Page 300 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 285

FIG. 6.35 Power, slide valve versus VFD for oil flooded screws.

Operational Guidelines
Capacity Control of Dry Screw Compressors
The most efficient capacity control method for dry screw compressors is speed
control, because the power consumption decreases linearly with the gas volume
flow and the speed. A typical turndown value may be between 40% and 60% of
rated flow. The lower values are valid for compressors with liquid injection
while the higher values are valid for compressors without liquid injection.
If speed control is not possible or the required flow is below the minimum
allowable speed the capacity is controlled by a cooled recycle line. With a
cooled recycle turndown to zero is possible. During recycling no power is saved
and therefore this method should be used only in cases where the compressor
operates at full capacity for most of the time. Typical input properties for the
capacity control are either suction pressure or discharge pressure.

Capacity Control of Flooded Screw Compressors
Oil-flooded screw compressors have an internal slide valve as described above.
Turndown to 10% or 20% of maximum flow is possible via the slide valve.
Operating at reduced capacity with the slide valve means less internal compres-
sion, and there is also a certain amount of mechanical and dynamic loss when
operating at full speed but reduced slide valve capacity. Thus, using a VFD for
capacity control is more efficient than using the slide valve, as discussed in Suc-
tion flow and power consumption versus compressor speed above. As with oil-
free screw compressors, consideration must be given to the amount of time the
machine will be operating at reduced load versus full load.

Starting of Dry Screw Compressors
Starting of dry screw compressors should always be unloaded, that is, with a
discharge pressure close to suction pressure. The compressor is separated from