Page 410 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressor Calculations 395

On multistage compressors, the actual compression ratios on each
stage are self-determining as all the gas from the low pressure cylinders
must pass through the higher pressure cylinders. The compression ratio
on each stage will not change in proportion to the system pressure ratio,
but it will change appreciably if the efficiency of any of the stages
decreases because of piston ring blow-by or valve failure.

Percent Clearance

Percent clearance is used to calculate the volumetric efficiency of a
cylinder. It is the ratio of clearance volume to piston displacement,
expressed as a percentage:
3
%q= Clearance Volume (in. ) xlQ()% (A g)
Piston Displacement (in. )

Clearance volume is the volume left in the cylinder end at position 3,
Figure A-2, including valves and valve ports.
Percent clearance varies between each end of the cylinder, so each is
considered separately in calculations, although it can be averaged. Per-
cent clearance is vital information for reciprocating compressor calcula-
tions. Therefore, it should be specified as name plate information
required on all purchases of compressors. For estimating purposes, if
clearance is unknown, a reasonable assumption is 15% clearance.

Volumetric Efficiency

Volumetric efficiency is the ratio of actual gas drawn into the cylinder
piston displacement, as a percent. Referring to Figure A-2, Position 3,
volumetric efficiency represents the distance 4 to 1, divided by the
stroke. Theoretically, this is expressed by the equation

=100-C1% (A-9)

In actual machines, other factors affect the volumetric efficiency.
These factors are leakage across the valves, across the piston rings, and
through the packing in addition to the heating effect of the incoming gas