ICFM, SCFM, ACFM, CFM What does it all mean!

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 SCFMACFM, 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)

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)

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)

Where:

ICFM = Inlet Cubic Feet Per Minute

P = 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.

Jordan Shouse
Application Engineer

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Calculating CFM of Air Needed for Cooling

It’s easy to know that EXAIR’s vortex tubes can be used to cool down parts and other items, but did you know that our air knifes can be used to cool down these same things? It’s the same process that we do every day to cool down hot food by blowing on it. Every molecule and atom can carry a set amount of energy which is denoted by physical property called Specific Heat (Cp); this value is the ration of energy usually in Joules divided by the mass multiplied by the temperature (J/g°C). Knowing this value for one can calculate the amount of air required to cool down the object.

Starting out you should note a few standard values for this rough calculation; these values are the specific heat of Air and the specific heat of the material. Using these values and the basic heat equation we can figure out what the amount of energy is required to cool. The specific heat for dry air at sea level is going to be 1.05 J/g*C which is a good starting point for a rough calculation; as for the specific heat of the material will vary depending on the material used and the composition of the material.

Heat Flow Equation
Using the standard heat equation above add in your variables for the item that needs to be cooled down. In the example I will be using a steel bar that is 25 kg in mass rate and cooling it down from 149 °C to 107 °C. We know that the specific heat of steel is 0.466 J/g°C therefore we have everything needed to calculate out the heat load using air temperature of 22 °C.
Calculating Joules/min
Using the heat rate, we can convert the value into watts of energy by multiplying the value by 0.0167 watts/(J/min) which gives us 16,537.18 watts. Furthermore, we can then convert our watts into Btu/hr which is a standard value used for cooling applications. Watts are converted into Btu/hr by multiplying by 3.41 Btu/hr/watt, giving us 56,391.77 Btu/hr.
Converting Joules to Btu/hr
Once you have Btu/hr you can plug the information into a re-arranged Cooling power formula to get the amount of CFM of air required for cooling.
Calculating CFM
As you can see in order to cool down this steel bar you only need to 343 CFM of air at 72°F. This can be done very easily and efficiently by using one of EXAIR’s Air Amplifiers or Air Knife. Sometimes you don’t need to use a vortex tube to cool down an object; sometimes simply blowing on it is good enough and its pretty simple to calculate out which product would fit your application the best.

If you have any questions about compressed air systems or want more information on any EXAIR’s of our products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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EXAIR Digital Flowmeters With Wireless Capability

“You can’t manage what you don’t measure” is a well-known axiom in engineering & process improvement circles.  We talk to callers every day who are keen on conserving compressed air use in their facilities by making a few tweaks, considering a complete overhaul, or more often, some point in between.  Bottom line (literally) is, compressed air isn’t cheap, so small gains in efficiency can add up.  And large gains can be complete game-changers…following our Six Steps To Optimizing Your Compressed Air System has resulted in users being able to shut down 50 and 100 HP air compressors, saving thousands of dollar A MONTH in operating costs.

Step #1 is measurement, and that’s where the EXAIR Digital Flowmeter comes in.  They’re easy to install, highly accurate, extremely reliable, and available for just about any size pipe used for compressed air distribution.  They can output a 4-20mA signal straight from their PCB board, or serial comms (RS485) through an optional control board.  USB Data Loggers and Summing Remote Displays have proven to be value-added accessories for data management as well.

Summing Remote Display (left) for remote indication and totalizing data. USB Data Logger takes data from the Digital Flowmeter to your computer and outputs to its own software (shown above) or Microsoft Excel.

If you want to go wireless, we can do that too: using ZigBee mesh network protocol, a radio module is installed in the Digital Flowmeter with wireless gateway to transmit data to an Ethernet connected gateway.  The transmitting range is 100 ft (30 meters,) and the data can be passed from one radio module to another, allowing for multiple Digital Flowmeter installations to extend the distance over which they can communicate with the computer you’re using for central monitoring.  Advantages include:

  • Wireless monitoring of EXAIR Digital Flowmeters throughout your plant.
  • Prevents unwanted joining upon the network.
  • Monitoring software is included at no extra charge.
  • Measures & transmits both current air usage, and cumulative air usage data.
  • 128 bit encryption for wireless transmissions.
  • Comes configured & programmed, out of the box, available for installation on 1/2″ to 4″ SCH40 iron pipe, or 3/4″ to 4″ Type L copper pipe.

Digital Flowmeter w/ Wireless Capability, Gateway, and Drill Guide Kit

If you’d like to find out more about how easy it is to measure, manage, and optimize your compressed air usage, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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Proper Air Supply Line Size Equals Proper Performance

Pipe_460top
Properly sized piping will allow your compressed air operated equipment to operate efficiently!

On any given day myself and my Application Engineering Brethren here at EXAIR have discussions with customers on air starvation of any given EXAIR Product.  The calls generally start off the same, “The Line Vac is not performing like it should”.  We at EXAIR absolutely want to help you get the most out of our products and we certainly want them to perform to your expectation.  However they must be supplied with clean/dry compressed air at sufficient pressure and volume.

Just the other day I was discussing a performance issue with a customer on a 1″ Line Vac.  The customer thought he needed a larger Line Vac.  I asked the questions regarding the diameter of his Supply Line and if he was using Quick Connect or Push Lock connectors.  He was attempting to feed this Line Vac with 1/4″ Poly Tubing through a elbow Push to Loc fitting.

This 1″ Line Vac was being severely starved for air and therefore not performing as expected.  The 1″ Line Vac require’s 14.7 SCFM @ 80PSI to reach the rated performance of 42″ of water column.

Below is a table for Pipe/Hose sizing from the Line Vac installation manual that you can use as a reference guide.  It is recommended that if using hose for the supply air to go up to the next size over the pipe recommendation.

Chart2

Don’t forget that quick connects and Push Lock fittings are not recommended and could restrict the air flow which will have a negative impact on performance.

If you would like to discuss Line Vacs or any EXAIR product,  I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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