Air Compressors: Power

Introduction Applications
Power Types
Selection
We thank www.about-air-compressors.com for permitting the use of information available therein for preparation of this article.

Compressor power

One of the factors used to designate compressor power is motor horsepower. However, this isn’t the best indicator. You really need to know the amount of air the compressor can deliver.

Two commonly used units to measure the amount of air the compressor can deliver are CFM and SCFM. Let us understand what these units mean:

CFM

This is the usual unit of measure for discharge air from a compressor.

CFM is the acronym for Cubic Feet per Minute. A compressor is said to have so many cubic feet of compressed air per minute (CFM) of flow from its discharge port.

When it comes to using compressed air in your plant or home workshop you will want to know how many cubic feet per minute you can expect from the discharge port of your compressor to help determine if that compressor has sufficient compressed air flow to power your air tools or other air-consuming applications.

To do that you need to know what CFM a particular device or tool will require to function within its design parameters. The device or tool will require a certain number of CFM at a specific air pressure, to work properly.

A rule of thumb is that 1 HP generates about 4 CFM @ 90 PSI.

This is an industry standard, though it doesn’t apply to most compressors under 10 HP. For compressors smaller than 10 HP, you will need to read the specifications for that particular unit to determine their flow and pressure rates or use the “guestimate” of 2 CFM at 90 PSI per HP of electric motor.

When you’ve sized all of your applications and totaled up all of the air you’re going to need now and for the future expansion you may be undertaking in the future, and you are out searching for the right air compressor, you would divide the number of CFM you need by 4, and that will give you a rough idea of the horsepower rating of the compressor required.

Be careful. Not all compressor manufacturers’ rate their compressor output the same way. You might see a compressor showing a discharge rate at what appears to be an acceptable CFM, but on closer inspection find that the figure is predicated on a much lower pressure than you might need.

Discharge rates in CFM at higher pressures are always quite a bit lower than discharge rates at lower pressures, for that same compressor.

Ensure that the unit you select will give you both the CFM you need, and the pressure your equipment demands to work properly for you

SCFM

Because atmospheric pressure plays a role in how fast air moves into the cylinder, CFM will vary with atmospheric pressure. Similarly CFM also varies with the Temperature and Humidity of the air.

In order to provide accurate flow measurement to an end user or to provide an “apples to apples” comparison of flow, that variability has to be removed.

So a “standard” cubic foot was created. For a simple definition of SCFM, the acronym stands for Standard Cubic Feet of air per Minute.

To set an even playing field, most experts agree that the air flow must meet the following set of rigid environmental circumstances:

    • The air temperature must be 68 degrees Fahrenheit
    • The relative humidity of the air must be 36%
    • The air itself must be measured at sea level

SCFM provides standardized correction to capacity claims at 110 psig versus those at 90 or 150 psig for instance

Note that a compressor will always show a higher discharge flow rate at a lower pressure than it will at a higher pressure and compressor manufacturers will often highlight a high flow rate and lowlight the CFM. Remember, it will be air flow at higher pressure that you will require for your application!

The CFM and PSI ratings are important because they indicate the tools that a particular compressor can drive. When choosing a compressor, make sure it can supply the amount of air and the pressure that your tools need. Make sure, when you are discussing your compressor size with your vendor, whether they measure the flow from their compressor in SCFM or CFM or whatever, that they assure you that you will get the CFM you need at the pressure you need to do the job you want done.

NOTE: Some compressor manufacturers use SCFM as their yardstick to measure free air flow into the intake filter on an air compressor, not the flow of compressed air from the discharge of their compressor.

Some compressor manufacturers will use the term CFM for the flow of air out of the compressor discharge to the receiver or to the plant air mains. Others will use the acronym ACFM (Actual CFM) as the measure for air from their compressors.

And some manufacturers use the acronym SCFM as their measurement of air from the discharge port on their compressors.

For us then, its extremely important to understand what exactly compressor manufacturers mean by terms CFM/SCFM when discussing compressor power/size/flow rate with them.

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We thank www.about-air-compressors.com for permitting the use of information available therein for preparation of this article.

References:
1. www.about-air-compressors.com
2. http://en.wikipedia.org/wiki/Gas_compressor

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