Page 304 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 289

Injection Below Saturation Temperature
By increasing the water injection flow the discharge temperature eventually
reaches saturation temperature and no more water will evaporate. In this mode
even a massive increase of injection water flow leads only to minor reduction of
discharge temperature. In this case the injection flow cannot be controlled by
discharge temperature but is controlled by a flowmeter and a control valve at
a predefined flow.
Injection below saturation temperature is often used for processes with dirty
gases like acetylene crack gas, lime kiln gas, or coke oven gas. A part of the
water does not evaporate but leaves the compressor in liquid form and washes
the rotors, casing, and piping. For these processes it should be noted that a prac-
tical discharge temperature is only approximately 1K below saturation temper-
ature. In case of polymerizing gases solvents may be injected in addition to
water to prevent sticking of the rotors or clogging of piping components.

Liquid Injection Flows for Dry and Oil-Injected Screw
Compressors/Liquid Hammer

The quantity of injected liquids differs very much between oil injected and dry
screw compressors. For dry screw compressors the volume flow of liquid is
determined by the evaporation of the liquid and is normally <0.01% by volume
of the gas flow at suction conditions. In cases with injection below saturation
temperature the injection flow may be 0.01% by volume or higher.
For oil-flooded screw compressors the cooling effect depends on the heat
transfer between gas and oil. Therefore the percentage of oil compared to
gas volume is much larger than in dry screw compressors. Fig. 6.37 shows
the relative relationship between gas discharge temperature and oil injection
flow. It can be see that in general, the relative cooling effect of the oil decreases

FIG. 6.37 Temperature versus oil injection flow for an oil flooded screw compressor.