Abstract:
A compressor apparatus ( 20 ) has a housing ( 22 ) having first ( 53 ) and second ( 58 ) ports along a flow path. One or more working elements ( 26; 28 ) cooperate with the housing ( 22 ) to define a compression path between suction ( 60 ) and discharge ( 62 ) locations along the flow path. An unloading valve ( 100 ) has a valve element ( 102 ) having a range between a first condition and a second condition, the second condition being unloaded relative to the first condition. Means ( 120, 160 ) bias the valve element toward a third condition intermediate the first and second conditions.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to compressors. More particularly, the invention relates to refrigerant compressors. 
         [0002]    Screw-type compressors are commonly used in air conditioning and refrigeration applications. In such a compressor, intermeshed male and female lobed rotors or screws are rotated about their axes to pump the working fluid (refrigerant) from a low pressure inlet end to a high pressure outlet end. During rotation, sequential lobes of the male rotor serve as pistons driving refrigerant downstream and compressing it within the space between an adjacent pair of female rotor lobes and the housing. Likewise sequential lobes of the female rotor produce compression of refrigerant within a space between an adjacent pair of male rotor lobes and the housing. The interlobe spaces of the male and female rotors in which compression occurs form compression pockets (alternatively described as male and female portions of a common compression pocket joined at a mesh zone). In one implementation, the male rotor is coaxial with an electric driving motor and is supported by bearings on inlet and outlet sides of its lobed working portion. There may be multiple female rotors engaged to a given male rotor or vice versa. 
         [0003]    When one of the interlobe spaces is exposed to an inlet port, the refrigerant enters the space essentially at suction pressure. As the rotors continue to rotate, at some point during the rotation the space is no longer in communication with the inlet port and the flow of refrigerant to the space is cut off. After the inlet port is closed, the refrigerant is compressed as the rotors continue to rotate. At some point during the rotation, each space intersects the associated outlet port and the closed compression process terminates. The inlet port and the outlet port may each be radial, axial, or a hybrid combination of an axial port and a radial port. 
         [0004]    It is often desirable to temporarily reduce the refrigerant mass flow through the compressor by delaying the closing off of the inlet port (with or without a reduction in the compressor volume index) when full capacity operation is not required. Such unloading is often provided by a slide valve having a valve element with one or more portions whose positions (as the valve is translated) control the respective suction side closing and discharge side opening of the compression pockets. The primary effect of an unloading shift of the slide valve is to reduce the initial trapped suction volume (and hence compressor capacity); a reduction in volume index is a typical side effect. Exemplary slide valves are disclosed in U.S. Patent Application Publication No. 20040109782 A1 and U.S. Pat. Nos. 4,249,866 and 6,302,668. The desired degree to which a compressor may be unloaded is often application-specific. High degrees of unloading (e.g., down to an exemplary 15% of full load capacity) may be preferred for some applications. 
       SUMMARY OF THE INVENTION 
       [0005]    According to one aspect of the invention, a compressor has housing having first and second ports along a flow path. One or more working elements cooperate with the housing to define a compression path between suction and discharge locations along the flow path. An unloading valve has a valve element having a range between a first condition and a second condition, the second condition being unloaded relative to the first condition. Means bias the valve element toward a third condition intermediate the first and second conditions. 
         [0006]    In various implementations, the means may comprise a first and second springs. The springs may be on opposite sides of a piston engaged to the valve element. 
         [0007]    The means may be introduced in a reengineering of an existing compressor configuration and/or a remanufacturing of an existing compressor. The reengineering may be an iterative process performed on hardware or as a simulation/calculation. The reengineering or remanufacturing may comprise adding a second spring to act against an existing first spring of the baseline compressor. 
         [0008]    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  
         [0009]      FIG. 1  is a longitudinal sectional view of a compressor. 
           [0010]      FIG. 2  is a transverse sectional view of a discharge plenum of the compressor of  FIG. 1 , taken along line  2 - 2 . 
           [0011]      FIG. 3  is a sectional view of a slide valve assembly of the discharge plenum of  FIG. 2  in a fully loaded condition, taken along line  3 - 3 . 
           [0012]      FIG. 4  is a view of the slide valve of  FIG. 3  in a relatively unloaded condition. 
           [0013]      FIG. 5  is a view of the slide valve of  FIG. 3  in a neutral condition more loaded than the  FIG. 4  condition and less loaded than the  FIG. 3  condition. 
       
    
    
       [0014]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION  
       [0015]      FIG. 1  shows a compressor  20  having a housing assembly  22  containing a motor  24  driving rotors  26  and  28  having respective central longitudinal axes  500  and  502 . In the exemplary embodiment, the rotor  26  has a male lobed body or working portion  30  extending between a first end  31  and a second end  32 . The working portion  30  is enmeshed with a female lobed body or working portion  34  of the female rotor  28 . The working portion  34  has a first end  35  and a second end  36 . Each rotor includes shaft portions (e.g., stubs  39 ,  40 ,  41 , and  42  unitarily formed with the associated working portion) extending from the first and second ends of the associated working portion. Each of these shaft stubs is mounted to the housing by one or more bearing assemblies  44  for rotation about the associated rotor axis. 
         [0016]    In the exemplary embodiment, the motor is an electric motor having a rotor and a stator. One of the shaft stubs of one of the rotors  26  and  28  may be coupled to the motor&#39;s rotor so as to permit the motor to drive that rotor about its axis. When so driven in an operative first direction about the axis, the rotor drives the other rotor in an opposite second direction. The exemplary housing assembly  22  includes a rotor housing  48  having an upstream/inlet end face  49  approximately midway along the motor length and a downstream/discharge end face  50  essentially coplanar with the rotor body ends  32  and  36 . Many other configurations are possible. 
         [0017]    The exemplary housing assembly  22  further comprises a motor/inlet housing  52  having a compressor inlet/suction port  53  at an upstream end and having a downstream face  54  mounted to the rotor housing downstream face (e.g., by bolts through both housing pieces). The assembly  22  further includes an outlet/discharge housing  56  having an upstream face  57  mounted to the rotor housing downstream face and having an outlet/discharge port  58 . The exemplary rotor housing, motor/inlet housing, and outlet housing  56  may each be formed as castings subject to further finish machining. 
         [0018]    Surfaces of the housing assembly  22  combine with the enmeshed rotor bodies  30  and  34  to define inlet and outlet ports to compression pockets compressing and driving a refrigerant flow  504  from a suction (inlet) plenum  60  to a discharge (outlet) plenum  62  ( FIG. 2 ). A series of pairs of male and female compression pockets are formed by the housing assembly  22 , male rotor body  30  and female rotor body  34 . Each compression pocket is bounded by external surfaces of enmeshed rotors, by portions of cylindrical surfaces of male and female rotor bore surfaces in the rotor case and continuations thereof along a slide valve, and portions of face  57 . 
         [0019]      FIG. 2  shows further details of the exemplary flowpath at the outlet/discharge port  58 . A check valve  70  is provided having a valve element  72  mounted within a boss portion  74  of the outlet housing  56 . The exemplary valve element  72  is a front sealing poppet having a stem/shaft  76  unitarily formed with and extending downstream from a head  78  along a valve axis  520 . The head has a back/underside surface  80  engaging an upstream end of a compression bias spring  82  (e.g., a metallic coil). The downstream end of the spring engages an upstream-facing shoulder  84  of a bushing/guide  86 . The bushing/guide  86  may be unitarily formed with or mounted. relative to the housing and has a central bore  88  slidingly accommodating the stem for reciprocal movement between an open condition (not shown) and a closed condition of  FIG. 2 . The spring  82  biases the element  72  upstream toward the closed condition. In the closed condition, an annular peripheral seating portion  90  of the head upstream surface seats against an annular seat  92  at a downstream end of a port  94  from the discharge plenum. 
         [0020]    For capacity control/unloading, the compressor has a slide valve  100  having a valve element  102 . The valve element  102  has a portion  104  along the mesh zone between the rotors (i.e., along the high pressure cusp). The exemplary valve element has a first portion  106  ( FIG. 3 ) at the discharge plenum and a second portion  108  at the suction plenum. The valve element is shiftable to control compressor capacity to provide unloading. The exemplary valve is shifted via linear translation parallel to the rotor axes. 
         [0021]      FIG. 3  shows the valve element at an upstream-most position in its range of motion In this position, the compression pockets close relatively upstream and capacity is a relative maximum (e.g., at least 90% of a maximum displacement volume for the rotors, and often about 99%).  FIG. 4  shows the valve element shifted to a downstream-most position. Capacity is reduced in this unloaded condition (e.g., to a displacement volume typically less than 40% of the  FIG. 3  displacement volume or the maximum displacement volume, and often less than 30%). In the exemplary slide valve, shifts between the two positions are driven by a combination of spring force and fluid pressure. A main spring  120  biases the valve element from the loaded to the unloaded positions. In the exemplary valve, the spring  120  is a metal coil spring surrounding a shaft  122  coupling the valve element to a piston  124 . The piston is mounted within a bore (interior)  126  of a cylinder  128  formed in a slide case element  130  attached to the outlet case. The shaft passes through an aperture  132  in the outlet case. The spring is compressed between an underside  134  of the piston and the outlet case. A proximal portion  136  of the cylinder interior is in pressure-balancing fluid communication with the discharge plenum via clearance between the aperture and shaft. A headspace  138  is coupled via electronically-controlled solenoid valves  140  and  142  (shown schematically) to a high pressure fluid source  144  at or near discharge conditions (e.g., to an oil separator). A port  146  is schematically shown in the cylinder at the headspace at the end of a conduit network connecting the valves  140  and  142 . In an exemplary implementation, the portions of the conduit network may be formed within the castings of the housing components. The exemplary main spring  120  acts with a force that is relatively insignificant in comparison to the net force which may developed by fluid pressures. During periods of non-operation, when fluid pressures are balanced, the main spring  120  acts as is described below. 
         [0022]    The loaded position/condition of  FIG. 3  can be achieved by coupling the headspace  138  to the source  144  and isolating it from drain/sink  150  by appropriate control of valves  140  and  142 . The unloaded position/condition of  FIG. 4  can be achieved by coupling the headspace  138  to the drain/sink  150  and isolating it from source  144  by appropriate control of valves  140  and  142 . Intermediate (partly loaded) positions, not shown, can be achieved by alternating connection of headspace  138  to either the source  144  or the drain/sink  150  using appropriately chosen spans of time for connection to each, possibly in combination with isolating the headspace  138  from both source  144  and drain/sink  150  for an appropriately chosen span of time (e.g., via appropriate modulation techniques). 
         [0023]    For some applications it is desirable to have the unloaded position/condition of  FIG. 4  be such that during operation the refrigerant mass flow through the compressor is as low as an exemplary 15% of the mass flow achieved when the slide valve is in the loaded position/condition of  FIG. 3 . Said another way, the displacement volume of the position of  FIG. 4  would be an exemplary 15-20% of the displacement volume of the position of  FIG. 3 . The displacement volume slightly above 15% would achieve the 15% flow rate due to internal leakage. At some start-up conditions, low rates of refrigerant mass flow may result in discharge pressure may not rising in a relatively short period of time. Many systems depend on discharge pressure in source  144  to deliver oil for actuating slide valve  100  as previously described and for lubricating rotors and bearings. An inability to rapidly develop adequate discharge pressure to accomplish these roles may be viewed as having a negative impact on system performance or may be detrimental to compressor reliability. The problem may be particularly serious when the system is started after it has not operated for a long period of time. In such situations, residual lubrication on rotors and in bearing cavities may be substantially diluted, owing to the tendency of many refrigeration oils to absorb refrigerant over time and thereby become diluted. During operation, this dilution tendency is countered by elevated temperatures and by high speed motion of parts, both of which tend to move refrigerant out of solution with oil. During a start-up after a long shutdown period it is therefore desirable to quickly deliver lubricant to the compressor. 
         [0024]    To provide rapid start-up it is desirable that the valve position at start-up be more loaded than the unloaded position of  FIG. 4 . Preferably, the start-up position would correspond to a mass flow rate that is in the range of 25-35% of that of the loaded position of  FIG. 3 . A displacement volume might be 25-50% that of  FIG. 3 . 
         [0025]    According to the present invention, means are provided for biasing the slide valve from the unloaded end of its range ( FIG. 4 ) at least partially toward the loaded end of its range ( FIG. 3 ). An exemplary means includes a spring  160 . An exemplary spring  160  is a compression coil spring within the headspace  138 . The exemplary spring  160  extends from a proximal end portion  162  to a distal end portion  164 . The proximal end portion  162  is engaged to a boss  166  of the valve case  130  in the headspace to securely retain the spring  160 . The exemplary spring  160  has dimensions and a spring constant such that the distal end  164  engages the face  168  of the piston  124  in the  FIG. 4  unloaded condition but disengages at some point in the range of travel to the  FIG. 3  loaded condition. 
         [0026]    The spring  160  may come into play, for example, during a shutdown condition. For example, in a shutdown condition, pressures may equalize in the suction plenum  60 , discharge plenum  62 , cylinder interior proximal portion  136 , and headspace  138 . In such a condition, the spring  160  will act to shift the valve element slightly away from the  FIG. 4  unloaded condition (e.g., to an intermediate condition of  FIG. 5 ). At shutdown, when pressures on each side of the piston are equal, spring  160  acts on piston  124  in opposition to spring  120 , moving piston  124  and attached slide valve  100  to the position of  FIG. 5  which is slightly more loaded than that of  FIG. 3 . The length and spring constant of spring  160  are chosen, possibly in combination with those of spring  120 , so that the resulting position shown in  FIG. 5  corresponds to a displacement volume that results in discharge pressure rising rapidly enough to ensure quick delivery of lubricant to the compressor. The displacement volume corresponding to the position of  FIG. 5  would typically be in the range of 25-35% of that of the loaded position of  FIG. 3 . After start-up, once discharge pressure has risen, the unloaded position of  FIG. 4  can automatically be achieved because the action of pressures acting on faces  168  and  134  of piston  124  and on sides  106  and  108  of slide valve  100  generates sufficient force to overcome the force provided by spring  160 . Alternatively, if desired, the unloaded position of  FIG. 4  can be prevented by coupling headspace  138  to source  144  as previously described as adequate pressure in source  144  has now been developed to allow delivery of fluid to headspace  138 . 
         [0027]    The spring  160  may be added in a reengineering or remanufacturing from a baseline compressor or configuration thereof. In the baseline, the main spring  160  could have sufficient length so that start-up would be in the fully unloaded condition. The main spring  160  may be preserved or modified in the reengineering or remanufacturing. One modification would be to shorten it. 
         [0028]    Among many alternatives to a headspace compression spring  160  would be to have the main spring  120  be neutral at the  FIG. 5  valve condition and go into tension between the  FIG. 4  and  FIG. 5  valve conditions. Rather than a coil spring, the spring  160  could be another form of spring (e.g., a Belleville washer spring). In another embodiment, the spring  160  could be attached to piston  124  rather than to boss  166  of valve case  130 . 
         [0029]    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 configuration may particularly influence or dictate details of the implementation. Accordingly, other embodiments are within the scope of the following claims.