Page 273 - Compression Machinery for Oil and Gas

258 SECTION II Types of Equipment

FIG. 6.5 p–v-Diagram simulated.

A certain mismatch of “built-in pressure ratio” and external pressure ratio is
often unavoidable and does not cause a real disadvantage. Large deviations
between both values, however, may have severe disadvantages such as a drop
of efficiency, gas pulsations, and internal overheating in case of overcompres-
sion. Undercompression is sometimes unavoidable, especially in oil-flooded
screw compressors where high-pressure ratios are possible; cases with an inter-
nal compression ratio of 10 and a pressure ratio of 25 have been known to oper-
ate without any issues. In any case a moderate undercompression is better than
overcompression.
The working phases are repeated for each lobe of the male rotor. Thus a
compressor with four lobes on the male rotor performs four compression cycles
during each rotation of the male rotor. The number of compression cycles per
second is called pocket passing frequency (PPF). Gas is discharged into the dis-
charge line discontinuously at the PPF. Table 6.1 shows examples of different
compressor types and speed with their respective pocket passing frequencies.
The PPF is much higher than the speed of reciprocating compressors which
means that the discontinuous flow pulsations occur at higher frequencies. The
discontinuous flow requires a careful discharge silencer design.

Comparison of Positive Displacement Machines (Screw
Compressor, Reciprocating Compressor) Versus Centrifugal
Compressors
Screw compressors are positive displacement machines with purely rotary
motion.