Page 126 - Air and gas Drilling Field Guide 3rd Edition

5.6 Prime Mover Input Power Requirements 117

5.6.2 Reciprocating Compressor Unit
A reciprocating compressor can adjust its output pressure to match the back pres-
sure on the machine. Thus, the reciprocating compressor is somewhat more flex-
ible than the rotary compressor and will tend to use less fuel for a given
application than a similarly configured rotary compressor [1, 11].
The intake volumetric flow rate of a real reciprocating compressor is slightly
smaller than the theoretical sweep volume (i.e., the calculated intake volumetric
flow rate). This is due to the fact that the piston compressor cannot be fabricated
without a clearance volume. This clearance volume at the top of the piston cylin-
der is necessary in order to have space for the valves and to keep the piston from
striking the top of the cylinder. This clearance volume results in a volumetric effi-
ciency term e v that is unique to the reciprocating compressor. This volumetric
efficiency is only applicable to the first stage of the reciprocating compressor.
The expression for the volumetric efficiency e v of a reciprocating compressor
can be approximated as

1
k
e v ¼ 0:96 1 cr s 1 ; (5-38)
where c is the clearance volume ratio for the compressor model. The clearance
volume ratio is the clearance volume divided by the sweep volume of the first-
stage piston. The range of values for the clearance volume term c is from 0.02
to 0.08 [1].
The mechanical efficiency term e m is used with the volumetric efficiency
term to determine the input power requirement. This mechanical efficiency term
is a measure of the friction losses in the mechanical application of the prime
mover power to the compressor. These losses are due to the friction in the
bearings and linkages in the compressor system. The values of mechanical effi-
ciency for typical reciprocating compressor systems can vary from about 0.84
to 0.99.
Illustrative Example 5.1 A two-stage reciprocating primary air compressor
system unit is rated to have a volumetric flow rate of 950 scfm (448.3 stan-
dard liters/sec) and a maximum pressure capability of 300 psig (206.9 N/cm 2
gauge) at API Mechanical Equipment Standards atmospheric conditions. The
compressor has a diesel prime mover that is rated to have a maximum power
of 350 hp (261 kW) at a prime mover output shaft speed of 1800 rpm (at API
Standard conditions). The prime mover is turbocharged. This reciprocating
compressor system has a clearance ratio of 0.02 and a mechanical efficiency
of 0.95.
Determine the horsepower required by the prime mover to operate the com-
2
pressor against a flow-line back pressure of 150 psig (103.5 N/cm gauge) for (a)
a surface location at sea level (use API Mechanical Equipment Standards atmo-
spheric conditions for mean sea level) and (b) a surface location elevation of
6000 ft (1829 m) (use average midlatitudes data in Table 5-1).