Understanding Compressed Air

In my job as an application engineer, I spend a lot of time explaining the principles of compressed air. When my clients run into trouble operating an air device, they overlook the flow requirements and focus mainly on pressure. The two are interrelated.

Let me offer a simple comparison to electricity. Air pressure is equivalent to voltage and air volume to amperage. Most people recognize that larger amperage draw, the larger gauge wire is required. Also the longer the wire, the larger wire gauge is required.

The same applies to compressed air. The more CFM required by the tool the larger the air line required. Also the longer the air line, the larger diameter pipe is required.

With electricity, if you pull more amps than what the wires can carry the lights dim because you are experiencing a voltage drop. With compressed air, if the air tool is demanding more air than the hose can carry, you get a pressure drop.

So you see pressure and volume are interrelated. On the EXAIR website in our knowledge base  we have a table of pressure drops for various sizes of pipe against the SCFM of draw.

In situations experiencing pressure drops with correct sized plumbing, you have to consider the capacity of your compressor as well as to how many other operations are drawing air from it. Again going back to the electricity analogy, you would not expect to run your entire house and your neighbor’s on a small portable generator. The same applies to the size of your compressor.

So many times when I ask a client the size of their compressor they will state the size of their receiver tank. Here again is another misconception. The receiver tank is only a reservoir to store air when demand is down. If the air device demand is greater than the capacity of the compressor, then it has to be idled for a period of time for the compressor to catch up. The length of this time depends on the differential of the air demand vs. compressor capacity. The size of the receiver tank will determine how long the demand can be on before you get ahead of the compressor.

The following equation enables one to calculate the air receiver volume in terms of gallons necessary to power an air operated components for a specified length of time, given an undersized compressor:

V = T(C-S)P0/ (P1-P2)

Where:

V is the receiver capacity in cubic feet
T is time in minutes the air component will operate
C is the air requirement of the air components (scfm)
S is the air being delivered to the receiver from a compressor (scfm)
P0 is the atmospheric pressure, psia (refer to table below)
P1 is the initial receiver pressure, psig
P2 is the workable pressure receiver is allowed to drop to, psig

 Joe Panfalone
Application Engineer
jopanfalone@exair.com

 

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