Calculating a cylinder's air consumption

What sets a cylinder's air consumption?

Air consumption is the volume moved per cycle, converted to normal conditions. Four quantities set it: the bore (piston area), the stroke, the working pressure and the cycle rate per minute. Because compressed air is delivered and then expands on exhaust, the geometric stroke volume must be multiplied by the compression factor. This yields the real demand in normal litres per minute (Nl/min).

What is the formula and a worked example?

The stroke volume of one piston side is circle area times stroke. For a double-acting cylinder, extend and retract are added, though the piston rod reduces the annular area on the back side. Multiplied by cycle rate and compression factor, this gives the normal volumetric flow. For a quick estimate the full piston area for both strokes is enough, without the rod deduction.

• Piston area A = Pi divided by 4 times bore squared.
• Stroke volume per direction V = A times stroke.
• Compression factor f = (pressure in bar + 1.013) divided by 1.013.
• Consumption Q = V times 2 times cycle rate times f (double stroke per cycle).

How do I size compressor and piping with it?

Add up the demand of all consumers running at the same time and add a margin for leakage and reserve. A usual 20 to 30 percent covers valve losses, couplings and later expansion. The compressor should supply the sum in continuous duty; the line must carry the peak flow without pressure loss, or pressure collapses at the cylinder and cycle time rises.

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