Abstract:
The volute is cast integrally with the gearbox base to allow closer radial tolerances to be used to improve efficiency. In the preferred embodiment, compressors for multi-stage compression are assembled with intercoolers and the integral volute is cast together with the impeller housing and the lower gearbox housing and the associated intercooler. Efficiency increases of 2% or more are achievable. In multistage applications, efficiency gains in the early stages are compounded in each subsequent stage. Speed changes that change center distance between bull gear and pinion are accommodated by an offset opening in the inlet and diffuser that can line up with the impeller shaft as well as an offset opening in the fluid seal despite a pinion gear change, by reorientation and remounting of those parts.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/US2008/059169 entitled “Integral Scroll and Gearbox for a Compressor with Speed Change Option”, filed on Apr. 2, 2008, which is herein incorporated by reference in its entirety, and which claims priority to U.S. Provisional Patent Application No. 60/921,547, entitled “Integral Scroll and Gearbox for a Compressor with Speed Change Option”, filed on Apr. 3, 2007, which is herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to centrifugal compressors, such as, for example, assemblies of compressors for multistage compression wherein the integral casting further includes a gearbox and intercooler housings. 
     BACKGROUND OF THE INVENTION 
     Centrifugal compressors supply oil-free compressed gas in a variety of industrial applications. A common application of a centrifugal compressor is in plant air systems, to supply a motive force for valve actuators and pneumatic cylinders used in robotic applications, as one example. Centrifugal compressors typically feature an impeller mounted in a closely-conforming impeller chamber. The chamber features an axial inlet port to allow fluid entry toward the center of the impeller. Fluid is drawn into the impeller due to its rotation at speeds that can exceed 75,000 revolutions per minute (RPM). The rotation of the impeller propels the fluid through an annular diffuser passageway and into a surrounding volute. The energy imparted into the fluid by the impeller&#39;s rotation increases the fluid&#39;s velocity and, consequently, pressure as the fluid passes the diffuser passageway into the scroll or volute. The diffuser passage way has inside and outside radial dimensions for each circumferential station of the impeller chamber and scroll. By definition, the inside radius of the diffuser section corresponds to the distance to the diffuser throat or the location at which the annular port or passageway has the smallest axial width for the given station, the diffuser section extending outwardly for the remainder of the annular passageway. 
     Traditionally, centrifugal compressors have featured a bolt on scroll/volute cover, which encompassed portions of the impeller chamber, the diffuser passageway and the volute-outlet passageway. U.S. Pat. No. 4,181,466 is illustrative of a bolt-on component featuring a fluid entry  51  and a volute  50  that is secured to the bearing housing  15  by a V-clamp  49 . A difficulty with the bolt-on scroll/volute cover incorporating the volute is the effective control of tip clearance between the impeller and the inlet passageway and the clearance between the impeller and the volute outlet. Due to the bolt-on construction previously employed, machining costs and assembly costs affected the finished cost of the product. The assembly of a plurality of components required the use of greater clearances around the impeller, which sacrificed compressor efficiency. This, in turn, required larger drivers and higher operating costs for electric power. Since each assembled component has a manufacturing tolerance, the final clearance near the impeller has to be sufficiently large to accommodate a situation where all the tolerances in the individual components of the assembly turned out within specification but all dimensions on the individual components were off from the ideal dimension and on the same side of the tolerance allowed. 
     Another problem with bolt-on volutes, i.e.,  24  and  26 , is the extra space and mass taken up by that type of assembly. Such space could become important in situations where ease of installation and maintenance is important to serviceability. For example, as will be explained below, use of bolt-on volutes (such as  24  and  26 ) hinders access to the driver shaft for an oil pump to be directly driven. The extra housing thickness for each stage in a multi-stage skid could preclude a direct drive on the oil pump and may necessitate a separate electrical drive for the oil pump. This would be undesirable in the event of an electrical failure. In an electrical failure, the impeller bearings need lubrication, as the impeller slows from its operating speed of 75,000 RPM or more. Bearing failure could result with an electrically driven oil pump if it stopped delivering oil too abruptly on power failure. A power takeoff from the main drive shaft, which could involve gears or belts, adds to the complication of packaged systems and tends to complicate access when maintenance is required. 
     One issue that remains unresolved by the integral casting of the volute as part of the gearbox is what can be done if the end user needs a capacity change that involves a speed change to one or more stages in a compressor assembly. Normally, such a speed change involves a gear ratio change. Typically, the end user prefers to simply change a pinion  34  shown in  FIG. 2  while retaining the much larger bull gear  32 . The reasons for this preference are cost and speed of getting the replacement parts. It is far easier for the original equipment manufacturer to stock a variety of pinions than to have a lot of cash tied up in very large parts such as different bull gears  32 . However, using a different sized pinion with the same bull gear changes the center to center distance between them and the scroll is integrally cast to a fixed center to center distance. 
     SUMMARY 
     In accordance with certain embodiments, the volute is cast integrally with the gearbox base to allow closer radial tolerances to be used to improve efficiency. In one embodiment, compressors for multi-stage compression are assembled with intercoolers and the integral volute is cast together with the impeller housing and the lower gearbox housing and the associated intercooler. Efficiency increases of 2% or more are achievable. In multistage applications, efficiency gains in the early stages are compounded in each subsequent stage. Speed changes that change center distance between bull gear and pinion are accommodated by an offset opening in the inlet and diffuser that can line up with the impeller shaft as well as an offset opening in the fluid seal despite a pinion gear change, by reorientation and remounting of those parts including the bearings that support them. Either the pinion or the bull gear or both can be moved to change the shaft center distance to accommodate the new gear pairing to get the desired speed. 
     The present invention, in the exemplary context of and integral scroll and gearbox, provides a way to accommodate the differing center to center distances of the gear drive from a speed change by allowing the scroll to accept an end plate with an eccentric opening that can be mounted in a variety of positions to accommodate different center to center distances for example. The bearing housing that is placed in the casting at the gearbox location is selected to also accommodate this offset and an insert shown in  FIGS. 7 and 8  can also be mounted to maintain the relation of the inlet opening  76  to the position of the shaft  38  that supports the impeller  36  and the pinion gear  34 . Alternatively, the present invention can accommodate changing the center distance between the bull gear and the pinion by moving the bull gear centerline using offset bearing housings for its shaft, for example. As yet another alternative and exemplary embodiment, the present invention, depending on the new desired speed, can have both shafts moved to increase or decrease the center distance to accommodate the new gearing combinations selected to get the desired speed. 
     These and other advantages of the present invention will become more apparent to a person of skill in the art from a review of the description of the exemplary embodiments described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view a prior art of a three-stage centrifugal compressor skid showing the first stage compressor housing with volute cast integrally with the lower gearbox housing and the first stage intercooler housing. 
         FIG. 2  is a section view along line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a close up view of the first-stage volute-type scroll housing shown at the top of  FIG. 2 . 
         FIG. 4  is a close up view of a prior art bolt on volute-type scroll housing typically used as an industry standard centrifugal compressor; 
         FIG. 5  shows an inlet assembly with the opening off center to address a shifted impeller shaft position from a change in pinion gear size on the impeller shaft; 
         FIG. 6  is an end view showing the inlet of  FIG. 5  assembled to a scroll: 
         FIG. 7  shows, the orientation of the inlet with the opening shifted right; 
         FIG. 8  shows the orientation of the inlet with the opening shifted left; and 
         FIG. 9  is an exploded view showing how a change in pinion size can be addressed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , this figure, which is in a perspective view, has portions removed for clarity. For example, the motor driver for the package is omitted. The illustrated package comprises a first stage  10 , a second stage  12 , and a third stage  14 . In essence,  FIG. 1  is a drawing of a casting, which further comprises a first-stage intercooler housing  16 , a second-stage intercooler housing  18  and the lower end of the gearbox  20 . An after-cooler (not shown) can be used after the third stage  14 . The first stage  10  has an inlet  22  omitted from  FIG. 1  but shown in section in  FIG. 2 . Second stage  12  has a differently configured inlet section  24  as compared to the first stage inlet  22 . Third stage  14  has an inlet  26  similarly configured to inlet  24 . The present invention, in certain instances, relates to the configuration of volute scroll  74  and its complementary inlet  22 . That configuration can also be used in second stage  12  and third stage  14  within the scope of the invention. 
     The first stage  10  has been configured differently than stages  12  and  14  to illustrate the difference between the prior-known technique (illustrated in stages  12  and  14 ) from the technique of the present invention exemplified in the first stage  10 . It is also different than another known technique as described in  FIG. 4 , item  90 . It should be noted that the invention does not presuppose multiple stages and the details of the first stage  10  can be employed in a single stage installation or in a multi-stage installation, on one or more of the stages, all within the scope of the invention. 
     Referring again to  FIG. 2 , shaft  28  has a coupling  30  connected at its end. The motor driver (not shown) is coupled to coupling  30 . Shaft  28  supports bull gear  32  in gear box  20 . Pinion gear  34  meshes with gear  32  to drive impeller  36  in first stage  10 . Shaft  38  supports the impeller  36  as well as pinion gear  34  and seals  40  along with bearings  42 , disposed on either side of pinion gear  34 . On the other side of the gear box  20 , another pinion gear  44  is supported on a shaft  46  to operate impellers  48  and  50  on the second stage  12  and third stage  14  in tandem. Similar seal and bearing arrangements are used on shaft  46  as on shaft  38 . Mounted to the end of shaft  28  is an oil pump  52 , directly connected by a coupling  54 . 
     As seen in  FIGS. 1 and 2 , the second and third stages,  12  and  14  are different than the first stage  10 . The housings  56  and  58  are cylindrically shaped and receive a combination inlet/volute  24  and  26  respectively. Bolts  64  and  66 , respectively secure the combination inlet/volute respectively to housings  56  and  58 . The housings  56  and  58  are cast integrally with the lower gear box  20  and the intercooler housings  16  and  18 . Second and third stages  12  and  14  illustrate the prior known technique and are included in the illustrated three-stage system to provide contrast for a clearer understanding of the advantages of the present invention.  FIG. 4  also illustrates a prior art technique, which provides further contrast and understanding of the advantages of the present invention. In the illustrative stage  90  of  FIG. 4 , the machined scroll/volute  90  is a complicated piece having numerous machined surfaces, each of which necessarily has a tolerance on one or both sides of the ideal dimension. Impeller  92  has a plurality of blades  96  extending radially from near its center. A clearance in the radial direction is required as between the blades  96  and surface  98  on inlet/volute  94 . The location and orientation of this clearance is also seen in  FIG. 3 , which is a close-up of first stage  10 , illustrating the clearance in the case of the present invention. The clearance  72  in the first stage  10  can be reduced to less than 0.020 inches as compared to the stage  90  where the counterpart clearance can run in the range of 0.024-0.035 inches or greater. The clearance  72  is obtained solely as a result of a casting followed by a machining process. To date, commercially available equipment of the type shown in  FIG. 4  has not been built with smaller clearances. While, theoretically, a coating process can be employed to further reduce clearances in the prior  FIG. 4  design below 0.024 inches, practically, these techniques have not been employed in centrifugal compressor applications for reasons of quality control problems and prohibitive cost. 
     The reason a smaller clearance is obtained in the first stage  10  is that it incorporates a volute type scroll as the housing  74 . Inlet  22  has an opening  76  made of a surface  78 , which conforms to the outer periphery of blades  80 . The radial clearance  72  eventually becomes an axial clearance in conformity to the shape of blades  80 . Since the volute is cast integrally to the housing  74  there are fewer surfaces to machine on the casting and on inlet  22  to fit them up. The ultimate blade clearance  72  can be smaller than in the stage  90  because there are fewer opportunities for the accumulated tolerances on the various machined surfaces to add up when the volute type scroll is cast integrally as housing  74 . There are also reduced man-hours for assembly of the first stage  10  as well as labor savings in reduced machining. The disadvantage of the second stage  12  is that by combining the volute into the inlet  60  and then inserting the inlet  60  into the cylindrically shaped housing  56  the outer profile of housing  56  is increased due to a near doubling of the wall thickness at the periphery. To illustrate the concept, had the first stage  10  been built in the same manner as the second stage  12  using the same exterior dimensions for the casting shown in  FIG. 1 , there would have been no room to mount the oil pump  52  and coupling  54  to the shaft  28  between the first and second stages  10  and  12  respectively. As previously stated, being able to power the oil pump  52  off of shaft  28  becomes an issue if there is a power failure from the perspective of protecting the bearings such as  42 . The other alternative of simply making the entire casting larger adds significant cost to the finished product. 
     It should be noted that with regard to the first stage  10 , that the diffuser plate  82  is secured to inlet  22  with bolts  84  before fitting up inlet  22  to the volute type scroll housing  74  using bolts  86 . The clearance  72  minimization allows the first stage to achieve an efficiency improvement of 1-2% and slightly more. This improvement is magnified in the subsequent stages of compression. Operating expenses can be reduced and a smaller driver utilized because of the reduction in internal leakage from use of smaller clearances. Of course, even greater efficiency can be obtained from using the volute type scroll cast integrally as the housing in all stages in a multi-stage assembly such as shown in  FIG. 1 . The oil pump  52 , even if there is a power failure, continues to deliver enough oil to all the bearings as shaft  28  slows down but continues to drive the oil pump  52 . 
     Referring to  FIG. 2  it can be seen that gearbox  20  is made integrally with housing  74  to accept a predetermined spacing between shaft  28  for bull gear  32  and shaft  38  for the pinion gear  34 . To make all the parts go together, the air seal  40  has a properly located bore (not shown) through which the pinion shaft  38  extends. The inlet assembly  22  has an inlet opening  76  to align it with the impeller  36 . Referring to  FIG. 3 , it can be seen that the diffuser  82  is part of the inlet assembly  22  and also features an opening (not shown) in which the impeller  36  resides. 
     The question comes up and the present invention addresses, what happens if a compressor assembly is constructed to a given speed for the first stage  10  for example and the operator has experienced a change in conditions and want to increase the speed. The original equipment manufacturer wants to be able to help the customer change the speed but issues arise as to what to stock in inventory and the cost of maintaining a variety of parts to accommodate a variety of selected driven speeds. Typically, the bull gear  32  is the largest gear in the gearbox and is very expensive to produce in a variety of teeth count configurations. If the bull gear  32  is not changed then a speed change involves changing the teeth count of the pinion  34  to reduce teeth for a speedup or increase teeth for a slowing down of the first stage  10  for example. However, fitting a new pinion gear  34  changes the center to center distance between shafts  28  and  38  and the integral casting of the gearbox  20  and the housing  74  was to a fixed center to center distance. 
     It is believed the present invention solves this problem. If the pinion  34  is replaced with a smaller gear to speed up the first stage  10 , for example, the bearings  42 ′ on either side of new pinion  34 ′ must be relocated within the housing  20  to adapt to the new center to center shaft distance.  FIG. 9  shows the bearing housings  43  with offset bores  45  to offset the bearings  42 ′ for a change in shaft center distance created by use of the new pinion  34 ′. In order to allow the shaft  38 ′ to enter the housing  74  through the air seal  40  the opening in it  47  is made eccentric to its center. This eccentric bore concept for the seal is also addressed for the inlet assembly  22 ′ and the diffuser  82 ′ connected to it.  FIGS. 5-8  show the inlet assembly  22 ′ with an off-center opening  76 ′ relative to the bolt circle diameter that accepts bolts  86  (see  FIG. 3 ).  FIG. 6  shows the modified inlet assembly  22 ′ mounted to housing  74  of the first stage, for example.  FIGS. 7 and 8  show the rotated left and rotated right positions for the inlet  76 ′, respectively. Those skilled in the art will appreciate that the opening  49  in the diffuser  82  is preferably aligned with opening  76 ′ in inlet  22 ′ so that the impeller  36  fits within diffuser  82  while the shaft  38 ′ goes through an eccentric opening relative to the center of seal  40  so that seal  40  fits in the same opening as before but now creates an offset bore for shaft  38 . The same result occurs for the bearings for shaft  38 ′. As a result of the offset opening  76 ′ in inlet assembly  22 ′ and the matching opening in the diffuser  82 ′ attached to it as well as a similar offset in air seal  40  there are now two positions at minimum for the center to center distance between shafts  28  and  38 ′ if they are kept at the same horizontal plane. Even more center to center distance changes are possible if the shafts are not limited to being in the same horizontal plane. Alternatively, the end user that requires a speed change can also buy a different inlet assembly  22 ′ with a diffuser  82  having a matching bore offset to accommodate the impeller  38  and just purchase those components with a new air seal  40  and relocate the bearings  42  closer or further away, as needed and everything can go back together and function in the existing equipment that has the integral gear box  74  and housing  74 . 
     As an alternative, offset bearing holders similar to those shown for the pinion shaft  38 ′ can be used for the bull gear shaft  28 . In that way, the bull gear can be relocated closer or further from a pinion shaft  38  that itself can be moved or left where it is. In a given gearbox and scroll combination housing the speed can be changed with a change of the pinion size, the bull gear size or by changing both depending on the desired end speed for the compressor stage that is desired. The center distance can be increased or decreased by moving one or both pinion and bull gear shafts. Offset mounting for the shafts are made possible by mounts with eccentric capabilities that allow the original components such as bearings and seals to be mounted in a variety of positions to accommodate the new shaft location within an existing gearbox. Speed changes are now possible for such precast units without major expense of obtaining new gearbox with scroll integrated housings. 
     While the invention has been described and illustrated in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the scope of the claims below are the full scope of the invention being protected.