Centrifugal compressor impeller cooling

Abstract

A centrifugal compressor including: a casing; at least one impeller supported for rotation in the casing and provided with a hub, a shroud and an impeller eye; an impeller-eye sealing arrangement, for sealing the impeller in the region of said impeller eye. The centrifugal compressor further includes at least one cooling-medium portlocated at the impeller-eye sealing arrangement, arranged for delivering a cooling medium around the impeller eye.

Claims

1. A centrifugal compressor comprising: a casing; at least one impeller supported for rotation in the casing, the at least one impeller comprising a hub, a shroud, and an impeller eye comprising an inner surface, an outer surface, external annular teeth formed on a portion of the outer surface, and at least one hole extending from the portion of the outer surface on which the external annular teeth have formed to the inner surface; an impeller-eye sealing arrangement disposed over the external annular teeth for sealing the at least one impeller in a region of the impeller eye; at least one cooling medium port located at the impeller-eye sealing arrangement, configured to deliver a cooling medium around the impeller eye.

2. The centrifugal compressor according to claim 1, further comprising at least one hole for each one of a plurality of blades provided between the hub and the shroud.

3. The centrifugal compressor according to claim 1, wherein the at least one cooling medium port is in fluid communication with a delivery duct of the compressor, through which a main stream of a working medium is caused to flow, a portion of the working medium being extracted from the main stream in the delivery duct and diverted towards the at least one cooling-medium port.

4. The centrifugal compressor according to claim 3, further comprising a heat exchanger, through which the portion of the working medium is cooled before being delivered to the cooling-medium port.

5. The centrifugal compressor according to claim 3, further comprising a pressure reducing arrangement, for reducing a pressure of the portion of the working medium, before being delivered to the cooling medium port.

6. The centrifugal compressor according to claim 1, further comprising a plurality of sequentially arranged compressor stages, each compressor stage comprising a respective impeller, at least one of the impellers being combined with the impeller-eye sealing arrangement and with the at least one cooling medium port.

7. The centrifugal compressor according to claim 1, further comprising at least one auxiliary cooling medium port configured to deliver an auxiliary cooling medium flow behind the hub of the at least one impeller.

8. The centrifugal compressor according to claim 7, further comprising a rotating shaft supporting the at least one impeller and a balance drum, the balance drum co-acting with a balance-drum sealing arrangement, and wherein the at least one auxiliary cooling medium port is configured to deliver the auxiliary cooling medium flow between the balance drum and the balance-drum sealing arrangement.

9. The centrifugal compressor according to claim 7, wherein the at least one auxiliary cooling medium port is in fluid communication with a delivery duct of the compressor, through which a main stream of a working medium is caused to flow, a portion of the working medium being extracted from the main stream in the delivery duct and diverted towards the at least one auxiliary cooling medium port.

10. A method of operating a centrifugal compressor comprising a casing and at least one impeller rotatingly arranged in the casing, the at least one impeller comprising an impeller hub, an impeller shroud, and an impeller eye comprising an inner surface, an outer surface, external annular teeth formed on a portion of the outer surface, and at least one hole extending from the portion of the outer surface on which the external annular teeth are formed to the inner surface; the method comprising: processing a working medium through the impeller; injecting a cooling medium into a gap around the impeller eye and circulating the cooling medium in the gap, for removing heat from the impeller eye; and delivering at least part of the cooling medium through the at least one hole towards the inner surface.

11. The method according to claim 10, wherein the gap is formed between the impeller eye and an impeller-eye sealing arrangement.

12. The method according to claim 10, wherein the cooling medium is a portion of the working medium.

13. The method according to claim 10, further comprising extracting a portion of the working medium as cooling medium.

14. The method according to claim 13, wherein a percentage from 0.5 to about 4% in volume of the working medium is extracted to cool the impeller eye.

15. The method according to claim 10, further comprising removing heat from the portion of the working medium before injecting into the gap.

16. The method according to claim 10, further comprising reducing the pressure of the portion of the working medium before injecting into the gap.

17. The method according to claim 10, further comprising delivering the cooling medium at least partly between the impeller shroud and the impeller hub.

18. The method according to claim 10, further comprising cooling the impeller hub by delivering a portion of the cooling medium behind the impeller hub.

19. The method according to claim 10, comprising: extracting a portion of the working medium as the cooling medium; injecting a first fraction of the portion of working medium in the gap around the impeller eye for cooling the impeller shroud; injecting a second fraction of the portion of working medium behind the impeller hub for cooling the impeller hub.

20. The method according to claim 10, wherein a percentage from 0.5 and 4% in volume of the working medium is extracted to cool the hub.

21. An impeller for a centrifugal compressor, the impeller comprising; an impeller hub and an impeller shroud forming an impeller eye, the impeller eye comprising a radially outer surface, a radially inner surface, external annular teeth formed on a portion of the radially outer surface, and at least one hole provided, extending from the portion of the radially outer surface on which the external annular teeth are formed to the radially inner surface, the at least one hole being arranged for conveying a cooling medium flow through the impeller eye.

22. The impeller according to claim 21, wherein the impeller eye comprises a plurality of the at least one hole.

23. The impeller according to claim 21, wherein the impeller eye further comprises at least one hole for each one of a plurality of blades arranged between the impeller shroud and the impeller hub.

24. The impeller according to claim 21, wherein each hole of the at least one hole has a hole outlet on the inner surface arranged substantially in front of a leading edge of the respective blade.

25. A centrifugal compressor comprising: a compressor casing; at least one impeller supported for rotation in the casing, the impeller comprising a hub with a front wall provided with a plurality of impeller blades, and a rear wall extending mainly radially, a shroud, and an impeller eye comprising an inner surface, an outer surface, external annular teeth formed on a portion of the outer surface, and at least one hole extending from the portion of the outer surface on which the external annular teeth are formed to the inner surface, wherein the at least one hole is configured to deliver a cooling medium to the inner surface: a space between the rear wall of the impeller and the compressor casing; and at least one cooling medium port, configured and arranged for delivering the cooling medium in the space, wherein the space is in fluid communication with a compressor diffuser at the outlet of the compressor impeller, and the cooling medium is delivered in the space between the compressor casing and the rear wall of the impeller flows in the diffuser.

26. The centrifugal compressor according to claim 25, wherein the cooling medium port is configured to deliver the cooling medium in a gap formed between a sealing arrangement and an axial rotary component, which rotates with the impeller, and wherein pressure of the cooling medium and the sealing arrangement are such that the cooling medium flows from the gap formed by the sealing arrangement and the axial rotary component, partly in the space between the rear wall of the impeller and the compressor casing, and partly in the opposite direction, towards the rear of the compressor casing.

27. The centrifugal compressor according to claim 25, wherein the axial rotary component is a balance drum arranged at the rear side of the impeller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

(2) FIG. 1 illustrates a longitudinal section according to a vertical plane of a multi-stage centrifugal compressor of the prior art;

(3) FIG. 2 diagrammatically illustrates a compressor with a cooling system in a first embodiment of the subject matter disclosed herein;

(4) FIG. 3 illustrates the diagrammatic representation of a different embodiment of the subject matter present disclosure;

(5) FIG. 4 illustrates longitudinal section of a compressor stage with an impeller eye-cooling system in combination with a hub-cooling system according to an embodiment of the present disclosure;

(6) FIG. 5 illustrates a perspective view of a shrouded impeller for a centrifugal compressor of FIGS. 4; and

(7) FIGS. 6 and 7 illustrate fragmentary perspective views of a portion of a shrouded impeller in an improved embodiment of the subject matter of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(8) The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

(9) Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

(10) FIG. 2 schematically illustrates a compressor assembly according to the present disclosure. In the diagrammatic representation of FIG. 2 a centrifugal compressor, designated 1 as a whole, is schematically represented. The centrifugal compressor 1 can comprise one or more compressor stages, each stage comprising one impeller similarly to the compressor 100 illustrated in FIG. 1. The working medium, for example air or any other gaseous medium, enters the compressor 1 at a compressor inlet 3 and exits the compressor 1 at a compressor outlet 5. As schematically represented in FIG. 2, a portion of the working medium flowing through the compressor outlet 5 is extracted and diverted along a duct 7 through a heat exchanger 9, wherein the portion of the diverted compressed working medium is cooled. The heat exchanger 9 can be a gas/air or gas/water heat exchanger, for example. The cooled working medium can then flow through a pressure reducing member, e.g. a throttling valve 11, and introduced again in one or more compressor stages through a duct 13. In other embodiments the pressure reducing member can be an expander.

(11) The pressure of the working medium flowing through the throttling valve 11 is reduced from a higher pressure P1 to a lower pressure P2. The pressure drop across the throttling valve 11 depends upon the pressure of the fluid at the compressor outlet and the pressure of the fluid in the point where the cooled working medium is reinjected in the compressor. In other embodiments, not shown, the working medium can be diverted from the main flow at a different location along the working medium path, e.g. at the outlet of an intermediate compressor stage.

(12) In some possible applications the working medium is air and the temperature of the air at the compressor outlet 5 can be around 650° C., while the temperature of the working medium at the outlet of the heat exchanger 9 can be around 450° C. These values are given by way of example only, and they should not be construed as limiting the scope of the present disclosure. A further temperature reduction can be achieved when the working medium flows through the throttling valve 11. In some embodiments, sufficient cooling could be achieved by throttling only, or by heat exchange only.

(13) A modified embodiment of the compressor assembly is shown in FIG. 3. The same reference numbers indicate the same or equivalent parts as in FIG. 2. In this embodiment, the cooling medium is not represented by a part of working medium diverted at the outlet of the compressor, but is delivered from a separate source, not shown. A compression device 14 can be provided to pump the cooling medium at the required pressure, depending upon the operating pressure of the compressor into which the cooling medium is to be injected.

(14) The embodiment of FIG. 2 does not require a separate pumping arrangement, even though the extraction of part of the working medium for cooling purposes reduces the overall efficiency of the compressor.

(15) The schematic layouts shown in FIGS. 2 and 3 are by way of example only, and it shall be understood that different arrangements can be provided, e.g. as far as the cooling medium source is concerned, or as far as the cooling of the fluid and/or the expansion thereof is concerned.

(16) The cooling medium flowing through the duct 13 and injected in the compressor is used for cooling some areas of one or more impellers of the compressor 1, as will be disclosed below, reference being made in particular to FIGS. 4 to 7. In the following description of the exemplary embodiment, reference will be made to an implementation according to FIG. 2, i.e. wherein a part of the working medium is used as a cooling medium, by diverting it from the main flow and re-introducing it in the compressor at a suitable temperature and pressure. However, as noted above, the cooling medium could be provided by an external source.

(17) Referring to FIGS. 4 to 7, reference will be made to the last compressor stage of a multi-stage centrifugal compressor. It shall be understood that the same features which will be described in connection with the impeller of the last compressor stage can be provided also in additional stages of the multi-stage compressor. It should further be understood that the features disclosed herein with respect to a multi-stage compressor can be implemented also in a single-stage compressor, if required.

(18) In FIG. 4 a portion of the compressor 1 is shown in a vertical section along a plane containing the axis A-A of the compressor rotor. The last compressor stage comprises an impeller 21 supported by a rotary shaft 22. The impeller is shown in isolation in FIG. 5. In the embodiment disclosed herein the impeller 21 comprises an impeller hub 23 and an impeller shroud 25. Blades 27 extend radially between the impeller hub 23 and the impeller shroud 25 forming impeller vanes 29 therebetween. The impeller shroud 25 comprises an impeller eye 31 extending around an impeller inlet 33.

(19) The impeller eye 31 can be provided with external annular teeth 35, cooperating with sealing lips 37 of an impeller-eye sealing arrangement 39 mounted in the compressor casing 41. The impeller-eye sealing arrangement 39 provides a sealing between the compressor stage containing the impeller 21 and the upstream compressor stage (not shown).

(20) The working medium processed by the impeller 21 is discharged radially from the vanes 29 in a diffuser 43 formed in the casing 41 and enters a volute 45 which is in fluid communication with the compressor outlet 5.

(21) A balance drum 47 is arranged behind the hub 23, i.e. on the side of the impeller 21 opposite the impeller eye 31. The balance drum 47 co-acts with a sealing arrangement 49, which seals the space where the impeller 21 is housed against the rear part of the compressor. In the diagrammatic section of FIG. 4 further sealing arrangements 51 co-acting with the rotary shaft 22 are also shown.

(22) In some embodiments one or more cooling medium ports 53 are arranged around the impeller eye 31. The cooling medium ports 53 are in fluid communication with the duct 13, through which the portion of suitably cooled working medium, extracted from the main compressor outlet 5, is re-introduced in the compressor casing, for cooling the impeller eye 31. In some embodiments a plurality of cooling medium ports 53 are uniformly arranged around the annular development of the impeller-eye sealing arrangement 39. For example, from 2 to 20 ports 53 can be provided. In some embodiments, between 8 and 15, and more particularly, between 10 and 14, cooling medium ports 53 can be provided. Through the cooling medium ports 53 a percentage of e.g. around 2% of the total outlet working medium flow exiting the compressor can be re-introduced in the compressor casing.

(23) The cooling medium flowing through each cooling medium port 53 enters the gap between the sealing lips 37 of the impeller-eye sealing arrangement 39 and the impeller eye 31. The cooling medium delivered through the cooling medium ports 53 has a pressure which is higher that the inlet pressure of the relevant compressor stage. For example, if the working medium pressure at the impeller inlet is around 55 Bars, the cooling medium can be delivered at around 60 Bars through the cooling medium ports 53. Consequently, the cooling medium will be forced to escape the gap between the lips 37 and the impeller eye 31. A fraction of the cooling medium will escape the gap according to arrow fA and another part of the cooling medium flow will escape the gap along arrow fB. The first part of the cooling medium, for example around 1.2 to 1.3% of the total working medium flowing through the compressor, will escape according to arrow fA and enter the upstream compressor stage, while the remaining part will flow along the outer surface of the shroud 25 of the impeller 21 along a gap 57 between the compressor casing 41 and the impeller shroud 25, finally entering the diffuser 43.

(24) The cooling-medium flow cools the outer surface of the impeller eye 31. The temperature of the impeller eye region, which is subject to particularly high mechanical stresses, will thus be reduced, thereby improving the creep life of the impeller.

(25) According to a further improvement of the subject matter disclosed herein, the impeller eye 31 is provided with a plurality of holes 61. In some embodiments at least one hole is provided for each blade 27. A clear illustration of one such hole is provided in FIGS. 6 and 7. These figures show a cross section of a portion of the impeller 21. In these figures a fragment of the impeller eye 31, of the hub 23 and of the shroud 25, as well as one of the blades 27 are shown. Each hole 61 extends from an inlet on the outer surface of the impeller eye 31 to an outlet on the inner surface of the impeller eye 31. In some embodiments, as shown in FIGS. 6 and 7, the hole 61 opens on the inner surface of the impeller eye 61 approximately in front of the leading edge 27A of a corresponding blade 27.

(26) With this arrangement at least part of the cooling medium delivered through the cooling medium ports 53 enters the holes 61. Each hole 61 generates a cooling medium flow, which flows along both sides of the respective blade 27. The cooling medium flow removes heat from the blade leading edge and the area where the blade 27 is connected to the impeller eye 31. This area is subject to high thermal and mechanical stresses. Removal of heat from this area reduces the temperature and alleviates creep, thus further increasing the creep life of the impeller.

(27) In some embodiments, additional reduction of overheating and creep problems can be achieved by providing a cooling medium flow also in the area of the hub 23.

(28) This is schematically shown in FIG. 4. One or more auxiliary ports 71 can be provided, which connect the duct 13 to the sealing arrangement 49. A fraction of the working medium, extracted from the compressor outlet 5, cooled in the heat exchanger 9 and expanded in the throttling valve 11, flows through the ports 71 into the gap between the sealing arrangement 49 and the balance drum 47. This cooling medium flow escapes the gap between the sealing arrangement 49 and the balance drum 47 and at least part of said flow enters the space between the stationary parts of the compressor casing 41 and the rear wall of the impeller 21 according to arrow fC. This part of the cooling medium flow will finally enter the diffusor 43. In some embodiments, the cooling medium delivered in the gap between the sealing arrangement 49 and the balance drum 47 can be approximately 2.0-2.2% of the overall compressor outlet flow and approximately ⅓ of this cooling medium flow will enter the space behind the impeller 23 and finally reach the diffusor 57, while the remaining part will escape the gap between the sealing arrangement 49 and the balance drum 47 at the opposite side.

(29) While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.