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

EXAIR Products Qualify for Energy Rebates

The use of compressed air can be found in almost any industry and is often referred to as a “fourth utility” next to water, gas and electric. The generation of compressed air accounts for approximately 1/3 of all energy costs in an industrial facility, in many cases, it’s the largest energy user in an industrial plant. With an average cost of $ 0.25 per every 1,000 SCF used, compressed air can be expensive to produce so it is very important to use this utility as efficiently as possible.

Many utility companies recognize the benefit of using engineered products to reduce compressed air usage, like the ones manufactured by EXAIR, and offers rebate incentives for making a switch. Duke Energy, who supplies power to sections of North Carolina, South Carolina, Ohio, Kentucky, Indiana and Florida offers several “Smart $aver Rebates” that focus around the generation and use of compressed air. (State and Location Dependent)

Duke Energy’s Smart Saver Program

However the best place to look at your states available programs is the DSIRE database. DSIRE is the most comprehensive source of information on incentives and policies that support renewable energy and energy efficiency in the United States. Established in 1995, DSIRE is operated by the N.C. Clean Energy Technology Center at N.C. State University. Follow the link above to read about the history of DSIRE, the partners on the project, and the research staff that maintains the policy and incentive data in DSIRE.

The Process is pretty easy! Visit https://www.dsireusa.org/ and type in your Zip Code!

After you get your results, search some key words, Like “Industrial” “Energy” “Commercial” “Energy Efficiency” “Compressed air”

Here you can see the two Programs that came up for 46077, you can then click the program name and it will take you a information page with the programs website and information!

Here at EXAIR, much of our focus is to improve the overall efficiency of industrial compressed air operating processes and point of use compressed air operated products. If you’d like to contact one of our application engineers, we can help recommend the proper engineered solution to not only save on your compressed air usage but also assist with possible energy rebates available in your area.

Jordan Shouse
Application Engineer

Send me an Email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_JS

Money Seed Creative Commons Images from Pictures of Money, Attribution 2.0 Generic (CC BY 2.0)

CFM, ICFM, ACFM, SCFM: Why so many volumetric flow rates?

Air Compressor

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 or liters per hour.  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 is CFM or Cubic Feet per Minute.  This term is commonly used for rating air compressors.  From history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of 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 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 with three areas: temperature, pressure, and relative humidity.  We can see this in the Ideal Gas Law: P * V = n * R * T or 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 value, 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 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 deg. F, and 0% RH.  Since we have a reference point, we still need to know the actual conditions for comparison.  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.   Similarly, we are “moving” the air from its actual 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 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.  We can correlate between SCFM and ACFM with 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 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 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 the ACFM to ICFM is with Equation 3:

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

Where:

ICFM = Inlet Cubic Feet Per Minute

Pf  = Pressure after filter or inlet equipment (PSIA)

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

Examples of these different types of flow rates can be found here in this EXAIR blog by Tyler Daniel.

To expand on my explanation above about SCFM and ACFM, a technical question comes up 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 required.  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 actually they do not.  To simplify Equation 2, we can compare the two nozzles at the same temperature and RH at 68 Deg. F and 0% RH respectively.  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 is the same amount, you are actually using 21% more air with the competitive nozzle that was reported at 60 PSIG.  So, when it comes to rating compressed air products or air compressors, always ask the conditions of pressure, temperature and RH.  The more you know about volumetric flow rates, the better decision that you can make.  If you need help, you can always contact our application engineers at EXAIR.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Advantages of Thermal Mass or Thermal Dispersion Flow Measurement

EXAIR’s Digital Flow Meter offers an easy way to measure, monitor and record compressed air consumption. The Digital display shows the current amount of compressed air flow, allowing for tracking to identify costly leaks and/or inefficient air users.

dfm

How exactly does the Digital Flow Meter work?  The unit falls under the category of Thermal Mass or Thermal Dispersion type flow meters.  Below shows the backside of a unit.

IMG_7387

Thermal mass flow meters have the advantage of using a simple method of measuring flow without causing a significant pressure drop. The EXAIR units have (2) probes that are inserted through the pipe wall and into the air flow.  Each of the probes has a resistance temperature detector (RTD.) One of the probes measures the temperature of the air flow.  The other probe is heated to maintain a preset temperature difference from the temperature measured by the first probe.  The faster the air flow, the more heat that is required to keep the second probe at the prescribed temperature.  From Heat Transfer principles, the heat energy input required to maintain the preset temperature is based on the mass velocity of the air.  Using basic physical properties for compressed air, the volumetric rate can be determined (SCFM), and displayed.

It is important to note that the compressed air should be filtered to remove oils, and dried to remove water, as these liquids have different physical properties from air, and will cause erroneous readings.

Advantages

  • Easy to install – No cutting or welding required
  • Summing Remote Display and Data Logger available
  • Sensitive at low flows
  • Rugged, reliable and no moving parts
  • No calibration or set-up required
  • Models from 1/2″ to 4″ schedule 40 iron pipe in stock
  • Short lead time for sizes up to 6″ Schedule 40 iron pipe
  • Available for size 3/4″ to 4″ copper pipe
  • New Wireless Capability

If you have any questions about the Digital Flow Meter or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

Send me an email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_BB