US3680973A

US3680973A - Compressor power recovery

Info

Publication number: US3680973A
Application number: US44463A
Authority: US
Inventors: Karol Pilarczyk
Original assignee: Carrier Corp
Current assignee: Carrier Corp; Elliott Turbomachinery Co Inc
Priority date: 1970-06-08
Filing date: 1970-06-08
Publication date: 1972-08-01
Anticipated expiration: 1989-08-01
Also published as: CH522826A; CA949043A; DE2128217C3; DE2128217B2; DE2128217A1; FR2096064A5; NL7107689A; GB1357182A

Abstract

The three stage centrifugal compressor of the present invention employs a one-piece cast casing having a cylindrical bore containing therein a removable barrel assembly, which assembly comprises three directly engaging clamped diaphragms, three diffusers, three shrouds and a rotor having three impellers, all in axially stacked relationship. Fluid passages are provided integrally cast in the diaphragms and casing for conducting the fluid between stages and from the last stage to a use connection, monitoring devices and a feedback line to the first stage nozzles for partially operating the first stage as a turbine. For starting, a valve in the feedback line is opened so that the first stage operating as a partial turbine will reduce the starting time and power required. On partial load, for example, the output pressure will rise to again open the feedback line valve to reduce power consumption.

Description

United States Patent Pilarczyk 1451 Aug. 1, 1972 [54] COMPRESSOR POWER RECOVERY FOREIGN PATENTS OR APPLICATIONS [72] In n r: K r Pilarczyk, Loudonville, NY. 536,890 5/1941 Great Britain ..415/53 [73] Assignee: m curporafion Syracuse NY 1,070,496 12/1959 Germany ..415/53 [22] Filed: June 8, 1970 Primary Examiner-Henry F. Raduazo Attorney-Harry G. Martin, Jr. and J. Raymond Cur- [21] Appl. No.. 44,463 tin 52 US. 01. ..415/53, 415/1 16, 415/146, 15 1 AB TRAC 415,179 The three stage centrifugal compressor of the present [51] Int.Cl ..F04d 27/00, F04c 29/04, F04d 17/08 invention employs a one piece cast casing having a Fleld of Search "415/ l 1 l6, cylindrical bore containing therein a removable barrel 415/146, 1316- l assembly, which assembly comprises three directly engaging clamped diaphragms, three diffusers, three References Cited shrouds and a rotor having three impellers, all in axially stacked relationship. Fluid passages are provided UNITED STATES PATENTS integrally cast in the diaphragms and casing for con- 1,915,997 6/1933 Hoffmann ..415/116 ducting the fluid between Stages a m the last 2 380,606 7/1945 Moody ..415/1 Stage a connecnonr momtonng devlces and a 2 656 096 10/1953 Schwarz ..4l5/DIG. 1 feedback line the first Stage mules Partially 2'660366 11/1953 Klein et al 415/53 operating the first stage as a turbine. For starting, a 2826l47 3/1958 Gauba'tz "415/53 valve in the feedback line is opened so that the first 2785634 3/1957 i'' 'g 5/143 stage operating as a partial turbine will reduce the 2850029 9/1958 Off d t l 415/10 starting time and power required. On partial load, for 3004494 10 1961 C e 5/143 example, the output pressure will rise to again open 3217'655 I 151965 g y g 'g "f "415/1 1 6 the feedback line valve to reduce power consumption. 3:462:071 8/1969 Garve ..415/1 16 7 Claims, 6 Drawing Figures MSKKMK PATENTEUAUG H972 3.680.973

sum 1 BF 3 'mmm fr? vent-or- ,far'a/ /larcayle PATENTEDAUG 1 m2 3. 680,973

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3 or 3 mvnmon KARoL PILARCZYK ATTORNEY BACKGROUND OF THE INVENTION Power recovery for multi-stage compression that employs pressure responsive valved feedback lines are known, for example in the US. patents to Moody US. Pat. NO. 2,380,606 and Alberger US. Pat. NO. 1,009,819. However, the needs for improved efficiency, decreased manufacturing costs, compactness and versatility are always present.

CROSS-REFERENCING TO RELATED APPLICATIONS The features of the invention of this application may be used in combination with the features of the inventions in applicants following related applications of the same filing date and assignee as the present application, the disclosures of which are incorporated herein in their entirety by reference: Compressor Barrel Assembly, Ser. No. 44,446; Variable Capacity Compressor, Ser. No. 44,263; Interchangeable Compressor Drive, Ser. No. 44,403; compressor Base and Intercoolers, Ser. No. 44,034 now issued as US. Pat. No. 3,644,054.

SUMMARY OF THE INVENTION It is an object of the present invention to improve multi-stage compressor efficiency, compactness and versatility while lowering manufacturing costs, with respect to compressor power recovery by feeding high pressure gas back to a lower stage employing impeller tip nozzles under the control of a pressure responsive and independently actuatable valve. The above is accomplished without a considerable amount of piping about the casing. The axial limitations with respect to length are of considerable advantage in providing a minimum length to the cantilevered portion of an overhung rotor so that high speed operation may be possible. The piping connections between stages are cast into removable diaphragms and a one piece casing.

The removable barrel assembly includes a diaphragm, a shroud, a diffuser and an impeller for each stage. The feedback lines are cast into the casing and diaphragms so that conversion to power recovery is accomplished by drilling one shroud and inserting nozzles, and by providing appropriate valving. The specific location of the nozzles with respect to the lower stage impeller blade tips is an important feature for efficien- BRIEF DESCRIPTION OF THE DRAWING Further objects, features and advantages of the present invention will become more clear from the following detailed description of a preferred embodiment of the present invention as shown in the attached drawing, in which:

FIG. 1 is a perspective view of a complete compressor employing the features of the present invention;

FIG. 2 is a schematic flow sheet showing the path of the fluid as it moves between stages and through the intercoolers;

FIG. 3 is a partial cross-sectional view taken on a vertical plane passing substantially through the axis Of rotation of the compressor of FIG. 1;

FIG. 4 is a side view of the first stage shroud;

FIG. 5 is an end view of the shroud of FIG. 4, looking toward the left as seen in FIG. 3; and

FIG. 6 is a cross-sectional view taken along line 6-6 in FIG. 5. 0

DETAILED DESCRIPTION OF THE DRAWING With reference to FIGS. l-3, the compressor base 1 securely mounts an electric drive motor 2, which has an output shaft 3 for driving the rotor 4 through gear train 5. The gear train 5 is mountedwithin a separate casing 6 that forms the end closure for the compressor casing 7. The

casings

6 and 7 are each cast in one piece from iron, and the base 1 is a welded sheet steel fabrication. Inlet fluid is provided for the compressor through an inlet housing 8 having mounted therein an inlet valve 9 controlled by a suitable mechanism 10.

Within the base 1, there are two separated intercooler chambers 11 and 12 for cooling the fluid between the first and second stages and between the second and third stages, respectively. For the purposes of the present invention, these intercooler chambers may be of any construction and include any type of conventional intercooling equipment, such as a parallel tube water-gas heat exchanger. As shown in FIG. 2, inlet fluid passes through the first stage impeller 13, the intercooler chamber 11, the second stage impeller 14, the intercooler chamber 12, the third stage impeller 15.

As shown in FIGS. 1 and 3, the cast iron casing 7 is provided with an axial cylindrical bore 16 and a planar surface 17, with a plurality of integrally cast passages therebetween for conducting the fluid between stages and the intercoolers. Particularly, passage 18 conducts fluid from the first stage output to the intercooler chamber 11, passage 19 conducts fluid from the intercooler charnber l l to the second stage, passage 20 conducts fluid from the second stage to the intercooler chamber 12, and passage 21 conducts fluid from the intercooler chamber 12 to the third stage.

In FIG. 3, a three stage removable barrel assembly is shown within the cylindrical bore of the casing 7, although according to the broader aspects of the present invention any number of stages may be employed. The first stage includes a diaphragm 22, a shroud 23, a diffuser 24 and the impeller 13; the second stage includes diaphragm 25, shroud 26, diffuser 27, and impeller 14; and the third stage includes diaphragm 28, shroud 29, diffuser 30, and impeller 15. Each of the diaphragms is a one piece iron casting, and each of the diffusers and shrouds is a one piece aluminum casting. Each of the

diaphragms

22, 25, 28 has an outer cylindrical surface in direct engagement with the inner cylindrical bore 16 of the casing 7. The end closure formed by the gear casing 6 has an adjacent inner cylindrical surface 31 that is flush with the cylindrical surface 16 of the compressor casing 7, with the outer cylindrical surface of the diaphragms 25 overlapping these flush inner cylindrical surfaces to accurately align the gear casing 6 with the barrel assembly for proper positioning of the rotor 4. The gear casing 6 determines the positioning of the rotor 4, by means of the radial bearing 32 and the combination radial-thrust bearing 33 that rotatably mount the rotor 4 in an overhung position. The rotor may be of any rigid type construction, but preferably the impellers l3, l4, 15 are integrally secured to the rotor shaft, with the interposition of suitable labrinth seals as shown.

In assembling the barrel assembly, the various components are assembled outside the casing and slid from left to right, as viewed in FIG. 3, into the

casings

6 and 7. Thereafter, the

diaphragms

22, 25, 28 are rigidly secured to the gear casing 6 by means of a plurality of tension bolts 34.

The

shrouds

23, 26, 29 and diffusers 24, 27, 30 radially engage the diaphragms to fix their radial position and axially engage, in only one direction, the diaphragms to fix their axial position while allowing free axial play or clearance movement in the opposite axial direction with respect to the diaphragms. This axial free play is taken up by biasing means including the axially compressed sealing O-rings 36, 37 and the piston action of the surfaces exposed to the pumped fluid on the diffusers and shrouds. Thus, the advantages of providing separate shrouds and separate diffusers with respect to manufacturing procedures and replacement for power recovery conversion are provided without also providing the heretofore correlated disadvantages of tolerance accumulation.

From FIG. 3, it is seen that the previously described passages l8, 19, 20, 21 are in communication with

annular chambers

38, 39, 40, 41, respectively, formed by opposed outwardly opening annular channels on the outer surfaces of the

diaphragms

22, 25, 28, and inwardly opening annular channels axially spaced along the bore 16 of the compressor casing 7. These annular chambers 38-41, are sealed with respect to each other by any appropriate means, for example, O-rings (not shown). The

diaphragms

22, 25, 28 are provided with integrally cast passages or externally configurated surfaces cooperating with surfaces of adjacent diaphragms to form passages for conducting fluid between the annular chamber 38-41 and respective ones of the shroud inputs and diffuser outputs.

In addition, the output from the last stage diffuser 30 is connected by the diaphragms into the annular chamber 42 formed by opposed annular channels in the diaphragm 25 and casing 7. From the annular chamber 42, the high pressure compressor output may pass upwardly through an outlet 43, as shown in FIG. 1, to the point of use, storage tank,- conventional blow-off device, or the like. Also, the compressor output may be conducted from the annular chamber 42 downwardly into the compressor base 1 to be connected to piping to the point of use and/or be connected to various pressure responsive control and monitoring devices, which for example may have meters, warning lights or the like on the control panel 45 as shown in FIG. 1. Also, in FIG. 1, the outlet 43 is shown with a sealing plug or cap that is used when'an excess pressure blow-off device is not employed and the compressor output is directed downwardly into the compressor base for connection with piping to the point of use. Further, the compressor output from annular chamber 42 is conducted by means of a generally axially extending passage that is cast into the casing 7, but not in communication with any of the previously described passages of the casing 7, which passage conducts a portion of the compressor output to annular chamber 47, which is shown in FIG. 3 as being formed-by opposed annular channels respectively in the casing 7 and diaphragm 22. From the annular chamber 47 the high pressure compressor output is directed into an annular chamber 48 formed between the shroud 23 and diaphragm 22. A plurality of nozzles 49 are inserted through the shroud 23 to direct the high pressure gases from chamber 48 against the blade tips of impeller 13 to produce a power recovery turbine action during starting and to take care of excess pressure on partial load. This capacity is built into the compressor so that the chamber exists with a shroud 23 not provided with nozzles, and if desired, appropriate pressure responsive valving may be incorporated into the feedback passage for dumping excess pressure into the annular chamber 48, and the shroud 23 may be drilled for receptionof nozzles to complete the conversion of the basic compressor into the power recovery compressor specifically shown in FIGS. 2-6. This capacity is built in whether used or not.

From the above, it is seen that the compressor of the present invention may be sold as a multi-stage compressor without power recovery, but with the power recovery piping integrally cast into the diaphragms and casing without extra cost so that conversion of the compressor for power recovery only involves removal and drilling of the shroud 23, the insertion of nozzles in the drilled bores of shroud 23, and the interposition of suitable valving controls in the feedback passage, as will be set forth in more detail below.

While the power recovery principles of the present invention may be used with a compressor having any number of stages, the only requirement is that compressed gas is fed back from a higher stage to a lower stage, either stage of which may be a terminal stage or intermediate stage, or combinations thereof. Specifically, the disclosed compressor, provides power recovery from the output of the third stage impeller 15, as shown in-FIG. 2, through feedback line 50, to the first stage impeller 13, with the interposition of a valve 51. Preferably, the valve 51 is a pressure responsive valve, that is, a valve normally spring biased in its closed position but which will open to allow passage of gas through feedback line 50 when a predetermined pressure is exceeded. Thus, during the operation of the compressor, if usage falls off so that the load drops and output pressure correspondingly increases, the pressure responsive valve 51 will open to allow the output of the third stage to be conducted back to the first stage to operate the first stage as a partial turbine to reduce the total input power required for the compressor. It is known to have pressure responsive blow-off valves that will vent excess pressure into the atmosphere, but the present invention provides power recovery so that this excess pressure is utilized to reduce the total power needed to drive the compressor during partial load. Numerous pressure responsive valves are commonly available, so that no specific valve has been disclosed.

During starting of the compressor, the power required for starting is reduced by connecting the starting circuit with a solenoid 52, so that the valve 51 will be open to allow pasage of compressed gas from the third stage to the first stage during starting regardless of the output pressure of the third stage. This will considerably reduce the starting time, reduce the starting power required and correspondingly a smaller drive motor may be employed. Preferably, thev solenoid 52 is connected with the valve 51 so that during normal operation of the compressor, the solenoid is energized to hold the valve in its spring-biased closed position in which it will operate as a pressure responsive valve.

This arrangement is desirable so that in the event of a power failure that may interrupt operation of the various controls, the solenoid 52 will be deenergized to automatically open the valve 51 so that the output pres sure of the third stage will not reach abnormally high levels. Thus, the compressor will fail safe.

From the above, it is seen that the power recovery system of the present invention provides for safe failure, reduced starting time, reduced starting power, and increased efficiency upon partial load.

As shown in FIGS. 4 6, the shroud 23 is provided with an axially aligned first drilled bore 53, an intermediate equal diameter or slightly smaller diameter drilled bore 54, a smaller diameter inner drilled bore 55, and an annular nozzle insert sleeve 56 received within the intermediate bore 54; all of which constitute a single nozzle assembly 49. As seen particularly in FIG. 5, a plurality of these nozzle assemblies 49 is provided with equal spacing around the circumference of the shroud 23.

FIG. 5 is a view of the shroud 23 looking toward the left in the axial direction of FIG. 3, so that reference line X is a diametric line, that is, perpendicular to and intersecting the axis of rotation of the rotor 4. Similarly, reference line Z is a radial line perpendicular to and intersecting the axis of rotation of the rotor 4. Line 6-6, on which is taken the cross section of FIG. 6, extends through the axis of symmetry of the elements 53-56 of one of the nozzles and thus gives a reference line for the projection of the jet passing through this nozzle. As seen in FIG. 5, line 66 forms an angle of approximately 57 with a radial line Z that passes through the nozzle output and forms an angle of approximately 33 with a line Y that passes through the nozzle outlet in a tangential direction, that is, perpendicular to line Z. Thus, it is seen that the jet issuing from this nozzle will have a velocity component in the tangential direction and the radial outward direction. Each nozzle assembly is correspondingly constructed although the details have been given for only one to avoid duplication.

From FIG. 6, it is seen that the line of symmetry, that is the direction of the nozzle jet, forms an angle of approximately 25 with a plane perpendicular to the axis of rotation of rotor 4. Thus, the jet for each nozzle also has an axial component of velocity.

From the above specific description of FIGS. 4-6, it is seen that the axial cross-sectional view of FIG. 3 has been broken away along the axis of symmetry of respective nozzles 49, for purposes of clarity in illustrating the relationship of the nozzle assemblies 49 with a chamber 48 and first stage impeller 13.

The compressor of the present invention is manufactured as a standard item without the power recovery feature, and at the time of ordering or at any later date it may be converted for power recovery. However, as a standard manufactured item, the annular output chamber 42 of the third stage is connected by a cast passage in the casing 7, which leads to the annular chamber 47, which annular chamber 47 is directly connected with the annular chamber 48 as shown in FIG. 3. Thus, the standard compressor without power recovery will provide feedback of high pressure gas to the annular chamber 48, where it will be contained and thus have no effect upon the compressor output. These feedback passages and chambers will not materially increase the cost of the basic compressor, but will greatly facilitate conversion to power recovery at any time, by completely eliminating the subsequent need for external piping.

To convert the compressor to power recovery, it is only necessary to remove the nut and washer on the bolt 34, the diaphragm 22 and the shroud 23. This will in no way disturb the mounting of the rotor 4 or the other elements of the barrel assembly. After the removal of the shroud 23, it is bored as shown in FIGS. 4-6, and thereafter provided with the nozzle inserts 56, which are converging type nozzles. Thereafter, the shroud 23, diaphragm 22 and nuts for bolts 34 are reassembled. As mentioned above, an integrally cast passage in the casing 7 extends from the annular chambers 42 to the annular chamber 47. Access to this passage for the insertion of the valve 51 may be obtained by drilling inlet and outlet bores, along with an intermediate plug bore, so that the plug bore may be used to block the through passage of fluid and the valve 51 may be inserted between the inlet and outlet bores. Further, it is contemplated that the standard compressor may only have short cast passages in the casing 7, respectively in communication with the

annular chambers

42 and 47, but not in communication with each other; that is, with this modification, there would be no fluid connection between the

chambers

42 and 47 in the standard compressor without power recovery. The mounting of the valve for this modification would require separate tapping of these passages with the insertion of the valve between the taps. In any event, an important feature of the present invention is the integral casting of internal passages for power recovery, and the interposition of the valve 51 may take on many forms with the above teachings.

When starting the compressor, the solenoid 52 is automatically deenergized so that the valve 51 will be held in its open position. Thus, the output of the last stage will be conducted back through the feedback passage 50, which includes

chambers

42, 47 48, to the nozzle assemblies 49 so that high pressure jets of gas will be directed against the outer tips of the blades on impeller 13. In this manner, the impeller 13 will partially operate as a turbine and simultaneously operate as the first stage of a multi-stage compressor. Thus, the output pressure will be reduced and power will be recovered in the first stage due to the turbine action so that the total power requirements for the compressor will be greatly reduced during start up. Since a drive system generally consumes its greatest amount of power during start up, the system will have the advantage of permitting a smaller and less powerful drive system to be employed.

During normal operation of the compressor, there will be periods when the demand for compressed gas will be reduced or temporarily halted. The shut down and start up of a compressor during these times is most unsatisfactory and it has been known to provide automatic pressure responsive valves for venting the compressor output to the atmosphere to reduce the power consumption. During these times, the valve 51 of the present invention will respond to this pressure build-up v to feed back high pressure gases to the first stage for the above-mentioned turbine operation, which will reduce the power requirement in the same way as a blow-off valve and additionally reduce the power requirement by the amount of power recovered in the turbine.

In the event of a power failure during normal operation, which might endanger the entire compressor system due to the shut down of various controls, the solenoid 52 will automatically be deenergized so that the valve 51 will automatically assume its open position to relieve the output pressure of the turbine and prevent excessively high pressures.

Since only the tips of the blades for impeller 13 are used for the turbine action, the impeller may be designed for efi'rcient compressor functioning, which as shown, takes on the preferred form of each blade having a radially extending inlet edge, an axially extending outlet edge, a curved twisted intermediate edge caused by circumferentially offsetting corresponding inlet and outlet edges. According to this type of blading the tip portion of each blade extends generally radially and will function quite well as a turbine blade with the arrangement of nozzles as shown in FIGS. 4-6.

While a preferred embodiment of the present invention has been specifically illustrated, further modifications, variations and embodiments are contemplated according to the broader aspects of this invention.

What is claimed is:

l. A compressor, comprising: a one piece cast casing; a plurality of separate diaphragms, separate shrouds, separate diffusers and drivingly connected open bladed centrifugal impellers axially stacked within said casing to form at least two serially connected stages of compression; and said diaphragms and casing having integrally cast passage means for conducting discharge gas from the higher stage of compression to the side of the lower stage shroud opposite from its impeller.

2. The compressor of claim 1, wherein said lower stage of compression shroud and diaphragm have cooperating surfaces forming an annular feedback gas chamber exteriorly surrounding said lower stage shroud.

3. The compressor of claim 2, wherein said passage means includes said lower stage diaphragm having integrally cast passage means extending radially from said annular chamber to the inner bore of said casing, said casing and lower stage shroud having cooperating annular. channels forming an annular chamber, said higher stage of compression diaphragm and said casing having cooperating annular channels forming an annular chamber, and said higher stage of compression diaphragm having integral passage means extending between its annular chamber and said higher stage of compression diffuser.

4. A multi-stage compressor with a power recovery means comprising a first open-bladed centrifugal impeller having an input and an output; a second openbladed centrifugal impeller having an input and an output; means for driving said impellers; means connecting said impellers to constitute said second impeller part of a higher pressure stage than said first impeller; gas feedback passage means for receiving high pressure gas from the output of said second impeller; nozzle means for receiving high pressure gas from said feedback passage means and discharging the gas directly against the o n bladin of said fi t im ller to decreas e energy output r epresente by LB: means for driviiig said impellers, said compressor including a stationarily mounted partition closely adjacent said first impeller, said partition having an inner surface facingsaid first open bladed impeller and an outer surface facing away from said first impeller, said nozzle means including a passage having a first bore extending inwardly from said second surface, and a second bore extending outwardly from said first surface and smaller in diameter than said first bore; and a nozzle insert extending between said bores.

5. A multi-stage compressor with power recovery means, comprising: a first open-bladed centrifugal impeller having an input and an output; a second openbladed centrifugal impeller having an input and an output; means for driving said impellers; means including a separate diaphragm for each of said stages, a separate shroud for each of said stages, and a separate difiuser for each of said stages constituting nested axially stacked fluid guide elements with said first impeller diaphragm and shroud forming therebetween an annular feed back chamber in fluid communication with a turbine nozzle means, connecting said impellers to constitute said second impeller part of a higher pressure stage than said first impeller; gas feed back passage means for receiving high pressure gas from the output of said second impeller to said annular feed back chamber; and turbine nozzle means in one of said annular feed back chamber forming members for receiving said high pressure gas from said feedback passage means and said annular feed back chamber and discharging the gas directly against the open blading of said first impeller to decrease the energy consumed for rotating said impellers by said driving means.

6. The compressor of claim 5, including a casing having a cylindrical inner bore receiving therein said diaphragms in radial engagement; said casing and first impeller diaphragm having integrally cast passages partially forming said feedback passage; and said second impeller diaphragm and said casing having integrally cast passage means at least partially forming said gas feedback passage means.

7. A multi-stage compressor with means power recovery, comprising: a first open-bladed centrifugal impeller having an input and an output; a second openbladed centrifugal impeller having an input and an output; means for driving said impellers; means connecting said impellers to constitute said second impeller part of a higher pressure stage than said first impeller; gas feedback passage means for receiving high pressure gas from the outputof said second impeller; and nozzle means for receiving high pressure gas from said feedback passage means and discharging the gas directly against the open blading of said first impeller to decrease the energy output represented by the means for driving said impellers; control means regulating flow of gas in said feedback passage means including a normally closed first pressure responsive control member and a second control member adapted to open said first member during start-up despite the absence of gas at a pressure required to open said control member

Claims

1. A compressor, comprising: a one piece cast casing; a plurality of separate diaphragms, separate shrouds, separate diffusers and drivingly connected open bladed centrifugal impellers axially stacked within said casing to form at least two serially connected stages of compression; and said diaphragms and casing having integrally cast passage means for conducting discharge gas from the higher stage of compression to the side of the lower stage shroud opposite from its impeller.

3. THe compressor of claim 2, wherein said passage means includes said lower stage diaphragm having integrally cast passage means extending radially from said annular chamber to the inner bore of said casing, said casing and lower stage shroud having cooperating annular channels forming an annular chamber, said higher stage of compression diaphragm and said casing having cooperating annular channels forming an annular chamber, and said higher stage of compression diaphragm having integral passage means extending between its annular chamber and said higher stage of compression diffuser.

4. A multi-stage compressor with a power recovery means comprising a first open-bladed centrifugal impeller having an input and an output; a second open-bladed centrifugal impeller having an input and an output; means for driving said impellers; means connecting said impellers to constitute said second impeller part of a higher pressure stage than said first impeller; gas feedback passage means for receiving high pressure gas from the output of said second impeller; nozzle means for receiving high pressure gas from said feedback passage means and discharging the gas directly against the open blading of said first impeller to decrease the energy output represented by the means for driving said impellers, said compressor including a stationarily mounted partition closely adjacent said first impeller, said partition having an inner surface facing said first open bladed impeller and an outer surface facing away from said first impeller, said nozzle means including a passage having a first bore extending inwardly from said second surface, and a second bore extending outwardly from said first surface and smaller in diameter than said first bore; and a nozzle insert extending between said bores.

5. A multi-stage compressor with power recovery means, comprising: a first open-bladed centrifugal impeller having an input and an output; a second open-bladed centrifugal impeller having an input and an output; means for driving said impellers; means including a separate diaphragm for each of said stages, a separate shroud for each of said stages, and a separate diffuser for each of said stages constituting nested axially stacked fluid guide elements with said first impeller diaphragm and shroud forming therebetween an annular feed back chamber in fluid communication with a turbine nozzle means, connecting said impellers to constitute said second impeller part of a higher pressure stage than said first impeller; gas feed back passage means for receiving high pressure gas from the output of said second impeller to said annular feed back chamber; and turbine nozzle means in one of said annular feed back chamber forming members for receiving said high pressure gas from said feedback passage means and said annular feed back chamber and discharging the gas directly against the open blading of said first impeller to decrease the energy consumed for rotating said impellers by said driving means.

7. A multi-stage compressor with means power recovery, comprising: a first open-bladed centrifugal impeller having an input and an output; a second open-bladed centrifugal impeller having an input and an output; means for driving said impellers; means connecting said impellers to constitute said second impeller part of a higher pressure stage than said first impeller; gas feedback passage means for receiving high pressure gas from the output of said second impeller; and nozzle means for receiving high pressure gas from said feedback passage means and discharging the gas directly against the open blading of said first impeller to decrease the enErgy output represented by the means for driving said impellers; control means regulating flow of gas in said feedback passage means including a normally closed first pressure responsive control member and a second control member adapted to open said first member during start-up despite the absence of gas at a pressure required to open said control member.