Page 34 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressors and Their Applications 21

PISTON RING LEAKAGE

This leakage allows gas from the compression chamber to escape past
the piston into the other end of the cylinder, which is taking suction with
the inlet valve open. Capacity is reduced because this hot leakage gas
heats up the incoming gas in that end of the cylinder.
Naturally, maximum piston leakage occurs as the piston approaches
the end of its stroke because differential pressure across the rings and the
time element are the greatest at this point. This leakage causes both a
volumetric and a horsepower loss as evidenced by an increase in dis-
charge temperatures.

VALVE SLIP

Valve slip means reversed gas flow through the valves before they
have had time to seat at the end of the piston stroke. Obviously, this vol-
ume loss can occur through both intake and discharge valves. Minimum
slippage occurs in a responsive valve; one that has minimum inertia so
that the moving element can easily be controlled by air flow.
Slippage is usually much less through intake valves than through dis-
charge valves. In the latter, differential pressure across the valve increas-
es rapidly as the piston reaches dead center, so that if the valve does not
respond instantaneously, high pressure gas naturally returns through the
valve before it seats.

EFFECTS OF MULTISTAGING

Multistaging has a marked effect on volumetric efficiency. Here, the
low pressure cylinder largely determines the entire machine's volumetric
efficiency because whatever volume this cylinder delivers to succeeding
stages must be discharged, with the exception of slight leakage that
occurs through packing boxes.
In other words, volumetric efficiency of a two-stage machine is the
same as if the low pressure cylinder were a single-stage compressor
delivering gas at intercooler pressure.
Figure 1-12 shows the pV combined diagram of a two-stage 100 psig
air compressor. Further stages are added in the same manner. In a recip-