Abstract:
A slide valve for use in a screw compressor comprises a main body portion configured for sliding in a pressure pocket of a screw compressor to regulate output of a working matter through screw rotors of the compressor. The main body of the slide valve includes a plurality of walls that define an enclosed interior cavity. The slide valve also includes a bore extending into a wall of the main body such that working matter discharged from the screw rotors has access to the enclosed interior cavity. The bore is sized to dampen pressure pulsations in the discharged working matter as the discharged working matter flows through the bore.

Description:
BACKGROUND 
       [0001]    This invention relates generally to the field of screw compressors. Specifically, it relates to screw compressor slide valve systems. 
         [0002]    Screw-type compressors are commonly used in refrigeration and air conditioning systems. Interlocking male and female rotors located in parallel intersecting bores define compression pockets between meshed rotor lobes. Compressors consisting of two rotors are most common, but other configurations having three or more rotors situated so as to act in pairs are known in the art. Fluid enters a suction port near one axial end of a rotor pair and exits near the opposite end through a discharge port. Initially, the compression pocket communicates with the suction port. As the rotors turn, the compression pocket becomes trapped between male and female rotor lobes and the rotor bore wall. The compression pocket becomes smaller as it is translated axially downstream, compressing the fluid within. Finally, the compression pocket rotates into communication with a discharge port and the compressed fluid exits. 
         [0003]    Volume V 1  is defined as the compression pocket volume at the instant the pocket first becomes sealed from the suction port. Volume V 2  is defined as the pocket volume just before the compression pocket first communicates with the discharge port. Compressor volumetric flow rate (capacity) depends on the magnitude of V 1 . The larger the value of V 1 , the greater the compressor capacity, assuming the rotors maintain a constant angular velocity. Rotor, inlet port, and rotor housing geometry define the initial size of the sealed compression pocket. Capacity is therefore fixed for a particular screw compressor operating at a fixed angular speed. 
         [0004]    Compressors limited to operating at fixed capacity sacrifice efficiency, particularly when operating under varying load conditions. Because compressor capacity is proportional to system cooling capacity, it is desirable to vary capacity to match dynamic cooling loads. To vary capacity while maintaining a constant rotor angular speed, screw compressors commonly incorporate a slide valve. In a conventional two-rotor screw compressor, the slide valve is located in the cusp of the bores housing the interlocking rotors. The slide valve is movable linearly in this sleeve along an axis parallel to the axis of the rotors, forming a portion of the bore wall. As each set of rotor teeth contact the slide valve, a new compression pocket is sealed and compression begins. Altering the axial position of the slide valve effectively changes the axial point at which compression begins. Due to screw rotor geometry, the compression pocket formed by intermeshing screw rotor lobes is largest at the rotors&#39; suction end and smallest at the discharge end. Changing the axial point where compression begins increases or decreases V 1 , and thereby increases or decreases compressor capacity. 
         [0005]    The axial position of the slide valve is commonly controlled by actuating a control piston. Conventionally, the control piston is attached to the slide valve by a rigid connecting rod. This allows the piston to transfer either compressive force to move the slide valve towards the suction port or tensile force to pull the slide valve towards the discharge port. It is common for the piston and slide valve assembly to reciprocate in a bore formed by multiple adjoining housing cases. To minimize wear and prevent binding, however, each of these housing cases must be carefully machined and precisely positioned so as to align their bores along a single axis. Such precision in machining and assembly greatly increases compressor cost. One known system, shown in U.S. Patent Publication 2005/0123422 A1, transfers motion to a piston using a relatively flexible rod attached at each end by non-rigid means, such as a ball joint. Another system, shown in U.S. Pat. No. 5,081,876, employs magnetic coupling to transfer control piston motion to an exterior sensor measuring slide valve position. Such systems, however, retain a rigid rod as the means for transferring control piston motion to the slide valve itself. 
       SUMMARY 
       [0006]    In exemplary embodiments of the invention, a screw compressor includes a linearly reciprocating slide valve system. The slide valve system includes a control piston axially movable in a piston sleeve, a biasing spring, a slide valve, and a flexible member connecting the control piston to the slide valve and capable of transmitting axial tensile force. In operation, screw compressor discharge pressure moves the slide valve in a first axial direction, while the flexible member moves the slide valve in a second axial direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a perspective view of a rotary screw compressor, partially cut away to reveal interior components. 
           [0008]      FIG. 2A  is a schematic view of the interior of the screw compressor, showing a slide valve in a fully unloaded position. 
           [0009]      FIG. 2B  is partial schematic view of the screw compressor, showing the slide valve in a partially loaded position. 
           [0010]      FIG. 2C  is partial schematic view of the screw compressor, showing the slide valve in a fully loaded position. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  provides a partial cut away perspective view of screw compressor  10 . Screw compressor  10  includes motor case  12 , rotor case  14 , outlet case  16 , slide case  18 , motor stator  20 , motor rotor  22 , male screw rotor  24   a,  female screw rotor  24   b,  slide valve  26 , control piston  28 , flexible connecting member  30 , suction inlet  32 , and discharge outlet  34 . Motor case  12  is attached to rotor case  14 , forming one end cap of screw compressor  10 . Motor case  12  and rotor case  14  together house motor stator  20 , motor rotor  22 , and male and female screw rotor set  24 . Motor rotor  22  drives male screw rotor  24   a  or female screw rotor  24   b.  Outlet case  16  is attached to the end of rotor case  14  opposite of motor case  12 . Outlet case  16  contains slide valve  26 . Slide case  18  is attached to the remaining end of outlet case  16 , forming the other end cap of screw compressor  10 . Control piston  28  reciprocates within slide case  18 , varying compressor capacity by changing the axial position of slide valve  26 . Flexible connecting member  30  connects control piston  28  to slide valve  26 . Low pressure working fluid enters suction inlet  32 , is compressed by male and female screw rotors  24   a  and  24   b,  and exits discharge outlet  34 . In the embodiment shown, screw compressor  10  comprises a two-screw compressor. However, in other embodiments, the present invention is readily applicable to compressors having three, four our more screw rotors that employ a reciprocating slide valve system. 
         [0012]      FIG. 2A  shows a schematic cross-sectional view of rotary screw compressor  10 . The end of rotor case  14  adjoining outlet case  16  includes suction chamber  40 , male and female screw rotors  24 , screw rotor lobes  42 , and screw rotor bore  44 . Working fluid enters through suction chamber  40  into a compression pocket formed between screw rotor lobes  42  and screw rotor bore  44 . As motor rotor  22  rotates male and female screw rotors  24 , compression pocket volume is reduced as the pocket is translated towards outlet case  16 . 
         [0013]    Outlet case  16  contains discharge port  46 , discharge chamber  48 , and slide valve  26 . Fluid exits the compression pocket formed between screw rotor lobes  42  through discharge port  46  and into discharge chamber  48 . Discharge port  46  may be radial or axial, depending on the shape and position of slide valve  26 . 
         [0014]    Screw compressor  10  controls capacity by altering the axial position of slide valve  26 . When slide valve  26  reaches the mechanical limit of its axial motion away from male and female screw rotors  24 , compressor  10  capacity is at a minimum. The present invention provides an innovative slide valve system  50 , where a means for connecting slide valve  26  to a control piston head is flexible rather than rigid.  FIG. 2A  shows slide valve system  50  in this fully unloaded configuration. 
         [0015]    In  FIG. 2A , slide valve system  50  includes control piston  28 , control piston sleeve  54 , biasing spring  56 , o-ring seal  58 , first piston chamber  60 , second piston chamber  62 , first sleeve lip  64 , second sleeve lip  66 , flexible connecting member  30 , connectors  70   a  and  70   b,  slide valve  26 , and means for controlling first piston chamber pressure  72 . Slide valve system  50  is now in an intermediate stage of loading, operating at some percentage of full capacity. The axial position of control piston  28  controls the axial position of slide valve  26  and therefore compressor capacity. Control piston  28  fits inside control piston sleeve  54  and is capable of reciprocating linearly along the vertical axis of sleeve  54 . Control piston  28  may be counter-bored from the underside to allow secure seating of biasing spring  56 . Control piston  28  is also sufficiently elongated in the axial direction to minimize torsional binding when the periphery of the head experiences asymmetric frictional forces. O-ring seal  58  prevents fluid leakage across control piston  28 , separating first piston chamber  60  from second piston chamber  62 . First sleeve lip  64  defines the limit of control piston  28  motion. When control piston  28  is pressed against first sleeve lip  64 , slide valve  26  is in the fully unloaded position. Second sleeve lip  66  is positioned at the base of control piston sleeve  54 . Second sleeve lip  66  is of dimensions sufficient to provide adequate retention of biasing spring  56  when control piston  28  is fully depressed. Biasing spring  56  is secured such that the lower end is pressed against second sleeve lip  66  and the upper end is seated in the underside of control piston  28 . Biasing spring  56  is designed to remain in compression even when released to its maximum length. Biasing spring  56  is at its maximum length when control piston  28  is pressed against first sleeve lip  64 , as shown in  FIG. 2A . 
         [0016]    Flexible connecting member  30  connects control piston  28  to slide valve  26 . Flexible connecting member  30  may comprise any non-rigid component capable of reliably transferring tensile loads, such as a wire rope or cable. Flexible connecting member  30  may be formed of any material, metallic or non-metallic, which has sufficient axial tensile strength and is capable of enduring cyclical loading. Flexible connecting member  30  is connected to control piston  28  by connector  70   a  and to slide valve  26  by connector  70   b.  Connectors  70   a  and  70   b  may include threaded connectors or any other means for securely attaching flexible connecting member  30 . 
         [0017]      FIG. 2B  shows slide valve system  50  in a partially loaded position. Slide valve system  50  is actuated by pressurizing first piston chamber  60  to overcome opposing force from biasing spring  56 . Biasing spring  56  is designed such that it overpowers ambient first piston chamber  60  pressure, pressing control piston  28  against first sleeve lip  64 . Means for controlling first piston chamber pressure  72  then increases pressure in first piston chamber  60 . Such means generally include at least one solenoid valve controlling the flow of a working fluid, such as oil. Solenoid valves allow for continuous, rather than stepwise control of chamber pressure. When pressure in first piston chamber  60  overcomes the force of biasing spring  56 , control piston  28  is driven axially towards male and female screw rotors  24 . This motion compresses biasing spring  56  and releases the tension on flexible connecting member  30 . Releasing tension on flexible connecting member  56  allows pressure in discharge chamber  48  to move slide valve  26  towards the partially loaded position shown in  FIG. 2B  and maintain flexible connecting member  30  in tension. 
         [0018]      FIG. 2C  shows slide valve system  50  in a fully loaded position. Flexible connecting member  30  remains in tension even with control piston  28  fully compressed. Slide valve  26  is located such that one axial end is always exposed to suction chamber  40  and the other end to discharge chamber  48 , acting as an effective seal between the two chambers. Due to the nature of screw compressors, discharge chamber  48  pressure is always higher than suction chamber  40  pressure. Pressure in discharge chamber  40  therefore biases slide valve  26  towards suction chamber  40 , maintaining tension in flexible connecting member  30  even when control piston  28  is driven to the fully loaded position. Biasing spring  56  and flexible connecting member  30  are sized so that when control piston  28  is in the fully loaded position as shown in  FIG. 2C , discharge pressure can drive slide valve  26  all the way to the position that allows rotary screw compressor  10  to operate at full design capacity. 
         [0019]    To unload compressor  10 , first piston chamber pressure control means  72  decreases first piston chamber  60  pressure until biasing spring  56  can force control piston  28  once again towards the unload position. Flexible connecting member  30  pulls slide valve  26  towards the unload position, and slide valve system  50  returns to the partially loaded state of  FIG. 2B  or the fully unloaded state of  FIG. 2A . 
         [0020]    A slide valve assembly often must reciprocate in multiple aligned bores. Slide valve assembly  50 , as shown in  FIGS. 2A ,  2 B, and  2 C, actuates in three separate mated bores: rotor case  14 , outlet case  16 , and slide case  18 . If control piston  28  and slide valve  26  were connected by a rigid rod as in prior art, the length of the assembly would require that the multiple bores be precisely aligned. Such precision requires expensive machining and manufacturing processes as well as costly alignment dowels. Flexible connecting member  30  allows system  50  to tolerate greater misalignment while retaining the ability to transfer control piston  28  motion in either direction to slide valve  26 . By increasing system tolerance of misalignment, slide valve system  50  decreases system cost. Because connecting member  30  is flexible, it does not translate misalignment into torsional forces on the control piston head and the slide valve. Therefore, the bores of slide valve assembly  50  need not be as precisely machined. This design also has the potential to increase useful life of screw compressors by decreasing wear in the slide valve assembly. Because the flexible member transfers only axial tensile forces, misalignment creates less friction between slide valve system components and the walls of the bores they reciprocate in. Furthermore, bushings designed to accommodate wear due to misalignment could be eliminated. Flexible connecting member  30  allows slide valve assembly  50  to tolerate greater misalignments between any number of multiple bores. Its use is not limited to the three mated bores shown in  FIGS. 2A ,  2 B and  2 C. 
         [0021]    Screw compressors commonly incorporate a slide valve system as a means to control compressor capacity. Such systems generally use rigid rods to connect the control piston to the slide valve, requiring precise and therefore expensive alignment of internal components. The present invention uses flexible connecting member  30  in place of a rigid rod. Controlling pressure in first piston chamber  60  causes control piston  28  and slide valve  26  move in unison in either direction, as if connected by a rigid member. In this manner, flexible connecting member  30  retains the functionality of a rigid connecting rod while tolerating greater misalignment. When integrated into a screw compressor, slide valve system  50  decreases both manufacturing costs and system wear and increases system reliability and lifetime. 
         [0022]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.