Page 28 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 1 5

However, since air or gas is elastic, compressor capacity varies widely
as pressure conditions change. For instance, with a given intake pressure,
machine capacity is considerably less when discharging at 100 psi than at
50 psi. This makes it impossible to rate a given compressor for a given
capacity. The only practicable rating is in terms of piston displacement—
volume swept by the moving piston during one minute.

PISTON DISPLACEMENT

The piston displacement is the net volume actually displaced by the
compressor piston at rated machine speed, as the piston travels the length
of its stroke from bottom dead center to top dead center.
In Figure 1-8, the entire stroke, and thus the piston displacement, is
represented by the travel of the piston from points B-H.
This volume is usually expressed in cubic feet per minute. For multi-
stage units, the piston displacement of the first stage alone is commonly
stated as that of the entire machine.
In the case of a double-acting cylinder, the displacement of the crank
end of the cylinder is also included. The crank end displacement is, of
course, less than the head end displacement by the amount that the piston
rod displaces.
The piston displacement (PD) for a single-acting unit is readily com-
puted by the following formulas:

1. Calculating PD for a single-acting cylinder:

PD = AHEx —xrpm
F
12
Where AHE = area of head end of piston in square feet
S = stroke in inches
rpm = revolutions per minute
PD = piston D is displacement in cubic feet per minute
2. Calculating PD for a double-acting cylinder:

S S
PD = AHE x — x rpm + ACE x — x rpm
F
F
12 12
Where ACE = area of crank end of piston in square feet
This can be approximated by the expression: