Page 131 - Air and gas Drilling Field Guide 3rd Edition

122 CHAPTER 5 Compressors and Nitrogen Generators

The rated volumetric flow rate into the compressor is 448.3 liters/sec. For this
example, the compressor is located at 1829 m above sea level, thus
3
Q i ¼ 0:448 m =sec:
With the aforementioned terms, the theoretical shaft horsepower required to
compress the air moving through the machine is given by Equation (5-36a). Thus,
the theoretical shaft horsepower is
2 3
ð0:4Þ

ð2Þð1:4Þ 6 1115720:0 ð2Þð1:4Þ 7
_ W s ¼ ð81170:0Þð0:4483Þ 6 1 7
ð0:4Þ 4 81170:0 5
_ W s ¼ 115660 watts:
For this example,

e m ¼ 0:95
and from Equation (5-39), the actual shaft power is
115660
_ W as ¼ ¼ 130947 watts:
ð0:95Þð0:930Þ
The above determined 131.0 kW is the actual shaft power needed by the com-
2
pressor to match the back pressure of 103.5 N/cm gauge (at the surface location
elevation of 1829 m above sea level). At this surface location, the input power
available from the prime mover is a derated value (derated from the rated
261 kW available at 1800 rpm). In order for the compressor system to operate
at this 1829-m surface location elevation, the derated input power available must
be greater than the actual shaft power needed. Figure 5-15 shows that for a
1829-m elevation the power of a turbocharged prime mover must be derated by
15%. The derated input horsepower available from the prime mover _ W i is
_ W i ¼ 261ð1 0:15Þ¼ 221:9 kW:

For this example, the prime mover derated input power is greater than the
actual shaft power needed, thus the compressor system can be operated at this
1829-m surface location elevation.

5.6.3 Rotary Compressor System Unit
Rotary compressors have fixed compressor ratios for each stage. These machines
have a constant pressure output. These compressors cannot adjust output pres-
sures to match the back pressure in the exit flow line. The rotary compressors
will inject compressed gas into the flow line at the compressor’s rated pressure
output. If the back pressure in the exit flow line is less than the rated pressure
output, the gas will expand in the flow line (or surge tank upstream of the flow
line) to match the back pressure [1, 7].
There is no volumetric efficiency term for the rotary compressor. There is a
mechanical efficiency term for these compressors (denoted by the term e m ).