Abstract:
A valve for varying volume ratio in a screw compressor to balance a compression pocket pressure and a discharge pressure in the screw compressor comprises a valve body and a reed valve. The valve body defines a duct and an auxiliary port. The duct includes an open end in communication with a discharge chamber of the compressor and thereby the discharge pressure. The auxiliary port extends from a rotor bore of the compressor to the duct and provides fluid communication therebetween for communicating the compression pocket pressure to the duct. The reed valve is disposed within the duct for regulating fluid flow between the compression pocket and the duct. The reed valve is operable via a pressure differential between the compression pocket pressure and the discharge pressure.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to screw compressors and more particularly to screw compressors with means for varying volume ratio. 
       BACKGROUND 
       [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 with 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 the rotor pair and exits near the opposite end through a discharge chamber. Suction and discharge ports may be located radially or axially with respect to the rotors. Initially, the compression pocket is in communication with the suction port. As the rotors turn, the compression pocket rotates past the suction port and becomes sealed between the male and female rotor lobes and the solid wall of the rotor bore. The enclosed pocket becomes smaller as it is translated axially downstream, compressing the fluid within. Finally, the compression pocket rotates into communication with the discharge chamber and the compressed fluid exits. 
         [0003]    Volume V b  is defined as the pocket volume at the instant the enclosed pocket first loses communication with the suction port, trapping fluid at pressure P b . Volume V f  is defined as the pocket volume just before the enclosed pocket first comes into communication with the discharge port and contains compressed fluid at pressure P f . Compressor volume ratio (V i ) is defined by the ratio of V b /V f . It is well known that volume ratio is an important feature of screw compressor design and operation. Its relevance to screw compressor design is described in references such as  Industrial Compressors: Theory and Equipment  (Peter A. O&#39;Neill, author; Butterworth Heinemann, publisher; 1993; ISBN 0750608706; pages 306-309) and 1996  ASHRAE Systems and Equipment Handbook  (Robert A. Parsons, editor; American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., publisher; 1996; ISBN 1-883413-34-6; pages 34.18-34.19). As is known, compressor discharge pressure P d  is determined by system operating conditions, while, pressure P f  in compression pocket just before it comes into communication with discharge port is determined by volume ratio V i  in combination with pressure P b  of gas in pocket volume V b . 
         [0004]    It is known that compression efficiency is optimum when P f  is equal to P d . If P f  is less than P d , the pocket fluid is under-compressed and discharge chamber fluid rushes into the pocket when they come into communication. If P f  is greater than P d , the pocket fluid is over-compressed and the compressed fluid rushes out of the pocket into the discharge chamber when pocket and discharge chamber come into communication. Both under-compression and over-compression are known to be inefficient. Compressor vibration and fluid pulsation amplitudes are also higher when under-compression and over-compression occur, resulting in higher levels of undesirable sound. 
         [0005]    Compressors that have a single built-in volume ratio will only operate without over-compression and under-compression at some operating conditions, not all. In these cases, the volume ratio is typically chosen to be optimum for a condition where compressor efficiency and sound levels are rated per industry standards. However, systems that use screw compressors, such as refrigeration systems, typically must operate over a wide range of conditions. For such systems, high energy efficiency and low sound levels are often important qualities. Considerable inventive effort has therefore been dedicated to developing systems with variable volume ratio so that over-compression and under-compression can be avoided, or at least diminished, at more operating conditions. 
         [0006]    Prior art methods of achieving variable volume ratio control include: the use of an axially movable slide valve and sensing and actuating means, as exemplified in U.S. Pat. Nos. 3,088,659, 3,936,239, Re. 29,283, 4,362,472, 4,842,501, 5,018,948 and 5,411,387; the use of an axially movable slide valve and slide stop and sensing and actuating means in combination, as exemplified in U.S. Pat. Nos. 4,516,914 and 4,678,406; the use of radial lift valves and sensing and actuating means, as exemplified in U.S. Pat. Nos. 4,737,082, 4,878,818, 5,108,269 and 3,151,806 and 5,044,909; the use of lift valves in discharge end wall with sensing and actuating means, as exemplified in U.S. Pat. No. 4,946,362; the use of pressure-actuated lift valves in discharge end wall, either self-acting or with sensing and actuating means, as exemplified in U.S. Pat. Nos. 2,519,913 and 5,052,901 and European Patent 0175354; the use of a discharge end wall slide valve and sensing and actuating means as exemplified in U.S. Pat. No. 4,457,681. Other prior art means of achieving some degree of variable volume ratio control include those exemplified in U.S. Pat. Nos. 4,234,296 and 4,455,131. 
         [0007]    In addition to differences of geometric form, these prior art methods can be distinguished by whether the variable volume control valve mechanism is actively controlled or self-acting. In actively controlled mechanisms, complicated sensing and actuating means are required to actuate the valve. In self-acting mechanisms, the valves are actuated directly by differential action of pressures P f  and P d . In the latter case, achieving some volume ratio variation without the need of independent sensing and actuating means such as sensors, control logic, actuating lines and servo or solenoid control valves is desirable, considering cost. 
       SUMMARY 
       [0008]    A valve for varying volume ratio in a screw compressor to balance a compression pocket pressure and a discharge pressure in the screw compressor comprises a valve body and a reed valve. The valve body defines a duct and an auxiliary port. The duct includes an open end in communication with a discharge chamber of the compressor and thereby the discharge pressure. The auxiliary port extends from a rotor bore of the compressor to the duct and provides fluid communication therebetween for communicating the compression pocket pressure to the duct. The reed valve is disposed within the duct for regulating fluid flow between the compression pocket and the duct. The reed valve is operable via a pressure differential between the compression pocket pressure and the discharge pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective cutaway view of a rotary screw compressor in which an automatic variable volume ratio valve of the present invention is used. 
           [0010]      FIG. 2  is a side sectional view of the screw compressor of  FIG. 1  showing an automatic variable volume ratio valve. 
           [0011]      FIG. 3  is a front sectional view of the screw compressor of  FIG. 1  showing an automatic variable volume ratio valve positioned between mating screw rotors. 
           [0012]      FIG. 4A  is a top view of a rotor housing having the automatic variable volume ratio valve of  FIGS. 2 and 3 . 
           [0013]      FIG. 4B  is a perspective view of a multi-fingered reed valve for use in the automatic variable volume ratio valve of  FIG. 4A . 
           [0014]      FIG. 5A  shows an end view of the automatic variable volume ratio valve of  FIG. 3  in which fingers of reed valves are closed. 
           [0015]      FIG. 5B  shows an end view of the automatic variable volume ratio valve of  FIG. 5B  in which the fingers of the reed valves are open. 
           [0016]      FIGS. 6A-6D  illustrate decreasing compression pocket volume as screw rotors translate a compression pocket past radial auxiliary ports of the automatic variable volume ratio valve. 
           [0017]      FIG. 7  is a side sectional view of a screw compressor having a slide valve including an automatic variable volume ratio valve of the present invention. 
           [0018]      FIG. 8  is a front cross sectional view of the screw compressor of  FIG. 7  showing the slide valve including an automatic variable volume ratio valve positioned between mating screw rotors. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a perspective cutaway view of rotary screw compressor  10  in which an automatic variable volume ratio valve of the present invention is used.  FIG. 2 , which is discussed concurrently with  FIG. 1 , is a side sectional view of screw compressor  10  taken at section  2 - 2  of  FIG. 1  showing automatic variable volume ratio valve  12  in hidden lines. Compressor  10  includes motor case  14 , rotor case  16 , outlet case  18 , rotor shaft  20 , motor stator  22 , motor rotor  24 , male screw rotor  26   a  and female screw rotor  26   b . In  FIG. 1 , motor case  14 , rotor case  16 , outlet case  18 , stator  22  and rotor  24  are partially cut-away to show shaft  20  and rotors  26   a  and  26   b . In  FIG. 2 , compressor  10  is sectioned at approximately the cusp between rotors  26   a  and  26   b , and rotor shaft  20 , motor rotor  24  and male screw rotor  26   a  are not shown for clarity. Motor case  14  includes intake port  28 , and rotor case  16  includes automatic variable volume ratio valve  12  and rotor bores  30 , in which rotors  26   a  and  26   b  rotate. Rotors  26   a  and  26   b  include screw rotor lobes  32 , and valve  12  includes pressure port or duct  34  and radial auxiliary ports  36 . Outlet case  18  includes discharge chamber  38 . Motor case  14  and outlet case  18  are fastened to rotor case  16  to form a housing in which shaft  20 , stator  22 , rotor  24  and screw rotors  26   a  and  26   b  are sealed such that a working fluid or gas, such as from a refrigerant, can be conducted between intake port  28  and discharge chamber  38 . 
         [0020]    As shown in  FIG. 2 , working fluid  40  at low pressure enters screw compressor  10  at intake port  28 , travels through motor case  14  and rotor case  16  and into rotor bores  30 . Within rotor bores  30 , low pressure working fluid  40  enters a compression pocket adjacent rotor  26   b  and rotor  26   a  ( FIG. 1 ) formed between screw rotor lobes  32  and walls of screw rotor bores  30 . Motor rotor  24  rotates male screw rotor  26   a  ( FIG. 1 ) and, by virtue of geared engagement, female screw rotor  26   b , reducing the volume of the compression pocket and compressing fluid  40  as the pocket translates towards outlet case  18  between lobes  32 . High pressure working fluid  40  is discharged from the pressure pocket into discharge chamber  38  through discharge port  41 . Discharge chamber  38  is in open communication with high pressure fluid  40  and the system discharge pressure in which compressor  10  is used. Therefore, pressure in discharge chamber  38  reflects changes in the operation of compressor  10 . Automatic variable volume ratio valve  12  of the present invention optimizes compression efficiency by balancing the pressure in the discharge pocket just before it comes into communication with discharge chamber  38  and the pressure in discharge chamber  38  over a range of operating conditions for compressor  10 . 
         [0021]      FIG. 3  is a front sectional view of screw compressor  10  taken at section  3 - 3  of  FIG. 1  showing a front surface of rotor case  16  and sections through support shafts for screw rotors  26   a  and  26   b . Automatic variable volume ratio valve  12  is integrated into rotor case  16  between male rotor  26   a  and female rotor  26   b . Thus, a portion of rotor case  16  comprises the body of valve  12 . Valve  12  includes male-side pressure port  34   a , female-side pressure port  34   b , male-side auxiliary port  36   a , female-side auxiliary port  36   b , male-side reed valve  42   a  and female-side reed valve  42   b . Male-side face  44   a  and female-side face  44   b  are part of male and female screw rotor bores  30 , and discharge end face  46  comprises a portion of rotor case  16 . Screw rotor bores  30  meet male-side face  44   a  and female-side face  44   b  to form bores in which male rotor  26   a  and female rotor  26   b  rotate, respectively. Male screw rotor  26   a  and female screw rotor  26   b  form compression pocket  48  between rotor lobes  32 , screw rotor bores  30  and faces  44   a  and  44   b . For parts of the compression process, either a suction or discharge end wall may also form part of the boundary of the compression pocket, as is discussed with respect to  FIGS. 6A-6D . 
         [0022]    Discharge end face  46  in rotor case  16  forms a discharge port through which fluid exits the compression pocket and enters discharge chamber  38  during the compression process. Valve  12  is formed by machining discharge end face  46 , pressure ports  34   a  and  34   b  and auxiliary ports  36   a  and  36   b  directly into rotor case  16 . In other embodiments, as shown in  FIGS. 7 and 8 , valve  12  can be incorporated into a slide valve that moves within rotor case  16 . Male-side and female-side pressure ports  34   a  and  34   b  comprise holes bored axially into discharge end face  46  parallel to the major axis of valve  12  and the axes of rotors  26   a  and  26   b . Auxiliary ports  36   a  and  36   b  comprise holes bored radially into axial surfaces of valve  12  along faces  44   a  and  44   b , respectively, perpendicular to pressure ports  34   a  and  34   b . Auxiliary ports  36   a  and  36   b  provide communication between compression pocket  48  and male and female side pressure bores  34   a  and  34   b , if permitted by deflection of reed valves  42   a  and  42   b . Pressure ports  34   a  and  34   b  comprise ducts that outlet to discharge chamber  38  ( FIGS. 1 and 2 ) to provide a shortcut or shunt around the full length of rotors  26   a  and  26   b . Reed valves  42   a  and  42   b  are inserted into pressure ports  34   a  and  34   b  to meter flow of compressed working fluid from compression pocket  48  to discharge chamber  38 . Working fluid from rotors  26   a  and  26   b  enters auxiliary ports  36   a  and  36   b  as the fluid is pressurized between lobes  32  of screw rotors  26   a  and  26   b . Reed valves  42   a  and  42   b  open at a threshold pressure to permit pressurized fluid to escape lobes  32  and enter pressure ports  34   a  and  34   b  to flow into discharge chamber  38 . The geometry of valve  12 , as well as the number and position of bores  34   a  and  34   b  and bores  36   a  and  36   b  can be varied to provide additional control over the flow of refrigerant through valve  12 . 
         [0023]      FIG. 4A  is a top view of a portion of rotor case  16  showing automatic variable volume ratio valve  12  of  FIGS. 2 and 3 . Valve  12  includes male-side pressure port  34   a , female-side pressure port  34   b , male-side auxiliary ports  36   a ,  36   c ,  36   e  and  36   g , female-side auxiliary ports  36   b ,  36   d ,  36   f  and  36   h , male-side reed valve  42   a , female-side reed valve  42   b , male-side face  44   a , female-side face  44   b  and discharge end face  46 . In the embodiment shown, faces  44   a  and  44   b  are each provided with four radial ports. In other embodiments, fewer or greater numbers of radial ports may be used. 
         [0024]    Pressure ports  34   a  and  34   b  comprise blind-end bores that extend into discharge end face  46  such that refrigerant is not permitted to pass axially through valve  12  or rotor case  16 . Radial auxiliary ports  36   a - 36   h  extend into faces  44   a  and  44   b , respectively, only so far as to intersect pressure ports  34   a  and  34   b . Pressure ports  34   a  and  34   b  are preferably positioned relative to faces  44   a  and  44   b  so as to minimize the volumes of fluid trapped in auxiliary ports  36   a - 36   h  between faces  44   a  and  44   b  and reed valves  42   a  and  42   b . It is desirable to minimize the trapped volumes to minimize deleterious effects on compressor efficiency. Specifically, fluid or gas trapped within these volumes escapes compression within compression pocket  48  as lobes  32  pass over them. Thus, pressure ports  34   a  and  34   b  are positioned close to faces  44   a  and  44   b  to minimize the volume of ports  36   a - 36   h . Reed valves  42   a  and  42   b , visible in phantom, are inserted into and secured in each of pressure ports  34   a  and  34   b.    
         [0025]      FIG. 4B  is a perspective view of multi-fingered reed valve  42   a  for use in automatic variable volume ratio valve  12  of  FIG. 4A . Reed valve  42   b  is identical to reed valve  42   a , differing only in orientation when assembled with valve  12 . Reed valve  42   a , as shown in  FIG. 4B , includes reed valve fingers  52   a - 52   d  and reed valve root member  54 . Reed valve root member  54  comprises a single, continuous body that connects with each individual reed valve finger  52   a - 52   d . Reed valve  42   a  is aligned and sized such that each individual reed finger completely covers a single radial auxiliary port  36   a ,  36   c ,  36   e  and  36   g  when the valve is inserted into pressure port  34   a . For valve  12  shown in  FIG. 4A , reed valve finger  52   a  covers radial  36   g , reed valve finger  52   b  covers auxiliary port  36   e , and so on. Reed valve fingers  52   a - 52   d  are capable of undergoing repetitive loading cycles in bending. Reed valve  42   a  is cylindrically configured so as to match the circumference and shape of pressure port  34   a  when installed as shown on  FIG. 3 . 
         [0026]    In practice, to avoid a loose fit for any assemblies that might result from slight variations in manufactured size in port  34   a  and reed valve  42   a , the nominal cross-section size of reed valve  42   a  prior to assembly with port  34   a  may be slightly larger than the nominal diameter of port  34   a  to provide slight interference for most assemblies. The amount of interference is chosen in combination with parameters that affect the stiffness of reed valve fingers  52   a - 52   d  to minimize any deleterious impact on the intended function. For example, valve fingers  52   a - 52   d  are configured to have stiffnesses such that fingers  52   a - 52   d  can be deflected by pressures generated within compressor  10 . 
         [0027]      FIGS. 5A and 5B  show axial end views of discharge end face  46  in rotor case  16  that illustrate the pressure differentials within compressor  10  that automatically operate reed valves  42   a  and  42   b . Valve  12  is formed in rotor case  16  of compressor  10  between rotors  26   a  and  26   b  ( FIG. 3 ) such that compression pocket  48  asserts pocket pressure P P  against faces  44   a  and  44   b , and discharge chamber exerts discharge pressure P D  against discharge end face  46 . Compression pocket pressure P P  extends through auxiliary ports  36   a  and  36   b  to act on outer surfaces of fingers  52   d  and  52   a  of reed valves  42   a  and  42   b . Discharge chamber pressure P D  extends through pressure ports  34   a  and  34   b  to act on inner surfaces of fingers  52   d  and  52   a  of reed valves  42   a  and  42   b . If compression pocket pressure P P  is less than discharge chamber pressure P D , then the discharge chamber pressure maintains the fingers pressed against the walls of pressure ports  34   a  and  34   b . Thus, compression pocket  48  remains sealed and working fluid continues to flow across faces  44   a  and  44   b . If discharge pressure P D  is less than compression pocket pressure P P , then the pocket pressure forces the fingers away from the walls of pressure ports  34   a  and  34   b . Thus, the seal of compression pocket  48  is broken and working fluid is permitted to travel through pressure ports  34   a  and  34   b  to reach discharge chamber  38 , after being partially compressed. As discharge pressure P D  changes under different operating conditions of compressor  10 , the position along valve  12  at which pocket pressure P P  equals discharge pressure P D  also changes. Thus, different fingers of reed valves  42   a  and  42   b  will deflect, as is illustrated in  FIGS. 6A-6D . 
         [0028]      FIGS. 6A-6D  illustrate a compression cycle and the method by which valve  12  automatically varies screw compressor volume ratio.  FIGS. 6A-6D  show portions of rotor bores  30  with successive compression pockets between screw rotor lobes  32  superposed. Valve  12  is shown in hidden lines beneath rotors  26   a  and  26   b . Screw rotors  26   a  and  26   b  are positioned between end walls  55   a ,  55   b  and  55   c , which assist in forming compression pocket  48  for portions of the compression process. For example, end walls  55   a  and  55   b  form a discharge port that regulates how long compression pocket  48  remains sealed, and end wall  55   c  comprises an end face seal that seals compression pocket  48  at the beginning of the compression process. Valve  12  is positioned between rotors  26   a  and  26   b  such that pressure ports  34   a  and  34   b  open to discharge port  41 . Auxiliary ports  36   a - 36   h , which are also shown in hidden lines, extend from pressure ports  34   a  and  34   b  and open through faces  44   a  and  44   b  to rotors  26   a  and  26   b  ( FIG. 3 ), respectively. In  FIG. 6A , the shaded area represents compression pocket  48  after having just been sealed by rotation of rotors  26   a  and  26   b . The initial volume of compression pocket  48  is designated as V b  and the initial pressure within pocket  48  is designated P b . As discussed in greater detail below with respect to  FIGS. 6B-6D , rotors  26   a  and  26   b  rotate to translate compression pocket  48  towards discharge port  41 , decreasing volume V b  and causing a corresponding increase in pressure P b . 
         [0029]    A conventional compressor would continue to compress the working fluid until compression pocket  48  comes into communication with discharge chamber  38 , as shown in  FIG. 6D , without, however, passing compression pocket  48  over valve  12  or auxiliary ports  36   a - 36   h . The shaded area represents the compression pocket volume at the moment it communicates with discharge port  41 . This volume is designated as V f . The volume ratio (V i ) is then V b /V f . If compression pocket pressure P f  of volume V f  is equal to discharge chamber pressure P D , no over or under compression occurs and the compressor is operating at peak efficiency. Discharge chamber pressure P D , however, often does not remain constant due to changes in system operating conditions. Therefore, mismatches between final compression pocket pressure P f  and discharge chamber pressure P D  typically occur. Valve  12  of the present invention provides a means for balancing final compression pocket pressure P f  and discharge chamber pressure P D  to facilitate operation of compressor  10  at peak efficiency. 
         [0030]      FIG. 6B  shows an intermediate stage of compression in which compression pocket  48  translates toward discharge port  41 . The volume of compression pocket  48  is reduced to intermediate volume V 2 , which is less than V b  but greater than V f . The pressure of compression pocket  48  rises to intermediate pressure P 2 , which is greater than P b  due to compression. In  FIG. 6B , compression pocket  48  has translated far enough along the axis of rotors  26   a  and  26   b  to contact auxiliary ports  36   h  and  36   g . At this point, the volume ratio is V b /V 2 . 
         [0031]      FIG. 6C  shows compression pocket  48  progressing further towards discharge port  41 . Compression pocket  48 , now at volume V 3  and with pressure P 3 , which is greater than P 2  due to further compression, is in contact with subsequent auxiliary ports  36   c - 36   f . If pressure P 3  is greater than discharge pressure P D , as is determined by the operating conditions of compressor  10 , fingers of reed valves  42   a  and  42   b  within pressure ports  34   a  and  34   b  will deflect, similar to those illustrated in  FIG. 5B . Reed valve fingers  52   b  and  52   c  ( FIG. 4B ) of valves  42   a  and  42   b  are deflected inward under the forces caused by the pressure differential between P 3  and P D , allowing some working fluid to exit compression pocket  48  by entering pressure ports  34   a  and  34   b  and then pass to discharge port  41 . As a result of this escape of fluid from compression pocket  48 , pocket pressure P P  of compression pocket  48  will not substantially exceed discharge pressure P D  so long as auxiliary ports  36  are sized large enough to not substantially restrict the flow rate of escaping fluid. 
         [0032]    As compression pocket  48  progresses towards discharge chamber  38 , the pressure within pocket  48  continues to build such that the action of successive auxiliary ports  36   a  and  36   b  and reed valve fingers  52   a  will be similar to that just described. Thus, fluid continues to discharge through pressure ports  34   a  and  34   b  at pressures not substantially exceeding discharge pressure P D . As a result, when compression pocket  48  finally connects with discharge port  41  as shown in  FIG. 6D , compression pocket pressure P P  will not substantially exceed discharge pressure P D  and refrigerant will also pass through port  41  at a pressure near P D . 
         [0033]    At almost any point during the compression cycle, working fluid can escape compression pocket  48  if compression pocket pressure P P  exceeds discharge chamber pressure P D . In this manner, the rotary screw compressor automatically varies V i  so as to discharge working fluid at a pressure closely matched to discharge chamber pressure. The specific point along valve  12  at which pocket pressure P P  exceeds discharge pressure P D  depends on the operating conditions of compressor  10 . The embodiments shown have depicted multi-fingered reed valves with four fingers and corresponding radial ports for exemplary purposes. In other embodiments, one, two, three or even more than four fingers may be used, depending on the compressor in which it is intended to be used and the intended application of such compressor. 
         [0034]    The automatic volume ratio variation means described herein acts only under conditions of over-compression, when compression pocket  48  pressure P P  exceeds discharge pressure P D . It may be useful for reducing occurrences of under-compression, when compression pocket  48  reaches discharge chamber  38  before pocket pressure P P  reaches discharge chamber pressure P D . For example, valve  12  can be used in combination with means for setting, e.g. increasing, the built-in or base V, of compressor  12 , such as end walls  55   a  and  55   b , slide valves, or other means to delay discharge of compressed fluid from the rotors as are known in the art. As such, the compression pocket pressure P P  will then reach the level of discharge pressure P D  before compression pocket  48  is connected to discharge chamber  38  for a greater portion of the operating conditions it is subjected to. As a result, the automatic volume ratio variation means described herein, such as valve  12 , will be activated for a greater portion of the operating conditions and provide its intended benefit. 
         [0035]    Other aspects of the present invention may also be varied to enhance the capability of valve  12  to match pocket pressure P P  with discharge pressure P D . For example, the embodiments shown have depicted reed valves on both male rotor side and female side of cusp for exemplary purposes. In other embodiments of the invention, however, placement of a single reed valve on only the male-side or only the female-side may offer acceptable automatic V i  variation at lower cost in compressors designed for some applications. Also, the embodiments shown have depicted uniformly spaced reed fingers and corresponding uniformly spaced radial ports. In other embodiments of the invention, however, non-uniformly spaced reed fingers and radial ports may be used for some applications. In other embodiments of the invention, the automatically variable V i  system may also be incorporated into compressors having a capacity control slide valve, as is shown in  FIGS. 7-8 . 
         [0036]      FIG. 7  is a side sectional view of screw compressor  56  having a slide valve  58  including an automatic variable volume ratio valve  60  of the present invention. Compressor  56  includes components similar to those of compressor  10  of  FIG. 1-FIG .  3 , with like components labeled accordingly. For example, compressor  56  includes motor case  14 , rotor case  16 , outlet case  18 , motor stator  22 , female screw rotor  26   b , intake port  28 , rotor bores  30 , lobes  32  and discharge chamber  38 . Rotor shaft  20 , motor rotor  24  and male screw rotor  26   a  are omitted for clarity. Compressor  56  also includes slide case  62  in which slide valve  58  reciprocates. Slide valve  58  (which is not shown in cross section for clarity) includes valve body  64 , in which valve  60  is placed, piston rod  66 , piston head  68  and biasing spring  70 . Slide valve  58  operates as is known in the art to vary the capacity of compressor  56 . Specifically, actuation means  72  directs a hydraulic fluid into piston chamber  74  to adjust the axial position of piston head  68 , which through piston rod  66  adjusts the axial position of valve body  64  relative to male and female rotors  26   a  and  26   b . As such, the length along which valve body  64  engages lobes  32  varies to adjust the amount of fluid compressed between rotors  26   a  and  26   b  and rotor bores  30 . Valve body  64  includes pressure port  76  and radial ports  78  similar to that of valve  12  of  FIGS. 2-6D . 
         [0037]      FIG. 8  is a front sectional view of screw compressor  56  of  FIG. 7  showing a front surface of rotor case  16  and sections through slide valve  58  and support shafts for screw rotors  26   a  and  26   b . Slide valve  58  includes automatic variable volume ratio valve  60  and is positioned between screw rotors  26   a  and  26   b . Valve body  64  comprises arcuate pressure surfaces to mate with screw rotors  26   a  and  26   b . Valve body  64  also includes a partially cylindrical bottom side for sliding along rotor housing  16  when actuated by piston rod  66  and piston head  68 . Valve  60  includes pressure ports  76   a  and  76   b , which comprise axial bores that extend discharge chamber  38  into valve  60 . Auxiliary ports  78   a  and  78   b  extend radially into the arcuate pressure surfaces to connect pressure pocket  48  with pressure ports  76   a  and  76   b . Reed valves  80   a  and  80   b  are inserted into pressure ports  76   a  and  76   b  to seal pressure ports  76   a  and  76   b  from auxiliary ports  78   a  and  78   b . Reed valves  80   a  and  80   b  permit fluid from pressure pocket  48  to escape to discharge chamber  38  when pressure inside pressure pocket  48  exceeds pressure within discharge chamber  38 . 
         [0038]    In any embodiment of the invention, a valve is provided for automatically varying compressor volume ratio in a rotary screw compressor, closely matching final compression pocket pressure to system discharge pressure without using electronic feedback control. At least one axial pressure port is positioned in a screw rotor housing or into a slide valve body so that the pressure port is adjacent a pressure pocket between screw rotors. The pressure port communicates the pressure pocket with system discharge pressure. A radial auxiliary port, or a series of auxiliary ports, extends from a portion of the screw rotor housing in contact with the compression pocket to the pressure port. A reed valve having a reed finger for each auxiliary port is inserted into each pressure port. The reed valve is cylindrically configured, sized and positioned such that the reed valve fits securely in the pressure port and individual reed fingers completely cover individual radial auxiliary ports. 
         [0039]    As the compression pocket travels down the axial length of the screw rotors, it sequentially contacts the radial auxiliary ports. As the compression pocket passes over a radial auxiliary port, compression pocket pressure within the auxiliary port acts on the topside of the reed valve finger covering the auxiliary port, while discharge pressure acts on the finger&#39;s underside within the pressure port. If the compression pocket pressure is greater than discharge pressure, the reed finger deflects, allowing working fluid to pass out of the compression pocket. Working fluid then flows through the axial pressure port into a discharge chamber of the compressor. The number and location of both radial ports and axial ports can be altered to match a variety of operating conditions. In this manner, the screw compressor is able to automatically vary the volume ratio so as to nearly match pocket pressure at the time of fluid exit more closely to discharge pressure. 
         [0040]    The combination of radial auxiliary ports and axial pressure ports having fitted reed valves is sufficient to largely prevent over-compression. Under-compression may be prevented over a wide range of operating conditions by configuring the screw compressor system to have a relatively high built in V i  such that fluid rarely reaches the discharge port under-compressed. 
         [0041]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.