Page 32 - Reciprocating Compressors Operation Maintenance
P. 32
Reciprocating Compressors and Their Applications 1 9
If a compressor is designed for a given capacity at a given condition,
the amount of normal clearance in the cylinder or cylinders has no effect
on power.
When a piston has completed the compression and delivery stroke and
is ready to reverse its movement, gas at discharge pressure is trapped in
the clearance space.
This gas expands on the return stroke until its pressure is sufficiently
below intake pressure to cause the suction valves to open. On a pV-dia-
gram, Figure 1-9 shows the effect of this re-expansion on the quantity of
fresh air or gas drawn in.
k—- ACTUAL CAPACITY-
-PISTON DISPLACEMENT-
FIGURE 1-9. Effect of clearance on the capacity of a reciprocating compressor.
Figure 1-10 shows a series of theoretical pV diagrams based on a pres-
sure ratio of 4.0 and clearances of 7, 14, and 21%. The effect of clearance
is clearly indicated.
Clearance
6O
114 100 80 6O 4O 2O
Percent Piston Displacement
FIGURE 1-1O. Quantitative effect of various cylinder clearances on the volu-
metric efficiency at constant compression ratio.
