Tag: Save the CFM

Volumetric Flow Rates: CFM, ICFM, ACFM, SCFM.  What Does it All Mean?

Published on by John BallLeave a comment

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 cover 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 definitions.

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 air compressors and pneumatic systems. Volume is measured in 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 fewer molecules of air are 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 find 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 use the ACFM value to calculate velocities and pressure drops in a system.  We can also 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 (14.5 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 (528oR) or (68oF + 460)

ICFM, or Inlet Cubic Feet per Minute, is one of the newer 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 rise 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 of an air compressor should be considered.  Equation 3 shows the relationship between ACFM and ICFM:

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)

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

Tact = Actual ambient air temperature (oR)

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 looking for 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.  At 1,000 feet above sea level, the atmospheric pressure is 14.2 PSIA.  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.  If they had to add any inlet equipment to the air compressor, then we could use the ICFM equation, Equation 3, to provide the proper volume of compressed air.

When it comes to rating compressed air products or air compressors, always ask about the conditions for the pressure, temperature and RH.  Your local compressor dealer can help you in selecting the proper unit.  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 with saving compressed air on your point-of-use blow-off devices, you can contact an Application Engineer at EXAIR.  We will be happy to help you with our efficient and safe engineered products. 

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

Photo: Air sign by Barney Moss.  Creative Commons 2.0

What’s with the “S” in SCFM

Published on by exaircorpLeave a comment
Photo by Joeclub_Ake, Licensed by Pixabay

So many times, when reviewing the air requirements for our products, I am met with psi. “I have 100 psi running to this Super Air Knife, but it has almost no air flow”. PSIG and SCFM work hand in hand, and both are critical in optimal performance of our products. These two measurements are related, but they are not immediate family. PSI is more about power, whereas SCFM is more about flow and consistency. When they both combine, force is the outcome. We need both PSI and SCFM to produce force and for products to perform. It seems that most have a solid grasp on PSIG, but the SCFM tends to cause some confusion. Mostly the “S” in the SCFM.

Once we redirect the conversation to SCFM, we almost always find the heart of the issue. But SCFM can be confusing and many times is mistakenly interchanged with CFM. CFM is the easiest to explain, so let’s start there.

Photo by tookapic, licensed by Pixabay

CFM is Cubic Feet per Minute of airflow. This is exactly as it sounds, 1 CFM = 1 12″x12″x12″ box full of air moving through the product (Air Knife, Nozzle, Vortex Tube, Air Wipe and so on) per minute. To visualize this, take a look at your office (or visualize one). Let’s say it is 12x12x8 feet, like the sample to the right. That is 1152 cubic feet of air inside that office. If we used a 48″ Super Air Knife, running at 80 psig, it will consume 139.2 standard cubic feet of air per minute, or in other words, it will use all of the air in that office in 8.27 minutes. Assuming that air was not replaced, the Super Air Knife would starve and not have any airflow even though your gauge may still show 80 PSIG. So, when we break it down to this simple form and example, CFM is pretty easy to comprehend.

Photo by Paulbr75, licensed by Pixabay

Now the confusing part: what is the S in SCFM? The S simply stands for Standard. I know, this is shocking, but what is “Standard”? Standard represents values that are a baseline of measurement, even though few of us reside or work in these “ideal” surroundings. Air volume can be altered by several things, the most common are atmospheric pressure, temperature, and relative humidity.

Some genius somewhere decided that the best place to measure for a Standard CFM at sea level. I imagine they chose Key West Florida, on the beach, which sits around 14.7 psia atmospheric pressure. Then, to be precise they measured this when the thermostat read 68°F, and the hygrometer showed 0% relative humidity. Outside of creating a lab with these settings, that beach, on that day, at that precise time, is where you can get a true CFM of air equal to the Standard.

To summarize the “S” or Standard conditions are 14.7 psi atmospheric pressure, 68°F, and 0% relative humidity.

Photo by Vectors Licensed by Pixabay

Since most of us will never work in that exact environment, SCFM is “mostly” accurate. This is the reference point that we all can use, and has become the “Standard” across the globe. As you can imagine, it would be impossible for companies such as mine to publish or calculate every Actual CFM (ACFM). But we can do this…

ACFM is just what is says – the exact CFM that you will use based upon your elevation (atmospheric Pressure), your temp, and your relative humidity. I have a peer that wrote a recent blog that goes into depth on this topic as well as ACFM and ICFM (Inlet) and how to calculate each of these. You can see that blog here.

Thank you again for stopping by to find out more about the S in SCFM. If you have any questions about this, or anthing else related to our products, please do not hesitate to reach out.

Thank you for stopping by,

Brian Wages

Application Engineer

EXAIR Corporation
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Cover photo by PIRO4D, and licensed by Pixabay

Watch EXAIR Webinars On-Demand

Published on by Brian FarnoLeave a comment

That’s right, just like your local cable or satellite TV provider, EXAIR offers On-Demand content that can be streamed and used for training, education, help with cost justification, or improve awareness around compressed air costs and safety.

The best part about this content is that you don’t have to pay for it, simply register on our website (where your information is not shared) and go to the Webinars section of our Knowledge Base.  Then gain access to the library of five webinars that have all been broadcast around compressed air safety, efficiency, and optimization.

EXAIR.com – Webinars On-Deman

The current On-Demand offering is listed below:

Intelligent Compressed Air Solutions for OSHA Compliance
Intelligent Solutions for Electrical Enclosure Cooling
Optimize Your Compressed Air System in 6 Simple Steps
Simple Steps for Big Savings
Understanding Static Electricity

The most recent webinar we created is currently only On-Demand for registered attendees and will soon be added to the Knowledge Base library.  If you did not get to see it live, the content was extremely helpful for anyone that works within a facility that uses compressed air.  Use This Not That – 4 Common Ways To Save Compressed Air In Your Plant, keep an eye out for the release date in our On-Demand section.

If you would like to discuss any of the webinar topics further, please feel free to reach out to an Application Engineer.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Compressed Air Can Be Costly – EXAIR Can Help You Save It

Published on by Brian FarnoLeave a comment

Growing up across the street from an old fashioned service station does have its perks.   This was one of the old 76  gas stations that still had a garage where a mechanic would fix your car, as well as you could get gas if you needed to.   They even had the rubber hose that would ring a bell when you drive over it.  One of the other things this station had was a compressed air hose that ran to the outside of the building and offered free compressed air to any passer by so they could fill up their tires as needed.   Normally it just had a very simple tire chuck on the end that would connect to the Schrader valve on your car tire.  This station is still there, they no longer sell gas, and they have also removed the bell ringing air hose that runs outside.  If you need a tire filled now, you have to either catch them during normal business hours or move on to the next gas station and hope they have some form of compressed air available.

The full service gas stations for the most part have been replaced by gas stations with convenience stores attached.   They carry more junk food than they do products for your car and they have no need to own or operate an air compressor.   Their customers still have that need though so there are now small stand alone compressed air stations.   These range from small shoebox sized units that you pay a few quarters for, up to the unit I noticed today while filling up my car.

Standalone air compressed for tire inflation.
Standalone air compressed for tire inflation.

This unit takes credit cards, regulates the compressed air to the user selected pressure, and all you have to do is hook it to your cars valve stem.  This is far from free, and the reason is, compressed air is a costly utility.  If you don’t believe us when we repeatedly talk about how much money you can save on compressed air by installing our engineered compressed air nozzles, Super Air Amplifiers or Super Air Knives (among others), then accept a challenge from me.  Contact an EXAIR Application Engineer to discuss any point of use compressed air applications that you have in your facility.   While you are waiting to get the in stock, shipped same day, Intelligent Compressed Air Products® received, monitor your compressed air use and the energy use on your compressor.  Install the EXAIR products once you receive them and continue to monitor your air and energy consumption.   If you don’t see a decrease within these two areas, call me and return the items.   We honor a 30 day guarantee on stock products just so you can make sure you are saving air, and getting the best performance out of your compressed air system.

Brian Farno
BrianFarno@EXAIR.com
@EXAIR_BF