Abstract:
A centrifugal compressor may comprise a wheel and a pipe diffuser. The pipe diffuser may comprise an annular pipe diffuser section and at least one cylindrical hole passing through the annular pipe diffuser section for compressing gas emerging from the wheel. The at least one cylindrical hole may have an axis that is slanted at an angle A relative to a plane that is orthogonal to an axis of rotation of the compressor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to centrifugal compressors and, more particularly, to centrifugal compressors with pipe diffusers. 
     Centrifugal gas compressors may be employed in numerous applications which may benefit from compact size of the compressor and a relatively low cost. It may be desirable to provide compact and low cost compressors in vehicles. For example, centrifugal compressors may be used as turbochargers in automotive applications or as air compressors in environmental control systems of aircraft. 
     A typical centrifugal compressor may employ a vaned wheel to draw gas from an inlet and accelerate the gas. As high-velocity gas emerges from an outlet of the wheel, the gas may enter a diffuser in which its velocity may be decreased and its static pressure may be increased. As the gas emerges from the diffuser, it may be in a compressed state. 
     Some centrifugal compressors may employ diffusers with vanes that form channels for reduction of gas velocity. These are known as vaned diffusers. Other centrifugal compressors may employ a solid diffuser in which tapered cylindrical openings act as velocity reduction channels. These tapered cylindrical openings are typically referred to as pipes. A diffuser which employs pipes is referred to as a pipe diffuser. 
     A typical pipe diffuser may be produced less expensively than a typical vaned diffuser. Consequently, it is desirable to build vehicular centrifugal compressors with pipe diffusers rather than vaned diffusers. The advantageously lower cost of a pipe diffuser has evolved largely as a result of fact that pipe diffusers may be fabricated from a single piece of metal with conventional machining techniques. While it is desirable to produce a centrifugal compressor at a low cost, it also important to assure that the centrifugal compressor may operate efficiently. It has been found that a smooth and controlled transition of gas from the wheel into the diffuser pipes is a key feature for providing high efficiency. 
     Referring now to  FIG. 1 , a cross-sectional view of a conventional centrifugal compressor  10  illustrates a relationship between a wheel  12  and a diffuser  14 . The diffuser  14  may comprise an annular pipe diffuser section  14 - 2  and an integral shroud  14 - 4 . In the compressor  10 , a transition region  16  may exist between outer extremities of blades  12 - 2  of the wheel  12  and inlets  18 - 2  of pipes  18  of the diffuser  14 . Gas  20  may enter the compressor  10  through an inlet  22 . Rotation of the wheel  12  may accelerate the gas  20  along the blades  12 - 2  and drive the gas  20  toward the diffuser  14 . The gas  20  may decelerate in the diffuser  14  as the gas  20  passes through the pipes  18 . Passage of the gas through the pipes  18  may result in an increase in static pressure of the gas  20 . 
     A design of the compressor  10  may be modeled mathematically to optimize its operational features. Such mathematical modeling may, for example, seek to minimize energy applied to the wheel  12  while maximizing static pressure produced in the pipes  18 . In the context of the modeling, consideration may be given to a shape of the blades  12 - 2 , a shape of the pipes  18  and also a shape of the transition region  16 . After a mathematical model has been completed, resultant shapes for the blades  12 - 2  and the pipes  18  may be implemented with conventional fabrication techniques. For example, the wheel  12  and its blades  12 - 2  may be produced as a single metal casting. The pipes  18  of the diffuser  14  may be produced by conventional machining techniques such as drilling and honing. 
     Referring now to  FIGS. 2 and 3 , it may be seen that a mathematically defined shape of the transition region  16  shown in  FIG. 1  may not be readily producible with conventional metal fabrication techniques.  FIG. 2  illustrates a portion of the diffuser  14  during a fabrication step in which one of the pipes  18  may be produced as a hole in the annular pipe diffuser section  14 - 2  with a drill (not shown) and a honing tool  24 . The honing tool  24  may produce a tapered hollow cylindrical shape for the pipe  18 . In order to provide a complete shaping of any one of the pipes  18 , the honing tool  24  may be required to project beyond an inner extremity or inlet  18 - 2  of the pipe  18  as illustrated in  FIG. 2 . When the honing tool  24  extends beyond the inlet  18 - 2 , the tool  24  may produce tool marks or indentations  26  in the shroud  14 - 4  of the diffuser  14  as shown in  FIG. 3 . The honing tool  24  may also produce indentations  28  in a backplane  14 - 6  of the diffuser  14 . The indentations  26  and  28  may result in a transition region  16 ′ having a shape that may be inconsistent with its mathematical model. Thus a configuration of the transition region  16 ′ of  FIG. 3  may differ from a shape of transition region  16  of  FIG. 1 . 
     Referring now to  FIG. 4 , a collective effect of the indentations  28  may be seen.  FIG. 4  may represent a cross-section of a portion of the diffuser  14  taken in a direction of the backplane  14 - 6 . A dashed-line circle  30  may represent a desired radius at which the inlets  18 - 2  of the pipes  18  are to be located. The indentations  28  may produce an irregular surface on the backplane  14 - 6  of the transition region  16 ′. 
     As can be seen, there is a need to provide high efficiency in a centrifugal compressors with pipe diffusers even though the diffuser may be produced with low-cost fabrication techniques. In that regard there is a need to tailor compressor design to accommodate low cost fabrication techniques in order to optimize compressor efficiency and cost of fabrication. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a centrifugal compressor may comprise a wheel and a diffuser comprising an annular pipe diffuser section; at least one cylindrical hole passing through the annular pipe diffuser section for compressing gas emerging from the wheel; and; the at least one cylindrical hole having an axis that is slanted at an angle A relative to a plane that is orthogonal to an axis of rotation of the compressor. 
     In another aspect of the present invention, a method for producing a centrifugal compressor may comprise the steps of producing pipes in an annular pipe diffuser, having an integral shroud, so that axes of the pipes slant at an angle A relative to a plane that is orthogonal to an axis of the compressor; positioning the pipe diffuser around a wheel; wherein a portion of the shroud forms a first boundary of a transition region between the wheel and the annular pipe diffuser; and wherein the first boundary is free of any indentations resulting from the step of producing the pipes. 
     In a further aspect of the present invention, a method for producing a pipe diffuser for a centrifugal compressor may comprise the steps of producing an annular diffuser integrally with a shroud; and producing pipes through the annular diffuser with axes of the pipes slanted relative to a plane that is orthogonal to an axis of the compressor and so that the pipes are produced without producing tool marks on the shroud. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional elevation view of a centrifugal compressor in accordance with the prior art; 
         FIG. 2  is an isometric view of a cut-away portion of a diffuser of the compressor of  FIG. 1  in accordance with the prior art; 
         FIG. 3  is an isometric view of portion of the diffuser of  FIG. 2  illustrating tool marks in accordance with the prior art; 
         FIG. 4  is a sectional view of the diffuser of  FIG. 3  taken along the section line  4 - 4  in accordance with the prior art; 
         FIG. 5  is a partial sectional elevation view of a centrifugal compressor in accordance an embodiment of the present invention; 
         FIG. 6  is an isometric view of a cut-away portion of a diffuser of the compressor of  FIG. 5  in accordance with an embodiment of the present invention; 
         FIG. 7  is an isometric view of a cut-away portion of the diffuser of  FIG. 6 , illustrating a honing tool in operation, in accordance with an embodiment of the present invention; 
         FIG. 8  is an isometric view of the cut-away portion of the diffuser of  FIG. 7 , with the honing tool removed, in accordance with an embodiment of the present invention; 
         FIG. 9  is a sectional view of the diffuser of  FIG. 8  taken through a pipe of the diffuser in accordance with an embodiment of the present invention; and 
         FIG. 10  is a flow chart of a method for constructing a centrifugal compressor in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Various inventive features are described below that can each be used independently of one another or in combination with other features. 
     Broadly, embodiments of the present invention generally provide a centrifugal compressor with a pipe diffuser in which a transition region between a wheel and the diffuser is not adversely distorted by fabrication steps. 
     Referring now to  FIG. 5 , a partial sectional view of a compressor  100  illustrates a design in which a transition region  116  may retain a desired mathematically modeled shape after pipes  118  are honed. In the compressor  100 , a diffuser  114  may be constructed with an annular pipe diffuser section  114 - 2  and an integral shroud  114 - 4 . Pipes  118  may be formed as tapered holes in the annular pipe diffuser section. The pipes  118  may have axes  118 - 4  oriented in a non-orthogonal relationship with an axis  100 - 2  of the compressor  100 . Additionally, the compressor  100  may be constructed so that a backplane  115 - 2  of the transition region  116  may be incorporated on a backplate  115  of the compressor  100 . In other words, unlike the backplane  14 - 6  of the diffuser  14  of prior art  FIG. 1 , the backplane  115 - 2  may be separate from the diffuser  114 . 
     Referring now to  FIGS. 5 through 8 , advantages of these structural features may be understood. In  FIG. 5 , it may be seen that the honing tool  24  may pass to a position adjacent the shroud  114 - 4  without contacting the shroud. Consequently, undesirable tool marks or indentations in the shroud  114 - 4  may be avoided. When indentations are avoided, the transition region  116  may remain free of distortions. This may be readily understood by referring to  FIG. 6  in which it may be seen that the honing tool  24  may produce no tool marks on the shroud  114 - 4 . 
       FIG. 7  also illustrates a portion of the diffuser  114  and the shroud  114 - 4  which partially comprise the transition region  116 . It may be noted that the shroud  114 - 4  may not be distorted with any indentation such as the indentations  26  of prior art  FIG. 3 . 
     This feature of an embodiment of the present invention may be further understood by referring to  FIG. 8 .  FIG. 8  shows a partial sectional view through one of the pipes  118 . It may be seen that the axis  118 - 4  of the pipe  118  is oriented at an angle A relative to a hypothetical plane  119  which may be orthogonal to the axis  100 - 2  of the compressor  100  of  FIG. 5 . The honing tool  24  of  FIG. 6  may pass through the pipe  118  without contacting the shroud  114 - 4 . Consequently, the shroud  114 - 4  may remain free of indentations such as the indentations  26  of prior art  FIG. 3 . 
     Referring back now to  FIG. 5 , it may be noted that the backplate  115  may be a compressor component separate from the diffuser  114 . The backplate  115  may be produced independently from production of the diffuser  114 . Consequently, the honing tool  24  may pass through the pipes  118  without contacting the backplate  115 . Thus the backplate  115  may remain free of indentations such as the indentations  28  of prior art  FIG. 3 . The compressor  100  may be assembled as shown in  FIG. 5  with the transition region  116  configured in accordance with a desired shape that is consistent with mathematical modeling. In other words, there may be no distortions of the transition region boundaries (i.e., the shroud  114 - 4  and the backplane  115 - 2 ) resulting from indentations such as tool marks from the honing tool  24 . 
     It may also be seen that the transition region  116  may be produced with a shape in which walls of the region  116  may diverge from one another. But, as illustrated in  FIG. 5 , this divergence may be limited to an amount that is merely sufficient to provide a width of an outlet  116 - 4  of the region  116  that may be about equal to a diameter of inlets  118 - 2  of the pipes  118 . In other words, a smooth gas flow transition may be provided in which only minimal gas turbulence is produced. This desirable configuration may be achieved by selecting the angle A so that the honing tool  24  produces the inlet  118 - 2  in a shape of a hole with an outer edge  118 - 2 - 2  adjacent the shroud  114 - 4 . 
     The transition-region backplane  115 - 2  may be incorporated directly onto the backplate  115 . Consequently, the compressor  100  may be constructed with a part count that is no greater than that of the compressor  10  of  FIG. 1 , i.e., the compressors  10  and  100  may each be constructed with a single diffuser and a single backplate. In other words the compressor  100  may be produced at a cost that is no greater than a cost of production of the prior art compressor  10 . 
     In one embodiment of the present invention, a method is provided for constructing a centrifugal compressor. In that regard, the method may be understood by referring to  FIG. 10 . In  FIG. 10 , a flow chart may portray various aspects of a method  200 . In a step  202 , a pipe diffuser may be produced with pipes having angled axes (e.g., the pipes  118  may be produced with the honing tool  24  oriented at an angle A relative to a plane  119  that is orthogonal to an axis  100 - 2  of the compressor  100 ). In a step  204 , a backplane of a transition region may be formed on a backplate (e.g., the backplane  115 - 2  of the transition region  116  may be formed on the backplate  115 ). In a step  206 , the backplate and the diffuser may be combined with a wheel to produce a compressor assembly (e.g., the wheel  112 , the diffuser  114  and the backplate  115  may be assembled on the axis  100 - 2 ). In a step  208 , the backplate may be axially aligned with the wheel and the diffuser to produce a transition region of a desired shape (e.g., the backplate  115  may be shimmed axially to produce the transition region  116  with a width of the outlet  116 - 4  that is about the same as a diameter of an inlet  118 - 2 ). 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.