Abstract:
A screw compressor having an unloader valve with a movable valve member, the unloader valve being capable of indicating when the valve member is installed incorrectly. The valve member is manufactured such that if it is installed incorrectly it provides a leakage path sufficiently large to be detected during full load testing of the compressor. When the screw compressor fails the load test, the compressor is partially disassembled, and the valve is reinstalled in the proper orientation and the compressor is re-tested. The valve member leakage path is provided while maintaining low costs for production and assembly of the compressor.

Description:
RELATED APPLICATIONS 
     This application claims priority to provisional application serial No. 60/225,352, filed on Aug. 15, 2000. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to screw compressors, and more particularly to axial unloading lift valves for screw compressors. 
     BACKGROUND OF THE INVENTION 
     Axial unloading lift valves are commonly used in screw compressors to vary the compression load produced by the screws. One or more valves are arranged axially towards the discharge side of the screws and the load is varied by selectively opening and closing the valves. Opening the valves to “unload” the compressor reduces the effective working length of the screws by opening communication pathways between portions of the screw and the low-pressure suction end of the compressor. The open pathways allow the pressure to equalize so that compression does not occur over the portions of the screw communicating with the suction end of the compressor. When the valves are closed to “load” the compressor, no pressure equalization occurs over the axial length of the screw. Therefore, the full working length of the screws is utilized for compression. The angular location of the valves around the discharge ends of the screws determines how much of the axial working length of the screws is used or eliminated when the valves are closed or opened. 
     FIGS. 1 and 2 are schematic representations showing a portion of a prior art compressor  10  having an axial unloading lift valve  14 . FIG. 1 shows the valve  14  in the loaded condition and FIG. 2 shows the valve  14  in the unloaded condition. The compressor  10  includes a pair of screws  15 ,  16  (only one is shown in FIGS. 1 and 2) mounted for rotation in a screw housing  22 . The interior of the screw housing  22  defines a compression chamber  24  where the fluid is compressed by the screws  15 ,  16 , as is understood by those skilled in the art. A discharge housing  26  supports the discharge end of the screws  15 ,  16  and is coupled to one end of the screw housing  22 . A suction housing  30  supports the suction end of the screws  15 ,  16  and is coupled to the other end of the screw housing  22 . 
     The axial unloading valve  14  typically includes a cylindrically-shaped valve member  34  housed in a valve chamber  38 . The valve chamber  38  is formed in the discharge housing  26  so that one end of the valve chamber  38  communicates both with the compression chamber  24  and with a vent passageway  42 . The vent passageway  42  is connected to a suction cavity  46  formed in the suction housing  30 . The other end of the valve chamber  38  communicates with a high-pressure fluid supply that controls the positioning of the valve member  34 . The high-pressure fluid supply is typically either high-pressure lubricating oil or refrigerant that has been discharged from the compressor. 
     To load the compressor  10 , the valve  14  is closed by flooding the valve chamber  38  with high-pressure fluid. The fluid in the valve chamber  38  forces the valve member  34  toward the screw housing  22  until the valve member  34  abuts the screw housing  22 , as shown in FIG.  1 . When the valve member  34  is in the position shown in FIG. 1, there is no communication, and therefore no pressure equalization, between the suction cavity  46  and the compression chamber  24 . Because there is no pressure equalization, the entire working length of the screws  15 ,  16  is utilized and maximum compression loading is generated by the compressor  10 . 
     To unload the compressor  10 , the valve  14  is opened by draining the fluid from the valve chamber  38 . The high-pressure fluid in the compression chamber  24  forces the valve member  34  away from the screw housing  22 , as shown in FIG.  2 . When the valve member  34  is in the position shown in FIG. 2, the passageway  42  provides communication, and therefore pressure equalization, between the compression chamber  24  and the suction cavity  46 . This pressure equalization reduces the effective working length of the screws  15 ,  16 , thereby reducing the compression load generated by the compressor  10 . 
     SUMMARY OF THE INVENTION 
     For the axial unloading valve  14  to function properly, the valve member  34  must be carefully manufactured and installed. FIG. 3 shows a prior art valve member  34  in greater detail. The valve member  34  is substantially cylindrical and includes opposing first and second axial sealing surfaces  50  and  54 , respectively. A radial sealing and positioning surface  58  extends between the axial sealing surfaces  50  and  54 . 
     With this symmetrical configuration, the valve member  34  could be installed in the valve chamber  38  in two ways. Therefore, both the first and the second axial sealing surfaces  50  and  54  must be machined to tight axial run-out tolerances to ensure that, regardless of how the valve member  34  is installed, proper axial sealing occurs when the valve  14  is closed. The term “run-out” is well-known to those in manufacturing and in this situation is generally understood to refer to the perpendicularity between a longitudinal axis  62  and each of the axial sealing surfaces  50  and  54 . In addition to sealing concerns, the tight run-out tolerance ensures that no portion of the valve member  34  will interfere with the 5 screws  15 ,  16  when the compressor  10  is operating at full load (i.e., when the valve  14  is closed). This is especially important on compressors having small axial screw endplay with respect to the discharge housing  26 . Maintaining the tight axial run-out tolerances requires expensive precision machining and, because both axial sealing surfaces  50  and  54  must be tightly toleranced, two separate machine setups are required for two separate precision machining operations. This significantly increases the manufacturing cost of the valve member  34 . 
     One way to eliminate the need for two tightly-toleranced axial sealing surfaces  50 ,  54  on the valve member  34  is to change the design. FIG. 4 illustrates an alternative prior art valve member  66  that has only one axial sealing surface  70 . Additionally, the radial sealing surface  74  and the radial positioning surface  78  are separate surfaces. This ensures that the valve member  66  can only be installed in one way, thereby eliminating the need for a second axial sealing surface with a tight run-out tolerance. 
     While only one precision machining setup is necessary for achieving the desired run-out tolerance on the single axial sealing surface  70 , a separate machining operation is still required to form the radial positioning surface  78 . This second operation need not be precision machining, but nonetheless requires a second machine setup. The two separate machine setups required to manufacture the different radial surfaces  74  and  78  can create tolerance stack-up problems and often mandate the use of a gasket  82  to prevent leakage. The use of the gasket  82  also adds to the cost of the compressor  10  and increases the number of parts that may require periodic replacement. 
     The present invention provides an improved valve member for an axial unloading lift valve. The improved valve member has only one axial sealing surface requiring a tight run-out tolerance. Therefore, only one machine setup is needed to produce the sealing surfaces of the improved valve member. Additionally, the valve member of the present invention includes features that facilitate proper assembly and ensure that the valve member is properly installed. No gaskets are required to seal the valve member. Thus, the valve member of the present invention provides a less-expensive and more reliable valve member than the prior-art valve members described above. 
     More specifically, the invention provides a screw compressor having a housing, a drive screw supported by the housing, and an idler screw supported by the housing. The drive screw and idler screw assembly have a low-pressure end and a high-pressure end. The drive screw, driven by an outside force, drives the idler screw, to which the drive screw is operably engaged. Rotation of the screws moves a fluid from the low-pressure end to the high-pressure end. The screw compressor further has at least one vent passageway with one end in fluid communication with the low-pressure region and a second end in selective fluid communication with the high-pressure end. In addition, the screw compressor has at least one valve having a valve member. Each valve member has a sealing surface, a non-sealing surface, and a radial sealing surface partially extending between the sealing surface and the non-sealing surface, the non-sealing surface having a recess. The valve is positioned such that the valve member is installable in a correct orientation and an incorrect orientation. When installed in the correct orientation the valve member is movable between a loaded position, at which the valve member substantially prevents flow from the high-pressure end to the low-pressure end, and an unloaded position, at which fluid passes from the high-pressure region through the vent passageway to the low-pressure region. When the valve member is installed in the incorrect orientation, the valve member provides a flow path from the high-pressure end through the vent passageway to the low-pressure end when in the loaded position and the unloaded position. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a prior art screw compressor with an axial unloading lift valve shown in the full load position. 
     FIG. 2 is a schematic illustration of the prior art screw compressor of FIG. 1 with the axial unloading lift valve shown in the partial load position. 
     FIG. 3 is a perspective view of a prior art axial unloading lift valve. 
     FIG. 4 is a perspective view of another prior art axial unloading lift valve. 
     FIG. 5 is a perspective view of an axial unloading lift valve embodying the present invention. 
     FIG. 6 is a partial section view of a screw compressor having the axial unloading lift valve shown in FIG. 5 installed incorrectly. 
     FIG. 7 is a section view of a screw compressor having the axial unloading valve shown in FIG. 5 installed correctly in the loaded position. 
     FIG. 8 is a section view of a screw compressor having the axial unloading valve shown in FIG. 5 installed correctly in the unloaded position. 
     FIG. 9 is a section view of a screw compressor showing both screws and two valves. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Screw type compressors  100  of the type described herein are commonly used to move fluids (liquid or gas) such as oil, water, refrigerant, or other like substances. Screw type compressors  100 , as shown in FIGS. 6-9, use a housing  105  and a pair of screws to increase the pressure of a fluid and move the fluid through the compressor  100 . The two screws are called the drive screw  10  and the idler screw  115 . In addition to these components, most systems in which a screw type compressor  100  is used contain an unloading valve  120 . The unloading valve  120  can be separate from the compressor  100 , however more typically the unloading valve  120  is incorporated into the compressor housing  105 , as in the present invention. In addition, multiple unloading valves  120  can be employed in the same compressor  100  to provide redundant functions or to perform different functions. For example, FIG. 9 shows a compressor  100  with two unloading valves  120 . One of the unloading valves  120  has a valve member  125  installed properly while the other unloading valve  120  is shown with the valve member  125  installed improperly. It should be noted that the unloading valves  120 , in the figures provided, are arranged around the drive screw  110  only. It is however, possible to arrange unloading valves  120  around either, or both screws  110 ,  115 . 
     In general, the compressor housing  105  is formed from three separate pieces, a suction end  130 , a discharge end  135 , and a screw housing  140 . The three pieces are then assembled to form a complete housing  105 . While it is possible to manufacture a housing  105  from less than three pieces, assembly of the other compressor components into the housing  105  becomes more complex as the number of housing pieces are reduced. For example, a housing  105  in which one of the end pieces  130  or  135  is combined with the screw housing  140  would require a very intricate casting or significant machining to complete. The three-piece arrangement, requires the same intricacy, however, with three pieces, access to the different regions requiring machining is simplified. Typically, the three pieces are cast into a rough shape, and then surfaces requiring a tighter tolerance or better surface finish are machined. The pieces are generally cast aluminum, steel, iron, bronze, or other material capable of containing fluid at the required operating pressures and temperatures. The end pieces  130 ,  135  each contain a chamber for the collection of a fluid. The suction end chamber or cavity  145  contains low-pressure fluid and defines a low-pressure region. The discharge end chamber  150  (see FIG. 9) contains high-pressure fluid and defines a high-pressure region. Generally, the regions are cast into the end pieces  130 ,  135  and require no additional machining. Each end piece  130 ,  135  further contains two bored regions, each sized to receive a bearing  155  which in turn supports either the drive screw  110  or the idler screw  115 . Any bearing type can be used to support the screws  110 ,  115  within the end pieces  130 ,  135  including roller bearings, ball bearings, needle bearings, and journal bearings. The illustration of FIG. 6 shows only one of the two screws  110 ,  115 , the one screw using needle bearings  155  for support within the housing  105 . The bearings  155  are of a known design; capable of operating effectively under the conditions experienced by the compressor. Each end piece  130 ,  135  attaches to the screw housing  140  using a known attachment, typically a series of bolts or screws. To improve the seal between the end pieces  130 ,  135  and the screw housing  140 , gaskets can be used. The gasket material should provide a superior seal throughout the operating temperature and pressure ranges of the compressor. 
     The discharge end piece  135  contains one or more bores or valve chambers  160 , sized to receive the unloading valve member  125 . A smaller bore  165  opens the valve chamber  160  to the outside surface of the end piece  135  allowing for the connection of a control fluid supply. The control fluid can be hydraulic oil, or any fluid compressed by the compressor, such as refrigerant. The use and function of the control fluid is well known in the art and will not be described in detail. 
     The screw housing  140  is manufactured in a manner very similar to that used to make the end pieces  130 ,  135 . In addition, similar materials are used. Two large bores placed in the screw housing  140  form a compression chamber  170 , which accommodates the screws  110 ,  115 . The bores are spaced apart a distance which allows the two screws  110 ,  115  to mesh while still providing enough clearance to allow free rotation of the screws  110 ,  115 . The size of each bore is precisely controlled to achieve a minimum operating clearance between the bore and the screws  110 ,  115  that rotate within the bore. Any excess clearance between the walls of the compression chamber  170  and the screws  110 ,  115  will reduce the compressor&#39;s efficiency, volumetric output, and maximum pressure output. A vent passageway  175 , parallel to the compression chamber  170 , provides a flow path from the high-pressure end of the screws  110 ,  115  to the low-pressure region, when the unloading valve  120  is in the unloaded position or is installed improperly. The vent passageway  175  can be any shape so long as it provides an adequate flow area, alone or in combination with other unloading valves  120 , to unload the compressor  100 . In addition, a wall  180 , typically formed as part of the housing  105 , exists between the vent passageway  175  and the compression chamber  170 . The function of the wall  180  will be described in detail in forthcoming paragraphs. While only one vent passageway  175  has been described, it is possible to have several vent passageways  175  spaced radially around the screws  110 ,  115 . The only limitation to the number of unloading valves  120  and vent passageways  175  is the radial space surrounding the screws  110 ,  115 . 
     A screw type compressor  100  uses two meshed screws  110 ,  115  to move and pressurize fluid. The screws  110 ,  115  are in fluid communication with two regions within the end pieces  130 ,  135 . The suction region, or low-pressure region, contains a supply of low-pressure fluid, which is drawn into the screws  110 ,  115  during operation. The discharge region, or high-pressure region, located in the discharge end piece  135 , collects the compressed fluid leaving the compressor  100 . 
     A screw type compressor  100  compresses a fluid by trapping the fluid in a series of pockets and then reducing the volume of the pockets, thus increasing the pressure therein. Rotation of the screws  110 ,  115  forces the fluid toward the high-pressure end of the screws  110 ,  115  where it is discharged producing a continuous flow of high-pressure fluid. Typically, one screw, the drive screw  110 , is coupled to an electric motor or other prime mover capable of turning the drive screw  110 . Rotation of the drive screw  110  forces the idler screw  115 , which is meshed with the drive screw  110 , to turn. The two screws  110 ,  115  working together trap and force the fluid to move toward the high-pressure region. The screws  110 ,  115  are sized to fit within the housing  105  such that there is very little endplay in the screws  110 ,  115 . This means that the gap between the high-pressure end of the screws  110 ,  115  and the housing  105  is small enough to prevent substantial leakage between adjacent pockets. 
     As the screws  110 ,  115  rotate, fluid is trapped in a pocket formed between the mesh point of the screws  110 ,  115  and the housing  105  at the high-pressure end of the screws  110 ,  115 . Continued rotation allows the end of the pocket to eventually pass over the discharge opening  150  and discharge the high-pressure fluid. If an unloading valve  120  is open at some point before the discharge opening  150 , the pressure within the pocket will prematurely discharge. For example, if an unloading valve  120  were open at a point one-half of a revolution before the discharge opening  150 , the fluid would discharge at that point. However, fluid remains within the pocket at a pressure approximately equal to the pressure in the low-pressure region. After the pocket passes the open unloading valve  120 , the high-pressure end will again seal and the pocket volume will continue to reduce. The continued rotation of the screws  110 ,  115 , after passing the open unloading valve  120 , will continue compressing the trapped fluid. Because the full rotation of the screws  110 ,  115  is not utilized in compressing the fluid, the outlet pressure will be less than the maximum achievable, and the effective length of the screws  110 ,  115  is reduced. 
     With this background in mind, the unloading valve  120  will now be discussed. Unloading valves  120  of the type described herein are capable of performing several known functions. For example, an unloading valve  120  can be used to maintain the pressure leaving the compressor  100  at a value below its maximum. The unloading valve  120  can be radially positioned such that the effective length of the screws  110 ,  115  is reduced a desired amount. The rotational angle between the unloading valve  120  and the discharge area  150  control the effective length of the screws  110 ,  115 . Shortening the effective length of the screws  110 ,  115  reduces the compressor&#39;s output pressure. This and other uses for unloading valves  120  are well known in the art and will therefore not be described in further detail. 
     FIG. 5 illustrates an embodiment of an unloading valve member  125  of the present invention. It should be pointed out that the relief area  185  shown in FIG. 5 is greatly exaggerated in the figure and does not appear in the other figures. The unloading valve  120  of the present invention contains a movable cylindrical valve member  125  housed in a valve chamber  160 . The valve chamber  160 , and thus the valve member  125 , is positioned such that the valve member  125 , in the loaded position, is in sealable contact with the wall  180 . The wall  180 , positioned between the screw bore and the vent passageway  175 , allows the valve member  125  to prevent flow therebetween. The valve member  125  has a sealing side  190  and a non-sealing side  195  along with a radial sealing surface  200 . The sealing side  190  and the radial sealing surface  200  are manufactured to very tight tolerances to ensure that they provide adequate seals. For example, the maximum allowable run-out on the sealing side surface  190  is approximately 0.008 mm (0.0003 in), while the allowable run-out on the non-sealing surface  195  is approximately 0.02 mm (0.0008 in). Run-outs as high as about 0.010 mm (0.0004 in) for the sealing surfaces  190  and  200 , and as low as about 0.015 mm (0.0006 in) for the non-sealing surface will function with the present invention. 
     The radial sealing surface  200  of the valve member  125  acts as a seal between the control fluid and the compression chamber  170 . In addition, the radial sealing surface  200  prevents leakage around the valve member  125  to the vent passageway  175  when in the loaded position. Further, the radial sealing surface  200  acts as a guide during assembly and during movement of the valve member  125 . To aid in the assembly process, the radial sealing surface  200  is relieved slightly as shown in FIG.  5 . The relieved portion  185  is inserted into the valve chamber  160  before bolting the discharge end piece  135  to the screw housing  140 . The relief area  185  allows the valve member  125  to slide into the valve chamber  160  more easily. In addition to simplifying assembly, the relieved portion  185  simplifies manufacturing by allowing for the creation of the relief or dimple  205  in the non-sealing axial surface  195  without upsetting the radial sealing surface  200 . Whether the dimple  205  is machined, stamped, or formed using other known processes, the relief area  185  allows for small movements of the relieved radial surface  185  without affecting the tight tolerance areas. To ensure that the dimple  205  does not affect the radial sealing surface  200 , the dimple  205  should extend no deeper than the relief area  185 . In other words, the axial length of the relieved area  185 , as measured from the non-sealing surface  195 , should be equal to or greater than the depth of the dimple  205 . 
     The sealing side  190  of the valve member  125  in the loaded position prevents flow between the high-pressure end of the screws  110 ,  115  and the vent passageway  175 . The sealing side  190  is forced against the wall  180  between the screw bore and the vent passageway  175  to form a seal. The seal area is relatively narrow and the pressure drop from the high-pressure side to the low-pressure side is potentially large requiring a good seal surface, thus the tight run-out requirements. 
     The valve member  125  of the present invention is simple and inexpensive to produce and assemble correctly. The sealing surfaces  190 ,  200  of the present invention can be machined in one setup, greatly reducing the cost of the component. Further, the dimple  205  can be produced in any number of ways available to typical manufacturing facilities. The valve member  125  is therefore inexpensive to manufacture. Assembly remains easy and the detection of an incorrect assembly is greatly simplified by the present invention. 
     The valve member  125  of the present invention uses a cylindrical-shaped body having a sealing side surface  190  manufactured to the rigid run-out requirements previously described. The non-sealing side  195  is dimpled to produce a leakage path if it is installed improperly and positioned in the loaded position. FIG. 6 illustrates the present embodiment of the valve member  125  installed incorrectly, and positioned in the loaded condition. One can see that a flow path  210  between the end of the screws  110 ,  115  and the low-pressure region exists even when the valve member  125  is in the loaded position. During load testing of this compressor  100 , before its shipment to a customer, this problem will be evident and can be easily corrected by reversing the orientation of the valve member  125 . The compressor  100  of FIG. 6 will be incapable of producing a pressure output corresponding to its maximum design output. 
     FIG. 7 shows the compressor  100  in the loaded position with the valve member  125  installed correctly. Clearly, no flow path exists between the high and low-pressure regions, and the compressor  100  output will correspond to the maximum design output. FIG. 8 shows the compressor  100  of the present invention in which the valve member  125  is installed correctly and the valve  120  is in the unloaded position. 
     While a non-sealing surface  195  having a dimple  205  has been described, many other shapes are possible. Any shape protrusion or recess  205  will function as long as it provides a leakage flow path  210 . In addition, the shape used should provide a relatively symmetric support that contacts the wall  180  so that there is no tendency for the valve member  125  to twist, bind, or stick. For example, a large hole drilled into the center of the non-sealing surface  195  would provide a leak path around the wall  180  while still allowing adequate support. In addition, a plurality of slots cut across the non-sealing surface  195  at different angles relative to one another would provide leakage paths  210  as well as adequate contact support. It should be clear to a person skilled in the art that there are many ways to adapt the non-sealing surface  195  to assure leakage if the valve member  125  is installed incorrectly. 
     The resulting valve member  125  should, when installed properly, seal the high-pressure end of the screws  110 ,  115  from the low-pressure region when in the loaded position and provide a substantial leakage path  210  when installed incorrectly. The leak path  210  should produce leakage that is detectable during a load test of the compressor  100 . Typically, the leakage will manifest itself as an inability to achieve the desired output pressure. When this condition is detected, it is a simple task to partially disassemble the compressor  100 , invert the valve member  125 , reassemble the compressor  100 , and retest the compressor  100 . 
     Although particular embodiments of the present invention have been shown and described, other alternative embodiments will be apparent to those skilled in the art and are within the intended scope of the present invention. Thus, the present invention is to be limited only by the following claims. 
     Various features of the invention are set forth in the following claims.