Abstract:
A casting technique is disclosed which improves efficiency and reduces fabrication and assembly cost for a centrifugal compressor. The volute is cast integrally with the impeller housing 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.

Description:
FIELD OF THE INVENTION 
     The field of this invention is centrifugal compressors featuring an integrally cast volute and more particularly to 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 is for plant air systems to supply a motive force for valve actuators and pneumatic cylinders for use in robotic applications, as one example. Centrifugal compressors 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 which 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 rotation increases the fluid velocity and consequently it pressure as the fluid passes the diffuser passageway and into the scroll or volute. The diffuser pas sage 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. 
     In the past, centrifugal compressors have featured a bolt on scroll/volute cover, which encompassed portions of the impeller chamber, and the diffuser passageway and the volute outlet passageway. U.S. Pat. No. 4,181,466 is illustrative of a bolt on component featuring the fluid entry 51 and the volute 50, which is also secured to the bearing housing 15 by a V-clamp 49. One main problem with the bolt on scroll/volute cover incorporating the volute was 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 had a manufacturing tolerance, the final clearance near the impeller had 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, ie. 24 and 26, is the extra space and mass taken up by that type of assembly which could become important in situations where ease of installation and maintenance is important to serviceability. For example, as will be explained below when the preferred embodiment is described, use of bolt-on volutes (such as 24 and 26) precludes 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 because it would stop 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. 
     What is needed is a technique to obtain better efficiency from a centrifugal compressor, whether running alone or connected to others in a multi-stage compression application. One of the objects of the present invention is to realize efficiency and operating cost improvements by integrally casting the volute as a part of the gearbox. Another objective is the reduction of radial clearance on the impeller, which results in an improvement of its efficiency. Another objective is to reduce production and assembly costs. 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 preferred embodiment described below. 
     SUMMARY OF THE INVENTION 
     A casting technique is disclosed which improves efficiency and reduces fabrication and assembly cost for a centrifugal compressor. 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view 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 lines  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. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the preferred embodiment is illustrated in a perspective view, with portions removed for clarity. The motor driver for the package is omitted. The 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 . 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 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 illustrated 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 bullgear  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 . 
     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.