Page 272 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 257

discharge temperature. In addition, the backflow of gas into the compression
chamber causes gas pulsations and noise in the chamber and the connected
discharge line.

Overcompression
The opposite case where the internal pressure p 2i is higher than the discharge
line pressure p 2 is called overcompression (Fig. 6.4). Here the gas is compressed
inside the closed compression chamber to a higher pressure than in the dis-
charge line. When the rotor lobes have reached the outlet port edges the cham-
ber opens and the gas expands rapidly into the discharge line until the pressures
equalize at p 2 . By further rotation the chamber volume is reduced to zero and the
gas is expelled into the discharge line at the pressure p 2 . The triangular pressure
peak in Fig. 6.4 shows that overcompression also leads to a larger work than in
Fig. 6.2 which indicates a higher power consumption and worse efficiency.
Again the rapid equalization of gas pressures may cause gas pulsations and
noise. The compression to a high internal pressure p 2i also leads to high internal
gas temperatures which are higher than the temperature in the discharge line. In
extreme cases, this may cause damages due to internal overheating that may not
be detected by the temperature sensors in the discharge line. In some cases,
overcompression can lead to internal pressures that exceed the pressure rating
of the machine, and can also cause very high loads on the radial and thrust bear-
ings, leading to reduced bearing life and possibly even shaft damage.
It should be noted that Figs. 6.2–6.4 are idealized cases. In reality, the com-
pression phase is not isentropic and also the pressure equalization in case of
undercompression or overcompression is not instantaneous but takes some
time. Therefore, Fig. 6.5 shows an example of a more realistic simulation.

FIG. 6.4 Overcompression.