators due to sound level restrictions.

Air Compressor
Antique Air Compressor (Not safe for use!)

That’s why your compressor manufacturer supplies a specific grade of air inlet/intake filter, and this is your first line of defense. If it’s dirty, your compressor is running harder, and costs you more to operate it.  If it’s damaged, you’re not only letting dirt into your system; you’re letting it foul & damage your compressor. It’s just like changing the air filter on your car, your car needs clean air to run correctly, so does your compressor and the entire demand side of your compressed air system.

According to the Compressed Air Challenge, as a compressor inlet filter becomes dirty, the pressure drop across the inlet increases, this is very similar to the point of use compressed air filters.  The inlet filter on the compressor is the only path the compressor has to pull in the air, when restricted the compressor can begin to starve for air very similar to if you only had a small straw to breath through and told to run a marathon.  A clogged inlet filter can give false symptoms to compressor technicians as well.

The effects can mimic inlet valve modulation which result in increased compression ratios. If we were to form an example based on a compressor with a positive displacement, if the filter pressure drop increases by 20″ H2O, a 5% reduction of the mass flow of air will be present without a reduction in the power being drawn by the compressor. This all leads to inefficiency which easily amounts to more than the cost to replace the depleted inlet air filter.

compressor
Compressed Air System

Where you place the filter is just as important as how often you replace it.  There are some tips to be used when mounting the inlet filter.

  1. The filter can be placed on the compressor, but the inlet pipe should be coming from an external area to the compressor room or even the building if possible. The inlet should be free from any contaminants as well.  Some examples that are easy to overlook are nearby condensate discharges, other system exhausts and precipitation.
  2. Depending on the type of compressor being used, a lower intake air temperature can increase the mass flow of air due to the air density.  A compressor that is lubricant injected is not susceptible to this due to the air mixing with the warmer lubricant before being compressed.

If you would like to discuss improving your compressed air efficiency or any of EXAIR’s engineered solutions, I would enjoy hearing from you…give me a call.

Jordan Shouse
Application Engineer
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Images Courtesy of  the Compressed Air Challenge and thomasjackson1345 Creative Commons.