Abstract:
A screw compressor ( 10 ) comprises a housing ( 12, 14, 16 ), a slide valve assembly ( 23 ) and a pulsation damper. The housing ( 12, 14, 16 ) receives a supply of working matter from a pair of intermeshing screw rotors ( 18, 20 ), and comprises a slide recess ( 51 ), a pressure pocket ( 32 ), and a piston cylinder ( 54 ). The slide valve assembly ( 23 ) regulates the capacity of the screw compressor ( 10 ), and comprises a slide valve ( 36 ) axially movable within the slide recess ( 51 ) and the pressure pocket ( 32 ), a piston head ( 40 ) axially movable within the piston cylinder ( 54 ), and a piston shaft ( 38 ) connecting the slide valve ( 36 ) with the piston head ( 40 ). The pulsation damper comprises a flange ( 58 ) for separating the pressure pocket ( 32 ) from the piston cylinder ( 54 ), a bore ( 60 ) for receiving the piston rod ( 38 ), and a damping channel ( 46 A) extending through the flange ( 58 ).

Description:
BACKGROUND 
       [0001]    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 leaves the screws. The screw compressors are typically turned, by motors operating at speeds such that 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. 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 do not address the underlying source of the noise and vibration and only provide after-the-fact countermeasures. In addition to adding cost and weight, such solutions provide only limited noise reduction and do not prevent wear on internal screw compressor components. Other solutions have proposed acoustic barriers that prevent pulsation damage to screw compressor components, but do not attenuate screw compressor noise or vibration. There is, therefore, a need for screw compressors having reduced effects from discharge pulsations. 
       SUMMARY 
       [0002]    Exemplary embodiments of the invention include a screw compressor comprising a housing, a slide valve assembly and a pulsation damper. The housing receives a supply of working matter from a pair of intermeshing screw rotors, and comprises a slide recess, a pressure pocket, and a piston cylinder. The slide valve assembly regulates the capacity of the screw compressor, and comprises a slide valve axially movable within the slide recess and the pressure pocket, a piston head axially movable within the piston cylinder, and a piston shaft connecting the slide valve with the piston head. The pulsation damper comprises a flange for separating the pressure pocket from the piston cylinder, a bore for receiving the piston rod, and a damping channel extending through the flange for damping pressure pulsations in the working matter discharged from the pair of intermeshing screw rotors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  shows a partially cutaway perspective view of a screw compressor in which the pulsation damper of the present invention is used. 
           [0004]      FIG. 2  shows a schematic diagram of the screw compressor of  FIG. 1  showing an outlet case incorporating the pulsation damper. 
           [0005]      FIG. 3  shows a partially cutaway perspective view of the outlet case of  FIG. 2  showing a plurality of damping channels comprising the pulsation damper. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]      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 assembly  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 (which 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 . Thus, the refrigerant is reduced in volume and pressurized as the refrigerant is directed into discharge pocket  32  by screw rotors  18  and  20 , before being discharged at pressure port  34  and released to, for example, a condenser or evaporator of a cooling system. Slide valve assembly  23 , which includes slide valve  36 , piston rod  38 , piston head  40  and spring assist  42 , regulates the discharge capacity of screw compressor  10 . In particular, piston head  38 , piston rod  40  and spring assist  42 , through a control system, translate slide valve  36  axially between rotors  18  and  20  to vary the volume of refrigerant compressed in the helical flow paths. Due to typically high speeds that motor  22  drives screw rotors  18  and  20 , the multiple sets of meshing grooves and lands comprising the helical flow paths discharge the refrigerant into pressure pocket  32  in a series of high frequency pulsations, which effectuates undesirable noise and vibration. Outlet case  14  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. 
         [0007]      FIG. 2  shows a schematic diagram of screw compressor  10  of  FIG. 1 , having pulsation damping means of the present invention. In particular, outlet case  14  includes damping channels  46 A and  46 B that attenuate the pulsation effects of refrigerant R within screw compressor  10 . Screw compressor  10  also includes rotor case  12 , slide case  16 , female screw rotor  20 , drive motor  22 , slide valve assembly  23  (including slide valve  36 , piston rod  38 , piston head  40  and spring assist  42 ) and control system  48 . Rotor case  12  includes slide recess  51 , slide stop  52  and recirculation passage  53 . Slide case  16  includes piston cylinder  54 , and outlet case  14  includes rod flange  58 . 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 . Refrigerant R from suction pocket  30  is compressed by male screw rotor  18  (not shown) and female screw rotor  20  and discharged into pressure pocket  32 . Female screw rotor  20  includes screw channels, or grooves,  50 A- 50 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 discharge pocket  32 . Accordingly, refrigerant R enters, for example, screw channel  50 A at suction pocket  30  having pressure P 1  and is discharged from the same screw channel  50 A at discharge pocket  32  having elevated pressure P 2 . Thus, each screw channel delivers a small volume of refrigerant R to discharge pocket  32 . As screw rotors  18  and  20  rotate, a series of discharge pulses of refrigerant R is released to discharge pocket  32 , which causes undesirable noise and vibration of screw compressor  10 . Outlet case  14  includes damping channels  46 A and  46 B, which act as pulsation dampers to reduce the noise and vibration effects of refrigerant R as it is discharged from screw rotors  18  and  20 . 
         [0008]    Outlet case  14 , which includes discharge pocket  32 , is disposed between rotor case  12  and slide case  16  such that it receives the high pressure side of screw rotors  18  and  20  at a first end, and piston rod  38  of slide assembly  23  at a second end. Slide valve  36  of slide assembly  23  is positioned within slide recess  51  of rotor case  12  such that it is disposed between male screw rotor  18  and female screw rotor  20 . Slide valve  36  is connected with piston rod  38  and piston head  40  such that slide valve  36  can be axially withdrawn from slide recess  51  and extended into pressure pocket  32  to control the amount of pressurized refrigerant R entrained within screw channels  50 A- 50 D. For example, slide valve  36  can be extended to the fully-loaded position (to the left in  FIG. 1 ) such that it abuts slide stop  52  and contacts the entire length of screw rotors  18  and  20 . Thus, the capacity of screw compressor  10  is maximized by maximizing the amount of refrigerant R compressed in the lands and grooves of screw rotors  18  and  20 . From the fully-loaded position, slide valve  36  is moved toward discharge pocket  32  (to the right in  FIG. 1 ) to open recirculation passage  53 , decreasing the discharge capacity of screw compressor  10 . 
         [0009]    Piston rod  38  extends through rod flange  58  to connect slide valve  36  within rotor case  12  to piston head  40  disposed within piston cylinder  54  of slide case  16 . Piston head  40  includes first pressure side  56 A, which is exposed to refrigerant R at pressure P 2 , and second pressure side  56 B, which is exposed to control oil at pressure P 3 . Pressure P 2  is dictated by refrigerant R and screw rotors  18  and  20 , while pressure P 3  is regulated by control system  48 . Based on the loading (i.e. cooling demands) of the refrigerator or air conditioner to which screw compressor  10  is connected, control system  48 , which comprises switches, valves, solenoids and the like, selectively provides control oil to piston cylinder  54 . Control oil is admitted into piston cylinder  48  to increase pressure P 3  to exert a force on second pressure side  56 B to move slide valve  36  toward slide stop  52  within slide recess  51 . Pressure P 3  is reduced by removing control oil from piston cylinder  54  such that slide valve  36  can be withdrawn from slide recess  51 . Spring assist  42  pushes on first pressure side  56 A to assist in withdrawing slide valve  36  from slide recess  51 . Piston head  40  is also in contact with refrigerant R, which exerts pressure P 2  on first pressure side  56 A to push piston head  40  away from rod flange  58 . Refrigerant R, is admitted into piston cylinder  54  through damping channels  46 A and  46 B disposed within rod flange  58 . Damping channels  46 A and  46 B, piston cylinder  54  and rod flange  58  are configured to attenuate vibration and noise associated with the discharge of refrigerant R from screw rotors  18  and  20 . Specifically, damping channels  46 A and  46 B act in concert with piston cylinder  54  to provide a Helmholtz resonator to absorb energy from the discharged pulses of refrigerant R. 
         [0010]      FIG. 3  shows a partially cutaway perspective view of slide valve assembly  23  of  FIG. 2 , in which damping channels  46 A- 46 C of rod flange  58  are shown. Slide valve assembly  23  also includes slide valve  36 , piston rod  38 , piston head  40  and spring assist  42 , which is omitted in  FIG. 3  for clarity. Slide valve assembly  23  extends axially through rotor case  12 , outlet case  14  and slide case  16  along an actuation path defined by slide recess  51 , pressure pocket  32  and piston cylinder  54 . Outlet case  14  is positioned within screw compressor  10  such that first end A connects with rotor case  12 , and second end B connects with slide case  16 . Slide valve  36  extends from slide recess  51  in rotor case  12  where it is disposed between rotor screws  18  and  20 , and into pressure pocket  32  within outlet case  14 . Piston rod  38  extends axially from slide valve  36  through central bore  60  in rod flange  58  of outlet case  14 , and into piston cylinder  54  of slide case  16  where rod  38  connects with piston head  40 . 
         [0011]    Rod flange  58  comprises a collar positioned on second end B of outlet case  14  such that central bore  60  axially aligns with slide recess  51  (in which slide valve  36  translates within rotor case  12 ) and piston cylinder  54  (in which piston head  40  translates within slide case  16 ). In the embodiment shown, rod flange  58  is integrally cast or formed with outlet case  14  along second end B. Rod flange  58  separates piston cylinder  54  from slide recess  51  and pressure pocket  32  to form two separate chambers for refrigerant R. Rod flange  58  is provided with seal or bearing ring  62  and is attached to rod flange  58  with snap rings  64 A and  64 B, which are disposed within grooves in ring  62 . In one embodiment, ring  62  comprises a seal and prevents refrigerant R from entering piston cylinder  54  between piston rod  38  and rod flange  58  at bore  60 . In another embodiment, seal ring  62  comprises a bearing that assists in sliding of piston rod  38  though rod flange  58  as well as performing sealing functions. Damping channels  46 A- 46 C, however, permit refrigerant R to enter piston cylinder  54  within slide case  16 . 
         [0012]    Slide case  16  comprises piston cylinder  54 , which forms an annular extension of outlet case  14  to accommodate piston rod  38  and piston head  40 . Piston head  40  divides piston cylinder  54  into discharge side  54 A and control side  54 B. Piston head  40  includes seal  65  to prevent flow of control oil and refrigerant R past piston head  40 . Piston cylinder  54 , therefore, comprises a sealed canister for actuating piston head  40 . Discharge side  54 A of this sealed canister, however, also acts as a resonance chamber, that along with damping channels  46 A- 46 C, absorb some of the vibrational and acoustical effects of the pulsed discharges of refrigerant R. 
         [0013]    As explained above, slide valve assembly  23  is connected with control system  48  ( FIG. 2 ) to actuate the position of slide valve  36  along rotor screws  18  and  20 . Slide valve  36  is translated to regulate the discharge capacity of refrigerant R from screw rotors  18  and  20 . Control system  48  regulates flow of the control oil into control side  54 B of piston cylinder  54  to vary pressure P 3 . Refrigerant R flows into damping channels  46 A- 46 C into piston cylinder  54  within slide case  16  to pressurize discharge side  54 A of piston cylinder  54  to pressure P 2 . Refrigerant R is compressed to pressure P 2  between screw rotors  18  and released in pulsed discharges into pressure pocket  32  at slide valve  36  as screw rotors  18  and  20  counter-rotate to open and close the helical flow paths formed by the lobes and channels. The pulsed discharges of refrigerant R flow past rod flange  58  before being discharged from screw compressor  10  at pressure port  34  ( FIG. 1 ). Damping channels  46 A- 46 C extend through rod flange  58  and permit refrigerant R to enter and pressurize piston cylinder  54  to pressure P 2 . 
         [0014]    In the embodiment shown in  FIG. 3 , rod flange  50  includes four damping channels: damping channels  46 A- 46 C, each disposed in a quadrant of rod flange  50 , and a fourth damping channel omitted due to the section taken out of  FIG. 3 . Damping channels  46 A- 46 C comprise hollowed-out chambers extending through rod flange  58  of outlet case  14 . The lengths of damping channels  46 A- 46 C are determined by the thickness of rod flange  58 , but can be altered by inserting hollow damping tubes  66 A- 66 C into damping channels  46 A- 46 C. Damping tubes  66 A- 66 C are inserted into damping channels  46 A- 46 C such that they extend into piston cylinder  54  and into pressure pocket  32 . As is illustrated in  FIG. 3 , damping tube  66 A is inserted into damping channel  46 A, and damping tube  66 B is inserted into damping channel  46 B. In the embodiment shown, damping tubes  66 A- 66 C each having the same length and the same diameter. In one embodiment, damping tubes  66 A- 660  comprise stainless steel tubes press fit into damping channels  46 A- 46 C. The specific quantity and geometry of damping channels  46 A- 46 C and damping tubes  66 A- 66 C, however, is selected to dampen the acoustic and vibrational pulsation effects of refrigerant R, and can thus vary depending on the specific design parameters of screw compressor  10 . Specifically, the number, length and diameter of damping tubes  66 A- 66 C are selected to extract the maximum amount of energy from refrigerant R as refrigerant R travels through tubes  66 A- 66 C into the resonance chamber formed by discharge side  54 A. 
         [0015]    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 . Outlet case  14  and slide case  16  are configured to function as a Helmholtz resonator, which comprises a container of fluid or gas having a necked opening, such as is produced by discharge side  54 A, refrigerant R and channels  46 A- 46 C. A Helmholtz resonator utilizes the spring-like compressibility of the fluid or gas to extract energy from a wave oscillating at a given frequency. Refrigerant R fills discharge side  54 A such that additional refrigerant attempting to enter discharge side  54 A through channels  46 A- 46 C must compress the volume of refrigerant R already present within discharge side  54 A. Thus, a pulsed wave of refrigerant R attempting to enter discharge side  54 A, compresses refrigerant R until the crest of the wave is reached. Then, the pressurized refrigerant R within discharge side  54 A will push back as the wave dissipates to the trough. As the pulsed wave propagates through crests and waves, the pressurized refrigerant R within discharge side  54 A 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. 
         [0016]    A Helmholtz resonator extracts the maximum amount of energy from the fluid or gas when the frequency of the wave matches the natural or resonance frequency of the Helmholtz resonator. Thus, the resonance frequency of the Helmholtz resonator produced by discharge side  54 A and damping channels  46 A- 46 C can be configured to match that of the pulsation discharges of refrigerant R as produced by motor  22 . Equation (1) illustrates the resonance frequency of an elongate tube used in a Helmholtz resonator, where f R  is the resonance frequency of the tube, v is the speed of sound in the medium filling the tube, A 0  is the area of the tube, L is the length of the tube and V 0  is the volume of resonance chamber. 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     R 
                   
                   = 
                   
                     
                       v 
                       
                         2 
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                         π 
                       
                     
                      
                     
                       
                         
                           A 
                           0 
                         
                         
                           
                             V 
                             0 
                           
                            
                           L 
                         
                       
                     
                   
                 
               
               
                 
                   equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
         [0017]    For the present invention, the tube or “necked opening” of the Helmholtz resonator comprises the aggregate of tubes  66 A- 66 C. Applying this equation to the embodiment of the present invention shown in  FIG. 3 , f R  is the resonance frequency of tubes  66 A- 66 C, v is the speed of sound in refrigerant R, A 0  is the total cross-sectional area of tubes  66 A- 66 C, L is the length of one of tubes  66 A- 66 C, and V 0  is the volume of discharge side  54 A. The dimensions of tubes  66 A- 66 C are selected such that the frequency of the discharge pulses of refrigerant R from screw rotors  18  and  20  at a given capacity matches the resonance frequency of the tubes. For example, in one embodiment of the invention, screw compressor  10  is configured to operate at 3,600 RPM at full load. Volume V 0 , therefore, comprises the volume of discharge side  54 A when piston head  40  is furthest away from rod flange  50  (all the way to the left in  FIG. 3 ), and frequency f R  is 60 Hz. Thus, the areas and lengths of tubes  66 A- 66 C are selected based on other design requirements, such as dimensional constraints of rod flange  58  and slide case  16 . Additionally, the number of tubes can be selected based on specific design considerations. In the embodiment shown, tubes  66 A- 66 C have the same lengths and diameters. Thus, screw compressor  10  is provided with a pulsation damper that is configured for damping pulsation effects of refrigerant R at a specific operating condition. However, in other embodiments, tubes  66 A- 66 C can have different geometries, such as different lengths and/or different diameters, such that the pulsation damper is tuned to one specific resonance frequency, or can attenuate vibration and acoustic effects over a range of frequencies. In other embodiments of the invention, rod flange  58  comprises a circular disk or annular ring that can be bolted or otherwise secured to piston cylinder  54  within slide case  16  such that pulsation dampers configured for different resonance frequencies can be interchangeably installed into screw compressor  10 . 
         [0018]    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.