BACKGROUND 
     The present invention relates generally to screw compressors. Screw compressors typically comprise a pair of counter-rotating, mating male and female screws that have an intermeshing plurality of lands and channels, respectively, that narrow from an inlet end to a discharge end such that an effluent working fluid or gas, or some other such working matter, is reduced in volume as it is pushed through the screws. The discharged working matter is released in pulses as each mating land and channel pushes a volume of the working matter out of the compressor. Each pulse comprises a burst of wave energy that propagates through the working matter and the screw compressor as the working matter decompresses. The screw compressors are typically turned by motors operating at elevated speeds such the wave pulsations are discharged at a high frequency. The pulsations not only produce vibration of the screw compressor, but also produce noise that is amplified by the working matter and the compressor itself. Such vibration is undesirable as it wears components of the compressor and produces additional noise as the compressor vibrates. Noise from the discharging working matter and vibrating compressor is undesirable as it results in loud operating environments. Previous attempts to counteract these problems have involved mufflers, padded mounts and clamps that are mounted external to the screw compressor. These solutions, however, rely on cumbersome add-ons that increase cost, weight and complexity of the screw compressor. Furthermore, these solutions do not address the underlying source of the noise and vibration, but only address the problem after it is produced. There is, therefore, a need for screw compressors having reduced effects from discharge pulsations. 
     SUMMARY 
     Exemplary embodiments of the invention include a slide valve for use in a screw compressor. The slide valve 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a partially cutaway perspective view of a screw compressor in which the present invention is used. 
         FIG. 2  shows a schematic diagram of the screw compressor of  FIG. 1  in which a slide valve having the pulsation damper of the present invention is used. 
         FIG. 3  shows a front view of the slide valve of  FIG. 2  nested between screw rotors of the screw compressor. 
         FIG. 4  shows a cross-sectional view of the slide valve of  FIG. 3 , in which a resonance chamber and damping tubes of the pulsation damper are shown. 
         FIG. 5  shows a top view of the slide valve of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a partially cutaway perspective view of screw compressor  10 , which compresses a working fluid or gas such as a refrigerant that is typically used in refrigeration or air conditioning systems. Screw compressor  10  includes rotor case  12 , outlet case  14 , slide case  16 , male screw rotor  18 , female screw rotor  20 , drive motor  22  and slide valve  23 . Male screw rotor  18  and female screw rotor  20  are disposed within rotor case  12  and include shafting and bearings such that they can be rotationally driven by drive motor  22 . For example, male screw rotor  18  includes shaft  24 A that extends axially through rotor case  12  and into motor  22  and rests on bearing  26 A, and shaft  24 B, which extends axially into outlet case  14  and rests in bearing  26 B. Refrigerant is introduced into rotor case  12  at suction port  28 , directed around motor  22  and into suction pocket  30  at the inlet of screw rotors  18  and  20 . Male screw rotor  18  and female screw rotor  20  include meshing grooves and lands that form helical flow paths having decreasing cross sectional areas as the grooves and lands extend from suction pocket  30 . Slide valve  23 , which is driven by a piston system disposed within slide case  16 , translates axially between rotors  18  and  20  to vary the volume of refrigerant compressed in the helical flow paths in order to regulate the discharge capacity of screw compressor  10 . Thus, the refrigerant is reduced in volume and pressurized as the refrigerant is directed into pressure pocket  32  before being discharged at pressure port  34  and released to, for example, a condenser or evaporator of a cooling system. Due to the multiple sets of meshing grooves and lands, the refrigerant is discharged into pressure pocket  32  in a series of high frequency pulsations, which effectuates undesirable noise and vibration. Slide valve  23  includes a pulsation damper that mitigates the pulsation effects of the discharged refrigerant. 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. 
       FIG. 2  shows a schematic diagram of screw compressor  10  of  FIG. 1 , having slide valve  23  of the present invention. Screw compressor  10  includes rotor case  12 , outlet case  14 , slide case  16 , female screw rotor  20 , drive motor  22 , slide valve  23 , control system  36 , slide rod  38 , piston  40 , cylinder  42  and spring assist  44 . Together, rotor case  12 , outlet case  14  and slide case  16  comprise a sealed flow path for directing refrigerant R through screw compressor  10 . Refrigerant R is directed into rotor case  12  at suction port  28 , and routed around motor  22  to suction pocket  30 . Male screw rotor  18  (not shown) and female screw rotor  20  compress refrigerant R from suction pocket  30  for discharge into pressure pocket  32 . Female screw rotor  20  includes screw channels, or grooves,  46 A- 46 D that mesh with mating lands or lobes on male screw rotor  18  to form a sealed, decreasing-volume flow path. The sealed flow path decreases in volume such that refrigerant R is pushed and compressed as it moves from suction pocket  30  to pressure pocket  32 . Accordingly, refrigerant R enters, for example, screw channel  46 A at inlet  26  having pressure P 1  and is discharged from the same screw channel  46 A at pressure pocket  32  having elevated pressure P 2 . Thus, each screw channel delivers a small volume of refrigerant R to pressure pocket  32 . As screw rotors  18  and  20  rotate, a series of discharge pulses of refrigerant R is released to pressure pocket  32 , which causes undesirable noise and vibration of screw compressor  10 . Slide valve  23 , which controls the capacity of screw compressor  10 , includes pulsation damper to reduce the noise and vibration effects of refrigerant R as it is discharged from screw rotors  18  and  20 . 
     Slide valve  23  is disposed within a slide recess within pressure pocket  32  and is configured to engage the crevice between male screw rotor  18  and female screw rotor  20 . As such, slide valve  23 , channels  46 A- 46 D of female rotor  20 , the lands of male rotor  18 , rotor case  12  and discharge case  14  define a sealed and pressurized flow path for refrigerant R. Slide valve  23  is connected with rod  38  and piston head  40  to axially traverse slide valve  23  within pressure pocket  32 . Slide valve  23  translates along screw rotor  20  to vary the volume of refrigerant R entrained within screw channels  46 A- 46 D. For example, when slide valve  23  is extended to the fully-loaded position (to the left in  FIG. 1 ) such that it contacts slide stop  48 , the output capacity of screw compressor  10  is increased such as to supply additional amounts of refrigerant R to a refrigerator or air conditioner. Slide valve  23  is moved toward pressure pocket  32  (to the right in  FIG. 1 ) to decrease the discharge capacity of screw compressor  10 . Rod  38  connects slide valve  23  to piston head  40 , which is disposed within piston cylinder  42 . Piston head  40  includes first pressure side  50 A, which is exposed to refrigerant R at pressure P 2 , and second pressure side  50 B, which is exposed to piston chamber  52  at pressure P 3 . Pressure P 3  is controlled by control system  36 , which comprises switches, valves, solenoids and the like to selectively provide pressure oil to piston chamber  52  to adjust the outflow of refrigerant R based on the loading (i.e. cooling demands) of the refrigerator or air conditioner. The pressure oil within piston chamber  52  exerts a force on second pressure side  50 B to move slide valve  23  toward slide stop  48  and the fully-loaded position. To move slide valve  23  away from slide stop  48 , pressure P 3  is reduced by removing pressure oil from piston chamber  52 . Spring assist  44  pushes piston head  40  to the right, which, through rod  38 , pulls slide valve  23 . Piston head  40  is also in contact with refrigerant R, which exerts pressure P 2  on first pressure side  50 A to pull slide valve  23  to the right. 
     Slide valve  23  is directly in contact with refrigerant R as refrigerant R flows through channels  46 A- 46 D of screw rotor  20  and out to pressure pocket  32 . Specifically, pressure face  54  of slide valve  23  is very near screw rotor  18  where refrigerant R is discharged into pressure pocket  32 . As such, the discharge pulsations of refrigerant R flow past pressure face  54 . Pressure face  54  includes pulsation damping channels  56  that permit refrigerant R to enter resonance chamber  58  such that the vibration and noise associated with the discharge of refrigerant R is attenuated. 
       FIG. 3  shows a front view of slide valve  23  of  FIG. 2 , in which pulsation damping channels  56 A- 56 E of pressure face  54  are shown. Slide valve  23  also includes actuation interface  60 , discharge pocket  62 , pressure discharge faces  64 A and  64 B, and outer surface  66 . Pressure discharge faces  64 A and  64 B of slide valve  23  together comprise a chevron-shaped head on slide valve  23  that seals the flow of refrigerant R along male screw rotor  18  and female screw rotor  20 . Slide valve  23  is connected to an actuation device, such as piston rod  38  and piston head  40  of  FIG. 2 , at interface  60  such that the position of slide valve  23  can be translated to regulate the discharge capacity of refrigerant R from screw rotors  18  and  20 . Refrigerant R is compressed in compression pocket  68 , which is formed between screw channels  46 A and  46 B of female screw rotor  20 , and screw lands  70 A and  70 B of male screw rotor  18 , respectively. Refrigerant R is released from compression pocket  68  in pulsed discharges into discharge pocket  62  as screw rotors  18  and  20  counter-rotate to open compression pocket  68  to slide valve  23 . The pulsed discharges of refrigerant R flow past pressure face  54  before being discharged from screw compressor  10  at pressure port  34  ( FIG. 1 ). Refrigerant R flows into damping channels  56 A- 56 E into internal resonance chamber  58  within slide valve  23 . In the embodiment shown, damping channels  56 A- 56 E are fitted with damping tubes  72 A- 72 E, which are explained in greater detail with respect to  FIG. 4 . 
       FIG. 4  shows a cross-sectional view of slide valve  23  of  FIG. 3 , in which damping tubes  72 A- 72 C and resonance chamber  58  of the pulsation damper of the present invention are shown. Damping tubes  72 A- 72 E are inserted into damping channels  56 A- 56 E, as is illustrated in  FIG. 4  with damping tube  72 C being inserted into damping cavity  56 C. Damping cavity  56 C comprises a hollowed out chamber formed in the interior of slide valve  23 . Slide valve  23  comprises a plurality of walls shaped to define a hollow canister having a chevron shaped head formed by pressure discharge faces  64 A and  64 B, and semi-cylindrical outer surface  66 , which are disposed between pressure face  54  and end cap  74 . As is shown in  FIGS. 4 and 5 , pressure discharge faces  64 A and  64 B come together to define apex  76 , which fits between screw rotors  18  and  20 . Thus, pressure discharge faces  64 A and  64 B are arcuate in shape. Pressure discharge faces  64 A and  64 B merge at the forward end of slide valve  23  to form discharge pocket  62 . Discharge pocket  62  comprises an arcuate, triangle shaped surface that forms both an axial and radial discharge port in pressure pocket  32  ( FIG. 1 ). Damping channels  56 A- 56 E are positioned generally below discharge pocket  62  such that refrigerant R after exiting discharge pocket  62 , flows past damping channels  56 A- 56 E. Discharge pocket  62  and pressure discharge faces  64 A and  64 B come together at pressure face  54 . Outer surface  66  wraps around pressure face  54  from first pressure discharge face  64 A to second pressure discharge face  64 B. End cap  74  is disposed between outer surface  66  and pressure discharge faces  64 A and  64 B to form resonance chamber  58 . Thus, resonance chamber  58  is enclosed within the walls of slide valve  23 . 
     Returning to  FIG. 4 , resonance chamber  58  is accessible within slide valve  23  through damping channels  56 A- 56 E. Damping channels  56 A- 56 E comprise bores extending through pressure face  54  such that refrigerant R is permitted to enter slide valve  23  to pressurize resonance chamber  58  to pressure P 2 . The lengths of damping channels  56 A- 56 E are determined by the thickness of pressure face  54 , but can be altered by inserting damping tubes  72 A- 72 E into damping channels  56 A- 56 E. In one embodiment, damping tubes  72 A- 72 E comprise stainless steel tubes press fit into damping channels  56 A- 56 E. The lengths and diameters of damping tubes  72 A- 72 E and damping channels  56 A- 56 E are selected to influence the acoustics and mechanics of refrigerant R as refrigerant R travels through channels  56 A- 56 E and tubes  72 A- 72 E. Specifically, the length and diameters of damping tubes  72 A- 72 E are selected to extract the maximum amount of energy from refrigerant R. 
     Refrigerant R is discharged from screw rotors  18  and  20  in pulses at regular intervals having a frequency dictated by the speed at which motor  22  drives screw rotors  18  and  20 . These pulses therefore produce undesirable sound waves that increase the noise generated by screw compressor  10 . The energy contained in these sound waves, however, can be used to do work to attenuate the propagation of the sound waves from screw compressor  10 . Slide valve  23  is configured to function as a Helmholtz resonator, which comprises a container of fluid or gas having a necked opening, such as is produced by refrigerant R, resonance chamber  58  and channels  56 A- 56 E. Refrigerant R fills resonance chamber  58  such that additional refrigerant attempting to enter resonance chamber  58  must compress the volume of refrigerant R already present within resonance chamber  58 . Thus, a pulsed wave of refrigerant R attempting to enter resonance chamber  58 , compresses refrigerant R until the crest of the wave is reached. Then, the pressurized refrigerant R within resonance chamber  58  will push back as the wave dissipates to the trough. As the pulsed wave propagates through crests and waves, the pressurized refrigerant R within resonance chamber  58  continues to compress and decompress, thus extracting energy from refrigerant R discharged from screw rotors  18  and  20 . The energy extraction reduces the amplitude of the pulsation wave, thereby reducing noise and vibration generated by the pulsed discharges of refrigerant R. The position of slide valve  23  is, however, unaffected by the wave pulsations of refrigerant R such that the performance of slide valve  23  is unaffected. The position of slide valve  23  is maintained constant through the rigid connection with piston rod  38  and piston head  40 , which is maintained by pressure P 3 . 
     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.
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.