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1. A screw compressor for intermittent operation and including a compressor housing ( 1 ) which comprises a casing wall (2) which encloses at least two radially overlapping compression chambers (5, 6) each of which accommodates a respective helical rotor (7, 8), and is terminated at respective ends thereof with a high-pressure end wall (4) and a low-pressure end wall (3), characterized in that the high-pressure end wall (4) incorporates a wall plate (14) which in the active compression state of the compressor is in sealing abutment with end surfaces on the casing wall (2) and which, for the purpose of disengaging the compressor is arranged for movement in the axial direction of the compressor in a space (16) incorporated in the high-pressure end wall and communicating with the compressor inlet (12) through a channel (12'), said plate movement being effected by means of an auxiliary drive arrangement (20) .

2. A compressor according to claim 1, characterized in that the high-pressure end wall (4) incorporates a cylinder chamber (18) in which a piston (19) connected to the plate (14) can be moved under the influence of pressure fluid.

3. A compressor according to claim 1 or 2, characterized in that the rotors (7, 8) are provided with journals (11) and bearing shafts (9, 10) solely at the low-pressure end (3) of the compressor; and in that the sealing surface of the wall plate (14) facing the end of the casing wall (2) is imperforate and extends sealingly across the compression chambers .

'4. A compressor according to any of claims 1-3, incorporating an arrangement (22, 23) for injecting oil into the compression chambers (5, 6) characterized by a valve means (24) for throttling or interrupting the flow of oil to the compressor chambers when the wall plate (14) occupies its compressor disengaging position.