Intelligent Compressed Air: SCFM, ACFM, ICFM, CFM – What do these terms mean?

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An old Ingersoll-Rand air compressor

Air compressors have come a long way over the years. When sizing a new system, a few terms are commonly used: CFM, SCFM, ACFM, and ICFM. The term CFM, simply put, stands for Cubic Feet per Minute. This term can often be confusing and impossible to define for just one condition. One definition will not satisfy the conditions that will be experienced in many of your applications due to 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)

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)

Let’s run through an example of a compressor operating at a “non-standard” condition:

Elevation – 5000 ft.

Temperature – 80°F (80+460=540) – 540°R

Saturation Pressure – .5069psia

Relative humidity – 80%

demand – 100 SCFM

ACFM = (100 SCFM) [(14.7 psia)/((12.23psia) – (0.5069 psia)(80/100))] ((540°R)/(520°R))

=129.1 ACFM

In this example, the actual flow is greater. To determine the total ACFM consumption of any of our products with your system, take the published total consumption of the product and plug in the values for your compressed air system along with the standard variables.

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

Pf  = Pressure after filter or inlet equipment (psia)

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

For this example, let’s say that we’re in Denver, Colorado. The barometric pressure, as of today, is 14.85 psi with current ambient temperature at 71°F. The compressor system in this example does not have any blower or device installed before the inlet, so there will be no temperature differential after filter or inlet equipment. The ICFM rating for the system is 1,000 ICFM.

ACFM = 1,000 (14.85/14.55)(530.67/530.67)

ACFM = 1,020

In order to maintain the 1,000 ICFM rating of the system, the ACFM is 1,020, about a 2% increase.

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.

Tyler Daniel
Application Engineer

E-mail: TylerDaniel@exair.com
Twitter: @EXAIR_TD

 

Compressor photo courtesy of David Pearcy via Creative Commons license.

3D Printing with Chocolate

Everyone seems to be talking about 3D printing lately. Last week, I received an email from a customer who had a new idea for 3D printing.

Chocolate!

Well I was intrigued. The customer wanted to modify current 3D Printing technology to work chocolate. There was obviously several hurdles. For instance, using a vat of molten chocolate as opposed to typical material, cleaning, and replacement parts to make a food safe low-cost printer. Her biggest problem was how to cool the chocolate after the application of each successive layer upon dispensing, so the chocolate didn’t pool into an amorphous blob.

She came to me asking about the Adjustable Spot Cooler. This product caught her attention because of the ease of installation with the magnetic base, the adjustable temperature control and instant cold air response. The magnetic base could be incorporated into her design fairly easily. The adjustable temperature control would allow her to decrease the temperature and decrease the cold flow at the same time. If she found that the force of the compressed air was damaging the printing process, reducing the cold flow would allow her to use a colder temperature to harden the layer that had just been used.  Finely the compressed air could be rapidly controlled with a solenoid to only run when the cold air is needed, which would limit compressed air cost.

Because of the high freezing point of chocolate and overall size constraints, I recommended that she first try a model 3204 Vortex Tube. A small Vortex Tube, which could use as little as 4 SCFM of compressed air and provide up to 3.2 SFCM of cold air at fifty degrees below the compressed air temperature, would be more than capable of forming a shell on the surface area of each extrusion. It is reasonable to assume that this air temperature would be around 20 degrees Fahrenheit, which could create a delicious chocolate shell for the next layer of chocolate be deposited.  She was able to buy the magnetic base, model 9029, separately to aid in her installation.Chocolate tools

Hopefully, you read this after lunch, because I made myself hungry looking for chocolate pictures, but I found what I would print for Christmas.

Dave Woerner
Application Engineer
davewoerner@exair.com
@EXAIR_DW

 

US Economy Growing and Energy Usage Lower

The National Resource Defense Council (NRDC) issued a report this October that America had used the same amount of energy (measured in BTU’s) as it had used in 1999.  We have reduced our energy usage per person, while still growing the economy, a feat that I would not have thought possible.

  Economy and Energy Growth

You can read the full report here: NRDC Energy Report. So often environmental news is dire and gloomy, but this news shows the power of energy efficiency.  As noted in the report, politicians and media members focus on where we are going to find new energy resources.  As opposed to opening up new energy reserves, we have reaped larger rewards from spending time and money conserving the energy over the last thirty-five years.  The energy report uses the household refrigerator as an example of an appliance, whose increasing energy efficiency greatly decreased our electrical load per person.  Refrigerators use 1/4 as much energy as the same size refrigerator used in 1975.  This decrease in energy usage is a huge gain for the user who replaces their refrigerator and for the power grid that doesn’t need to build a new power plant to keep up with the increased load. Average Household Refrigerator Energy Use As an EXAIR employee, I can not help but notice that EXAIR opened in 1982, which is one of the first years in the graph above were economic growth was not directly tied to energy usage.  At EXAIR, we realize the impact of conserving compressed air can have on your compressed air system.  By replacing home-made blow offs like open tubes or holes drilled in pipe with Super Air Nozzle or Super Air Knife engineered solutions, you conserve compressed air and save money. This also reduces wear on the your compressor and can extend its life. A model 1100 Super air nozzle uses 14 SCFM when fed with 80 PSIG, which is a 58% reduction from 1/4″ open copper tube, which uses 33 SCFM when fed with 80 PSIG.  Go to our Air Savings Calculator to see how much compressed air and money you can save by replacing those home made blow offs.

Dave Woerner
Application Engineer
davewoerner@exair.com
@EXAIR_DW

Replacing Home Made Blow Offs

Yesterday, I took a call from a customer, who had successful replaced a low cost blow off station with (2) of 6″ Super Air Knife.  The low cost blow off station was made up of two flexible lines with plastic liquid coolant nozzles to dry two surfaces after a roll forming process.  At the beginning of every production run, the nozzles would have to be positioned to hit the surface of the part because the flexible line would not hold its position.  On top of that even though these nozzles were blowing 74 SCFM of compressed air at 80 PSIG, the blow off was ineffective and left moisture on the parts after the run, which lead to corrosion on the parts during storage.

Before

  image

The (2) six inch Air Knives improved this process in three ways.  First, he never needed to reposition the nozzles after a production run.  Using the 1/4-20 threads in the bottom of the air knife, the customer was able to mount the air knife directly to the machine, which was a more robust system than the flexible liquid coolant hose. Second, the knives only consumed 35 SCFM of compressed air reducing his air consumption by half.  By utilizing the 40:1 amplification ration of the air knife, the customer had a much lower demand on the compressed air system, while getting the job done.  Finally, he left his customers happy with no corrosion marks on the parts after they had been stacked and delivered.  With no complaints about the quality at his customer, our customer will be able to improve his relationship with his customers and improve his business.

AFTER

after

Dave Woerner
Application Engineer
davewoerner@exair.com
@EXAIR_DW