Page 303 - Compression Machinery for Oil and Gas

288 SECTION II Types of Equipment

Liquid Injection for Dry Screw Compressors
Liquid injection is essential for oil-injected screw compressors and is often used
for dry screw compressors as well. Dry screw compressors use liquid injection
for cooling and washing purposes. The most common injection liquid is demi-
neralized water but other liquids like methanol or heavy hydrocarbons may also
be used if they are compatible with the process.
Fig. 6.36 shows the effect of water injection on the discharge temperature of
a large dry screw compressor at two different discharge pressures. Two modes
can be identified which are separated by the saturation temperature. The satu-
ration temperature depends on the humidity of the suction gas and the discharge
pressure.

Injection Above Saturation Temperature
For low injection flows there is a sharp linear decrease of discharge tem-
perature with increasing injection flow. The cooling effect is caused by
evaporation of the injected liquid during compression. This characteristic
is valid until the saturation temperature is reached. In this mode the injec-
tion flow is controlled by a control valve to adjust a defined discharge
temperature. Due to temperature measurement precision this mode is prac-
tically limited to a discharge temperature of at least 10K above saturation
temperature.
This injection mode is used if the discharge temperature must be reduced
due to process or compressor design limits.

250
Discharge pressure 5 bar abs
Discharge temperatrure with water injection [°C] 150
200
Discharge pressure 3 bar abs

100

50

0
0 2000 4000 6000 8000 10,000
Injection water flow [kg/h]
FIG. 6.36 Temperature versus injection flow for a dry screw compressor.