Screw compressors are positive displacement machines with combine the advantages of piston compressors and turbomachinery.
Their reliability and virtually wear-free operation have led to their use in almost all industrial applications.
Compression is achieved by the intermeshing of two helical rotors with a minimum of play or clearance in a suitable casing.
On rotation, gas is drawn in to fill the space between adjacent lobes, the volume increasing from zero initially to a maximum (induction process).During this phase, the interlobal space is connected to the gas intake of the compressor.
On further rotation of the rotors, the gas is transported onwards and separated from the intake space by the rotor lobes. The gas volume trapped between the rotors, which are enclosed by the casing with very small clearances, is reduced progressively on the delivery side of the compressor until the interlobal space is exposed to the outlet port , through with the gas is discharged.
The location and size of the outlet port determines the compression ratio and the magnitude of the discharge pressure that can be achieved.The gas is subsequently completely discharged against the backpressure prevailing on the delivery side.
This process is repeated several times on each rotation of the male rotor, so that in contrast to compression, by piston compressors, virtually continous and pulse – free compression takes place without any dead spaces.
VPT offers capacity control systems ranging from 0 to 100 % delivery at constant discharge pressure.
The speed control via frequency converter processes a 4…20 mA signal and provides continous control of the main drive motor speed and thus delivery capacity at constant discharge pressure.
Speed control range: 40 – 100 %
With this method of regulation, which is used only for oil-injected units, the measuring signal of the pressure sensor drives a servo valve.The volume of the interlobal space is reduced or increased by hydraulic actuation of the capacity control slide valve located underneath the rotors, thus effecting stepless adjustment of the compressor flow rate.
Slide valve capacity control range : 20 – 100 %
Spillback control should only be used for constant operating conditions, as the power consumption of the compressor during operation is always constant.
If a drop occurs below the permitted minimum value of the speed or slide valve control range, the superfluous gas can be returned to the intake side by means of a spillback controller, thus realizing a control range of the compressor unit of
0 to 100 %