Page 110 - Air and gas Drilling Field Guide 3rd Edition

5.4 Intermittent (Positive Displacement) Compressors 101

In Line L-type V-type W-type

Engine

Compressor
Horizontal Opposed Horizontal with Stepped Piston Integral L-type
(Four-stage)

FIGURE 5-8. Double-acting (crosshead type) reciprocating piston compressor [1].

cylinder) either through an oil-lubricated intake gas stream or via an oil pump and
injection of oil to the piston sleeve. There are some applications where oil must
be omitted completely from the compressed air or gas exiting the machine. For
such applications where a reciprocating piston type of compressor is required,
there are nonliquid lubricated compressors. These compressors have piston rings
and wear bands around the periphery of each piston. These wear bands are made
of special wear-resistant dry lubricating materials such as polytetrafluorethylene.
Trunk type nonlubricated compressors have dry crankcases with permanently
lubricated bearings. Crosshead type compressors usually have lengthened piston
rods to ensure that no oil wet parts enter the compression space [1, 7].
Most reciprocating compressors have inlet and outlet valves (on the piston
heads) and are actuated by a pressure differential. These are called self-acting
valves.
The main advantage of the multistage reciprocating piston compressor is the
positive control of both the volumetric flow rate, which can be put through
the machine, and the pressure of the output. Many reciprocating piston compres-
sors allow for the rotation to be adjusted, thus changing the throughput of air or
gas. Also, provided that there is adequate input power from the prime mover,
reciprocating piston compressors can adjust to any back pressure changes
and maintain proper rotation speed (which in turn maintains a given volumetric
flow rate).
The main advantage of this subclass of compressor is extremely high pressure
output capability and reliable volumetric flow rates (see Figures 5-2 and 5-3). The
main disadvantage to multistage reciprocating piston compressors is that they
cannot be practically constructed in machines capable of volumetric flow rates
much beyond 1400 actual cfm (660 actual liters/sec). Also, the higher capacity