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Cylinder air consumption calculations are complex. Why bother?

The need to find out cylinder air consumption on a machine, towards determining  the amount of compressed air a fixture will use would be one reason.

Another would be to determine if an air compressor has enough capacity to supply all the air requirements of a larger machine… and so on.

As part of figuring that out, you either know already or you are about to find out, that Pi equals approximately 3.14.

I say Pi is approximately 3.14 because the number actually has been solved out to 2,000,000 places past the decimal point, and the number is still growing! I opted not to put all the numbers available behind the 3.14 here.

Why Knowing Pi Is Needed For Cylinder Air Consumption?

Iso Air Cylinder
Iso Air Cylinder

We need to know what Pi means to help figure out cylinder compressed air consumption. To do that we first need to determine the surface area of the piston inside the air cylinder. Then, if we multiply that surface area by the working stroke we’ll get that cylinder air volume.

Cylinder Retraction Uses Less Air

It is true that the retraction stroke of a single rod air cylinder will use less volume of compressed air than the extension of that same air cylinder. The reason is that part of the surface area on the retract side of the piston is taken up by the piston rod. That means less room on the retract side for air.

For simplicity’s sake in determining air cylinder consumption though, I have chosen to ignore that difference.

Determine The Area Of A Piston

The formula to use to determine the area of a circle is:

Pi x r2 (Pi time radius squared)

or, 3.14 x the radius squared

Our example will be a 2.5″ bore air cylinder.

  • A 2.5″ bore cylinder will have: 3.14 x r2 ( which is 1.25 x 1.25) or, 4.90 square inches of surface area on the piston face.
  • We will make the stroke 10″. That means that this cylinder holds 4.9 x 10, or 49 cubic inches of compressed air.
  • If we extend and retract this cylinder one cycle that is a total of 98 cubic inches of air we would need for one extension and one retraction.
  • 98 cubic inches of air x 10 complete cycles per minute = 980 cubic inches of air consumed per minute
  • A cubic foot of air contains 1,728 cubic inches

Therefore, 980 cu inches divided by 1728 = .6 cubic feet of air.

One More Step In Cylinder Air Consumption

You still have to allow for the compression ratio, a factor that affects the flow and volume of air under pressure. Those calculations are beyond this page.

So, my rule of thumb about air cylinder consumption:

Multiply the net cubic feet of air found using the above formula x 5 to get a very rough estimate of the actual CFM needed to supply the cylinder.

That being the case, this cylinder (2.5″ bore x 10″ stroke – 10 cycles per minute) will need approximately 3 CFM of compressed air to run those 10 complete cycles per minute.

Since 1 HP of compressor motor generates about 4 CFM at 90 PSI, you can see that the use of air cylinders will quickly eat up compressor capacity.

On a high speed machines with multiple cylinders, air cylinder consumption can be staggering.

Is Cylinder air consumption critical?

If you require detailed and accurate air cylinder air consumption figures, figure on getting help from an engineer, or, contact the air cylinder manufacturer.

That’ll get you the straight goods about their air cylinder air consumption.