WO2017059495A1

WO2017059495A1 - A turbine

Info

Publication number: WO2017059495A1
Application number: PCT/AU2016/050943
Authority: WO
Inventors: Hugh Russell; Ingo Jahn; Stephen GWYNN-JONES; David Stevens; Joshua KEEP; Phil SWANN
Original assignee: The University Of Queensland
Priority date: 2015-10-07
Filing date: 2016-10-07
Publication date: 2017-04-13

Abstract

A turbine able to be used with an high-pressure and dense working fluid, the turbine comprising: a housing a shaft mounted for rotation with respect to housing; a rotor assembly attached to the shaft; at least one inlet to allow a working fluid to pass through the housing to rotate the rotor; an outlet to allow a working fluid to pass through the housing after rotating the rotor; wherein the rotor assembly includes two sets of rotor vanes, the rotor vanes located on opposite sides of the rotor assembly.

Description

TITLE

A TURBINE

FIELD OF THE INVENTION

[0001 ] This invention relates to a turbine. In particular, the invention resides in a turbine that is operable using dense and high pressure working fluids such as supercritical carbon dioxide.

BACKGROUND OF THE INVENTION

[0002] Thermal power plants utilises many different types of fuel such as burning coal, concentrating sunlight or splitting atoms. However, most thermal power plants use fuel for a sole purpose i.e. heating water into steam to drive a turbine. Steam-based generation produces 80 percent of the world's electricity.

[0003] After more than a century of incremental improvements in the steam cycle, engineers have improved the efficiencies considerably. However, there is now a cost versus return issue. For every percentage point of improvement in efficiency, millions of dollars are being spent on this improvement. Accordingly, engineers are looking to replace steam with alternative working fluids that can yield improvements in thermodynamic cycle efficiency and that result in smaller turbomachinery. One way to enhance thermodynamic cycle efficiency is to operate partially in the supercritical regime and to exploit non-linear fluid properties to enhance the ratio of compression work to power generated. This has been successfully employed with supercritical steam turbines.

[0004] By using fluids or other gases (e.g. refrigerants or carbon dioxide), that have a low critical point (31 degrees Celsius and 73 atmospheres for supercritical carbon dioxide) at sufficiently high pressures, it is possible to operate cycles entirely under supercritical conditions. Under these conditions, supercritical fluids have a density similar to water, but maintain gas like physical properties. The net result is that using supercritical fluids or other dense fluids, it is possible to design more efficient power cycles that will operate with turbines that can be 10 times smaller than its steam equivalent due to the high densities. [0005] However, the difficulty with producing a turbine that operates with a dense and high-pressure fluid (such as supercritical carbon dioxide) is that large forces are imparted on comparably small bearings and seals need to be more compact to suit the smaller turbines. This is largely due to the axial loads that are developed. Accordingly, achieving commercially viable turbines that utilise dense high-pressure fluids (e.g. supercritical carbon dioxide) is an engineering challenge.

OBJECT OF THE INVENTION

[0006] It is an object of the invention to overcome and/or alleviate one or more of the above disadvantages or provide the consumer with a useful or commercial choice.

SUMMARY OF INVENTION

[0007] In one form, although not necessarily the broadest or only form, the invention resides in a turbine able to be used with an high-pressure and dense working fluid, the turbine comprising:

a housing

a shaft mounted for rotation with respect to housing; a rotor assembly attached to the shaft;

at least one inlet to allow a working fluid to pass through the housing to rotate the rotor;

an outlet to allow a working fluid to pass through the housing after rotating the rotor;

where in the rotor assembly includes two sets of rotor vanes, the rotor vanes located on opposite sides of the rotor assembly.

[0008] The working fluid may be carbon dioxide. However, other advance suitable working fluids may include helium, supercritical steam, refrigerants and other organic fluids.

[0009] The shaft normally extends through the rotor assembly. Alternatively the shaft extends only one side of the rotor assembly. That is, the rotor assembly is cantilevered by the shaft. [0010] The shaft normally extends through one end of the housing. The shaft may be connected to a generator. Preferably the shaft is directly connected to the generator. Normally the generator is a generator that operates at speeds in excess of ordinary synchronous generators.

[001 1 ] Alternatively, a magnetic coupling is connected to the shaft and connects the shaft to a generator using non mechanical means.

[0012] The rotor assembly may be formed using a single rotor having rotor vanes located on opposite sides of the rotor. Alternatively, the rotor assembly may be formed from two rotors that are linked together, each rotor having a set of rotor vanes.

[0013] The rotor design and construction would be suitable for operation at temperatures in excess of 150°C. The rotor or rotors normally have a diameter of less than 400 mm. Normally the rotors will have a diameter of between 150- 250mm. The fluid temperature at the inlet to the rotor would likely be in excess of 150°C.

[0014] The rotor may be made from high chromium and high nickel alloys (e.g. Inconel) due to high strength and corrosion resistance at high temperatures. The rotor may be cast with the addition of post machining or direct machining from solid.

[0015] One or more bearings may be used to support the shaft. The bearings may be used to support the shaft on both sides of the rotor. Alternatively, one or more bearings may be used to support the shaft on one side of the rotor where the shaft exits the housing. Alternatively the bearing on the non-outlet side may be of the film riding type and operating within the working fluid.

[0016] One or more shaft seals may be used to seal the outlet on the side where the shaft passes through the housing.

[0017] One or more shaft seals may be used to isolate the working fluid from one or more bearings. Alternatively, shaft seals may be used to isolate the working fluid from one or more bearings solely on the side where the shaft extends through the housing. The one or more shaft seals may be labyrinth seals.

[0018] Normally there are one or more set of shaft seals including at least two shaft seals. The first shaft seal may be spaced from the second shaft seal. A sealing fluid may be injected between the first shaft seal and the second shaft seal. One or more gas injection ports may be used for this purpose. The sealing fluid may be at a pressure in excess of the turbine outlet pressure to prevent leakage. The fluid may be also be at a lower temperature. Preferably the sealing fluid is the same as the working fluid. More preferably, the sealing fluid is carbon dioxide. Each set of shaft seals may include a third seal to isolate lubricating oil used in the bearings from the first shaft seal and the second shaft seal.

[0019] The fluid injected between the seals may be directed in such a direction that it opposes the pressure gradient, thereby improving seal performance.

[0020] Sealing between the working fluid and a bearing chamber may be achieved through a single dry gas seal.

[0021 ] Normally the turbine includes a single inlet and dual outlets. The inlet may comprise a volute with a single stator. The single stator may include single stator channel may be used to introduce the working fluid into both sets of rotor vanes.

[0022] A plenum chamber may be located upstream of the stator. This plenum chamber may be fed by a single or multiple inlets that can be off-set both in the axial direction and relative to the radial direction.

[0023] One or more tip seals may be located adjacent a radial end of each of the sets of vanes of the rotor assembly. The one or more tip seals may be labyrinth seals. The tip seals may be incorporated in the shroud and be positioned axially or radially adjacent to the stator.

[0024] Each of the sets of vanes may include a shroud which is located adjacent the tip seal. The shroud may extend along the full or partial length of each set of vanes of the rotor assembly.

[0025] The housing may be made from high grade stainless steels or nickel stainless steels are a preferred option (e.g. 253 MA or Incoloy 800H).

[0026] The housing and the rotor may have appropriately matched coefficients of thermal expansion, so that the rotor is maintained central to the housing. This may eliminate axial offset.

[0027] A secondary flow of the working fluid may be introduced at a low temperature to provide cooling of the shaft and housing between the rotor and the bearings and part or all of the seal. [0028] Alternatively an actively controlled axial positioning system may be employed keep the rotor central with respect to the housing. The axial positioning system may measure the axial position of the rotor during operation. This measurement is fed back to an actuator which controls the axial position of the shaft. One embodiment to achieve this would be to use a magnetic thrust bearing which allows control of the axial position.

[0029] The shaft may be made of a material with low thermal conductivity.

[0030] A thermal barrier or other corrosion resistant coating may be applied to the components to improve performance and to allow operation at higher temperatures.

[0031 ] Laser powder deposition/sintering (or other rapid manufacturing process) may be used to achieve complex geometries such as the housing. As material properties improved, the use of a printed rotor may become viable.

[0032] In another form, the invention resides in a method of operating a turbine, the method including the steps of:

rotating a rotor assembly attached to a shaft using a high pressure and dense working fluid such that that the forces imparted on the shaft in an axial direction are substantially balanced.

[0033] The method may further include the step of rotating a rotor assembly attached to a shaft across a range of operating conditions (e.g. changing inlet pressure, temperature and rotational speeds) such that the force imparted on the shaft in an axial direction are substantially balanced at all conditions.

[0034] The method may further include the step of introducing the working fluid onto the rotor at a temperature in excess of 150°C.

[0035] The method may include the step of the shaft power being less than 40 mega watts.

[0036] Further features of the invention will become apparent from the detailed description below. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Embodiments of the invention will now be described in reference to the accompanying figures in which:

[0038] Figure 1 is a sectional view of a turbine according to a first embodiment of the invention;

[0039] Figure 2 is a schematic sectional view of a tip seal according to an embodiment of the invention;

[0040] Figure 3 is a schematic sectional view of sealing fluid being injected between two shaft seals;

[0041 ] Figure 4 is a sectional view of a turbine according to a second embodiment of the invention;

[0042] Figure 5 is a sectional view of a turbine according to a third embodiment of the invention;

[0043] Figure 6 is schematic sectional view of a turbine according to a fourth embodiment of the invention;

[0044] Figure 7 A is a schematic sectional view according to a first embodiment of a rotor;

[0045] Figure 7B is a schematic sectional view of according to a second embodiment of rotor;

[0046] Figure 7C is a schematic sectional view of according to a third embodiment of rotor;

[0047] Figure 7D is a schematic sectional view of according to a forth embodiment of rotor;

[0048] Figure 8 is schematic sectional view of a turbine according to a fifth embodiment of the invention;

[0049] Figure 9A is a schematic sectional view according to a first embodiment of a rotor;

[0050] Figure 9B is a schematic sectional view of according to a second embodiment of rotor;

[0051 ] Figure 9C is a schematic sectional view of according to a third embodiment of rotor;

[0052] Figure 10 is schematic sectional view of a turbine according to a six embodiment of the invention;

[0053] Figure 1 1 is schematic sectional view of a turbine according to a seventh embodiment of the invention;

[0054] Figure 12 is schematic sectional view of a turbine showing split lines; and [0055] Figure 13 is schematic sectional view of a turbine showing multiple concentric circular elements.

DETAILED DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 shows a first embodiment of a turbine 10 that is operated using a dense and high pressure working fluid, in particular supercritical carbon dioxide. The turbine 10 includes a rotor assembly 20, a housing 30, a number of bearings 40, a shaft 50 and a set of shaft seals 60.

[0057] The rotor assembly 20 is formed by a single rotor 21 having rotor vanes 22 located on opposite sides of the rotor 21 . In this embodiment the rotor 21 is machined from a single block of metal. The rotor 21 may be unshrouded or may also include a shroud 23 located adjacent the end of the rotor vanes 22 as shown in Figure 2.

[0058] The housing 30 houses the rotor assembly 20 and provides a single inlet plenum 31 to feed the rotor assembly 20. The inlet 31 is in the form of a single volute with a stator 33 having number of stators vanes (not shown) arranged in a ring surrounding the rotor 21 . The stator 33 has a series of channels (not shown) located between the stator vanes (not shown) through which fluid is fed to both sets of vanes 22 on the rotor 21 . According this stator channel can be twice as wide as a stator channel intended to serve the two sets of vanes independently. This reduces viscous losses in the stator 33 and may also simplify manufacture.

[0059] The housing 30 also includes two outlets 34 located on respective sides of the rotor 21 . Each outlet includes two outlet chambers 35 where carbon dioxide is accumulated after passing through the rotor assembly 20 and two or more respective outlet pipes 36 to pass carbon dioxide from the housing 30. The shaft 50 extends through the housing 30 and is attached to the rotor 21 . The shaft 50 is supported by the housing 30 via bearings 40. [0060] The shaft 50 extends through the housing 30 and is attached to the rotor 21 . The shaft 50 is supported by the housing 30 via bearings 40. The bearings 40 may be of a type suitable for radial and thrust loads, or may include both radial and thrust bearings separately. A set of three shaft seals 61 , 62, 63, are used to seal the shaft 50 with the housing 30 to minimise any leakage of carbon dioxide from the output chamber 35.

[0061 ] A gas injection port 70 is located between the first shaft seal 61 and the second shaft seal 62 on the output-side of the rotor 21 . The gas injection port 70 is used to introduce carbon dioxide between the respective first shaft seal 61 and the second shaft seal 62 as shown in Figure 3. The introduction of carbon dioxide between the first shaft seal 61 and the second shaft seal 62 minimises leakage across the first shaft seal 61 and also has a benefit of cooling the shaft 50. A bleed port 71 is located between the second shaft seal 62 and the third shaft seal 63.

[0062] In a variation a fluid other than the working fluid is injected through the injection port 70. The mixture of this fluid and the working fluid are then scavenged through the bleed port 71 .

[0063] A further variation of this seal arrangement is to replace the second shaft seal 62 by a dry gas seal or other high performance seal. If such a seal requires lower operating temperatures, the fluid injected through the injection port 71 can be at pressures above the rotor exit pressure, resulting in a flow reversal for the first shaft seal 61 .

[0064] To improve efficiency, the option exists to split seal 62 into multiple seals, with intermediary bleed ports to allow recovery of leakage fluid at higher pressures than the bleed port between seal 62 and 63.

[0065] If available the complete seal arrangement may be replaced by a single high performance seal that can operate at the operating conditions.

[0066] A tip seal 80 is located adjacent the stator 33. The tip seal 80 is located adjacent the shroud 23 of the rotor 21 as shown in Figure 2. As the tip seal 80 is located adjacent the end of the shroud 23 adjacent the tip of the rotor vanes 22, axial movement of the rotor 21 will not substantially effect leakage of the working fluid. [0067] The shaft is may be directly connected to a high speed generator (not shown) to create a DC current that can be rectified. This increases efficiency of the production of electricity. Alternatively the shaft 50 could be connected to the generator via a reducing gearbox.

[0068] In use, supercritical carbon dioxide passes through the volute 32, and into the stator 33. The carbon dioxide then feed into the different sides of the rotor 21 by the stator channels causing the rotor to spin the shaft 50. The shaft 50 drives the generator to produce electricity. The low pressure carbon dioxide then passes into respective outlet chambers 35 and out of respective outlet pipes 36. Minimal losses of the working fluid are experienced adjacent the tip seal 80 and the shaft seals 60. Further, as the rotor vanes 22 are located on opposite sides of the rotor 21 , the axial loads are mostly balanced. Accordingly, the size of the thrust bearings 40 are able to be reduced.

[0069] Figure 4 shows a second embodiment of the invention in which shaft seals 60 are only used to isolate the output side bearing 40 from the working fluid. A fluid element bearing 41 is used on the non-output side. In this embodiment, losses due to seal windage and leakage are minimized but a specialized bearing is required.

[0070] Figure 5 shows a third embodiment of the invention in which the rotor 20 is cantilevered off of the shaft 50. Accordingly there are no bearings or seals on the one side of the rotor 21 . In this embodiment, losses due to seal friction are minimized and the construction of the casing is simplified.

[0071 ] Figure 6 shows a fourth embodiment of the invention. It would be appreciated by a person skilled in the art that this representation is a schematic representation only. However, the representation would provide sufficient information for a person skilled in the art of put the invention into practise. The fourth embodiment of the invention includes a shaft 50 that is supported by two bearings 40 that are in direct contact with the working fluid. The shaft 50 is connected to a magnetic coupling 80 which is connected to an external shaft 90. Accordingly, rotation of the shaft 50 causes rotation of the magnetic coupling which in turn causes rotation of the external shaft 90. This embodiment does not require seals and therefore the loss of working fluid across the seals is eliminated. [0072] It would be appreciated by a personal skilled in the art that various modification to the embodiments described in this specification can be made. For example, the convention rotor 20 shown in Figure 7A may be replaced with a rotor 20 with forward facing vanes as shown in Figure 7B, a rotor 20 with rear facing vanes as shown in Figure 7C or a rotor with scalloped vanes as shown in Figure 7D (the vanes may be conventional, forward facing or rear facing).

[0073] Figure 8 shows fifth embodiment of the invention in which a scalloped rotor 20 replaces the conventional rotor 20 in the first embodiment. The scalloped rotor 20 enables increased blade height and improves shroud clearance ratio (B/c1 or B/c2).

[0074] It would also be appreciated by a person skilled in the art that the inlet 30 may also take various forms. For example, Figure 9A shows a conventional inlet 31 which includes a plenum chamber 37 with a large cross-sectional area (normal to tangential direction) upstream of the stator. This plenum chamber may be fed from multiple ports 38. While the inlet 30 shown in Figure 9 is easy to manufacture, a downside of this design is that non uniform flow is created. Channels, located between stator vanes of the stator, immediately below a port 38 receive increased mass flow and channels half way between the ports 38 receive reduced flow.

[0075] Figure 9B an inlet 30 having a volute 39 with of reducing cross-sectional area is designed and wrapped around the stator. The design intent is to maintain a near uniform tangential velocity around the circumference, thereby providing a uniform supply to all the channels.

[0076] Figure 9C shows a hybrid of the inlets shown in Figure 9A and 9B. A large cross-sectional area plenum chamber 37 is provided. However the inlet ports are off-set from the symmetry plane by a given distance. This distance is a fraction of a radius of the plenum chamber. The resulting fluid delivery system has several distinct advantages. The geometry is easier to manufacture than a volute design. The fluid conditions at the inlet to the stator are more uniform than both conventional designs. Finally, the losses within this design are reduced relative to both conventional designs.

[0077] Figure 1 0 shows a sixth embodiment of the invention in which the inlet 31 shown in Figure 9C has been utilised in the turbine shown in Figure 1 . Further, the rotor 20 is shrouded using a shroud 23 to reduce the losses due to tip clearances. The stator vanes are designed with reduced axial width to reduce leakage at the inlet to rotor 20 ie N < B. This leakage can be further reduced by fitting a tip seal 80 as shown in Figure 2.

[0078] For designs operating with a shrouded rotor the cavity 28 can be created between the shroud 23 so to reduce windage effects and associated losses. Preferably the shape of the cavity 28 is such that recirculation and induced losses are minimised.

[0079] Figure 1 1 shows a seventh embodiment of the invention which include flow defectors 100. An issue with some of the embodiments described above such as the embodiment shown in Figure 1 is that the working fluid exiting the rotor 20 will impinge on the first seal 40. This effect can be mitigated by adding flow deflectors 100 located on the shaft 50 to deflect the flow towards the outlet. These deflectors 100 are designed such that the flow is directed away from the seal, preferably in radially outwards direction. The flow deflectors 100 operation benefits the seal performance firstly as the seals will now experience static pressure instead of total pressure and as flow approaching the seals 40 may have reduced swirl. If required the deflectors 100 may also include features on the sides facing towards the seal, which actively pumps fluid away from the seal 40.

[0080] A diffuser 1 10 is located adjacent the outlets 34. The turbines described above restrict the design of any diffuser to accommodate the shaft and restrict axial extent due to the seal and bearing system. With these restrictions, an annular-radial geometry diffuser 1 10 offers the highest potential for static pressure recovery.

[0081 ] Swirl in the exhaust stream is a key flow feature of turbines, and it can have a significant impact on the performance of a diffuser 1 10. This is particularly true for annular diff users where performance degrades significantly with increasing inlet swirl. The current diffuser 1 10 maintains passage area constant along both the annular and annular-radial sections in order to eliminate this degradation. By design static pressure recovery is confined to the radial-radial section of the diffuser 1 10. Radial-radial diffusers possess a smaller space claim than alternative geometries.

[0082] The assembly of a turbine that utilises a double sided rotor 20 is not straight forward. That is, the double side rotor 20 shown in the embodiments above requires special consideration during assembly. To allow assembly of the rotor 20 between two casing halves which form the shrouds 23 (as shown in Figure 1 for example) the casing has to be split. Such a split can be along a plane perpendicular to the rotor axis (see Split Line 1 ) or along a plane parallel to the rotor (see Split Line 2) both of which are shown in Figure 1 2.

[0083] Further the assembly of the entire turbine requires consideration. In is envisaged that the best way to assembly the turbine is to divide it into elements that primarily consists of multiple concentric circular elements. Such a design lends itself to an axially stacked assembly. In this context a preferred assembly method for the turbine is shown in Figure 13 which represents the embodiment shown in Figure 3. (Note only major components are shown and it is envisaged that other elements are inserted as additional layers to the stack).

[0084] A variation to the assembly method shown in Figure 13 is to assemble the turbine as a number slices stacked inside a tubular casing. This casing is sealed at one or both ends with an end plug secured by a threaded ring or an alternative arrangement. Advantages of such an assembly are the possibility to carry pressure and other structural loads by this casing that, thereby reducing the strength requirements for internal components.

[0085] The turbine can be designed as impulse type, reaction type, or a hybrid. The current invention primarily relates to reaction and hybrid turbines. However the underlying concept of a double sided rotor can equally be applied to impulse turbines.

[0086] The main advantage of the current invention resides with reaction turbines, where a large fraction (>20%) of the total pressure drop occurs across the rotor. For such turbines the double sided rotor has the additional benefit of minimising axial thrust load variations during off-design operation. As the thrusts acting on both sides always balance there is no change in net thrust load as a consequence of changes in conditions at the rotor inlet. Contrary for a single sided rotor, variations in pressure at the rotor inlet manifest as changes in pressure acting on the rear of the rotor and thus thrust load. Thus the current invention simplifies the thrust load management task (or allows elimination of the system used to manage loads). [0087] This invention also mitigates transient bearing un-loading as may exists for single sided rotors during off-design operation.

[0088] In this specification, the terms "comprise", "comprises", "comprising" or similar terms are intended to mean a non-exclusive inclusion, such that a system, method or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.

[0089] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

[0090] It should be appreciated that various other changes and modifications may be made to the embodiments described without departing from the spirit or scope of the invention.

Claims

CLAIMS:

1 . A turbine able to be used with an high-pressure and dense working fluid, the turbine comprising:

a housing

a shaft mounted for rotation with respect to housing;

a rotor assembly attached to the shaft;

wherein the rotor assembly includes two sets of rotor vanes, the rotor vanes located on opposite sides of the rotor assembly.

2. The turbine of claim 1 wherein the working fluid is carbon dioxide.

3. The turbine of claim 1 or claim 2 wherein the shaft extends through the rotor assembly.

4. The turbine of claim 1 or claim 2 wherein the shaft only extends on one side of the rotor assembly.

5. The turbine of any one of the preceding claims wherein the shaft is connected to a generator.

6. The turbine of any one of the preceding claims wherein the rotor assembly is formed using a single rotor having rotor vanes located on opposite sides of the rotor.

7. The turbine of any one of claims 1 to 5 wherein the rotor assembly is formed from two rotors that are linked together, each rotor having a set of rotor vanes.

8. The turbine of anyone of the preceding claims wherein the rotor design and construction would be suitable for operation at temperatures in excess of 150°C.

9. The turbine of any one of the preceding claims wherein the rotor or rotors have a diameter of less than 400 mm

10. The turbine of any one of the preceding claims wherein one or more bearings may be used to support the shaft.

1 1 . The turbine of claim 10 wherein one or more bearings support the shaft on both sides of the rotor.

12. The turbine of claim 10 wherein one or more bearings support the shaft on one side of the rotor where the shaft exits the housing.

13. The turbine of any one of the preceding claims wherein one or more shaft seals are used to seal the outlet on the side where the shaft passes through the housing.

14. The turbine of any one of the preceding claims wherein one or more shaft seals are used to isolate the working fluid from one or more bearings.

15. The turbine of any one of the preceding claims wherein the rotor vanes have an integral shroud.

16. The turbine of claim 1 5 wherein one or more tip seals are located adjacent a radial end of each of the sets of shrouds.

17. The turbine of any one of the preceding claims wherein a secondary flow of the working fluid is introduced at a low temperature to provide cooling of the shaft and housing between the rotor and the bearings and part or all of the seal.

18. The turbine of any one of the preceding claims wherein a plenum or volute is used to evenly distribute flow around the stator inlet.

19. A method of operating a turbine, the method including the steps of:

rotating a rotor assembly attached to a shaft using a high pressure and dense working fluid such that the so that the forces imparted on the shaft in an axial direction are substantially balanced.

20. The method of claim 19 further including the step of:

rotating a rotor assembly attached to a shaft using a high pressure and dense working fluid such that the so that the forces imparted on the shaft in an axial direction are substantially balanced while the operating conditions are altered.

21 . The method of claim 19 or 20 further including the step of introducing the working fluid onto the rotor at a temperature in excess of 150°C.

22. The method of any one of claim 19 to 21 further including the step of the shaft power being less than 40 mega watts.