Page 220 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 209

bore and other items. Friction manifests itself as heat, for example, causing the
crankcase oil to get hot.

Compression Efficiency
Compression efficiency is usually defined as the adiabatic efficiency also
known as isentropic efficiency and so is the ratio of the adiabatic power to
the PV card indicated power:
EFF ¼ AP=IP 100%
where
EFF¼compression efficiency
AP¼adiabatic power
IP¼indicated power
Fig. 5.28 is a plot of compression efficiency vs. compression ratio for a
given compressor cylinder when compressing two different gases, hydrogen
and nitrogen:
Note how efficiency increases as compression ratio increases. The effi-
ciency curve will have this shape for any reciprocating compressor cylinder.
Also note how much higher the efficiency is for hydrogen (with a very low
MW of two) as compared to natural gas (with a medium MW of 18). Everything
else being equal, compressing hydrogen will have one-ninth (11%, 89% less)
the VLP, hence much better efficiency.

Pulsation Bottle Losses
The losses in a pulsation bottle can be considered in two parts, the relatively
static portion which is the pressure drop at the line connection and the choke

FIG. 5.28 Plot of compression efficiency vs. compression ratio.