Patent Publication Number: US-6709243-B1

Title: Rotary machine with reduced axial thrust loads

Description:
TECHNICAL FIELD 
     The present invention relates to an improved helical flow compressor design modified so as to produce very low bearing thrust loads without a loss in efficiency. 
     BACKGROUND ART 
     A helical flow compressor is a high-speed rotary machine that accomplishes compression by imparting a velocity head to each fluid particle as it passes through the machine&#39;s impeller blades then converting that velocity head into a pressure head in a stator channel that functions as a vaneless diffuser. While in this respect a helical flow compressor has some characteristics in common with a centrifugal compressor, the primary flow in a helical flow compressor is peripheral and asymmetrical, while in a centrifugal compressor, the primary flow is radial and symmetrical. The fluid particles passing through a helical flow compressor travel around the periphery of the helical flow compressor impeller within a generally horseshoe-shaped stator channel. Within this channel, the fluid particles travel along helical streamlines, the centerline of the helix coinciding with the center of the curved stator channel. This flow pattern causes each fluid particle to pass through the impeller blades or buckets many times while it travels through the helical flow compressor, each time acquiring kinetic energy. After each pass through the impeller blades, the fluid particle reenters the adjacent stator channel where it converts its kinetic energy into potential energy which, in turn, produces a peripheral pressure gradient in the stator channel. 
     The multiple passes through the impeller blades (regenerative flow pattern) allows a helical flow compressor to produce discharge heads of up to fifteen (15) times those produced by a centrifugal compressor operating at equal tip speeds. Since the cross-sectional area of the peripheral flow in a helical flow compressor is usually smaller than the cross-sectional area of the radial flow in a centrifugal compressor, a helical flow compressor would normally operate at flows which are lower than the flows of a centrifugal compressor having an equal impeller diameter and operating at an equal tip speed. The high-head, low-flow performance characteristics of a helical flow compressor make it well suited to a number of applications where a reciprocating compressor, a rotary displacement compressor, or a low specific-speed centrifugal compressor would not be as well suited. 
     A helical flow compressor can be utilized as a turbine by supplying it with a high pressure working fluid, dropping fluid pressure through the machine, and extracting the resulting shaft horsepower with a generator. Hence the term “compressor/turbine” which is used throughout this application. 
     The flow in a helical flow compressor can be visualized as two fluid streams which first merge and then divide as they pass through the compressor. One fluid stream travels within the impeller buckets and endlessly circles the compressor. The second fluid stream enters the compressor radially through the inlet port and then moves into the horseshoe-shaped stator channel which is adjacent to the impeller buckets. Here the fluids in the two streams merge and mix. The stator channel and impeller bucket streams continue to exchange fluid while the stator channel fluid stream is drawn around the compressor by the impeller motion. When the stator channel fluid stream has traveled around most of the compressor periphery, its further circular travel is blocked by the stripper plate. The stator channel fluid stream then turns radially outward and exits from the compressor through the discharge port. The remaining impeller bucket fluid stream passes through the stripper plate within the buckets and merges with the fluid just entering the compressor/turbine. 
     The fluid in the impeller buckets of a helical flow compressor travels around the compressor at a peripheral velocity which is essentially equal to the impeller blade velocity. It thus experiences a strong centrifugal force which tends to drive it radially outward, out of the buckets. The fluid in the adjacent stator channel travels at an average peripheral velocity of between five (5) and ninety-nine (99) percent of the impeller blade velocity depending upon the compressor discharge flow. It thus experiences a centrifugal force which is much less than that experienced by the fluid in the impeller buckets. Since these two centrifugal forces oppose each other and are unequal, the fluid occupying the impeller buckets and the stator channel is driven into a circulating or regenerative flow. The fluid in the impeller buckets is driven radially outward and “upward” into the stator channel. The fluid in the stator channel is displaced and forced radially inward and “downward” into the impeller bucket. 
     The fluid in the impeller buckets of a helical flow turbine travels around the turbine at a peripheral velocity which is essentially equal to the impeller blade velocity. It thus experiences a strong centrifugal force which would like to drive it radially outward if unopposed by other forces. The fluid in the adjacent stator channel travels at an average peripheral velocity of between one hundred and one percent (101%) and two hundred percent (200%) of the impeller blade velocity, depending upon the turbine discharge flow. It thus experiences a centrifugal force which is much greater than that experienced by the fluid in the impeller buckets. Since these two centrifugal forces oppose each other and are unequal, the fluid occupying the impeller buckets and the stator channel is driven into a circulating or regenerative flow. The fluid in the stator channel is driven radially outward and “downward” into the impeller bucket. The fluid in the impeller buckets is displaced and forced radially inward and “upward” into the stator channel. 
     While the fluid is traveling regeneratively, it is also traveling peripherally around the stator-impeller channel. Thus, each fluid particle passing through a helical flow compressor or turbine travels along a helical streamline, the centerline of the helix coinciding with the center of the generally horseshoe-shaped stator-impeller channel. While the unique capabilities of a helical flow compressor would seem to offer many applications, the low flow limitation has severely curtailed their widespread utilization. 
     Permanent magnet motors and generators, on the other hand, are used widely in many and varied applications. This type of motor/generator has a stationary field coil and a rotatable armature of permanent magnets. In recent years, high energy product permanent magnets having significant energy increases have become available. Samarium cobalt permanent magnets having an energy product of twenty-seven (27) megagauss-oersted (mgo) are now readily available and neodymium-iron-boron magnets with an energy product of thirty-five (35) megagauss-oersted are also available. Even further increases of mgo to over 45 megagauss-oersted promise to be available soon. The use of such high energy product permanent magnets permits increasingly smaller machines capable of supplying increasingly higher power outputs. The permanent magnet rotor may comprise a plurality of equally spaced magnetic poles of alternating polarity or may even be a sintered one-piece magnet with radial orientation. The stator would normally include a plurality of windings producing rotatable electro-magnet poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. In the motor mode, alternating electrical current is passed through the coils which will cause the permanent magnet rotor to rotate. 
     U.S. Pat. No. 5,899,673 provides an example of a helical flow compressor/turbine integrated with a permanent magnet motor/generator, and is hereby incorporated by reference in its entirety. 
     In a multi-stage helical flow compressor, multiple impellers are arranged along a common shaft to achieve a desired pressure rise. The impeller wheels are generally very thin and relatively large in diameter. If there is any leakage of pressurized fluid between compression stages, such as through the radial gap between the rotating impeller spacer rings and the compressor housing, a pressure differential will develop across the impeller wheel in each stage. Each stage&#39;s pressure differential, acting on the large area of the impeller wheel, applies a thrust load to the compressor shaft. The thrust loads generated in each stage are cumulative, normally resulting in high thrust loads being applied to the bearings supporting the compressor shaft and impeller wheels. These loads may induce unwanted bearing deflections, wheel rubbing and bearing damage or failure. These problems may occur in single-stage or multi-stage helical flow compressors. 
     Accordingly, it is desirable to provide an improved helical flow compressor wherein thrust loads applied to the impeller(s) are minimized. 
     DISCLOSURE OF INVENTION 
     The present invention provides an improved helical flow compressor wherein thrust loads applied to the impeller(s) are minimized in various embodiments of the invention by providing axially oriented vent holes through the impeller(s), eliminating the radial flow splitter, providing labyrinth seals between adjacent impellers and between the motor cavity and the impeller adjacent to it, as well as by providing at least one bypass vent around the shaft support bearing adjacent to the motor cavity. 
     More specifically, in a preferred embodiment, the present invention provides a rotary machine including a helical flow compressor/turbine and a permanent magnet motor/generator mounted and operated within a common housing. A shaft is rotatably supported within the housing. A permanent magnet rotor is mounted on the shaft and operatively associated with the motor/generator stator. Disk shaped impeller wheels are mounted on the shaft each having a plurality of impeller blades extending therefrom. The compressor/turbine section of the housing includes a generally horseshoe-shaped fluid flow stator channel on each side of each impeller wheel with an inlet at a first end and an outlet at a second end for each wheel/stage. The fluid in each generally horseshoe-shaped fluid flow stator channel proceeds from the inlet to the outlet while following a generally helical flow path with multiple passes through the impeller blades. Each impeller disk has a plurality of axially-oriented vent holes formed therethrough to minimize a pressure differential across the impeller, thereby minimizing thrust loads applied to the impeller. 
     The vent holes in the impeller disk are preferably chamfered to reduce local pressure drop where fluid enters or exits the holes. A ratio of hole diameter to outer chamfer diameter is optimized based on the axial clearance between the impeller disk and the adjacent housing so as to minimize flow restrictions and minimize vent hole volume. 
     The commonly-used radial flow splitter, such as that described in U.S. Pat. No. 5,899,673, is eliminated from the housing adjacent the periphery of the impeller blades, thereby providing a radial gap between the periphery of the impeller blades and the housing to allow increased axial flow around the periphery of the impeller blades to further minimize the pressure differential across the impeller. 
     In one embodiment, the shaft is supported by ball bearings, and at least one bypass vent is formed through the housing around the ball bearing closest to the large gas storage volume of the motor in order to provide fluid communication between opposing sides of the bearing which minimizes the flow of contaminant-laden gas through the bearing. 
     In a multi-stage helical flow compressor, a labyrinth seal is disposed between any or all adjacent impellers to minimize leakage between impellers, thereby decreasing thrust loads on the impellers. (For example, there would be three seals for four impeller wheel/disks.) 
     Accordingly, it is a principal object of the invention to provide an improved helical flow compressor wherein thrust loads applied to the impeller(s) are minimized. 
     It is another object of the invention to provide a helical flow compressor having features which reduce the thrust load applied to the compressor&#39;s shaft by the impeller wheels and to the compressor&#39;s bearings by the compressor shaft. 
     It is another object of the invention to provide a helical flow compressor with decreased pressure differentials across its impeller wheels by decreasing restrictions for axial flow of fluid through or around each impeller wheel, and increasing restrictions for axial flow of fluid between the impeller wheels. 
     It is yet another object of the invention to provide a helical flow compressor with a pattern of axially-oriented vent holes that pass through the compressor&#39;s impeller wheels in order to reduce the pressure differential across the wheel and reduce the thrust load applied to the wheel, shaft, and bearings. 
     It is still another object of the invention to provide a helical flow compressor with a pattern of axially-oriented vent holes passing through the compressor&#39;s impeller wheels, with chamfers provided for each of the holes where the holes meet the surfaces of the wheel in order to reduce the local pressure drop where the flow enters or exits the holes. 
     It is another object of the present invention to provide a helical flow compressor with a pattern axially-oriented vent holes with chamfers wherein the ratio of the hole diameter to the outer chamfer diameter is optimized to minimize flow restrictions and minimize vent hole/chamfer volume. 
     Still further, it is another object of the invention to provide a helical flow compressor with an increased axial flow area radially outboard of the impeller wheel and radially inboard of the housing by deleting the radial flow splitter which normally occupies the entire periphery of the pump, except in the area of the flow stripper. 
     It is yet another object of the invention to provide a helical flow compressor wherein vent holes, vent chamfers, and the elimination of the radial flow splitter combine to minimize the flow restriction from one side of an impeller wheel to the other side. 
     It is another object of the present invention to provide a helical flow compressor wherein labyrinth seals are located between the impeller wheels to minimize leakage between compressor stages, thereby minimizing the thrust load applied to the wheels, the shaft, and the bearings. 
     It is a further object of the invention to provide a helical flow compressor wherein decreasing axial flow restrictions for each wheel with vent holes, vent chamfers, and the elimination of the radial flow splitter combined with increasing axial flow restrictions between adjacent wheels with labyrinth seals minimizes the pressure differentials across the impeller wheels and thus minimizes the thrust load applied to the wheels, the shaft, and the bearings. 
     It is a further object to provide a helical flow compressor impeller with axial vent holes which are smaller at outer row(s) to limit swept through volume penalties at the stripper plate location. 
     It is another object of the invention to provide a helical flow compressor wherein axial flow restrictions are decreased across the bearing adjacent to the motor by providing vent holes around the bearing to minimize the undesired flow of fluid through the bearing. 
     It is a further object to provide a helical flow compressor with a labyrinth seal between the motor cavity (on either side of the bearing) and adjacent the impeller wheels. 
     The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Having thus described the present invention in general terms, reference will now be made to the accompanying drawings in which: 
     FIG. 1 is an end view of a two-stage helical flow compressor/turbine permanent magnet motor/generator of the present invention; 
     FIG. 2 is a cross-sectional view of the helical flow compressor/turbine permanent magnet motor/generator of FIG. 1 taken along line  2 — 2 ; 
     FIG. 3 is a cross-sectional view of the helical flow compressor/turbine permanent magnet motor/generator of FIG. 1 taken along line  3 — 3 ; 
     FIG. 4 is an enlarged sectional view of a portion of the low pressure stage of a prior art helical flow compressor/turbine permanent magnet motor/generator; 
     FIG. 5 is an enlarged sectional view of a portion of the low pressure stage of the helical flow compressor/turbine permanent magnet motor/generator of FIG. 3; 
     FIG. 6 is an enlarged sectional view of the helical flow compressor/turbine permanent magnet motor/generator of FIGS. 1-3 illustrating the cross-over of fluid from the low pressure stage to the high pressure stage; 
     FIG. 7 is an enlarged schematically-arranged partial plan view of the helical flow compressor/turbine impeller having straight radial blades and illustrating the flow of fluid therethrough; 
     FIG. 8 is an enlarged partial plan view of a helical flow compressor/turbine impeller having curved blades; 
     FIG. 9 is an exploded perspective view of a stator channel plate of the helical flow compressor/turbine permanent magnet motor/generator of FIGS. 1-3; 
     FIG. 10 is an enlarged sectional view of a portion of FIG. 2 illustrating fluid flow streamlines in the impeller blades and fluid flow stator channels; 
     FIG. 11 is a schematic representation of the flow of fluid through a helical flow compressor/turbine; 
     FIG. 12 is a cut-away perspective view of a partially disassembled four-stage helical flow compressor/turbine permanent magnet motor/generator in accordance with a second embodiment of the invention; 
     FIG. 13 is a longitudinal cross-sectional view of the four-stage helical flow compressor/turbine permanent magnet motor/generator of FIG. 12; 
     FIG. 14 is a partially cut-away, partially disassembled perspective view of a thrust disk, shaft, and a plurality of impellers corresponding with the embodiment of FIG. 12; 
     FIG. 15 shows a perspective view of a labyrinth seal in accordance with the embodiment of FIG. 12; 
     FIG. 16 shows a longitudinal cross-sectional view of a four-stage helical flow compressor/turbine permanent magnet motor/generator in accordance with a third embodiment of the invention; 
     FIG. 17 shows a perspective view of an impeller in accordance with the invention; 
     FIG. 18 shows a plan view of an alternative impeller in accordance with the invention; and 
     FIG. 19 shows a partial cross-sectional view of the impeller of FIG.  18 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A two-stage helical flow compressor/turbine permanent magnet motor/generator  15  is illustrated in FIGS. 1-3 and includes a fluid inlet  18  to provide fluid to the helical flow compressor/turbine  17  of the helical flow compressor/turbine permanent magnet motor/generator  15  and a fluid outlet  16  to remove fluid from the helical flow compressor/turbine  17  of the helical flow compressor/turbine permanent magnet motor/generator  15 . The helical flow machine is referred to as a compressor/turbine since it can function as both a compressor and as a turbine. The permanent magnet machine is referred to as a motor/generator since it can function equally well as a motor to produce shaft horsepower or as a generator to produce electrical power. 
     The helical flow compressor/turbine permanent magnet motor/generator  15  includes a shaft  20  rotatably supported by duplex ball bearings  21  and  31  at one end and single ball bearing  22  at the opposite end. The bearings are disposed on either side of low pressure stage impeller  24  and high pressure stage impeller  23  mounted at one end of the shaft  20 , while permanent magnet motor/generator rotor  27  is mounted at the opposite end thereof. The duplex ball bearings  21  and  31  are held by bearing retainer  28 , while single ball bearing  22  is disposed between high pressure stator channel plate  32  and the shaft  20 . Both the low pressure stage impeller  24  and high pressure stage impeller  23  include a plurality of blades  26 . 
     Low pressure stripper plate  37  and high pressure stripper plate  36  are disposed radially outward from low pressure impeller  24  and high pressure impeller  23 , respectively. Following the general description, it will be explained that the stripper plates  36 , 37  have been modified in accordance with the present invention. 
     The permanent magnet motor/generator rotor  27  on the shaft  20  is disposed to rotate within permanent magnet motor/generator stator  48  which is disposed in the permanent magnet housing  49 . The low pressure impeller  24  is disposed to rotate between the low pressure stator channel plate  34  and the mid-stator channel plate  33 , while the high pressure impeller  23  is disposed to rotate between the mid-stator channel plate  33  and the high pressure stator channel plate  32 . Low pressure stripper plate  37  has a thickness slightly greater than the thickness of low pressure impeller  24  to provide a running clearance for the low pressure impeller  24  between low pressure stator channel plate  34  and mid-stator channel plate  33 , while high pressure stripper plate  36  has a thickness slightly greater than the thickness of high pressure impeller  23  to provide a running clearance for the high pressure impeller  23  between mid-stator channel plate  33  and high pressure stator channel plate  32 . 
     The low pressure stator channel plate  34  includes a generally horseshoe-shaped fluid flow stator channel  42  having an inlet to receive fluid from the fluid inlet  56 . The mid-stator channel plate  33  includes a low pressure generally horseshoe-shaped fluid flow stator channel  41  on the low pressure side thereof and a high pressure generally horseshoe-shaped fluid flow stator channel  40  on the high pressure side thereof. The low pressure generally horseshoe-shaped fluid flow stator channel  41  on the low pressure side of the mid-stator channel plate  33  mirrors the generally horseshoe-shaped fluid flow stator channel  42  in the low pressure stator channel plate  34 . The high pressure stator channel plate  32  includes a generally horseshoe-shaped fluid flow stator channel  38  which mirrors the high pressure generally horseshoe-shaped fluid flow stator channel  40  on the high pressure side of mid-stator channel plate  33 . 
     Each of the stator channels includes an inlet and an outlet disposed radially outward from the channel. The inlets and outlets of the low pressure stator channel plate generally horseshoe-shaped fluid flow stator channel  42  and mid-helical flow stator channel plate low pressure generally horseshoe-shaped fluid flow stator channel  41  are axially aligned as are the inlets and outlets of mid-helical flow stator channel plate high pressure generally horseshoe-shaped fluid flow stator channel  40  and high pressure stator channel plate generally horseshoe-shaped fluid flow stator channel  38 . 
     The fluid inlet  18  extends through the high pressure stator channel plate  32 , high pressure stripper plate  36 , and mid-stator channel plate  33  to the inlets of both of low pressure stator channel plate generally horseshoe-shaped fluid flow stator channel  42  and mid-helical flow stator channel plate low pressure generally horseshoe-shaped fluid flow stator channel  41 . The fluid inlet  18  extends from the outlets of both the mid-helical flow stator channel plate high pressure generally horseshoe-shaped fluid flow stator channel  40  and high pressure stator channel plate generally horseshoe-shaped fluid flow stator channel  38 , through the high pressure stripper plate  36 , and through the high pressure stator channel plate  32 . 
     The cross-over from the low pressure compression stage to the high pressure compression stage is illustrated in FIG.  6 . Both of the outlets from the low pressure stator channel plate generally horseshoe-shaped fluid flow stator channel  42  and mid-helical flow stator channel plate low pressure generally horseshoe-shaped fluid flow stator channel  41  provide partially compressed fluid to the cross-over  88  which, in turn, provides the partially compressed fluid to both inlets of mid-helical flow stator channel plate high pressure generally horseshoe-shaped fluid flow stator channel  40  and high pressure stator channel plate generally horseshoe-shaped fluid flow stator channel  38 . 
     The impeller blades or buckets are best illustrated in FIGS. 7 and 8. The radial outward edge of the impeller  23  includes a plurality of low pressure blades  26 . While these blades  28  may be radially straight as shown in FIG. 7, there may be specific applications and/or operating conditions where curved blades may be more appropriate or required. 
     FIG. 8 illustrates a portion of a helical flow compressor/turbine impeller having a plurality of curved blades  44 . The curved blade base or root  45  has less of a curve than the leading edge  46  thereof. The curved blade tip  47 , at both the root  45  and leading edge  46  would be generally radial. 
     The fluid flow stator channels are best illustrated in FIG. 9, which shows the mid-stator channel plate  33 . The generally horseshoe-shaped stator channel  41  is shown along with inlet  55  and outlet  56 . The inlet  55  and outlet  56  would normally be displaced approximately 30°. Outlet  56  connects with cross-over  58 . An alignment or locator hole  57  is provided in each of the low pressure stator channel plate  34 , the mid-stator channel plate  33 , and the high pressure stator channel plate  32 , as well as stripper plates  37  and  36 . The inlet  55  is connected to the generally horseshoe-shaped stator channel  40  by a converging nozzle passage  51 , but converts fluid pressure energy into fluid velocity energy. Likewise, the other end of the generally horseshoe-shaped stator channel  40  is connected to the outlet  56  by a diverging diffuser passage  52  that converts fluid velocity energy into fluid pressure energy. 
     The depth and cross-sectional flow area of fluid flow stator channel  40  are tapered preferably so that the peripheral flow velocity need not vary as fluid pressure and density vary along the fluid flow stator channel. When compressing, the depth of the fluid flow stator channel  40  decreases from inlet to outlet as the pressure and density increases. Converging nozzle passage  41  and diverging diffuser passage  42  allow efficient conversion of fluid pressure energy into fluid velocity energy and vice-versa. 
     FIG. 10 shows the flow through the impeller blades and the fluid flow stator channels by means of streamlines  53 . FIG. 11 schematically illustrates the helical flow around the centerline of the impeller and fluid flow stator channel. The turning of the flow is illustrated by the alternating solid and open flow pattern lines in FIG.  11 . 
     In a helical flow compressor/turbine, fluid enters the inlet port  18 , and is accelerated as it passes through the converging nozzle passage  51 , splits into two flow paths (formerly by a radial flow splitter), then enters the end of the generally horseshoe-shaped fluid flow stator channels  41  and  42  axially adjacent to the low pressure impeller blades  26 . The fluid is then directed radially inward to the root of the impeller blades  26  by a pressure gradient, accelerated through and out of the blades  26  by centrifugal force, from where it reenters the fluid flow stator channel. During this time, the fluid has been traveling tangentially around the periphery of the helical flow compressor/turbine. As a result of this, helical flow is established as best shown in FIGS. 7,  10  and  11 . 
     While the duplex ball bearings  21  and  23  are illustrated on the permanent magnet motor/generator end of the helical flow compressor/turbine and the single ball bearing  22  is illustrated at the opposite end of the helical flow compressor/turbine, their positions can readily be reversed with the single ball bearings  22  at the permanent magnet motor/generator end of the helical flow compressor/turbine and the duplex ball bearings  21  and  31  at the opposite end of the helical flow compressor/turbine. Likewise, while the low pressure impeller  24  is shown at the permanent magnet motor/generator end of the helical flow compressor/turbine and the high pressure impeller  23  at the opposite end, their relative positions can also be readily reversed. 
     Returning to FIG. 4, prior art helical flow compressors included a stripper plate  37 ′ with a radial flow splitter  39 ′ positioned between the stator channels  41 , 42  to split the fluid into two flow paths. Surprisingly, it has been discovered that the radial flow splitter  39 ′ shown in FIG. 4 is not needed, and has therefore been eliminated, as shown in FIG.  5 . 
     Accordingly, a radial gap (g) is provided between the periphery  60  of the impeller blades  26  and the radially inboard side  62  of the stripper plate  37 , which is part of the compressor/turbine housing. This radial gap (g), shown in FIG. 5, allows increased fluid flow around the periphery  60  of the impeller blades  26  to minimize the pressure differential across the impeller  24 , thereby reducing thrust loads acting upon the impeller  24 . 
     The radial gap (g) is preferably between approximately 0.047 and 0.049 inch. The radial gap (g) is preferably proportional to the impeller&#39;s bucket depth (i.e., the impeller blade length) and can be unrelated to the impeller diameter. 
     Another feature of the invention is illustrated in FIGS. 2,  3 ,  5  and  6 . As shown, each impeller  23 , 24  includes a pattern of axially-oriented vent holes  64 , 66  therethrough in order to provide fluid communication between opposing sides of the impeller wheels to reduce the pressure differential across the impeller wheels, and thereby reduce the thrust load applied to the impeller wheels, the shaft, and bearings. 
     Turning to FIG. 17, a perspective view of the impeller  24  is shown, illustrating the axial holes  66  therein. 
     FIGS. 18 and 19 show an alternative impeller  70  having three rows of differently sized axial holes  72 , 74 , 76 . Preferably, the larger holes  72  are positioned near the center of the impeller  70 , and the smaller holes  76  are positioned near the periphery of the impeller  70 . Also, each hole  72 , 74 , 76  includes a chamfer  78 , 80 , 82  to reduce local pressure drop where fluid enters or exits the holes. A ratio of the hole diameter (d) to outer chamfer diameter (c) is optimized to minimize flow restrictions and minimize vent hole volume. This optimization is effected by wheel-to-housing axial clearance, which is commonly 0.005 inch adjacent each face of the impeller. 
     The present invention is applicable to single impeller flow machines, as well as two-stage, three-stage, four-stage, etc. flow machines. FIGS. 12-15 illustrate features of the present invention incorporated in a four-stage helical flow compressor/turbine permanent magnet motor/generator  90  in accordance with a second embodiment of the invention. 
     The four-stage helical flow compressor/turbine permanent magnet motor/generator  90  shown in FIGS. 12-15 is in all respects generally similar to the two-stage machine described previously with reference to FIGS. 1-11 except for the addition of third and fourth impellers, and items associated with such structure. The details of the structure and functionality of such a four-stage helical flow compressor/turbine permanent magnet motor/generator is also described in commonly assigned U.S. patent application Ser. No. 09/295,238, which is hereby incorporated by reference in its entirety. The details thereof will not be repeated here. Rather, distinguishing features of the invention will be described. 
     Accordingly, FIGS. 12,  13  and  14  show a four-stage helical flow compressor/turbine permanent magnet motor/generator  90  having four impellers  92 , 94 , 96 , 98  within a housing  100 . Similarly to the embodiment described above with reference to FIGS. 1-3 and  5 - 11 , a radial gap (g), shown in FIG. 13, is implemented between the periphery of each impeller  92 , 94 , 96 , 98  and the corresponding inner surface  102 , 104 , 106 , 108  of the housing  100 . As described with reference to the earlier embodiment, the gap (g) is operative to minimize the pressure differential across each impeller  92 , 94 , 96 , 98 , thereby reducing thrust loads acting upon each impeller. 
     In order to further reduce such thrust loads, leakage between the four stages of the motor/generator  90  is reduced by providing stainless steel labyrinth seals  110 , 112 , 114 , 116  between each impeller, and between the high pressure impeller  98  and the thrust disk  118 . One such labyrinth seal  110  is illustrated in FIG.  15  and includes a central aperture  120  to receive the compressor shaft  122 . A plurality of spaced rings  124  provide a near perfect seal between adjacent impellers by requiring fluid traveling therethrough to expand and compress multiple times before bypassing the seal. The rings  124  are preferably approximately 0.005 inch wide at their respective tips with a 15° to 20° taper angle, and spacing between rings of approximately 10 times the width of the rings. These labyrinth seals minimize flow leakage between compressor stages, thereby further minimizing pressure differential across each impeller to further minimize thrust load applied to the wheels, the shaft and the bearings. 
     As shown in FIG. 13, a plurality of vent holes  126  (only one is shown) are formed through the bottom part  128  of the housing  100  adjacent the air bearing  130  and arranged symmetrically with respect to the shaft  122 , thereby communicating the low pressure stage associated with impeller  92  with the outside of the housing  128  to bypass the air bearing  130 , thereby reducing the pressure differential across the air bearing  130 . Accordingly, the undesired flow of gaseous process fluid through the bearing  130  adjacent the motor  132  is minimized when gas pressure at the compressor inlet changes, gas pressure at the compressor outlet changes, compressor speed changes, turbogenerator speed changes, or turbogenerator power operating level changes. 
     FIG. 16 shows a four-stage helical flow compressor/turbine permanent magnet motor/generator  140  in accordance with a third embodiment of the invention. This embodiment is in most respects similar to the embodiment shown in FIG. 13 except that the air bearings have been replaced by roller bearings  142 , 144 . A plurality of bypass vents  146  are provided to reduce fluid flow through the roller bearings  142 , 144 . The bypass vents  146  also prevent grease from being forced out of the roller bearings  144  by movement of fluid through the bearings  144 . Also, labyrinth seals  148  are provided to minimize leakage between impellers, and a radial gap (g) is provided between the periphery of the impellers  150 , 152 , 154 , 156  and the inward-facing surface of the flow splitters  158 , 160 , 162 , 164  to balance pressures on opposing sides of the impellers  150 , 152 , 154 , 156 . Axial holes  151 , 153 , 155  are provided through the impellers to further balance pressures on opposing sides of the impellers  150 , 152 , 154 , 156  to reduce axial forces on the impellers. 
     These features provide the same benefits as described above with reference to the earlier embodiments. Also, the various aspects of the invention may be provided in various combinations in any sized rotary machine (one-stage to four-stage). 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.