Tag: acfm

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

When You Get To Asheville

Published on by Brian FarnoLeave a comment
1 – Steve Martin & Edie Brickell – “When You Get To Asheville”

Over the past week, my amazing wife and I traveled to Asheville, NC for a long weekend away. This is our second year going down, and I can most certainly say that we will be going back. Our days consisted of going to a small mom-and-pop type diner for breakfast, loading the cooler with water, and then picking a hike to hit up. This time we hiked mostly in the Pisgah National Forest and while we did not hit the same elevation as last year, we still managed to double the first hike of the week on the second day and felt great once we reached the end. I also chose to make the hikes hard on myself by carrying my trusted GO-RUCK GR1 to carry our water, first aid kit, and a 30 lb. steel plate, because you should always choose the harder thing.

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While we weren’t at elevations like Pikes Peak in Colorado, we still felt the difference in the air between being in Cincinnati and being in the mountains. Maybe it was just the fact it was cleaner. When we crested a hill on the trail and stopped to take a quick break, we looked around and realized that after all the switchbacks we had just gone through, we looked over the valley we had just climbed out of and were at the tree tops of the valley and still nowhere near the top of the mountain. This got me to thinking about how I was working harder because I had a steel plate, walking too many lunches where I just sit for 30 minutes instead of walking and that is immediately connected to the ACFM calculations for an air compressor and just how a compressor will have to work harder to produce the same volume of air when elevated because the air is thinner. This is going to change the air density, which results in a lower atmospheric pressure due to higher altitude.

Altitude is just one of the factors that matters in the calculation to determine a compressor’s output at different locations. The other factors include relative humidity, which was way better in the mountains than here in Cincinnati, and the actual temperature, again better in Asheville than Cincinnati.

If you are wondering about the equations I am referencing, we’ve blogged about them many times and even have a Webinar that touches on the math and reasoning behind these different values. Check the equation below and the links above.

In case you were wondering, the post-Ruck/Hike hydration is always better after events, it also always helps to have a good partner in crime to enjoy all the experiences with you. Thankful for the ability to connect all these hobbies and my knowledge of compressed air on top of sharing it with others. If you want to discuss how to calculate some ACFM or SCFM consumption and outputs of your compressor or application, or if you want to talk about rucking, hiking, or any of your favorite trails, give me a call, chat, or tweet.

Brian Farno, MBA – CCASS Application Engineer

BrianFarno@EXAIR.com
@EXAIR_BF

1 – Steve Martin & Edie Brickell – “When You Get To Asheville” – CBS, Retrieved from https://www.youtube.com/watch?v=4RzhTN9zW3w

Acronyms & Horses On the Hill

Published on by Brian FarnoLeave a comment

I’ve discussed how I volunteer in previous blogs. Sometimes it is during work hours, others, it is outside and on the weekends. The men’s group that I am part of at church has a smaller offshoot that goes out into our community and helps however possible. The name of our group is B4. It stands for Barbecue, Beer, Bible and Brotherhood. Four things most men appreciate, and again we call it B4 for short. My nerd-self argues it should be B 4. One of the projects we just wrapped up in our local community was for an organization called BLOC at their HOH facility. More acronyms. The HOH stands for Horses On the Hill and is a horse farm that is in an urban setting. This has working gardens that they sell fruit and vegetables to the community from, as well as a working horse farm where members learn how to care for the animals and ride, as well as work on some of their own items. While this blog isn’t about horse farms, it is about acronyms and how they can easily get thrown around when dealing with compressed air and air compressors. The picture is from our last day working there when we were wrapping up some trim work and watching a storm roll in. (Now tell me you’re from the MidWest with a single picture. )

Acronyms are something that comes with almost any field of study or professional position. Professionals everywhere love to use them and many even use them with redundant words. In my previous life, I took care of MSDS for the shop I worked in. What is MSDS? Material Safety Data Sheet is what it stands for, and any chemical should have one of these to accompany it in order to know how to handle the chemical safely. Even something like a window cleaner has one. It’s very easy to ask for an MSDS Sheet on a chemical, see that redundant word there? Times have changed, and now they are known as SDS, Safety Data Sheets. Still the same premise and still just as valuable when trying to be safe. When it comes to efficiency, though, acronyms used with redundant words are just not as efficient. You know that EXAIR is all about efficiency, and we deal with quite a few acronyms, so let’s talk about how we can ensure efficiency and lack of redundancy can be used when dealing with some of our acronyms.

CFM is the most basic one out of the bunch. It stands for Cubic Feet per Minute, and it is a unit of flow over time. This flow rate is generally used when dealing with gases, hence why we use it when talking about airflow, and really plays into many aspects of our products as well as the compressed air system in a facility. The metric equivalent is often M3/min. which, for an engineer-minded person, is easier to decipher, which doesn’t help a lot. It stands for Cubic Meters per Minute and is used by everyone else in the world outside the US. Both of these units of measure are thrown around a lot in the industry though, when a different unit is actually needed in order to meet the information needed.

CFM (M3/min.) is often stated when someone may actually be looking for SCFM, which is Standard Cubic Feet per Minute. This can also be a confusing unit, as the unit revolves around a reference point in order to be able to adapt it to your point of use conditions. We use a “Standard” condition that is accepted in the industry and that is 14.5 PSIA at 69° F and 0% RH. The Ideal Gas Law is used quite a bit to help determine how the RH, temperature, and pressure all affect the volume of the gas in question.

The least used version is ACFM, which is Actual Cubic Feet per Minute and is the ACTUAL flow rate under the conditions at the point of use. If we look at the horse farm mentioned above, on that day our relative humidity was through the roof while the temperature was low, so our volume would be impacted. While you believe this may be what you need more, generally it all has to be taken to standard conditions in order to calculate across different items anyway.

The newest of the acronyms is ICFM which is based around the Inlet Cubic Feet per Minute for the air’s conditions prior to entering the compressor at all. This, again, has to do with ACFM because it is at a specific pressure, relative humidity, and temperature.

The largest benefit to discussing consumption or flow rates with any Application Engineer at EXAIR is that we know the math behind converting each of these units of measure and can often get you the information you need right then and there as you talk with us. If you have some acronym fatigue from not knowing which to use, or you are trying to determine what the “Actual” output of your compressor is, contact one of our team members today.

Brian Farno, MBA – CCASS Application Engineer

BrianFarno@EXAIR.com
@EXAIR_BF

Webinar Replay: SCFM, ACFM, ICFM, CFM – Why So Many Terms For Air Flows?

Published on by Tyler DanielLeave a comment

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.

Tyler Daniel, CCASS

Application Engineer

E-mail: TylerDaniel@EXAIR.com

Twitter: @EXAIR_TD