Abstract:
A vaned diffuser for a centrifugal compressor has provision for selectively adjusting the pitch of the vanes in order to accommodate variable load conditions. Each of the vanes is rotatable about a pivot pin near its leading edge and is engaged with an actuation member near its trailing edge. The actuation members are attached to a common ring which can be selectively rotated to move to the vanes in unison. The ring is supported by rollers at its outer periphery and is positioned at the outer periphery of a diffuser wall such that there is no forward facing step projecting into the flow stream.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to centrifugal compressors and, more particularly, to a diffuser structure for centrifugal compressors.  
           [0002]    One of the major problems arising in the use of centrifugal vapor compressors is that of maintaining flow stabilization when the compressor load varies over a wide range. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate. Accordingly, when there is a relatively low volumetric flow rate through such a compressor, the flow becomes unstable in the following manner. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there occurs a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are complete flow reversals in the diffuser passage, destroying the efficiency the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates are desirable in most compressor applications, numerous modifications have been suggested to improve flow stability and machine efficiencies at low volumetric flow rates.  
           [0003]    In U.S. Pat. No. 3,362, 625, a vaneless diffuser is provided with flow restrictors which serve to regulate the flow within the diffuser in an effort to improve stability at low volumetric flow rates. In U.S. Pat. Nos. 2,996,996 and 4,378,194, there are described variable width vane diffusers wherein the diffuser veins are securely affixed, as by bolting, to one of the diffuser walls. The vanes are adapted to pass through openings formed in the other wall, thus permitting the geometry of the diffuser to be changed in response to changing load conditions. Although a vaned diffuser is preferred over a vaneless diffuser because a vaned diffuser is more efficient at design incidence than a vaneless diffuser, the variable width vane diffusers presented a number of problems, particularly in regard to the manufacture, maintenance and operation of the machine.  
           [0004]    Such problems were overcome in the vaned diffuser shown in U.S. Pat. No. 5,807,071, wherein a pair of interconnected rings are provided to jointly define the flow passages which can be selectively varied by rotating one of the rings.  
           [0005]    Another approach to a variable vaned diffuser is that shown in U.S. Pat. No. 5,683,223, wherein the individual vanes are selectively rotated in unison by way of a mechanism connected thereto to thereby accommodate the variable load conditions. Generally, such an arrangement is problematic in two respects. First, it is difficult to obtain the precise control that is needed in order to maintain uniformity in the positioning of the individual vanes. That is, for example, if it is desired that all vanes are closed, any inaccuracies in the positioning mechanism may well allow one or more of the vanes to be in a partially open position, thereby introducing inefficiencies that are undesirable. These nonuniformities are further complicated by the existence of various tolerances and the wear of components that are typical of such machines. Secondly, the substantial forces that are exerted on the leading edges of such variable position vanes, tend to cause vibration of the leading edges thereof to thereby affect dynamic stability. In order to control and or eliminate these vibrations it is necessary to provide a very strong, durable and stable vane positioning mechanism which is designed with these considerations in mind.  
         SUMMARY OF THE INVENTION  
         [0006]    The object of the present invention is to provide, in a centrifugal compressor, a vaned diffuser, with the vanes being variably positioned and selectively controlled in order to effectively and accurately vary the pitch of the vanes in order to accommodate the variable load levels in the compressor.  
           [0007]    In a preferred embodiment, a vane mounting means is provided with each vane having a pivot pin disposed near its leading edge and acting to position its vane, an actuation mechanism engaging each of the vanes near its trailing edge and operable to rotate the vane on the axis of its pivot pin, and a slot in each of the vanes to allow for relative movement between the vane and mounting means when they are relatively rotated.  
           [0008]    Such an arrangement provides for a positive and accurate positioning of the vanes so as to maintain a stable flow of gases therethrough.  
           [0009]    In accordance with another aspect of the invention, the actuation mechanism includes a shaft and an associated eccentric cam surface which engages said vane, with the shaft being rotatable to cause the vane to rotate.  
           [0010]    By another aspect of the invention, the pivot pin is integral with the vane.  
           [0011]    By yet another aspect of the invention, the slot is located near the trailing edge of the vane and the cam surface is disposed in the slot.  
           [0012]    In accordance with another aspect of the invention, the pivot pin is disposed in the slot.  
           [0013]    In accordance with another aspect of the invention, the cam surface is round and is mounted in a round opening in the vane.  
           [0014]    By another aspect of the invention, the actuation mechanism includes a ring which is interconnected to each of the vanes by way of actuation pins, and means for rotating the ring to move the vanes in substantial unison.  
           [0015]    In accordance with another aspect of invention, the actuation pins are integral with the rotatable ring and are disposed in the openings formed in the vanes.  
           [0016]    By yet another aspect of the invention, the vane openings are elongated to allow reciprocal movement between the actuation pins and the vanes.  
           [0017]    By still another aspect of the invention, the actuation pins are integral with the vanes and are disposed in openings in the rotatable ring.  
           [0018]    The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments considered in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a perspective view of a centrifugal compressor with one embodiment of the present invention incorporated therein.  
         [0020]    [0020]FIG. 2 is an exploded perspective view of the vane and cam portion thereof in accordance with the preferred embodiment.  
         [0021]    [0021]FIGS. 3A and 3B show an alternative embodiment of the vane and cam member thereof.  
         [0022]    [0022]FIG. 4 is a sectional view of the vane and cam members as seen along lines A-A of FIG. 3A.  
         [0023]    [0023]FIG. 5 is a partial cut-away view of the vanes and actuation ring thereof in accordance with the preferred embodiment.  
         [0024]    [0024]FIGS. 6 and 7 are front and rear perspective views thereof.  
         [0025]    [0025]FIG. 8 is a schematic illustration of a side view of the present invention as installed in a centrifugal compressor in accordance with the preferred embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    Referring to FIG. 1, the invention is shown generally at  10  as incorporated into a centrifugal compressor having an impeller  11  for compressing refrigerant gas to a high-pressure, high kenetic energy state, after which it passes to the diffuser  12  where the kenetic energy is converted to potential energy or static pressure, and finally it is passed to the collector  13  where the pressure is caused to become uniform prior to entering the discharge line.  
         [0027]    Initially, the refrigerant is caused to enter the suction housing  14  and to pass through the inlet guide vanes  16 . The flow volume is selectively controlled in a rather conventional manner by adjustment of the pitch of the inlet guide vanes  16  by way of pulleys  17  and cables  18  as driven by a drive motor  19 . In a similar but unconventional manner, the pitch of the diffuser vanes  21  are selectively varied by an actuation mechanism which includes a drive motor  22  and crank linkage which includes a drive shaft  23 , a collar with an actuation arm  24 , a linkage arm  26 , and a drive pulley  27 . In operation, the drive motor  22  selectively rotates the drive shaft  23  along with the collar  24  so as to thereby cause the linkage arm  26  to translate and rotate the drive pulley  27  to which it is connected. The rotation of the drive pulley  27  causes the cable  28  to move because of the mechanical engagement therewith, and the other pulleys  29  are then caused to rotate in unison with the drive pulley  27 . Since each pulley  29  is connected to an actuation shaft  31 , a rotation of the pulleys  29  causes rotation of the actuation shafts  31 , which will bring about a movement of the diffuser vanes  21  in a manner to be more fully described hereinbelow.  
         [0028]    It should be recognized that the pulley and cable drive arrangement shown and described herein is merely one of many approaches that can be employed for the purpose of actuating the vane movement mechanism and should therefore be considered merely a simple mechanical representative of the many possibilities which could include various alternatives of mechanical, hydraulic or electrical drive systems, for example. A rack and pinion drive arrangement will later be described as a preferred mechanical approach.  
         [0029]    Referring now to FIG. 2, the diffuser vane  21  and actuation shaft  31  are shown in greater detail. For simplicity, the diffuser vane  21  is shown to be triangular in shape but in actuality would be optimized for aerodynamic performance and would therefore be generally triangular in shape but could be of various specific shapes. It has a leading edge  32  and a trailing edge  33 , with the fluid flow on either side of the vane  21  flowing from the leading edge  32  toward the trailing edge  33 . Located near the leading edge  32  is a pivot pin  34  extending outwardly from one side  36  thereof for mounting and positioning of the vane  21 . In the preferred embodiment, the pivot pin  34  is rotatably mounted on a fixed axis so as to permit a rotary movement of the vane  21  about the axis in a manner to be more fully described hereinafter.  
         [0030]    Located near the trailing edge  33  of the vane  21  is a slot  37  extending along a longitudinal plane extending between the leading edge  32  and the trailing edge  33 . The actuation shaft  31  has an offset pin  38  extending eccentrically from its one end as shown. With the offset pin  38  installed in the slot  37 , rotation of the actuation shaft  31  causes a side-to-side movement of the trailing edge  33 , with any relative movement between the offset pin and the vane  21  being accommodated by the longitudinal movement of the offset pin  38  within the slot  37 . The forward placement of the pivot pin  34  as shown provides for dynamic stability with minimal vibration at the leading edge  32  of the vane  21 . Clearance and alignment problems are minimized by the fact that the actuation shaft  31  is designed to engage, but is not attached to, the vane  21 . Finally, the cam action of the offset pin  38  makes it possible to make minute adjustments in the vane position since relatively large rotational movements of the actuation shaft  31  are required in order to effect relatively small rotational movements of the vane  21 .  
         [0031]    An alternative embodiment of the vane and its associated mounting and actuation means is shown in FIGS. 3 and 4. Here, the vane  41  has a longitudinally extending slot  42  located near the leading edge  43  of the vane  41 , and a circular opening  44  located near the trailing edge  46  thereof. The mounting arrangement includes a fixed pivot pin  45  that fits into the slot  42  such that the vane  41  can rotate about its axis. The actuation mechanism includes a rotatable shaft  47  which has a disk  48  rigidly attached to its end in an eccentric manner as shown. A rotation of the shaft  47  within its bearings  49  and  51  causes a rotation of the disk  48  within the circular opening  44  so as to thereby bring about a rotation of the vane  41  about the axis of the pivot pin  45 . Any radial movement of the vane  41  disk caused by the eccentric action of the disk  44  will be accommodated by the longitudinal movement of the pivot pin  45  within the slot  42 . Although the slot  42  is shown to be linear and longitudinally aligned in form, it may be angled from the longitudinal direction or even curved in order to optimize the control of the leading edge  43 .  
         [0032]    Returning now to the preferred embodiment, reference is made to FIGS.  5 - 7  wherein more detail is shown with respect to the actuation system for varying the pitch of the vanes. A diffuser housing  52  is made up of a pair of annular components, a flange plate  53  and a bearing ring  54  fastened together by a plurality of bolts  56  and spacers (not shown) in spaced relationship such that a diffuser channel  57  is defined therebetween for locating the diffuser vanes  21  and for conducting the flow of fluid which flows radially outwardly from the impeller (not shown) mounted in a central opening therein. Rigidly attached to and extending from an inner surface  59  of the flange plate  53  are a plurality of pivot pins  34  on which the diffuser vanes  21  are rotatably mounted. The clearance between the pivot pins  34  and the openings in the vanes  21  are sufficient to permit easy rotation of the vanes on the pivot pins  34  but not so great as to allow for any significant translational or vibrational movement between the components.  
         [0033]    The bearing ring  54  has an annular channel  61  formed therein for rotatably receiving a coordinating ring  62  therein (see FIGS. 6 and 7 ), with bearings  63  being provided for smooth and easy rotation of the ring  62 . One side  64  on the ring  62  has a plurality of circumferentially spaced actuation pins  66  extending therefrom for engagement with the respective slots  37  of the diffuser vanes  21  (see FIGS. 5 and 7). A rotation of the ring  62  therefore causes all of the vanes  21  to uniformly change their pitch by rotating about the respective axes of their pivot pins  34 . During such rotation, the actuation pins  66  will move in the radial direction with respect to their respective vanes, and this relative movement is accommodated by the movement of the actuation pins  66  within their respective slots  37 .  
         [0034]    It should be recognized that, because the coordinating ring  62  is mounted internally within the diffuser, and is closely coupled to the vanes  21  in a very simple, robust, and cost-effective manner as described, the potential for wear, looseness and inaccuracies in the positional control of the vanes is minimized. Further, because the motion of the pins and the vanes closely approximate each other, sliding motion is minimized, and the adjustment of individual vanes is made unnecessary, thereby making the mechanism easy to assemble and service.  
         [0035]    Turning now to a preferred approach as to how the coordinating ring is selectively made to rotate, a coordinating ring is shown at  67  in FIG. 8 to include a gear rack  68  secured by bolts  69  to the indent  71  of the coordinating ring  67 . The rack is operably engaged with a pinion  72  as shown in FIG. 10, with the pinion  72  being driven by the drive motor  22  and drive shaft  23  as shown in FIG. 1. The coordinating ring  67  is supported by three circumferentially spaced rollers  73  disposed at its inner diameter and being rotatably secured to the machine framework by securing apparatus  74  as shown in FIG. 8. Axial support of the coordinating ring  67  is provided by a plurality of circumferentially spaced pads  76  which frictionally engage one side  77  of the coordinating ring  67 . The positioning of the pads  76  is fine-tuned by the adjusting threaded shaft  78  to enable a proper positioning and axial support of the coordinating ring  67 .  
         [0036]    Before going into further details of the present invention, it would be well to revisit the design as shown in FIGS. 6 and 7. There, the coordinating ring  62  is disposed in an annular channel  61  of the bearing ring  54 . If the dimensioning of the components and the fit of one within the other is precise, then there is no problem with respect to the loss of efficiency because of drag that may be caused by a forward facing member. However, if one of the components has an edge that extends axially into the stream of fluid flow as it passes radially outwardly, then the efficiency will be reduced. For example, if the forward face (i.e. the face not seen in FIG. 6 but seen in FIG. 7) of the coordinating ring  62  extends axially beyond the forward face of the bearing ring  54 , then its radially inner edge will be projecting into the flow stream to provide an unnecessary restriction to the flow. If, on the other hand, the forward face of the coordinating ring  62  does not extend as far forward as the corresponding face of the bearing ring  54 , then the radially outer edge of the annular channel  61  will be exposed to the flow stream. This problem is overcome by the design of FIGS.  8 - 10  wherein the coordinating ring  67  is not recessed within an annular channel  61  as shown in FIG. 6, but is rather located radially outwardly at the outer edge of the bearing ring  54  has shown in FIGS. 9 and 10. Here, as will be seen, there is no surrounding portion of the bearing ring  54  structure that can affect the performance as described hereinabove. Accordingly, in order that the coordinating ring  67  may not extend axially beyond the face of the bearing ring  54  so as to create the problem as described hereinabove, it is deliberately made with a smaller axial thickness as shown in FIG. 9 so that it will never project into the flow stream. The problem that this would have created with the FIG. 6 design, as described above, is alleviated since there is no bearing ring structure which can project into the flow stream. Such an arrangement also simplifies the machining process as compared with that required for the annular channel  61  of FIG. 6. As will be seen in FIG. 9, as a result of the coordinating ring  67  being located radially outwardly, the radially outer surface  79  of the coordinating ring  67  is also preferably substantially radially aligned with the trailing edges  33  of the vanes  21 .  
         [0037]    While the present invention has been described with reference to a number of specific embodiments, it should be understood that the spirit and scope of the present invention is determined with reference to the appended claims.