Method for operating dual slide valve rotary gas compressor

A method for operating a rotary screw type gas compressor and control means therefor to prevent undesirable compression of oil in the gas compression chambers during compressor start-up,

• said compressor comprising a housing having a cylindrical bore therein, a motor-driven helically grooved single main rotor mounted for rotation in said bore, and a pair of star-shaped gate rotors rotatably mounted in said housing and engageable with said grooves in said main rotor to define a plurality of compression chambers, one chamber at each groove,
• said compressor further comprising a suction port (55) in said housing and confronting said main rotor to admit low pressure uncompressed gas to said compression chambers and a discharge port (58) in said housing and confronting said main rotor to release high pressure compressed gas from said compression chambers,
• said compressor also comprising slide valve members (47, 48) for regulating both compressor capacity and compressor power input, one being a suction slide valve member (47) which is slidably positionable to control the extent to which said suction port (55) is open to thereby function as a suction by-pass to control compressor capacity, the other being a discharge slide valve member (48) which is independently slidably positionable to control the position at which said discharge port (58) is open to threby control the volume ratio and thereby the input power to the compressor, said slide valve members (47, 48) being disposed in side-by-side sliding relationship in a recess in said housing, which recess extends alongside and is in communication with said cylindrical bore, and each slide valve member (47, 48) having a face which is complementary to and confronts the main rotor surface in sliding sealed relationship, said slide valve members (47, 48) being movable independently of each other by said control means,
• said control means including a separate actuator for each slide valve member (47, 48) and sensing means, said control means being operable to position the slide valve members for compressor start-up, said control means also being responsive to the capacity of the compressor and to the volume ratio while the compressor is running to operate the actuators to appropriately position the slide valve members and thereby enable the compressor to operate at a predetermined capacity and a predetermined volume ratio;
• each of said dual slide valves (47, 48) being movable independently of the other between two extremes positions and to intermediate positions therebetween, said method comprising the steps of:
• operating said control means to dispose said suction slide valve (47) in one of its extreme positions so that said suction port (55) is fully open and operating said control means to dispose said discharge slide valve (48) in one of its extreme positions so that said discharge port (58) is fully open while said compressor (10) is being started;
• operating said control means to move said suction slide valve (47) from its said one extreme position to a predetermined position, including any of its intermediate positions and its other extreme position, to maintain a predetermined suction pressure at said gas suction port (55) or in said gas compression chambers while said compressor (10) is operating at normal speed;
• operating said control means in response to said sensing means to move said discharge slide valve (48) from its said one extreme position to a position, including any of its intermediate positions and its other extreme position, whereat fluid pressure at said discharge port (58) is equal to the fluid pressure in said compression chambers, while said compressor (10) is operating at normal speed;
• and operating said control means to dispose said suction slide valve (47) in its said one extreme position and to dispose said discharge slide valve (48) in its said one extreme position, while said compressor is being stopped.