Page 219 - Compression Machinery for Oil and Gas

208 SECTION II Types of Equipment

Fundamentally, a compressor cylinder is made more efficient by using
larger compressor valves for a given cylinder bore diameter (everything else
being equal). But something else happens with the compressor valves being
larger—the fixed clearance becomes larger. Larger fixed clearance results in
lower VE which is lower capacity. A given cylinder diameter with more
and/or larger compressor valves will compress less gas, but compress that
gas with better energy efficiency (lower power per capacity).
The cylinder designer must make a trade-off between compression effi-
ciency and VE by optimizing valve flow area and clearance. The cylinder clear-
ance is the ratio of fixed clearance to swept volume. Most of the fixed clearance
2
is in the valves and valve clearance C. The swept volume is πD S/4, and the
2
resultant ratio is 4C/πD S, so the percent clearance is proportional to the inverse
00
of stroke. For example, a 10 diameter cylinder on a 6 stroke machine might
00
have 20% clearance and operate at 900rpm. However, the same diameter cyl-
00
inder with the same valves on a 12 stroke would operate at 450rpm and would
have the double the swept volume per stroke but the same displacement per
minute. The clearance would be only half or 10%. In practice, however, the cyl-
inder designer makes the short stroke machine a nonlinear design which cuts
clearance considerably. On long stroke cylinders with small bores more flexi-
bility is available to maximize the valves.

Friction
A reciprocating compressor is a mechanical device and as such encounters and
must overcome friction. Friction is accounted for very simply:
IP
BP ¼
M:E:
where
BP¼brake power
IP¼indicated power
M.E.¼mechanical efficiency, typically 95%–97%
A definition of indicated power is:
Adiabatic Power
+Suction Valve Loss Power
+Discharge Valve Loss Power
Indicated Power
Indicated power is all the power derived from the P-V diagram. Brake power
is then the total power required to be input to the compressor to get the indicated
power to the gas (for the P-V diagram).
Friction develops due to the crankshaft turning in the bearings, driving oil
pumps, windage, crossheads sliding in the crosshead guides, packing rubbing
against the piston rod, piston rings, and wearbands rubbing against the cylinder