Page 33 - Reciprocating Compressors Operation Maintenance

2O Reciprocating Compressors: Operation and Maintenance

It can now be seen that volumetric efficiency decreases as

1. The clearance increases.
2. The compression ratio increases.

Figure 1-11 illustrates the effect of clearance at moderate and high
compression ratio condition. A theoretical pV diagram for a ratio of 7 is
superimposed on a diagram for a ratio of 4, all else being the same. A rel-
atively high clearance (14%) has been used for illustrative purposes. The
clearance for a commercial compressor designed for a ratio of 7 would
be less than 14%.

too

80 60 40 20 0
PERCENT PISTON DISPLACEMENT
FIGURE - l i . Effect of different compression ratios on the volumetric efficien-
i
cy of a given cylinder.

Just as clearance in a cylinder has predominant control over volumet-
ric efficiency, the valve area has predominant control over compression
efficiency.
To obtain low clearance and a high volumetric efficiency, it is neces-
sary to limit the size and number of valves. This may tend to lower the
efficiency of compression and raise the horsepower. Both factors must be
evaluated and compromises made.