Abstract:
A variable capacity screw compressor for use in a refrigeration system is provided. Compressed refrigerant gas from the compressor is expelled into a discharge port in fluid communication with the refrigeration circuit. The volume associated with the discharge port can be periodically varied, allowing the efficiency of the compressor to be varied periodically. The discharge port volume includes a penetration that houses a movable member or plug that permits the volume to be periodically varied. This movable member is accessible from the exterior of the compressor housing to adjust the position of the movable member within the discharge port volume. The movable member may be adjusted to a full open position in which the discharge port volume is maximized, to a full closed position in which the discharge port volume is minimized, and to any position between full open and full closed.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/579,687 filed on Dec. 23, 2011 and U.S. Provisional Application No. 61/548,304 filed on Oct. 18, 2011. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This application is directed to screw compressors. More particularly, the present invention is directed to screw compressors having a variable volume capacity. 
       BACKGROUND OF THE INVENTION 
       [0003]    In positive-displacement compressors, capacity control may be obtained by both speed modulation and suction throttling to reduce the volume of vapor or gas drawn into a compressor. Capacity control for a compressor can provide continuous modulation from 100% capacity to less than 10% capacity, good part-load efficiency, unloaded starting, and unchanged reliability. In some positive-displacement compressors, capacity can also be controlled by a slide valve employed in the compressor. The slide valve can be operated to remove a portion of the vapor from the compression chamber of the compressor, thereby controlling the capacity of the compressor. Besides the slide valve, other mechanical devices, such as slot valves and lift valves, may be employed in positive-displacement compressors to control capacity. Adjustments to capacity control valves or variable displacement mechanisms can meet the demands of the system. In a refrigeration system, capacity can be regulated based upon a temperature set point for the space being cooled. In other systems where the compressor is processing gas, capacity may be regulated to fully load the torque generator or prime mover (turbine or engine drive) for the compressor. However, all of the currently available methods are expensive and add to the initial cost of investment in the equipment. 
         [0004]    In chiller applications where economy is desired both in the initial cost of the system and in operation of the system, a variable volume ratio application is desired. Volume ratio (V i ,) is the ratio of the volume of a groove at the start of compression to the discharge volume when the discharge port begins to open. Hence, the volume ratio is determined by the size and shape of the discharge port, since the groove volume is a fixed volume at the start of compression. 
         [0005]    For maximum efficiency, the pressure generated within the grooves during compression should exactly equal the pressure in the discharge volume when the discharge port begins to open. If this is not the case, either overcompression or undercompression occurs, both resulting in internal efficiency losses. Furthermore, overcompression can harm the compressor. Such losses increase power consumption and noise, while reducing efficiency. Volume ratio selection should be made according to operating conditions. 
         [0006]    If the operating conditions of the system seldom change, it is possible to specify a fixed-volume ratio compressor that will provide good efficiency. Because overcompression can damage a compressor, a compressor is designed so that it does not frequently operate in an overcompression mode. As a result, such a compressor is designed to run at maximum compression under the most severe operating conditions, meaning that such a compressor runs in undercompression modes as service conditions dictate operation in maximum compression mode. What is needed is a system that permits adjustments to the volume ratio that changes the volume ratio depending on the conditions that the compressor experiences. This will allow the compressor volume to be adjusted to change the volume, and hence the volume ratio, as operating conditions change, allowing the compressor to operate at maximum efficiency. A variable volume ratio screw compressor that meets one or more of the above needs would be desirable in the art. 
         [0007]    A variable volume ratio screw compressor that meets one or more of the above needs would be desirable in the art. 
       SUMMARY OF THE INVENTION 
       [0008]    A screw compressor for use in a refrigeration system is provided. The screw compressor has a variable capacity and includes a motor connected to a power source. A control panel controls operation of the compressor, including the motor and power source. The screw compressor has a variable volume capability. The screw compressor comprises a pair of meshing helical lobed rotors rotating within a housing, the rotating rotors being driven by a drive shaft connected to the motor. The housing encloses the rotors or screws, which operate in a working chamber located within the housing. 
         [0009]    Refrigerant gas enters the compressor inlet from the suction or low pressure side of the refrigerant circuit through an inlet port when the rotors are arranged in the chamber to maximum length. The space between the lobes of the rotors, the interlobe region, is filled with refrigerant and the inlet port is closed. The refrigerant is compressed between the rotors in the interlobe region as the rotors rotate with respect to one another, compressing the refrigerant gas and raising its pressure. The highly compressed gas is ejected from the rotor interlobe region as a high pressure gas, which is expelled into a discharge port in fluid communication with the refrigeration circuit. 
         [0010]    The volume ratio is a measure of the efficiency of operation of a positive displacement compressor. The present invention permits efficiency of operation of a compressor to change with climate, which may be naturally variable with seasons. The volume ratio is determined by the size and shape of the discharge port. The volume associated with the discharge port, referred to as the discharge port volume, can be periodically varied in the present invention, also allowing the efficiency of the compressor to be varied periodically. The efficiency of a variable capacity screw compressor is determined by the position of a plug in the discharge port and by ambient temperature. 
         [0011]    The discharge port volume includes a penetration that houses a movable member or plug. The movable plug or member allows the discharge port volume to be varied. This movable member is accessible from the exterior of the compressor housing when the compressor is shut down. The movable member can be accessed from the exterior of the housing to adjust position of the movable member within the discharge port volume. The movable member may be adjusted to a full open position in which the discharge port volume is maximized, to a full closed position in which the discharge port volume is minimized and to any intermediate position between full open and full closed. By adjusting the position of the movable member from one position to another, the volume in the discharge port can be modified, thereby modifying the volume ratio of the compressor, even as all other operating parameters remain constant. 
         [0012]    An advantage of a screw compressor of variable volume having a volume adjustment mechanism is that a machine can be manufactured and the volume ratio periodically can be adjusted to maximize efficiency based on the climate of the area in which it is used with minimal disassembly of the compressor after shipment. 
         [0013]    Another advantage of a screw compressor having a volume adjustment mechanism is that a machine can be procured based on a maximum volume ratio for the most severe conditions, but the volume ratio can be adjusted based on seasonal variations by using the volume adjustment feature with minimal disassembly of the compressor so that undercompression can be avoided when conditions are not severe. 
         [0014]    In one embodiment, a variable capacity screw compressor, having a volume ratio adjustment mechanism, can be manufactured and the volume ratio periodically adjusted to maximize efficiency based on the climate of the area in which the compressor is used with minimal disassembly of the compressor. 
         [0015]    In another embodiment, a variable capacity screw compressor having a volume ratio adjustment mechanism can be procured based on a maximum volume ratio for the most severe conditions. The volume ratio can be adjusted based on seasonal variations so that undercompression is avoided when conditions are not severe. 
         [0016]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  depicts a refrigeration cycle. 
           [0018]      FIG. 2  schematically illustrates a typical screw compressor used in the refrigeration cycle of claim  1 . 
           [0019]      FIG. 3  depicts the screw compressor of  FIG. 2  showing the interior components of the screw compressor through the housing, the view further showing the discharge port with a plug in the discharge port. 
           [0020]      FIG. 4  depicts a partial horizontal cross sectional view of the screw compressor through its center, viewed from above, providing a detail view of the discharge port, viewed from above, with the movable plug positioned in the discharge port to provide it with maximum volume. 
           [0021]      FIG. 5  depicts a partial horizontal cross sectional view of the screw compressor through its center, viewed from above, providing a detail view of the discharge port, viewed from above, with the movable plug positioned in the discharge port to provide it with minimum volume. 
           [0022]      FIG. 6  depicts a worker inserting a tool into the screw compressor to modify the location of the movable plug within the discharge port. 
           [0023]      FIG. 7  depicts the compressor in cross section, with the cover removed, showing the tool partially inserted into the compressor. 
           [0024]      FIG. 8  depicts the plug of  FIG. 7  in cross section. 
           [0025]      FIG. 9  is a perspective view of the plug of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Provided is an exemplary variable capacity screw compressor for use in a refrigeration system. A volume ratio adjustment mechanism allows the volume ratio to be adjusted based on seasonal variations thus improving efficiency so that undercompression is avoided. 
         [0027]    Referring to  FIG. 1 , an exemplary refrigeration cycle is shown. The refrigeration cycle is a closed loop system  21  in which refrigerant, the working fluid, is compressed by a positive displacement compressor  23  that increases the pressure of the refrigerant gas. Compressor  23  is driven by a power source  10  that is controlled by a control panel  22 . The high pressure refrigerant from compressor  23  is discharged through a compressor discharge port which is in fluid communication with a condenser  25  that condenses the high pressure gas into a pressurized fluid. In one embodiment, compressor  23  is a screw type. Alternatively, compressor  23  is a reciprocating, rotary, scroll, or centrifugal type compressor. 
         [0028]    Condenser  25  is in heat exchange communication with a first heat transfer medium that removes heat of condensation resulting from the change of state of refrigerant from hot, high pressure gas to liquid. This heat transfer medium may be the atmosphere (air of forced air) or a liquid, preferably water. The various ways of removing this heat are well know and do not contribute to the inventive nature of this invention. The condensed fluid from condenser  25  is in fluid communication with an expansion valve  31  that expands at least some of the pressurized fluid into a gas as it flows within closed loop system  21 . The closed loop system  21  from the discharge port  48  of compressor  23  to the expansion valve  31  is termed the high pressure side of the refrigeration system or circuit  21 . 
         [0029]    After the refrigerant passes through expansion valve  31  as a mixture of gas and liquid, its pressure is reduced. Evaporator  27  receives the refrigerant from expansion valve  31 . Evaporator  27  is in heat exchange communication with a second heat transfer medium. The heat of absorption is absorbed by the refrigerant in evaporator  27  from the second heat transfer medium, as the liquid refrigerant undergoes a change of state to a vapor. As this heat is absorbed, the heat transfer medium is cooled. The heat transfer medium may be used directly to cool or refrigerate an area, for example when the heat transfer medium is air, or it may be used to cool a liquid, such as water and conveyed to another heat transfer device in an area or room, such as in water cooled chiller applications. In such applications, the chilled water is sent to the chiller and then set o heat transfer devices in separate areas of a building on demand. The refrigerant gas from evaporator  27  is then returned to the suction side of compressor  23  to complete the circuit. Closed loop system  21  immediately after expansion valve  31  to the suction side of compressor  23  is termed the low pressure side of the system circuit. 
         [0030]    Referring to  FIGS. 2 and 3 , there is depicted a screw compressor  38  that may be used as compressor  23  in closed-loop refrigeration system  21  of  FIG. 1 .  FIG. 3  depicts, in cut-away, some the interior components of screw compressor  38  through the compressor housing. Screw compressor  38  includes control panel  22  connected to a power source (not shown in  FIG. 2 ), which is used to power a motor  43  that drives screw compressor  38 . Although not shown, the screw compressor  38  includes a lubrication system as one known in the art. Lubrication systems include lubricating oils, (usually mineral oils which are completely dehydrated, wax-free and non-foaming), an oil pump to deliver oil under pressure to all bearing surfaces, and an oil separator. Oil is separated from the refrigerant gas as the refrigerant gas exits the high pressure side of the compressor. Oil is then returned to the low pressure side of the compressor to seal the clearance between the rotors, and between the rotors and the cylinder. Screw compressor  38  is in fluid communication with an oil separator (not shown). Refrigerant gas from evaporator  27  and lubricating oil are introduced into the suction side of screw compressor  38  at inlet port  44  to lubricate the rotors of the compressor. A lubricant is also introduced into the screw compressor to lubricate the rotors of the compressor. Once compressed within screw compressor  38 , the mixture of high pressure refrigerant gas and lubricating oil is discharged into an oil separator where the mist of lubricating oil in the form of finely divided particles entrained in the refrigerant gas is separated from the refrigerant gas. After separation, the refrigerant gas exits the oil separator through its discharge port  48  and is provided to condenser  25  in the closed loop system  21 . 
         [0031]    In  FIG. 3 , the internal mechanisms of the screw compressor  38  can be seen. A shaft  50  extending from motor  43  is connected to at least one of a pair of helically-grooved rotors  52 . One rotor may be stationary or both rotors may be driven by the use of rotor-synchronized timing gears that synchronizes their rotation. Refrigerant enters screw compressor  38  through inlet port  44  and is compressed between the lobes of the rotors  52 . The compressed gas is discharged into discharge port  48  which is in communication with the downstream condenser  25  in closed loop system  21 . As can be seen in  FIG. 3 , a plug  54  is positioned within discharge port  48 . Although plug  54  is locked into position, it may be unlocked and moved from a first position to a second position. 
         [0032]      FIG. 4  is a partial horizontal cross-sectional view of the screw compressor  38  through its center viewed from above, providing a detail view of discharge port  48 . In  FIG. 4 , rotors  52  are not visible, as the view is taken below the rotors. However, this view clearly shows the path taken by refrigerant into discharge port  48 . Plug  54  is depicted as threaded into discharge port  48  in  FIG. 4 , although the method of inserting plug  54  into discharge port  48  is not limited to threading, and any other well-known method of assembling and locking plug  54  into a bore may be used. When threading is utilized, plug  54  and discharge port  48  have matching threads. In  FIG. 4 , plug  54  is shown fully inserted into discharge port  48 , providing discharge port with the maximum possible volume. 
         [0033]      FIG. 5  is a partial horizontal cross sectional view of the screw compressor  38  through its center viewed from above, providing a detail view of discharge port  48 .  FIG. 5  is identical to  FIG. 4 , except that plug  54  is threaded into discharge port  48  so that the discharge port has a minimum volume. As shown in  FIG. 5 , plug  54  is threaded to a second position that provides discharge port with a minimum volume and exposed threads  49  of discharge port  48  are visible as plug  54  is not fully threaded into discharge port  48  in  FIG. 5 . 
         [0034]      FIG. 4  and  FIG. 5  depict plug  54  inserted into discharge port  48  in two positions, a first position in which the discharge port  48  has a maximum volume ( FIG. 4 ) and a second position in which the discharge port  48  has a minimum volume ( FIG. 5 ), respectively. It will be understood by those skilled in the art, that plug  54  may be inserted into discharge port  48  at any position from the first position depicted in  FIG. 4  to a second position depicted in  FIG. 5  to provide a variable volume dependent on the location of plug  54  in port  48 . Plug  54  generally may be fabricated of a relatively dense material, such as steel, so that the inertia of plug  54  within bore  48  is sufficient to prevent movement during operation of screw compressor  38 . Additionally, plug  54  has a self locking feature, for example, a spring, a chemical additive, a prevailing torque feature having a deflective or distorted thread type, or any combination thereof. 
         [0035]    As previously noted, volume ratio V i  is related to the discharge volume. More specifically, the volume ratio is provided as: 
         [0000]      V i =ε 1/κ 
 
         [0000]    where
 
V i  is the volume ratio
 
ε is compression ratio and
 
κ is a refrigerant constant. For refrigerant  134 A, K is 1.8.
 
         [0036]    When plug  54  is in its first position, as shown in  FIG. 4 , in which discharge port has its maximum volume, high pressure refrigerant gas is discharged from compressor lobes into discharge port  48  and achieves its minimum volume ratio. The volume ratio V i  is the ratio of the suction volume to the discharge volume. In this first position, the suction volume is the volume of the interlobal region before compression. The discharge volume is the volume of the meshing rotors just prior to the opening to the discharge port area. Since the discharge port volume  48  is at its maximum due to the position of plug  54 , the volume ratio V i  for the system is at a minimum. In this position, the operation of the screw compressor  38  and the refrigeration system  21  is most efficient in cooler climates and during cooler months in late autumn, winter and early spring. 
         [0037]    When plug  54  is in its second position, as shown in  FIG. 5 , in which discharge port  48  has its minimum volume, high pressure refrigerant gas is discharged into the discharge port  48  and achieves its maximum compression ratio. The volume ratio V i  is the ratio of the suction volume to the discharge volume. In this second position, the suction volume is the volume of the interlobal region before compression. The discharge volume is the volume of the meshing rotors just prior to the opening to the discharge port area. Since the volume of the discharge port  48  is at its minimum, the volume ratio V i  for the system is at a maximum. In this position, the operation of the screw compressor  38  and the system is most efficient in warmer climates or during warmer months in late spring, summer, and early autumn. 
         [0038]    It will also be recognized by those skilled in the art that the volume ratio V i  may also be adjusted, if desired, between the extremes shown in  FIGS. 4 and 5 . An intermediate adjustment may be more desirable for seasonal changes and provide improved efficiency for autumn and spring rather than an adjustment selection using one of the extreme positions depicted in either  FIG. 4  or  5 . 
         [0039]    In operation, plug  54  is moved to its second position depicted, in  FIG. 5 , to provide the discharge port with its minimum volume for warmer climates and/or summer conditions. In a season in which warmer weather is expected or wherein the compressor is in a system located in a warmer climate, a higher volume ratio V i  is required. Higher temperatures require higher operating pressures, and the minimum discharge port volume provides higher pressures. The discharge port pressure dictates the downstream pressure at the evaporator. The increase in pressure represents an increase in work performed by the compressor  38 . The increase in work represents an increase in energy usage by the screw compressor  38 , but the screw compressor  38  is operated in a more efficient manner. In one embodiment, by matching the volume ratio V i  to the season or the climate, not only is the compressor operated more efficiently, but noise from screw compressor  38  operations is also reduced. 
         [0040]    In operation, plug  54  is moved to its first position depicted in  FIG. 4  to provide the discharge port  48  with its maximum volume for cooler climates and/or winter conditions. In a season in which cooler weather is expected or wherein the screw compressor  38  is in a system located in a cool climate, a lower volume ratio V i  is required. Cooler ambient temperatures permit lower operating pressures, and the larger discharge port volume provides lower pressures. The reduced discharge port pressure decreases the downstream pressure at the evaporator, which in turn provides less cooling capacity. In one embodiment, the reduction in pressure represents a decrease in work performed by screw compressor  38 , which results in increased screw compressor  38  efficiency in cold conditions. 
         [0041]    Discharge port  48  and movable plug  54  are located in the interior of screw compressor  38 , as can be seen in  FIGS. 2 and 3 . Plug  54  is not readily accessible from the exterior of screw compressor  38 . In order to access plug  54  to move it from a first position to a second position, or any intermediate position, compressor cover  33  must be removed to access the interior of the screw compressor  38 . In one embodiment, prior to removing compressor cover  33 , screw compressor  38  must be shut down and the pressure within screw compressor  38  must be allowed to equalize to atmospheric pressure and the refrigerant must be removed from the system. In another embodiment, the screw compressor  38  is isolated from closed loop system  21  and refrigerant must be removed from screw compressor  38 . Because this is not a simple procedure and requires shutting screw compressor  38  down, repositioning plug  54  cannot be done on a daily or even a weekly basis. Repositioning of plug  54  is best done on a seasonal or even a semi-annual basis. 
         [0042]    Referring now to  FIG. 6 , screw compressor  38  is depicted with cover  33  removed. A worker is shown inserting a tool  56  to engage plug  54 . Tool  56  is inserted into the end of screw compressor  38  and engages the end of plug  54 . The worker then moves plug  54 , such as by rotating it either clockwise or counterclockwise, from a first position to a second position.  FIG. 7  depicts tool  56  being inserted into screw compressor  38  after cover  33  has been removed from screw compressor  38 . Tool  56  is shown as partially inserted, and the tool head is visible out of contact with plug  54  in discharge port  48 , with plug  54  being in first position to provide discharge port  48 , plug  54  being in a first position to provide discharge port  48  with its maximum volume. 
         [0043]    Plug  54  is shown in greater detail in  FIG. 8  a cross-section view, and in  FIG. 9  a perspective view. In a preferred embodiment as shown in  FIG. 8 , plug  54  is threaded with external threads while discharge port  48  includes internal threads  49 . However, plug  54  may be positioned within discharge port  48  using any other mating method. For example, plug  54  may include spring loaded projections that can be aligned into a series of mating apertures at different locations along discharge port  48 . Alternatively, plug  54  may be slidable from a first position to a second position along discharge port  48 , and locked into position by rotating, for example, 90°. The exact mechanism used to position plug  54  within discharge port  48  is not critical to the operation of the invention. Plug  54  further includes at least one o-ring groove  60  for insertion of an o-ring. O-rings consist of neoprene, chloroprene, and other fluid-resistant elastomeric compounds. Positioning an o-ring in this groove when assembled into discharge port  48  prevents leakage of refrigerant around plug  54 . Additionally, seals to prevent leakage of refrigerant for use in combination with plug  54  include compression seals, mechanical seals, and the like. 
         [0044]    The position of plug  54  within discharge port  48  can be determined by any convenient method. For example, the total number rotations that plug  54  can make within discharge port  48  is known. When plug  54  is fully inserted into discharge port  48  as shown in  FIG. 4 , discharge port  48  has a maximum volume suitable for summer conditions and plug  54  is at maximum travel within discharge port  48  (i.e. fully inserted). The number of rotations required to move plug  54  from the position shown in  FIG. 4  to the position shown in  FIG. 5  is known or can be determined, and the worker can count the number of rotations when changing the position of plug  54 . A position of the plug  54  for autumn or spring conditions can also be determined on the basis of rotations from hot or warm positions. In another embodiment, screw compressor  38  and plug  54  include index lines or marks adjacent to the aperture wherein tool  56  is inserted to interface with plug  54 . The index lines can be related to the position of the plug within the aperture, which can be related to the discharge port volume. In yet another embodiment, tool  56  includes a series of scribe lines which can be matched to the index line, each scribe line corresponding to a position of the plug  54  for a particular season. In another embodiment, tool  56  may be provided in different lengths, each length corresponding to a different season, corresponding to a desired location of plug  54 . Plug  54  first may be moved o the position shown in Figure  4  and then the appropriate tool can be inserted to move plug  54  to its proper position. 
         [0045]    In one embodiment, plug  54  includes a feature that mates with a corresponding feature on tool  56 . As shown in  FIGS. 8 and 9 , this feature is by way of example, but not limited to, a hex shaped aperture  62  on plug  54 , and tool  56  includes, a corresponding hex-shaped head, that can be inserted into the aperture on plug  54 . The shapes are not limited and any other shape may be used. In addition, the head and aperture may be reversed so that plug  54  includes the head and tool  56  includes the aperture. Any other configuration for mating tool  56  to plug  54  to facilitate movement of plug  54  within discharge port  48  may be used. 
         [0046]    By use of a discharge port, such as discharge port  48  with a plug  54  to provide a variable volume discharge port, a screw compressor  38  may be fabricated for use in cold climates or warm climates. The volume ratio V i  of the screw compressor  38  can be adjusted manually to provide the volume ratio V i  most suitable for the climate in which the screw compressor  38  is used. Additionally, a screw compressor  38  having a manually variable volume ratio V i  can be adjusted seasonally to provide screw compressor  38  with the volume ratio V i  most suitable for the season, while also providing improved efficiency. 
         [0047]    While the invention has been described with reference to a preferred embodiment, 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 disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.