Rotary screw compressor having a thrust balancing piston device and a method of operation thereof

A rotary screw compressor comprising:

• a pair of rotors meshing in a working space, said rotors being subject to a gas force (F.sub.G) in a first axial direction during operation of the rotary screw compressor;
• wherein at least one of said rotors has a main thrust bearing and is provided with a thrust balancing piston device having a first pressure surface mechanism establishing a force in a second axial direction opposite to said first axial direction, and a second pressure surface mechanism establishing a force in said first axial direction;
• wherein said first pressure surface mechanism includes at least one first pressure surface, and a first conduit connecting said at least one pressure surface to a high pressure source;
• and
• wherein said second pressure surface mechanism includes at least one second pressure surface, and a second conduit having a valve for selectively connecting said at least one second pressure surface to one of a low pressure source and an intermediate pressure source.