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Cylinder Air Consumption; you know already, or you're about to find out if you don't, that Pi equals approximately 3.14.
I say approximately because we understand that Pi has been solved out to 2,000,000 places past the decimal point, and the number is still growing! We opted not to put all the numbers available behind the 3.14 here. Hope you’ll understand :-) !
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. If we multiply that figure by the working stroke we'll get that cylinder's volume of air.
The retraction of a single rod cylinder will use less compressed air than the extension of that same air cylinder, as the retract side will have a piston rod taking up some of the surface area on one side of the piston. For simplicity's sake in determining air cylinder consumption we've chosen to ignore that difference. Also, given our ongoing suggestion that you err on the side of caution when calculating the size of the compressor you'll need, ignoring the lower volume of air on the rod side will help in ensuring that your compressor is large enough for your all of your applications.
You can 'dial down' the air from a too-big compressor. You can't 'dial-up' air supply from one that's too small. When it comes to compressors, bigger is better, in most cases!
The formula to use to determine the area of a circle is:
Pi x r2 or 3.14 x the radius squared
We'll use a 2.5" bore air cylinder for demonstration purposes. A 2.5" bore cylinder will have: 3.14 x r2(1.25 x 1.25) or, 4.90 square inches of surface area on the piston face.
Using this same 2.5" bore cylinder, let's 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's 98 cubic inches of air we'll 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 compressed air has 1,728 cubic inches in it, therefore:
980 cu inches divided by 1728 = .6 cubic feet of air.
There's one more step!
You still have to allow for the "compression ratio", a factor that affects the flow and volume of air under pressure.
Rule of Thumb: 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.
Since 1 HP of compressor 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 machine with multiple cylinders, air consumption can be staggering.
We hope the reference charts below will help fast-track determining your air cylinder air consumption!
CHARTS
The first chart shows air consumption for a 1" stroke cylinder, in a variety of standard NFPA bore sizes, and with a specific cycle rate.
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Then, multiply that number by a factor of 5, and you'll have a decent "guestimate" of what the actual CFM you'll need to run that cylinder.
It's a pain in the neck to have to do that for every cylinder. After you've done a few, you can likely extrapolate for the rest trying to overestimate usage a bit, rather than underestimating consumption.
Or...
I've just had developed and made available here a quick cylinder air consumption calculator. Interested? Here it is.
And certainly, your air cylinder vendor should be able to provide air consumption figures for the air cylinders they provide you.
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