Abstract:
A compressor has first ( 26 ) and second ( 28; 30 ) enmeshed rotors rotating about first ( 500 ) and second ( 502; 504 ) axes to pump refrigerant to a discharge plenum ( 42 ). The compressor includes a muffler system ( 200 ) comprising a sound absorbing first element ( 232 ) and a sound absorbing second element ( 236 ). The second element at least partially surrounds the first element and defines a generally annular flow path portion ( 230 ) between the first element and the second element. A wall ( 250 ) at least partially surrounds the second element. A space ( 259 ) optionally containing a sound absorbing third element ( 261 ) surrounds the wall.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Benefit is claimed of U.S. patent application Ser. No. 60/670,499, filed Apr. 11, 2005, and entitled “Compressor Muffler”, the disclosure of which is incorporated by reference herein as if set forth at length. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to compressors. More particularly, the invention relates to sound and vibration suppression in screw-type compressors. 
         [0003]    In positive displacement compressors, discrete volumes of gas are: trapped at a suction pressure; compressed; and discharged at a discharge pressure. The trapping and discharge each may produce pressure pulsations and related noise generation. Accordingly, a well developed field exists in compressor sound suppression. 
         [0004]    One class of absorptive mufflers involves passing the refrigerant flow discharged from the compressor working elements through an annular space between inner and outer annular layers of sound-absorptive material (e.g., fiber batting). US Patent Application Pub. No. 2004/0065504 A1 discloses a basic such muffler and then improved versions having integral Helmholtz resonators formed within the inner layer. The disclosure of this &#39;504 publication is incorporated by reference herein as if set forth at length. 
         [0005]    International Applications PCT/US04/34946 and PCT/US05/03403 disclose further muffler configurations. Exemplary embodiments of these mufflers use inner and outer stacked rings of sound absorbing material. Exemplary ring material is expanded polypropylene beads (e.g., material known as porous expanded polypropylene (PEPP)). The disclosures of these applications are incorporated by reference herein as if set forth at length. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, one aspect of the invention involves a compressor having first and second enmeshed rotors rotating about first and second axes to pump refrigerant to a discharge plenum. The compressor includes a muffler system comprising a sound absorbing first element and a sound absorbing second element. The second element at least partially surrounds the first element and defines a generally annular flow path portion between the first element and the second element. A wall at least partially surrounds the second element. A sound-absorbing third element at least partially surrounds the wall within a muffler case. 
         [0007]    In various implementations, the wall may be essentially imperforate. The wall may have a thickness in excess of 0.5 cm. The thickness may be 0.8-1.2 cm. The wall may consist essentially of steel. The case may consist essentially of steel or cast iron. At least one of the first, second, and third elements may comprise a number of rings of porous expanded polypropylene. Along majorities of total longitudinal spans of the first and second elements, the first and second elements may have inboard and outboard surfaces that are essentially non-convergent and non-divergent. At least one foraminate metallic element may be between the first and second elements. A first such foraminate metallic element may be at an inboard boundary of the generally annular flowpath portion and a second may be at an outboard boundary. The third element may have a median thickness of 0.5-2.0 cm (more narrowly 1.0-1.5 cm). The second element may have a median thickness of 3.0-8.0 cm (more narrowly 4.0-6.0 cm). 
         [0008]    Such a muffler may be provided in a remanufacturing of an existing compressor or a reengineering of an existing configuration of the compressor. The initial/baseline compressor or configuration may lack at least one of the wall and the third element. 
         [0009]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a longitudinal sectional view of a compressor. 
           [0011]      FIG. 2  is a view of a case and muffler assembly for installation on the compressor of  FIG. 1 . 
           [0012]      FIG. 3  is an upstream end view of the assembly of  FIG. 2 . 
           [0013]      FIG. 4  is a downstream end view of the assembly of  FIG. 2 . 
           [0014]      FIG. 5  is a longitudinal sectional view of the muffler of the assembly of  FIG. 2 . 
           [0015]      FIG. 6  is a partially exploded view of the muffler of  FIG. 5 . 
       
    
    
       [0016]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0017]      FIG. 1  shows a compressor  20  as in PCT/US05/03403 having a housing or case assembly  22 . The exemplary compressor is a three-rotor, screw-type, hermetic compressor having rotors  26 ,  28 , and  30  with respective central longitudinal axes  500 ,  502 , and  504 . In the exemplary embodiment, the first rotor  26  is a male-lobed rotor driven by a coaxial electric motor  32  and, in turn, enmeshed with and driving the female-lobed rotors  28  and  30 . In the exemplary embodiment, the male rotor axis  500  also forms a central longitudinal axis of the compressor  20  as a whole. The rotor working portions are located within a rotor case segment  34  of the case assembly  22  and may be supported by bearings  36  and sealed by seals  38  engaging rotor shafts at each end of the associated rotor working portion. When driven by the motor  32 , the rotors pump and compress a working fluid (e.g., a refrigerant) along a flowpath from a suction plenum  40  to a discharge plenum  42 . The flowpath is divided along distinct compression pockets or compression paths defined by associated pairs of the rotors between the suction and discharge plenums. Thus, the flow splits in the suction plenum and merges in the discharge plenum. 
         [0018]    In the exemplary embodiment, the suction plenum  40  is located within an upstream end of the rotor case  34  and the discharge plenum is located generally within a discharge case  46  separated from the rotor case by a bearing case  48  and having a generally downstream-convergent interior surface  49 . In the exemplary embodiment, a bearing cover/retainer plate  50  is mounted to a downstream end of the bearing case  48  to retain the bearing stacks. Downstream of the discharge case  46  is a muffler  52  in a muffler case  54 . Downstream of the muffler  52  is an oil separator unit  60  having a case  62  containing a separator mesh  64 . An oil return conduit  66  extends from the housing  62  to return oil stopped by the mesh  64  to a lubrication system (not shown). An outlet plenum  68  having an outlet port  69  is downstream of the mesh  64 . 
         [0019]    The exemplary main muffler  52  includes annular inner and outer elements  70  and  72  separated by a generally annular space  74 . These elements may be formed of sound absorption material. In the exemplary embodiment, the inner element  70  is retained and separated from the space  74  by an inner foraminate sleeve  76  (e.g., wire mesh or perforated/expanded metal sheeting) and the outer element  72  is similarly separated and retained by an outer foraminate sleeve  78 . In the exemplary embodiment, the outer element  72  is encased within an outer sleeve  80  telescopically received within the housing  54 . The sleeves  80  and  78  are joined at upstream and downstream ends by annular plates  82  and  84 . In the exemplary embodiment, the upstream end of the sleeve  76  is closed by a circular plate  86  and the downstream end closed by an annular plate  90 . In the exemplary embodiment, a non-foraminate central core  94  (e.g., steel pipe) extends through the inner element  70  and protrudes beyond a downstream end thereof. At the upstream end of the main muffler, radially-extending connectors  96  join the circular plate  86  to the annular plate  82 . At the downstream end, radially-extending connectors  98  connect the annular plates  84  and  90  to hold the inner and outer elements concentrically spaced apart to maintain the annular space  74 . 
         [0020]    In operation, compressed gas flow exits the compression pockets of the screw rotors  26 ,  28 ,  30  and flows into the discharge plenum  42 . Upon exiting the compressor discharge plenum, the gas flows down the annular space  74 . Upon exiting the muffler, the gas flow, which typically has entrained oil droplets, flows through the oil separating mesh  64 . The mesh  64  captures any oil entrained in the gas and returns it to the oil management system by means of the conduit  66 . The gas leaves the oil separating mesh and enters the plenum  68  and exits the outlet  69  toward the condenser (not shown). 
         [0021]    It may be desirable to further limit the sound transmitted by the muffler case. One method is to thicken the muffler case. PCT/US04/34946 shows a relatively thick combined discharge and muffler case. Yet further sound limitation may be desired. According to the present invention further means are used to isolate the muffler case from the refrigerant flow.  FIG. 2  shows an improved case and muffler assembly  200 . The assembly  200  uses a case  202  that serves as a combined muffler case and discharge case (e.g., as in PCT/US04/34946), although muffler case-only implementations are also possible. 
         [0022]    The exemplary case  202  has an upstream mounting flange.  204  (also in  FIG. 3 ) for bolting to the bearing case. A generally circular cylindrical body or sidewall  206  is welded to and extends downstream from the flange  204 . A downstream end plate  208  (also in  FIG. 4 ) is welded to a downstream end of the body  206 . A periphery of the end plate includes an array of threaded holes for bolting to an upstream flange of the separator case/housing  62 . 
         [0023]    A muffler unit  210  ( FIG. 6 ) may be installed to the case  202  through the open upstream end of the body  206 . A structural core assembly  212  of the muffler includes an upstream metal end member  214 . The exemplary member  214  is approximately bat- or butterfly-shaped, having a central hub area  216  positioned to cover the male rotor bearing compartment and two wings  218  positioned to cover the female rotor bearing compartments while leaving the discharge ports open. 
         [0024]      FIG. 5  shows a central core pipe  220  having an upstream end welded to a downstream face of the member  214 . A foraminate centerbody sleeve  222  (e.g., metallic mesh or perforated sheet metal) has an upstream end welded to a downstream face of the member  214  at the periphery of the hub  216 . A metal frustoconical discharge plenum wall member  224  has a large upstream end welded to a downstream face of the member  214  slightly inboard of the wing periphery. A foraminate outer element liner  226  (e.g., metallic mesh or perforated sheet metal) has an upstream end welded to a small downstream end of the wall member  224 . 
         [0025]    An annular flow passageway  230  is defined between the liner  226  and the sleeve  222 . To form the inner element  232 , a stack of PEPP rings  234  is received in the annular space between the pipe  220  and sleeve  222 . To form the outer element  236 , a stack of PEPP rings  238 ,  240 , and  242  is accommodated over the liner  226 . The upstream ring  238  has a frustoconical upstream surface for engaging a downstream surface of the member  224  via a neoprene seal  244 . A plurality of rectangular sectioned rings  240  follow to a downstreammost ring  242 . 
         [0026]    An additional annular wall  250  may be placed over the outer element rings  238 ,  240 , and  242 . The exemplary wall  250  is a continuous, imperforate metallic (e.g., steel) tube/pipe intended to acoustically float relative to the case  202  (e.g., not being rigidly structurally connected to the case  202 . The exemplary floating is accommodated by allowing the upstream end  252  to rest against the seal  244 . An inboard surface  254  rests against the outer surface  256  of the outer muffler element. In the exemplary embodiment, the upstream end  252  is beveled to minimize contact pressure against the downstream surface of the seal  244 . 
         [0027]    The annular space  259  between the wall outboard surface  258  and the inboard surface  260  of the body may be filled by further sound-absorbing material  261  such as a stack of PEPP rings  262 ,  264 , and  266 . The upstreammost ring  262  may be rebated to accommodate the wings  218 . An isolation seal  270  may engage the downstream rim area  272  of the wall  250  and may have a portion extending outward between the downstreammost ring  266  and a downstreammost one of the rings  264  to prevent infiltration of refrigerant pulsations into the space  259 . Thermal isolation gaskets  274  and  276  ( FIG. 6 ) are inserted between the downstream ends of the inner and outer polypropylene rings, respectively, and the end plate  208  to protect the polypropylene material from heat caused by welding operations during final muffler assembly. 
         [0028]    When assembled, the muffler may be inserted into the case  202 . When fully inserted, an end portion of the pipe  220  is received in a central aperture  280  in the end plate  208 . The end plate further includes outlet apertures  282  aligned with the passageway to pass the refrigerant to the separator. 
         [0029]    The combined effect of the case sidewall  206  and the floating wall  250  is greater sound reduction than a single wall of the same mass or combined thickness (although not necessarily greater than a much more massive wall—e.g., whose thickness equals the combined wall thickness plus the thickness of the space  259 ). The particular relative dimensions may be engineered to provide maximal or other desired degree of sound/vibration suppression at one or more frequencies (e.g., the frequencies of compression pocket opening/closing at nominal operating speed or a range thereof). 
         [0030]    The floating wall may operate to keep noise from reaching the outer case and then propagating downstream through piping to the condenser (not shown, which may act as an acoustical radiator). Sound propagating radially outward through the outer element  236  is deflected by the floating wall  250  back toward the center of the muffler where it can be further attenuated. 
         [0031]    In the absence of the floating wall  250 , the sound would travel directly to the outer muffler case  54 . The sound would then either radiate into the room or travel downstream along the housing and discharge the pipe (not shown) to the condenser (not shown) and then radiate into the room. 
         [0032]    In alternative embodiments, the floating wall can be of a non-steel or non-metal heavy/dense material which can exist in a refrigerant environment. The floating wall may have multiple layers (e.g., as multiple floating walls). Other materials may be used for the inner, outer and exterior elements (e.g., glass fiber batting). 
         [0033]    The inventive system may be implemented in a remanufacturing of a given compressor system or a reengineering of a configuration thereof. One area of possibilities involve preserving an existing case. This may involve a new muffler whose annular flow space is shifted inward to provide room for the floating wall. Another area involves preserving an existing basic muffler element while expanding the case to accommodate the floating wall. In the reengineering of a baseline system having a thick-walled case, the case could be thinned with the floating wall making up for the thinning (e.g., to maintain or reduce an overall weight while not adversely affecting noise control). 
         [0034]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, in a reengineering or remanufacturing situation, details of the existing compressor may particularly influence or dictate details of the implementation. Accordingly, other embodiments are within the scope of the following claims.