Compressors (e.g., rotary screw gas compressors) are used, for example, in compression systems (e.g., refrigeration systems) to compress refrigerant gas, such as “Freon”, ammonia, natural gas, or the like. One type of rotary gas compressor employs a housing in which a motor-driven single main rotor having spiral grooves thereon meshes with a pair of gate or star rotors on opposite sides of the rotor to define gas compression chambers. The housing is provided with two gas suction ports (one near each gate rotor) and with two gas discharge ports (one near each gate rotor). Two dual slide valve assemblies are provided on the housing (one assembly near each gate rotor) and each slide valve assembly comprises a suction (also referred to as a “capacity slide valve”) and a discharge slide valve (also referred to as a “volume slide valve”) for controlling an associated suction port and an associated discharge port, respectively.
During operation of the compressor, a small amount of oil is typically continuously supplied to the compression chambers to provide an oil seal at points where the main rotor meshes with the gate rotors and with the housing to thereby effectively seal the chambers against gas leakage during gas compression. The oil flows out through the discharge ports and is recovered and recirculated. When the compressor is shut down and coasting to rest, excess oil can collect or settle in the compression chambers. When the compressor is restarted, the residual oil in the compression chambers, plus fresh oil entering the compression chambers, must be expelled through the discharge ports. U.S. Pat. Nos. 4,610,612, 4,610,613 and 4,704,069, all of which are assigned to the same assignee as the present application, disclose a dual-slide valve rotary gas compressor of the kind described above. The teachings and disclosures of each of these patents are incorporated by reference in their entireties herein.
Additionally, U.S. application Ser. No. 11/677,868 which is also assigned to the same assignee as the present application is directed to a compressor having a dual slide valve assembly that includes: i) a volume slide valve mechanism that is slidably movable to control compressor volume ratio and power input to the compressor; and ii) a capacity and volume slide valve mechanism that is in operational association with the volume slide valve mechanism. The capacity and volume slide valve mechanism is slidably movable to control compressor capacity and to control volume ratio and power input to the compressor. The disclosure of this application is also incorporated by reference here in its entirety.
The electric motors or engines employed to drive rotors in rotary compressors are usually of a type which requires the compressor to be unloaded while being started and brought up to some predetermined normal constant speed. Loading and unloading is accomplished by positioning of slide valves which control admission and discharge of gas into and from the compression chambers.
Often a discharge-suction pressure differential exists within the compressor during operation. When the discharge-suction pressure differential reaches and/or exceeds a certain threshold differential, the slide valve mechanisms can have a tendency to seize up and, in some instances, be damaged. For example, it has been found that certain screw-type compressors (e.g., single screw compressors) currently have threshold discharge-suction pressure differentials of about 400 p.s.i. Accordingly, it would desirable to provide a compressor, and more particularly a slide valve assembly, that can function in a high pressure environment, for example, when threshold pressure differentials are at or exceed about 400 p.s.i.