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.
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""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 xe2x80x9ccompressor/turbinexe2x80x9d 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 xe2x80x9cupwardxe2x80x9d into the stator channel. The fluid in the stator channel is displaced and forced radially inward and xe2x80x9cdownwardxe2x80x9d 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 xe2x80x9cdownwardxe2x80x9d into the impeller bucket. The fluid in the impeller buckets is displaced and forced radially inward and xe2x80x9cupwardxe2x80x9d 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""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.
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""s shaft by the impeller wheels and to the compressor""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""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""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.