Page 283 - Pipeline Rules of Thumb Handbook

270 Pipeline Rules of Thumb Handbook

Compression efficiency is the ratio of the theoretical thermal, which was used in establishing the compression
horsepower to the actual indicated horsepower required to efficiency.
compress a definite amount of gas. The efficiency, expressed Piston rod gas load is the varying, and usually reversing,
in percent, should be defined in regard to the base at which load imposed on the piston rod and crosshead during the
the theoretical power was calculated, whether adiabatic or operation, by different gas pressures existing on the faces of
isothermal. the compressor piston.
Mechanical efficiency is the ratio of the indicated horse- The maximum piston rod gas load is determined for each
power of the compressor cylinder to the brake horsepower compressor by the manufacturer, to limit the stresses in the
delivered to the shaft in the case of a power driven machine. frame members and the bearing loads in accordance with
It is expressed in percent. mechanical design. The maximum allowed piston rod gas load
Overall efficiency is the product, expressed in percent, is affected by the ratio of compression and also by the cylin-
of the compression efficiency and the mechanical efficiency. der design; i.e., whether it is single or double acting.
It must be defined according to the base, adiabatic iso-

Performance calculations for reciprocating compressors

Piston displacement Let L = 0.3 for lubricated compressors
Let L = 0.07 for non lubricated compressors (6)
Single acting compressor:
These values are approximations and the exact value may
P d =[ S t ¥ N ¥ 3 1416 ¥ D ] [4 1 728 ] (1) vary by as much as an additional 0.02 to 0.03.
2
.
¥
,
Note: A value of 0.97 is used in the volumetric efficiency
Double acting compressor without a tail rod: equation rather than 1.0 since even with 0 clearance, the
cylinder will not fill perfectly.
2
2
P d =[ S t ¥ N ¥ 3 1416 ¥ (2 D - )] [4 1 728 ] (2)
d
.
,
¥
Double acting compressor with a tail rod: Cylinder inlet capacity
P d =[ S t ¥ N ¥ 3 1416 ¥ ¥ ( D - )] [4 1 728 ] (3) Q 1 = E v ¥ P d (7)
2
2
¥
2
d
,
.
Single acting compressor compressing on frame end only:
Piston speed
2
P d =[ S t ¥ N ¥ 3 1416 ¥ ( D - )] [4 1 728 ] (4)
2
¥
.
,
d
PS =[2 ¥ S t ¥ N] 12 (8)
where P d = Cylinder displacement, cuft/min
S t = Stroke length, in.
N = Compressor speed, number of compression Discharge temperature
strokes/min
D = Cylinder diameter, in. T 2 = ( ( k 1) k ) (9)
-
d = Piston rod diameter, in. T r p1
where T 2 = Absolute discharge temperature °R
T 1 = Absolute suction temperature °R
Volumetric efficiency
Note: Even though this is an adiabatic relationship, cylin-
.
-
E v = 097 - ( [ 1 f r p ) 1 k - ] 1 C L (5) der cooling will generally offset the effect of efficiency.
where E v = Volumetric efficiency
f = ratio of discharge compressibility to suction Power
compressibility Z 2/Z 1
(
P Q 33 000
1
r p = pressure ratio W cyl =[144 1 1 , n cyl] ¥[ k k - )] ¥[ r p k-1 k - ] 1 (10)
k = isentropic exponent
C = percent clearance where n cyl = efficiency
L = gas slippage factor W cyl = Cylinder horsepower