Page 139 - Air and Gas Drilling Manual

Chapter 4: Compressors 4-25
Illustrative Example 4.1 A three-stage reciprocating primary air compressor
system unit is rated to have a volumetric flow rate of 900 scfm and a maximum
pressure capability of 240 psig at API Mechanical Equipment Standards standard
atmospheric conditions. The compressor has a diesel prime mover that is rated to
have a maximum power of 300 horsepower at a compressor shaft speed of 1,000 rpm
(at API Standard conditions). The 1,000 rpm is the shaft speed required for the
compressor volumetric flow rate of 900 scfm. The prime mover is naturally
aspirated. This reciprocating compressor system has a clearance ratio of 0.06 and a
mechanical efficiency of 0.90.
Determine the horsepower required by the prime mover to operate the
compressor against a flow-line back pressure of 150 psig for, a) a surface location at
sea level (use API Mechanical Equipment Standards standard atmospheric conditions
for sea level) and for, b) a surface location elevation of 6,000 ft (use average mid
latitudes data in Table 4-1).
a) Surface location at sea level.
This is a reciprocating piston compressor and, thus, has a volumetric efficiency
term. The volumetric efficiency, ε , can be determined using Equation 4-39.
v
n = 3
s
p = 14 696. psia
i
.
p o = 150 + 14 696 = 164 696 psia
.
Equation 4-36 becomes

1

.
r =  164 696 3 = 2 238
.
s
 14 696 
.
The specific heat ratio for air is
k = 14
.
The clearance volume ratio for this compressor is
.
c = 006
With the above values, Equation 4-37 becomes
  1 
ε v = 096 1 − 006  (2283 . −1  

.
.
.
)14
  
ε = 0 915
.
v