1. IN SCREW COMPRESSOR APPARATUS OF THE CHARACTER DESCRIBED HAVING A CASING WITH A WORKING SPACE DISPOSED THEREIN IN THE SHAPE OF TWO INTERSECTING CYLINDRICAL BORES WITH PARALLEL AXES, A HIGH PRESSURE END WALL AT ONE END THEREOF, A LOW PRESSURE END WALL AT THE OTHER END THEREOF, INTERMESHING MALE AND FEMALE ROTORS DISPOSED IN SAID WORKING SPACE IN SEALING ENGAGEMENT WITH SAID CASING AND THE END WALLS THEREOF AND HAVING HELICAL LANDS AND INTERVENING GROOVES WITH A WRAP ANGLE OF LESS THAN 360*, SAID MALE ROTOR HAVING THE MAJOR PORTION OF ITS LANDS AND GROOVES OUTSIDE THE PITCH CIRCLE THEREOF AND WITH ITS FLANKS SUBSTANTIALLY CONVEX, AND SAID FEMALE ROTOR HAVING THE MAJOR PORTION OF ITS LANDS AND GROOVES INSIDE THE PITCH CIRCLE THEREOF AND WITH ITS FLANKS SUBSTANTIALLY CONCAVE, A LOW PRESSURE PORT ADJACENT SAID LOW PRESSURE END WALL, AN AXIAL HIGH PRESSURE PORT AT THE END OF SAID WORKING SPACE OPPOSITE FROM SAID LOW PRESSURE PORT AND WITH SAID PORTS BEING LOCATED SUBSTANTIALLY ON OPPOSITE SIDES OF A PLANE THROUGH THE AXES OF SAID BORES, LOW PRESSURE CHANNELS IN SAID CASING IN FLOW COMMUNICATION WITH SAID LOW PRESSURE PORT AND HIGH PRESSURE CHANNELS IN SAID CASING IN FLOW COMMUNICATION WITH SAID HIGH PRESSURE PORT, THE COMBINATION WHICH COMPRISES AN AXIALLY EXTENDING RECESS DISPOSED IN SAID CASING AND EXTENDING FROM SAID HIGH PRESSURE END WALL TO A FIXED END EDGE OF SAID RECESS AND BEING IN FLOW COMMUNICATION WITH SAID HIGH PRESSURE CHANNELS, SAID RECESS AND THE WALL OF SAID WORKING SPACE INTERSECTING ALONG TWO STRAIGHT EDGES PARALLEL WITH THE AXES OF SAID BORES AND ON THE SAME SIDE OF SAID PLANE THROUGH THE AXES OF SAID BORES, WITH AT LEAST ONE OF SAID STRAIGHT EDGES BEING LOCATED IN THE CASING WALL PORTION OF THE BORE SURROUNDING SAID FEMALE ROTOR, A VALVE AXIALLY MOVABLY DISPOSED IN SAID RECESS AND IN SEALING ENGAGEMENT THEREWITH, SAID VALVE HAVING A PROFILE ON THE SURFACE THEREOF FACING SAID WORKING SPACE CORRESPONDING THERETO, AND WITH A LOW PRESSURE CONTROL EDGE PARALLEL WITH AND FACING THE END EDGE OF THE SAID RECESS, AND WITH A HIGH PRESSURE CONTROL EDGE FACING THE HIGH PRESSURE CHANNELS, THE PORTION OF SAID RECESS DEFINED BY SAID STRAIGHT EDGES, BY THE END EDGE OF SAID RECESS, AND BY SAID LOW PRESSURE CONTROL EDGE FORMING A BLEED PORT PROVIDING FLOW COMMUNICATION FROM SAID WORKING SPACE TO THE LOW PRESSURE END OF SAID CASING WITH THE PORTION OF THE RECESS DEFINED BY THE SAID STRAIGHT EDGES, THE HIGH PRESSURE END WALL AND THE HIGH PRESSURE CONTROL EDGE FORMING A RADIAL HIGH PRESSURE PORT, SAID VALVE BEING ADJUSTABLE BETWEEN A FIRST POSITION IN WHICH THE BLEED PORT IS CLOSED, AND A SECOND POSITION IN WHICH AT LEAST A PORTION OF SAID HIGH PRESSURE CONTROL EDGE EXTENDS OUTSIDE THE HIGH PRESSURE END WALL INTO SAID HIGH PRESSURE CHANNEL, AND AN AXIAL HIGH PRESSURE PORT DISPOSED IN SAID HIGH PRESSURE END WALL, SAID AXIAL HIGH PRESSURE PORT BEING LIMITED BY TWO OUTER EDGE PORTIONS, EACH EXTENDING GENERALLY RADIALLY FROM THE WALL OF ONE BORE TO THE BOTTOM CIRCLE OF THE GROOVES OF THE CORRESPONDING ROTOR, AND BY AN INTERMEDIATE EDGE PORTION, AT LEAST THE MAJOR SECTION OF EACH OF SAID OUTER EDGE PORTIONS BEING BROUGHT INTO REGISTER WITH THE TRAILING FLANK OF THE TRAILING LAND OF A ROTOR GROOVE IN COMMUNICATION WITH THE HIGH PRESSURE CHANNEL AFTER THE TRAILING EDGE OF THE CORRESPONDING LAND TIP HAS BEEN IN REGISTER WITH THE AXIALLY OUTERMOST POINT OF THE HIGH PRESSURE CONTROL EDGE OF SAID VALVE WHEN THE VALVE IS IN SUCH A POSITION THAT SAID AXIALLY OUTERMOST POINT IS LOCATED IN THE PLANE OF THE HIGH PRESSURE END WALL OF SAID WORKING SPACE.