Wednesday, December 22, 2010

Computing the Air Consumption of a Reciprocating Air Cylinder

In order for me to properly size the air compressor on my asparagus harvester I need to determine how much air is consumed each time an air cylinder is extended and retracted to cut a spear of asparagus.

My air cylinders have a one inch bore and a 5/8” diameter piston rod.

The stroke is 24 inches.

To extend the cylinder requires filling the cylinder until it is fully extended, and vice versa for retracting it. Therefore a first step is to calculate the total cubic inches of air contained in the cylinder when it is completely extended and again when it is completely retracted.

The area of the piston is .785 square inches. Multiplying the area times the stroke gives us a volume of .785 x 24 = 18.84 cubic inches.

The area of the piston rod is .306 square inches. We subtract the area of the rod from the area of the piston and multiply times the stroke to get the volume of the retracted cylinder. .785 - .306 = .479 square inches x 24 inches = 11.5 cubic inches.

Adding the two volumes gives us 30 cubic inches per stroke of the air cylinder. Converting to cubic feet: 30/1728 = .017361 cubic feet per cycle.

This is the free air volume, in other words the air is not compressed. I now need to determine how much “compressed” air is used.

The formula is ((100 psi) – (14.7 psi))/14.7 psi. This is known as the “compression factor”. Here the factor is 5.8.

Multiplying the cubic feet per minute times the compression factor gives: 1 cubic foot of air per stroke at 100 psi.