Page 31 - Reciprocating Compressors Operation Maintenance

1 8 Reciprocating Compressors: Operation and Maintenance

VOLUMETRIC EFFICIENCY

A volumetric efficiency, which varies for different compression ratios,
must then be applied to the piston displacement to determine actual free-
air capacity. Volumetric efficiency also varies to some extent with the "n"
value, and molecular weight, of the gas being compressed.
Greatest volumetric loss occurs because of clearance within the com-
pressor cylinders. However, other losses, while of lesser importance, also
affect compressor capacity.

CLEARANCE Loss

When the compressor reaches the end of its stroke and has discharged
all the gas it can, a small amount remains in the valve pockets and in the
clearance space between piston and cylinder head.
When the piston starts its return stroke, this clearance gas at discharge
pressure must expand to intake pressure before inlet valves can open;
thus, no air enters the cylinder for that portion of the stroke, which
reduces the intake volume by that amount.
Since the volume for this clearance gas, expanded to intake pressure,
varies with the compression ratio, it follows that compressor volumetric
efficiency, and hence its actual capacity, varies with compression ratio
instead of with pressure.
Cylinder clearance cannot be completely eliminated. Normal clear-
ance is the minimum obtainable in a given cylinder and will vary
between 4% and 16% for most standard cylinders. Some special low-
ratio cylinders have normal clearance much greater than this. Normal
clearance does not include volume that may have been added for other
purposes, such as capacity control.
Although clearance is of little importance to the average user (guaran-
tees are made on actual delivered capacity), its effect on capacity should
be understood because of the wide application of a variation in clearance
for control and other purposes. There are many cases where extra clear-
ance is added to a cylinder:

1. To reduce capacity at fixed pressure conditions.
2. To prevent driver overload under variable operating pressure condi-
tions by reducing capacity as compression ratio changes.