STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

486 Auxiliary Equipment
The main advantage of the multistage reciprocating piston compressor is that there
is nearly total positive control of the volumetric flowrate which can be put through
the machine and the pressure of the output. Many reciprocative piston compressors
allow for the rotation to be adjusted, thus, changing the throughput of air or gas.
Also, providing adequate power from the prime mover, reciprocating piston
compressors will automatically adjust to back pressure changes and maintain proper
rotation speed. These compressors are capable of extremely high output pressure
(see Figure 3-68).
The main disadvantages to multistage reciprocating piston compressors is that they
cannot be practically constructed in machines capable of volumetric flowrates much
beyond 1,000 actual cfm. Also, the highercapacity compressors are rather large and
bulky and generally require more maintenance than similar capacity rotary compressors.
In a compressor, like a liquid pump, the real volume flowrate is smaller than the
displacement volume. This is due to several factors. These are:
pressure drop on the suction side
heating up of the intake air
internal and external leakage
expansion of the gas trapped in the clearance volume (reciprocating piston
compressors only)
The first three factors are present in compressors, but they are small and on the
whole can be neglected. The clearance volume problem, however, is unique to
reciprocating piston compressors. The volumetric efficiency eV estimates the effect
of clearance. The volumetric efficiency can be approximated as
e, = 0.96[1- E(rf - l)] (3-76)
where E = 0.04-0.12. Figure 3-73 gives values of the term in the brackets for various
values of E and the rt.
= 0.04
= am
= 0.08
= 0.10
= 0.12
1 2 3 4
pressure ratiopJp,
= 0.14
Figure 3-73. Volumetric efficiency for reciprocating piston compressors (with
clearance) [4].