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.
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.
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.
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.
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EXAIR’s latest addition to the Fall Webinar series was a discussion on the topic of volumetric air flow terms: SCFM, ACFM, ICFM, and CFM. In the compressed air world, these terms are used often to quantify the performance of a compressor or the point-of-use equipment on the supply side of your system. Since conditions will vary from one site location to another, it’s important that we understand how certain variables can change the performance of your system. The webinar is available to view on demand on the EXAIR.com.
The term SCFM (Standard Cubic Feet Per Minute) is used to allow us to make an apples to apples comparison across different equipment. The performance is rate at a set of “standard” conditions to remove any potential variables from the equation. CAGI, or the Compressed Air and Gas Institute, uses the standard conditions of: 14.5 psia, 0% relative humidity (RH), and 68°F. This allows us to compare different devices without needing to make any sort of adjustments.
Variables such as elevation (barometric pressure), relative humidity, and temperature all change the performance and must be considered.
With elevation, we’re looking at the atmospheric or barometric pressure at the location of operation. One way to illustrate this to consider a balloon. If you inflated a balloon at sea-level, or 14.5 psia, then carry that same balloon up to the top of Mt. Everest what would happen? Using Boyle’s Law (P1 x V1 = P2 x V2), we’re able to calculate the exact volume of the balloon. At the peak of Mt. Everest, pressure is significantly lower at roughly 4.5 psi. The balloon when taken to the peak at 4.5 psi would become 3.2x it’s original size as the pressure acting on the outside of the balloon decreases.
Relative humidity tells us how much moisture content is contained within a specific volume of air. Water molecules cannot be compressed, so when the air is compressed this water takes up the same volume. The water condenses in the inter-coolers and after-coolers or is removed via drains and dryers downstream. So, 1 cubic foot of air coming into the compressor weigh more than 1 cubic foot of air out due to this water vapor loss.
As temperature increases, so does air pressure as the molecules in the air speed up and come into contact with one another and the walls of its container at a more rapid pace. Air can also hold a greater volume of moisture at higher temperatures. So, the balance between RH and temperature is an important consideration when determining actual performance, or ACFM.
In the webinar, we walked through two different examples to highlight the changes in these variables and how it impacts the performance of a compressed air system. If you were unable to attend live, the webinar is available to view on demand on the EXAIR website. We have this latest webinar posted there on the website along with all prior webinars as well! There, we talk about topics ranging from compressed air system optimization, static electricity, OSHA Compliance, and more! Check out the available webinars on the Resources tab of the EXAIR.com page today for all the knowledge you’ll need about your compressed air system and processes.
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.
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:
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:
ICFM = ACFM * (Pact / Pf) * (Tf / Tact)
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:
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.
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.
A common question we get asked is “What does SCFM mean?” Most people are aware of CFM but the “S” in front seems to be less known about! Well strap on your seat belt, we are about to go into a compressed air worm hole all about volumetric flow rates!
Here at EXAIR we rate all of our products air consumption in SCFM at a given supply pressure. CFM stands for Cubic Feet per Minute, but one definition will not satisfy the conditions that will be experienced in many applications by a number of variables (altitude, temperature, pressure, etc.). Air by nature is a compressible fluid. The properties of this fluid are constantly changing due to the ambient conditions of the surrounding environment.
This makes it difficult to describe the volumetric flow rate of the compressed air. Imagine you have a cubic foot of air, at standard conditions (14.696 psia, 60°F, 0% Relative Humidity), right in front of you. Then, you take that same cubic foot, pressurize it to 100 psig and place it inside of a pipe. You still have one cubic foot, but it is taking up significantly less volume. You have probably heard the terms SCFM, ACFM, and ICFM when used to define the total capacity of a compressor system. Understanding these terms, and using them correctly, will allow you to properly size your system and understand your total compressed air consumption.
SCFM is used as a reference to the standard conditions for flow rate. This term is used to create an “apples to apples” comparison when discussing compressed air volume as the conditions will change. EXAIR publishes the consumption of all products in SCFM for this reason. You will always notice that an inlet pressure is specified as well. This allows us to say that, at standard conditions and at a given inlet pressure, the product will consume a given amount of compressed air. It would be nearly impossible, not to mention impractical, to publish the ACFM of any product due to the wide range of environmental conditions possible.
ACFM stands for Actual Cubic Feet per Minute. If the conditions in the environment are “standard”, then the ACFM and SCFM will be the same. In most cases, however, that is not the case. The formula for converting SCFM to ACFM is as follows:
ACFM = SCFM [Pstd / (Pact – Psat Φ)](Tact / Tstd)
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)
The last term that you’ll see floating around to describe compressed air flow is ICFM (Inlet Cubic Feet per Minute). This term describes the conditions at the inlet of the compressor, in front of the filter, dryer, blower, etc. Because several definitions for Standard Air exist, some compressor manufacturers have adopted this simpler unit of measure when sizing a compressor system. This volume is used to determine the impeller design, nozzle diameter, and casing size for the most efficient compressor system to be used. Because the ICFM is measured before the air has passed through the filter and other components, you must account for a pressure drop.
The inlet pressure is determined by taking the barometric pressure and subtracting a reasonable loss for the inlet air filter and piping. According to the Compressed Air Handbook by the Compressed Air and Gas Institute, a typical value for filter and piping loss is 0.3 psig. The need to determine inlet pressure becomes especially critical when considering applications in high-altitudes. A change in altitude of more than a few hundred feet can greatly reduce the overall capacity of the compressor. Because of this pressure loss, it is important to assess the consumption of your compressor system in ACFM. To convert ICFM to ACFM use the following formula:
ICFM = ACFM (Pact / Pf) (Tf / Tact)
ICFM = Inlet Cubic Feet Per Minute
Pf = Pressure after filter or inlet equipment (psia)
Tf = Temperature after filter or inlet equipment (°R)
If you’re looking into a new project utilizing EXAIR equipment and need help determining how much compressed air you’ll need, give us a call. An Application Engineer will be able to assess the application, determine the overall consumption, and help recommend a suitably sized air compressor.