Page 133 - Air and gas Drilling Field Guide 3rd Edition

124 CHAPTER 5 Compressors and Nitrogen Generators

The actual shaft power _ W as is actual power needed to compress the air to a pres-
sure of 300 psig. Actual shaft power from Equation (5-39) is
_ W s
_ W as ¼ :
e m e v
For this example,
e m ¼ 0:95

and Equation (5-39) becomes
234:3
_ W as ¼ ¼ 246:6 hp:
ð0:95Þð1:0Þ
The above determined 246.6 horsepower is the actual shaft power needed by
the compressor to give a fixed design pressure output of 300 psig. This power
level is less than the prime mover’s rated input power of 350 hp, thus this com-
pressor system is capable of operating at a sea level surface location.

(a) Surface location at sea level (SI units)
The rotary compressor is a fixed ratio machine. Therefore, the output of the com-
pressor will be 300 psig.
n s ¼ 2
2
p i ¼ 101360 N=m abs
2
p o ¼ 2069000:0 þ 101360:0 ¼ 2170360:0N=m abs:
Thetotal fixedcompression ratioacrossthe twostagesofthe compressor
r tf is
2170360:0
r tf ¼
101360:0
r tf ¼ 21:414:
The rotary compressor volumetric efficiency e v is
e v ¼ 1:0:
The rated volumetric flow rate into the compressor is 448.3 liters/sec. For this
example, the compressor is located at mean sea level (API Standard conditions),
thus
3
Q i ¼ 0:4483 standard 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