US3390545A - Boundary layer control on interstage guide vanes of a multistage centrifugal compressor in a refrigeration system - Google Patents

Description

July 2, 1968 n. c. HOFFMAN 3390,55

BOUNDARY LAYER CONTROL ON INTERSTAGE GUIDE VANES OF A MULTISTAGE: CENTRIFUGAL COMPRESSOR IN A REFRIGERATION SYSTEM Filed June 28, 1967 2 Shee'cs-Sheeil l INVENTOR.

DAVID C. HOFFMAN @uw Qa July 2, 1968 D, c. HOFFMAN 3,390,545

BOUNDARY LAYER CONTROL ON INTERSTAGE GUIDE VANES OF A MULTISTAGE CENTRIFUGAL COMPRESSOR IN A REFRIGERATION SYSTEM Filed June 28, 1967 2 Sheets-Sheet 2 INVENTOR. DAVID C. HOFFMAN nited States Patent C 3,390,545 BOUNDARY LAYER CONTROL N INTERSTAGE GUIDE VANES 0F A MULTISTAGE CENTRIF- UGAL COMPRESSOR IN A REFRIGERATION SYSTEM David C. Hoffman, Stoddard, Wis., assignor to The Trane Company, La Crosse, Wis., a corporation of Wisconsin Filed June 28, 1967, Ser. No. 649,583 8 Claims. (Cl. 62-510) ABSTRACT 0F THE DISCLOSURE Separation and stall at the interstage guide vanes of a multistage centrifugal compressor are reduced by using a relatively high pressure jet of fluid to energize the boundary layer adjacent each guide vane in the region where separation normally occurs. Flash gas formed in an intermediate pressure economizer chamber located in the refrigeration system in which the centrifugal compressor is installed is employed as the boundary layer energizing fluid.

Background of the invention In gas pumping machinery a flow condition known as separation almost invariably occurs. This is the phenomenon according to which fluid streamlines leave the solid bounding walls of a passage 0r body rather than following the bounding surface. As streamlines separate from a wall, they divert the boundary layer from the wall, causing sheets of vorticity to leave the wall. The fluid in the separated region normally takes the form of a bubble. Separation reduces performance efficiency because of the pressure losses which it produces in the main fluid stream. The most significant loss is the increased shearing force produced as the main flow passes over a separated region. A separated bubble will normally cause a shear loss in the main flow stream many times that caused by a wall. Other flow losses are generated as the separated fluid mixes with the main flow downstream from the region of separation. The condition of extreme separation causing the accumulation of large quantities of stagnant fluid, and often unsteadiness adjacent the portion of the bounding surface from which the flowing fluid has separated, is known as stall. The periodic shedding of the accumulated stagnant fluid causes the unsteadiness.

Separation and stall are limitations on the performance of any fluid handling device. Most gas handling machinery is designed with the flow on the verge of, but not quite separated, for this is where the highest performance is usually achieved. Separation in compressors is probably responsible for the onset of surge at a flow slightly below the peak efiiciency range. If the load point where stall commences can be increased, then a concomitant increase in performance will invariably be realized.

Boundary layer energizing is one method by which stall and separation may be eliminated or minimized. This involves supplying additional energy to the particles of fluid which are being retarded in the boundary layer. One way in which this may be accomplished is to inject a secondary stream of relatively high pressure fluid to increase the momentum of the boundary layer near the wall or surface over which the fluid is flowing. It is known that this can be accomplished by discharging fluid from the interior of a moving impeller blade or wing over which the fluid is moving, or by injecting a portion of the high energy fluid being pumped into the low energy separation zone thru slots correctly placed in the fluid handling device. See for example, Streeter, Handbook of Fluid Dynamics, McGraw-Hill, 1961, and U.S. Patent Nos. 3,069,072 and 3,237,850.

Brief summary of the invention I have applied the basic concept of boundary layer energization by injecting economizer gas in a unique way to improve the fluid flow efliciency of the interstage, stationary guide vane portion of a multiple stage centrifugal compressor. The centrifugal compressor of my invention is employed to compress refrigerant gas in a refrigeration system consisting of a compressor, condenser, economizer, and an evaporator interconnected in refrigerant flow relationship. The economizer is a vessel or chamber interposed between the condenser and the evaporator to effect a pressure reduction of the liquid refrigerant before it enters the evaporator. As the liquid is reduced in pressure in the economizer a portion of it flashes to vapor, cooling the remaining liquid. The flash gas from the economizer is normally directed to an intermediate pressure stage of the refrigerant compressor. According to my invention, an interstage wall of the compressor is provided with a plurality of angled nozzles each of which has its outlet disposed adjacent the suction side of one of a number of fixed guide vanes located in the crossover passage between stages of the compressor. Refrigerant gas from the economizer 4is directed through the nozzles into the crossover passage. The location and angle of the nozzles is such that economizer gas is injected into the separated flow region near the trailing surface of each fixed guide vane. In this way, the boundary layer adjacent each fixed guide vane is energized and the losses normally associated with separation and stall are greatly minimized if not eliminated.

Brief description of the drawings FIGURE 1 is a front elevation view, partly in section, showing the improved compressor of my invention.

FIGURE 2 is a vertical section view taken along line 2-2 of FIGURE 1, showing the fixed interstage guide vanes and the economizer injection nozzles.

FIGURE 3 is a schematic view showing the manner in which my improved compressor is located in a refrigeration system utilizing an economizer.

FIGURE 4 illustrates the gas flow pattern between interstage guide vanes Without boundary layer control.

FIGURE 5 illustrates the improved gas flow pattern between guide vanes achieved by using the economizer gas injection nozzles of this invention.

Description of the preferred embodiment With reference to FIGURES l and 2 of the drawings, the multiple stage compressor 1 of my invention is comprised of an outer casing 4 through which a drive shaft 2 extends. Shaft 2 may be driven by any convenient means such as a turbine or engine, not shown. Mounted upon drive shaft 2 for rotation therewith are a first-stage impeller wheel 6 and a second stage impeller wheel 8. A crossover passage generally indicated by reference numeral 13, having a discharge portion 14 and a return and diffusing portion 16, connects the outlet of first stage mpeller 6 with the inlet of second stage impeller 8. Suction gas is introduced into the inlet 12 of first stage impeller 6 from a suction gas pipe 10. Gas compressed by first stage centrifugal impeller 6 is discharged into passage 14 from which it is directed into diffusing and return passage 16 towards suction inlet 18 of second stage impeller 8. Blades 19 of impeller wheel 8 add energy to the gas to obtain the final discharge pressure. The `gas flows through discharge passage 20 into chamber 21 from which it is `conducted out of the compressor by discharge conduit 22.

Refrigerant gas ow into first stage impeller wheel 6 is controlled and the capacity of impeller 6 is regulated by a first set of adjustable guide vanes 24. A similar set of adjustable guide vanes 26 are disposed upstream of the suction inlet 18 to second stage impeller 8. Linkage mechanism generally indicated by reference numeral actuates first stage guide vanes 24 and is driven by shaft 31 which extends outside of the compressor casing 4 to a point of connection with control means not shown. A similar linkage mechanism 32 is employed to actuate second stage guide vanes 26. Linkage mechanisms 30` and 32 are interconnected for synchronous operation by common drive shaft 28.

As is indicated in FIGURES 1 and 2, plate 36 which serves to define interstage return passageway 16 is provided with a plurality of fixed guide vanes 34 mounted thereon. The purpose of guide vanes 34 is to turn the gas radially inwardly towards second stage suction inlet 18, and to assist in diffusing the gas delivered from the first stage of compression. The gas coming from the first stage of compression has a relatively high tangential component of velocity. This tangential component of velocity is substantially removed as the gas is guided through passage 16 by vanes 34 and is turned radially inwardly. The conversion of kinetic energy into static pressure is accomplished by the turning action of vanes 34 promoting diffusion of the total velocity of the compressed gas as it passes through the areas of gradually increasing cross section between adjacent vanes 34.

Because the refrigerant gas is being diffused inwardly in the direction of decreasing radius, blades 34 are highly loaded and are therefore quite susceptible to separation and stall. FIGURE 4 illustrates the fiow condition which would normally develop between adjacent vanes 34. As is shown in that figure by reference numeral 15, a separated or stalled region exists on the suction or low pressure side of guide vanes 34 indicated by minus signs. Such a fiow condition has an adverse effect on compressor etliciency as a result of the additional pressure losses which it creates in the gas stream flowing to the second stage suction inlet 18. Furthermore, the zone of substantially no flow which separation produces adjacent `the trailing side of each vane 34 causes a poor flow distribution into second stage suction inlet 18.

In order to overcome these difiiculties and improve compressor efficiency as well as to provide a method of introducing economizer gas, I have provided a plurality of injection nozzles 40 in vane plate 36. As is indicated in FIGURES 1 and 2, nozzles 40 are angled in order to direct economizer gas from chamber 38 generally parallel to the unseparated 'flow direction in return passage 16. Nozzles 40 are circumferentially spaced about plate 36, each of said nozzles having its outlet disposed adjacent the trailing side of one of the guide vanes 34 in the region where separation normally occurs.

The manner in which compressor 1 is connected in a standard refrigeration system to receive a supply of economizer gas for injection through nozzles is illustrated in FIGURE 3. Refrigerant gas discharged by second stage impeller 8 passes through discharge conduit 22 to condenser 42 where the refrigerant is condensed to a liquid. From condenser 42 the refrigerant liquid passes through pipe 44 to economizer vessel 46. In economizer vessel 46 the refrigerant liquid is lflashed to an intermediate pressure as the result of which the main body of liquid is further chilled and a certain amount of flash gas is formed. The chilled refrigerant liquid is directed through conduit 52 to evaporator 54 where it vaporizes to produce the desired cooling effect on a secondary liquid such as water flowing through coil 53. The vaporized refrigerant gas then passes through suction line 10 to the inlet of first stage impeller 6. Flash gas formed in economizer 46 passes through separator 48 and then into pipe 50 which is connected to compressor 1.

With reference again to FIGURES 1 and 2, economizer gas flowing out of pipe 50 is directed into plenum chamber 38 formed within compressor casing 4 between vane plate 36 and second stage diffuser plate 25. From plenum chamber 38, the economizer gas is discharged through nozzles 40 into the main stream of refrigerant flowing through return passage 16. The location and angular disposition of nozzles 40 are such that economizer Igas is injected tangentially with respect to curved trailing side 35 of each vane 34. The relatively high energy economizer gas serves to energize the stagnant or low energy boundary layer adjacent each vane 34 and to prevent separation, thus establishing ow in a zone where there is normally substantially no flow. Nozzles 40 are tapered, and are sized and shaped so as to produce jets of economizer gas having substantial velocity without creating such a back pressure as to significantly dampen the vaporization of refrigerant in economizer 46.

The improved flow pattern realized by economizer gas injection in the aforesaid manner is illustrated in FIGURE 5. The injection of high energy economizer gas in the normally stalled region adjacent each vane 34 serves to settle and reattach the flow of gas over the trailing side 35 of vanes 34. As may be seen by the streamlines in FIGURE 5, separation is substantially eliminated and refrigerant gas follows the bounding surface of vanes 34 throughout their entire length. Gas is thus discharged uniformly from the entire cross sectional area between adjacent vanes 34, and an even distribution of refrigerant gas all of the way around second stage suction inlet 18 is realized.

Thus the specific method of injecting economizer gas so as to energize the `boundary layer on fixed guide vanes 34 provides several important benefits. First of all, separation and stall are substantially reduced if not e1iminated, thus greatly minimizing the flow losses normally associated therewith. Secondly, theimproved flow pattern producing uniform distribution of refrigerant gas into second stage suction inlet 18 significantly improves the performance of second stage impeller 8. Thirdly, the noise generating tendencies of second stage impeller 8 are reduced by the uniform fiow of refrigerant gas therethrough.

Although I have shown and described a two stage centrifugal compressor, a compressor having any number of stages could obviously be employed, with economizer gas being injected at the appropriate intermediate pressure stage. I anticipate that various other modifications will occur to those skilled in the tart which will be within the spirit and scope of my invention as defined by the following claims.

I claim:

1. In a refrigeration machine having a compressor, condenser, economizer, refrigerant Iliquid flow regulating means, and an evaporator interconnected in a closed circuit, said economizer comp-rising a flash chamber interposed between said condenser and said evaporator, and said compressor being of the multiple stage centrifugal type having axially spaced impe-ller wheels with crossover passage means therebetween, and guide vane means in said crossover passage means to turn the refrigerant gas owing from one of said impeller wheels and cause it to flow in a radial direction, the improvement comprising:

means for injecting refrigerant gas from said economizer into said crossover passage means near said guide vane means in such a way as to energize the boundary layer adjacent the surface of said guide vane means, thereby minimizing flow separation from said guide vane means.

2. Apparatus as defined in claim 1 wherein:

said crossover passage means comprises a discharge passage portion and a return passage portion leading inwardly towards the inlet of the next one of said impeller Wheels;

and wherein said guide vane means comprises (a plurality of fixed, curved vanes circumferentially spaced within said return passage portion.

3. Apparatus as defined in claim 2 wherein:

said means for injecting refrigerant gas from said economizer in'to said crossover passage is so disposed and arranged as to direct said gas into the normally stagnant boundary layer near the trailing side of each of said guide vanes in a direction general-ly parallel to the main stream of refrigerant gas flowing `between said guide vanes.

4. Apparatus as defined in claim 2. wherein:

said compressor comprises an outer casing within which said impeller wheels are rotatably mounted;

and further including wall means within said casing and cooperating therewith to form an economizer gas plenum chamber, said chamber vbeing connected to said economizer by a conduit extending through said casing;

and wherein said wall means includes a dividing wall separating said return passage Iportion from said economizer gas plenum chamber;

and wherein said means for injecting refrigerant gas from said economizer into said crossover passage means comprises a plurality of circumferentially spaced openings in said dividing wall communicating said economizer gas plenum chamber with said return passage portion.

5. Apparatus as dened in claim 4 wherein:

said openings are in the form of nozzles, and each of said nozzles is disposed with its outlet directed towards the stagnant boundary layer on the trailing surface of the low pressure side of one of said xed guide vanes.

6. Apparatus as defined in claim 4 wherein:

each of said openings is angled inwardly towards the axis of said compressor in a direction such that econ-omizer gas discharging from each of said openings flows in a direction generally tangent to the curved trailing surface of one of said guide varies.

7. Apparatus as defined in claim 4 wherein:

said openings are in the form of tapered nozzles capable of increasing the velocity of economizer gas owing therethrough.

8. Apparatus as dened in claim 4 wherein:

said dividing wall is in the form of a vane plate to which said guide vanes are secured.

References Cited UNITED STATES PATENTS 2,277,647 3/1942 Jones 62-498 3,011,322 12/1961 Tanzberger 62--510 MEYER PERLIN, Primary Examiner.

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