Page 296 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 281

400 vi=1.25

Power consumption (kW) 300 vi=2.0

vi=2.5
vi=3.15
200

100

0
1 1.5 2 2.5 3 3.5 4 4.5 5
Discharge pressure (bar abs)
FIG. 6.28 Power versus discharge pressure for a dry screw compressor.

FIG. 6.29 Power versus discharge pressure for an oil flooded screw compressor.

consumption at pressure ratio 1 (discharge pressure¼suction pressure) is high.
Therefore, compressors with large built-in volume ratio are well suited for large
pressure ratios but have a high torque in unloaded operation, for example, dur-
ing start-up.
With oil-flooded screw compressors, the power curves with relation to dis-
charge pressure are less steep in the normal operating range. This is because oil-
flooded screw compressors are generally used for higher pressure applications
than oil-free, and there is less inertia-based loss as a percentage of total power
consumption (refer to Fig. 6.29).