By: Aerzen Canada  09-12-2011
Keywords: Compressors, Air Compressors, Screw Compressors

Volume control for Aerzen Screw Compressors

In principle, it is necessary to consider the problems of volume control for dry-running and for oil injection type screw compressors separately.

Volume control for dry screw compressors

Control by variable speed:
In consequence of the fact that screw compressors displace the medium positively, the most advantageous method of achieving volume control is that obtained by variable speed. This may be done in any of the following ways:

  • by variable speed electric motors
  • by use of a torque converter
  • by steam turbine drive

Speed may be reduced to about 50 % of the maximum permissible speed. Intake volume flow and power consumption are in this manner reduced in approximately the same proportion.
Control by bypass: Using this method, the surplus gas volume is allowed to flow back to the intake side by way of a governor that is controlled by the admissible final pressure. An intermediate cooler cools back the surplus gas volume down to the intake temperature.

Full load/idling speed governor:
As soon as a predetermined final pressure is attained a pressostat operates a diaphragm valve which opens up a bypass between discharge- and suction side of the compressor.
When this occurs, the compressor idles until pressure in the system drops to a predetermined minimum value, whereupon the valve will close once again on receiving an impulse from the pressostat, thereby bringing the compressor back onto full load once again.

Suction throttle control:
This method of control is suitable for air compressors only. As in the case of the full load/idling speed control method, a predetermined maximum pressure in the system, for example in a compressed air receiver, causes pressure on the discharge side to be relieved down to atmospheric pressure. Simultaneously, the suction side of the system is throttled down to about 0.15 bar absolute pressure. When pressure in the whole of the system has dropped to an admissible minimum value, full load is restored once again.

Control of screw compressors equipped with oil injection

Suction throttle control:
Since final compression temperature is governed by the injected oil, a greater range of compression ratios such as may arise when the induced volume is throttled down, can be safely coped with. This permits the main flow volume to be varied infinitely within wide limits. This particular type of control is the one to be preferred for compressors of the VMX series.

Keywords: Air Compressors, Compressors, Screw Compressors

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