WO2016005834A1

WO2016005834A1 - Turbine and method for expanding an operating fluid with high isentropic enthalpy jump

Info

Publication number: WO2016005834A1
Application number: PCT/IB2015/054505
Authority: WO
Inventors: Roberto Bini; Mario Gaia
Original assignee: Turboden S.R.L.
Priority date: 2014-07-11
Filing date: 2015-06-15
Publication date: 2016-01-14
Also published as: EP3167158A1

Abstract

A turbine is described for the expansion of a compressible operating fluid, for example gas or steam. The turbine comprises a first group of stages, named stages of first expansion, that extends in a radial centripetal direction in order to carry out a first centripetal expansion and a second group of stages, named stages of second expansion, that extends downstream of the stages of first expansion in order to carry out a second centrifugal expansion. The arrays of rotor blades of the stages of first expansion are constrained to a first shaft and the arrays of rotor blades of the stages of second expansion are constrained to a second shaft coaxial with respect to the first shaft, and are interposed between additional arrays of rotor blades constrained to the first shaft. The first shaft rotates at a speed higher than the second shaft, and in an opposite way. The rotors carrying the arrays of rotor blades are constrained in a 'cantilevered' configuration to the respective shafts at an end thereof.

Description

"Turbine and method for expanding an operating fluid with high isentropic enthalpy jump"

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Field of the invention

The present invention relates to a turbine in which the operating fluid expands, at the beginning, in a centripetal direction and then in a centrifugal (and eventually axial) direction, and to a method for expanding an operating fluid in such a turbine, in particular an organic fluid in a Rankine cycle.

Background of the Invention

In every Rankine cycle and for a given operating fluid in the subcritical zone, the isentropic enthalpy drop provided by the expansion in the turbine, i.e. the maximum work per mass unit the expanding operating fluid can produce, expressed for example in kJoule/kg, most of all depends from fluid characteristics and, generally, is a function of the difference between evaporation and condensation temperatures of the fluid. On the other hand the characteristics of the operating fluid itself strongly affect the enthalpy drop, which is higher for fluids having simple molecule and low value of molecular mass.

The computation of the isentropic enthalpy drop is a well known topic of the theory of Rankine cycles; the turbine designer implements the turbine design by using, as starting values, the composition of the operating fluid, the inlet values of temperature, flow rate, pressure and titer of the fluid, as well as the value of exhaust pressure. From these data the value of the isentropic enthalpy drop can be easily calculated with known methods, therefore such a value has to be interpreted as a characterizing parameter in turbine design.

The same data take part of the turbine design also in case in which the turbine itself is included in a power cycle different from the Rankine cycle (for example a Kalina cycle or a Brayton cycle), or even if it is not part of a cycle but belongs to a thermodynamic process of different nature (the example of an expander of natural gas placed at the end of a distribution duct of the gas itself counts).

Another aspect calculable from afore mentioned data is the expansion ratio, both defined as the ratio between the inlet pressure and the exhaust pressure, and as the volume expansion ratio, i.e. the ratio between the volumetric flow rate at the exhaust zone and that one at the turbine inlet zone.

The acronym ORC "Organic Rankine cycle" , as everyone knows, identifies the thermodynamic cycles of Rankine type that use an organic operating fluid preferably provided with high molecular mass, much higher than that of the water vapor used in most of the Rankine power cycles.

For example, ORC plants are used for the combined production of electrical and thermal power starting from solid biomass; alternatively waste heats of industrial processes, heat recovery from prime movers or geothermal heat sources are used.

For example an ORC plant fed with biomass usually comprises:

- a combustion chamber fed with fuel biomass;

- a heat exchanger provided to give part of the heat of combustion fumes/ gases to a heat-transfer fluid, such as a diathermic oil, delivered by an intermediate circuit;

- a heat exchanger provided to transfer part of the heat of the intermediate heat-transfer fluid to an operating fluid to be evaporated;

- a turbine fed with an operating fluid in the vapor state; and

- an electric generator activated by the turbine for the production of electric power.

In the combustion chamber the heat-transfer fluid, for example diathermic oil, is heated up to a temperature usually equal to about 300°C. The heat-transfer fluid circulates in a closed loop, passing through the afore mentioned heat exchanger in which the organic operating fluid evaporates. The steam of the operating fluid expands in the turbine, producing mechanic power, then is transformed into electrical power by the generator connected to the shaft of the turbine itself. As the respective expansion in the turbine will end, the steam of the operating fluid condenses in an appropriate condenser, giving heat to a cooling fluid, usually water, used downstream of the plant as thermal carrier at about 80°C - 90°C, for example for the district heating. The operating fluid is fed to the heat exchanger crossed by the heat-transfer fluid, completing the cycle in closed loop.

The present invention applies in Rankine cycles in general, both of ORC type or else of steam type, in Kalina cycles and in general in industrial processes where the expansion of an operating fluid is provided, in the cases the isentropic enthalpy drop of the turbine is high relative to the squared peripheral speed of the turbine, and therefore in connection with an external diameter of the arrays approximately by a meter, i.e. in the cases considered in the present application:

- greater than 40 kJ/kg, for a turbine rotating at 1500 revolutions per minute (and is therefore suitable for the direct coupling with a 50 Hz electric generator having 4 poles), or

- greater than 57.6 kJ/kg for a turbine rotating at 1800 revolutions per minute, or

- greater than 160 kJ/kg for a turbine rotating at 3000 revolutions per minute, or

- greater than 230.4 kJ/kg for a turbine rotating at 3600 revolutions per minute, and so on.

In particular, it applies in cycles characterized by a high ratio of volumetric expansion of the operating fluid when passing through the entire turbine, i.e. higher than 100 in the cases considered in the present application.

In case of turbines with shaft power up to about 20 MW, a so-called

"cantilevered" solution is preferably adopted, meaning that the bearings supporting the shaft are at the same part with respect to the rotor in which the produced power is collected. In fact it is an easier solution from the implementation point of view, needing only one rotating seal per each shaft, it is cost-effective and can be maintained more easily than a solution with a rotor comprised between the bearings.

Applications WO 2010/106569 and WO 2010/106570, in the name of the Applicant, describe cantilevered solutions.

The Application EP 2699767, in the name of Exergy S.p.A., describes a radial centrifugal turbine for applications in ORC Rankine cycles.

The International Application WO 2013/108099, in the name of the Applicant, describes a third solution than can be considered the closest prior art with regard to the present invention. In particular, WO 2013/108099 describes a turbine with only one shaft in which the fluid expands in radial centrifugal stages and in axial stages, in succession. At least one array of stator or rotor blades, name angular blades, is arranged between the radial stages and the axial stages in order to divert the operating fluid. The enthalpy drop of the operating fluid expanded through the angular blades is equal to at least 50% of the average enthalpy drop provided to complete the fluid expansion in the whole turbine. From a structural point of view the rotor or the rotor pool supporting radial and axial stages can be assembled on the shaft at an end, extending substantially in a cantilevered configuration, so that the turbine has extremely compact size in comparison with the other known solutions, and the bearings, the reduction gear and the electric generator are on the same side, easily accessible for maintenance. From a thermodynamic point of view this solution provides the highest level enthalpy drop occurring at the pool of the angular blades and the subsequent axial blades.

The Applicant found that known solutions can be improved as regard to the efficiency.

It is known that the efficiency of a turbine stage is optimal for a rather narrow field of combined values of angular velocity, volumetric flow rate, machine diameter, and isentropic enthalpy drop to be performed in the stage. In particular, for an extended expansion in a wide pressure field, i.e. a relatively high input pressure at the turbine (for example 20 bars abs) at a moderate or even sub-atmospheric exhaust pressure (for example 0.5 bars abs), higher efficiencies are obtained if the first stages rotate at higher angular velocity with respect to the final stages of the machine.

Object and Summary of the Invention

It is an object of the present invention to provide a turbine characterized by a division of the enthalpy drop on an optimal number of stages and by a high efficiency also in the first expansion stages of the operating fluid, where the volumetric flow rate of the operating fluid is typically lower with respect to the rest of the turbine and a good efficiency is more difficult to achieve. All of the foregoing by keeping relatively reduced bulks and a simple structure.

It is a further object of the present invention to disclose a method for expanding a compressible operating fluid, for example a gas or steam, in a turbine, overcoming the limits of the known art.

Therefore, the present invention relates, in a first aspect thereof, to a turbine according to the claim 1 for the expansion of a compressible operating fluid, for example gas or steam.

The turbine comprises at least two groups of expansion stages, each defined by arrays of stator blades and/or arrays of rotor blades. A first group of stages is named group of stages of first expansion, and a second group of stages is named group of stages of second expansion.

The group of stages of second expansion is positioned downstream of the group of stages of first expansion with respect to the direction of motion of the operating fluid through the turbine.

The arrays of rotor blades of the group of stages of first expansion are constrained to a first shaft. Some of the arrays of rotor blades of the group of stages of second expansion are constrained to the first shaft and the others to a second shaft, so to be alternated and fluidically interconnected.

The first shaft and the second shaft are aligned on a common rotation axis X- X, i.e. they are coaxial, and rotate in a way opposite one to another.

The rotation speed of the first shaft is greater than the rotation speed of the second shaft. For example if the second shaft rotates at a speed of 3000 revolutions per minute, the first shaft rotates at a speed comprised between 4500 and 6000 revolutions per minute.

The group of stages of first expansion extends in a radial centripetal direction with respect to the rotation axis X-X and the group of stages of second expansion extends in a radial centrifugal direction with respect to the same axis.

The described configuration allows obtaining a high efficiency also in the group of stages of first expansion.

The proposed solution allows obtaining high efficiencies without complicating the turbine design, which remains simply to be maintained and can be manufactured with held down costs. As a matter of fact the addition of centripetal stages (first expansion), upstream of the centrifugal stages (second expansion), where the volumetric flow rate of the operating fluid is typically moderate, allows carrying out a first expansion from an outer radial position but anyway not much exceeding that one of the first centrifugal stages (second expansion), maintaining a high efficiency as the moderate value of the insertion diameter of blades causes a relatively great blade height.

Hence, the so-called secondary losses and the losses due to leakage at the blade end, or in the corresponding labyrinth, are acceptable.

The turbine is anyway compact and robust, since the centrifugal stages of second expansion extend substantially radially and have very little bulk in axial direction. Exploiting the enthalpy drop available at the turbine is more efficient than what can be ascertained in known solutions, where the expansion ratio per stage is excessive and/or the aerodynamic load on blades is excessive; the proposed solution allows distributing the enthalpy drop on an optimal number of stages, almost with the same bulks with respect to known solutions, to efficiency advantage.

These advantages are particularly apparent in case the turbine is included in a thermodynamic cycle characterized by high enthalpy drops. In fact, the enthalpy drop corresponding to the expansion of the operating fluid in the group of stages of second expansion, defined by counter-rotating and fluidically interconnected rotor arrays, can be relatively high, i.e. the group of stages of second expansion is able to exploit an enthalpy drop in percent equal to 30% - 50% with respect to the overall enthalpy drop of the turbine.

In addition, the centrifugal stages of the group of stages of second expansion, and possibly of additional stages downstream, are positioned on a diameter corresponding to or larger than the diameter of the second shaft and the respective bearings; this allows partially disassembling the turbine, for example partially taking out the shaft and/or bearings to get access to the other rotor disks possibly constrained to the second shaft, in order to carry out inspections or maintenance operations without having to completely disassemble the turbine. For example, the volute of the turbine can be realized with appropriate joints and flanges, to allow a simple removal of the first shaft and the respective bearings, extracting the first rotor as well.

For example, the group of stages of second expansion comprises from 1 to 10 stages, depending on the turbine size.

Preferably, the rotation speed of the first shaft is equal or greater than 1.5 times the rotation speed of the second shaft. Preferably it is lower than 4 times the rotation speed of the second shaft.

In the preferred embodiment, the arrays of rotor blades of the group of stages of first expansion are assembled in a supporting disk keyed to the first shaft at an end thereof, anyway not in the area between the bearings, according to a so called 'cantilevered' configuration. The arrays of stator blades of the group of stages of first expansion are fastened to a stationary portion of the turbine, for example a volute.

The group of stages of second expansion is defined by only counter-rotating rotor blades fastened to the first supporting disk and to the second supporting disk, respectively, and then fastened alternately to the first shaft and the second shaft.

Preferably, also the second supporting disk is cantileverly assembled with respect to the bearings of the second shaft.

In one of the possible configurations the first shaft can be directly coupled, i.e. with no reduction gear, with an electric generator having two poles and the second shaft can be directly coupled with an electric generator having four poles. One of the advantages of having shafts rotating at different speeds also consists in being able to couple corresponding generators sized upon the number of revolutions of the respective shaft.

Another configuration provides for a reduction gear on one or both the shafts. Still another configuration provides for a generator having a rotor between the bearings, preferably with generated frequency higher than the mains frequency.

For example, the fastest electric generator can simply have 2 poles at 50 Hz, i.e. it rotates at 3000 revolutions per minute (rpm), or 60 Hz at 3600 rpm, or else can be connected to the shaft through a revolutions' reduction gear or can even be a generator having a frequency different from the mains frequency and be directly assembled on the shaft between the bearings.

Preferably, the rotor blades of the group of stages of first expansion and some rotor blades of the group of stages of second expansion are assembled on a common first supporting disk provided with a reversing blade assembly or channel at which the operating fluid reverses its own way of expansion from radial centripetal to radial centrifugal one.

Preferably, the first supporting disk has, in a meridian section, a U-profile and the reversing channel or blade assembly is arranged in the most radially internal part of the U, i.e. the portion closest to the first shaft.

In an alternate embodiment at least one array of stator blades in the group of stages of first expansion is of the type having variable pitch angle. This characteristic allows adjusting the inlet volumetric flow rate and possibly rapidly stopping the adduction of operating fluid.

Downstream of the group of stages of second expansion, additional expansion stages of the operating fluid can be provided that extend, with respect to the axis X-X, in a radial centripetal direction and/or axial direction, anyway at diameters greater than the last array of blades of the group of stages of second expansion.

In an embodiment, the turbine comprises an array of rotor or stator blades, named angular blades, intermediate between the last stage of the group of stages of second expansion and the first axial stage, if it is present. At the angular blades the expansion direction of the operating fluid switches from substantially radial centrifugal to substantially axial and tangential with respect to an observer integral with the same angular blades. Preferably, the turbine comprises at least one adduction or extraction duct (or channel) of operating fluid that opens at the inlet of the arrays of angular blades.

Preferably, if the array of angular blades is present, the operating fluid is herein subjected to a considerable enthalpy drop (meant as production of kinetic energy at the expense of the fluid expansion) with respect to the average enthalpy drop per stage, for example equal to at least 10% of the average enthalpy drop per stage.

Preferably, the turbine comprises an expansion chamber free of stages downstream of the group of stages of second expansion (and upstream of the additional expansion stages, if they are present), wherein at least one adduction or extraction duct of operating fluid opens into said chamber.

Preferably, the turbine is designed to fulfill the following condition:

k'(is) = Ah(i ot) / ( i²/2),

and where Ah₍i_{S; tot}) is the overall enthalpy drop performed in the groups of radial stages, either centripetal or centrifugal, calculated as the difference between the overall enthalpy drop of the turbine and the enthalpy drop performed in the groups of axial stages downstream of the groups of radial stages, and where ui is the peripheral speed at the average diameter of the first axial stage.

The described criterion does not take account, to all intents and purposes, of the single radial stage but of the overall behavior of the radial stages, the physical limits of the number of radial stages having been taken account, which cannot be arranged in succession. Reference enthalpies have to be intended as overall enthalpies and not as static enthalpy.

In general, the turbine can be realized with the first shaft and the second shaft spaced apart and facing one another head to head. Preferably the operating fluid is organic and its expansion occurs in a Rankine cycle, or in a Kalina cycle or, in general, in a thermodynamic cycle providing for the expansion of the operating fluid. Alternatively, the method can concern the expansion of every fluid in a process, for example within a process of liquefaction and/or regasification of natural gas.

The present invention, in its second aspect, relates to a method according to claim 17 for expanding a compressible operating fluid, for example gas or steam, in a turbine.

The method comprises the steps of:

- providing a turbine comprising a first shaft and a group of stages of first expansion defined by arrays of stator blades constrained to a stationary part of the turbine and arrays of rotor blades assembled on a first supporting disk fastened to said first shaft, and comprising a second shaft and a group of stages of second expansion defined by arrays of rotor blades assembled on said first supporting disk and arrays of rotor blades assembled on a second supporting disk fastened to the second shaft;

- carrying out a first centripetal expansion of the operating fluid in the group of stages of first expansion, in a radial direction towards the first shaft;

- inverting the way of the expansion direction of the operating fluid, and

- carrying out a second centrifugal expansion of the operating fluid in the group of stages of second expansion, in a radial direction away from the second shaft;

so that the first shaft rotates in an opposite way with respect to the second shaft and at a speed higher than the latter.

The advantages of the method are the same as described afore in connection with the turbine.

Preferably, the method further comprises one or more of the following steps:

- the operating fluid being equal, assigning the rotation speed of the turbine (depending on the generators that have to be coupled with the shafts) and sizing the arrays of blades so that the first shaft rotates at a speed equal to at least 1.5 times the rotation speed of the second shaft;

- carrying out additional expansions of the operating fluid downstream of the second expansion, in further radial and/or axial stages, which are anyway positioned at diameters greater than the group of stages of second expansion;

- downstream of the second expansion, switching the expansion direction of the operating fluid from radial to axial, preferably at an array of blades named angular blades;

- injecting or extracting a flow rate of operating fluid just downstream of the second expansion and/or just upstream of the expansion in the angular blades, if they are present;

- downstream of the last centrifugal expansion, diverting the operating fluid and carrying out an expansion in axial direction so that the following conditions are fulfilled:

k'(is) = Ah(i ot) / ( i²/2), where Ah(i_{S; to}t) is defined above.

In a different embodiment of the turbine and the method according to the present invention, for which the Applicant reserves the right of filing a divisional patent Application, the group of stages of first expansion is not provided. In other words, in this variation at the beginning the operating fluid proceeds along a radial centripetal path, but inside a channel free of expansion stages and defined by stationary parts of the turbine, for example its volute, and possibly parts constrained to the first shaft and thereby rotating at the speed of the latter. The other characteristics described above can also be present in this variation of the turbine and the method.

More in detail, the embodiment for which the Applicant reserves the right of filing a divisional patent Application comprises at least one group of expansion stages defined by arrays of only rotor blades. One or more arrays of rotor blades are constrained to the first shaft of the turbine and, alternated to these, one or more arrays of rotor blades are constrained to the second turbine shaft. The first and second shaft rotate in opposite way on the common rotation axis; the first shaft rotates at a speed higher than the second shaft. In practice, the group of expansion stages is defined by counter- rotating rotor arrays defining a radial centrifugal expansion path: the fluid expands in the radial direction away from the rotation axis of the shafts.

Downstream of the group of expansion stages other radial and/or axial expansion stages are preferably provided.

Brief Description of the Drawings

Further details of the invention will be evident anyway from the following description course made with reference to the attached drawings, in which:

figure 1 is a schematic sectional view of a first embodiment of the turbine according to the present invention;

figure 2 is a schematic sectional view of the turbine shown in figure 1, with the stator parts and the rotor parts highlighted;

figure 3 is a schematic view, enlarged and in section, of a detail of the turbine shown in figure 1 ;

- figure 4 is a schematic and sectional view of a detail of a second embodiment of the turbine according to the present invention;

figure 5 is a schematic and sectional view of a detail of a third embodiment of the turbine according to the present invention;

figure 6 is a schematic sectional view of a fourth embodiment of the turbine according to the present invention.

Detailed Description of the Invention

Figure 1 is a partial view, in a meridian section, of a turbine 1 according to the present invention for the expansion of a compressible operating fluid, for example an organic fluid in a Rankine cycle.

The turbine comprises a first shaft 2, whose longitudinal rotation axis is denoted by X-X, a second shaft 3 distinct from the first shaft but coaxial with respect to the latter, and an outer case 4 or volute, and at least two groups El, E2 of expansion stages, and preferably also the groups E3, E4.

In particular, the group El of stages of first expansion is the first group of stages the operating fluid encounters along its path inside the turbine. It is a group comprising at least one stage defined by arrays of stator blades and arrays of rotor blades alternated in radial direction with respect to the rotation axis X-X of the first shaft 2.

As shown in figure 1, the arrays of rotor blades are in practice assembled on a supporting disk 5 keyed to the first shaft 2. The arrays of rotor blades are on the contrary fastened to a stationary part of the turbine 1, as its volute 4.

Downstream of the group El of stages of first expansion with respect to the expansion direction of the fluid there is the group E2 of stages of second expansion, which will be now described in detail.

The operating fluid is diverted between the groups El and E2 so that its expansion switches from radial centripetal to radial centrifugal, i.e. it moves away from the axis X-X.

In the solution shown in figure 1, the inversion occurs through appropriate passages obtained in the supporting disk 5. The passages can be channels, spokes with no substantial expansion or an appropriate blade assembly characterized by an expansion with considerable enthalpy drop, preferably equal at least to 5% of the average enthalpy drop per stage.

The group E2 of stages of second expansion is only defined by arrays of rotor blades, i.e. no arrays stator blades are provided.

Figure 3 is an enlargement of the area of groups El and E2, where different arrays of blades are clearly denoted.

In the example shown in figures 1 and 3, the group El of stages of first expansion comprises a first stator array SI fastened to the volute 4, followed by a rotor array Rl assembled on the supporting disk 5, in its turn followed by a second stator array S2 also fastened to the volute 4.

The flow inversion occurs in channels C obtained through the supporting disk 5.

The group E2 of stages of second expansion alternatively comprises the rotor arrays R2, R3, R4 and R5. The arrays R3 and R5 are constrained to the supporting disk 5 keyed on the first shaft 2, and therefore rotate on the axis X-X at the speed thereof. The arrays R2 and R4 are constrained to a second supporting disk 6 keyed on the second shaft 3, and therefore rotate on the axis X-X at the speed of the shaft 3, in opposite way with respect to the arrays R3 and R5.

The rotation speed of the first shaft 2 is higher than the rotation speed of the second shaft 3, preferably at least 1.5 times and more preferably at least twice.

Optionally, as shown in figures 1 and 3, the turbine 1 is preferably provided with groups E3 and E4 of expansion stages downstream of the second group E2. The group E3 is defined by arrays of stator blades S3 and S4 alternated with arrays of rotor blades R6 and R7 in radial centrifugal direction.

Downstream of the group E3 an array of angular blades AB is provided as diverting the operating fluid flow in axial direction, i.e. parallel to the axis X-X and also preferably tangential, i.e. incident with respect to the plane of the drawing. In the example of figures 1 and 3 the angular blades AB are stator blades, but it is generally possible providing angular rotor blades.

The group E4 is defined by arrays of rotor blades R8, R9, RIO alternated to arrays of stator blades S5, S6 and defines an axial path.

As it can be observed, the group El is on average at a diameter close to the diameter of the group E2 with respect to the axis X-X and at a smaller diameter with respect to the groups E3 and E4.

Figure 2 shows the turbine 1 also represented in figures 1 and 3, with the difference that the stator parts, the rotor part rotating with the first shaft 2 and the rotor part rotating with the second shaft 3 are highlighted by corresponding fillings. In addition, in the turbine shown in figure 2 the electric generators Gl and G2 are configured in this way: the generator Gl is directly assembled on the shaft 2 with no reduction gear, and the generator G2 is external to the shaft 3 and is coupled with the same by means of the reduction gear 11.

It has to be noted that the first supporting disk 5 and the second supporting disk 6 are constrained to the respective shafts 2 and 3 externally with respect to the corresponding bearings, according to a so-called 'cantilevered' configuration. The bearings of the first shaft 2 are denoted by the numerals 7 and the first supporting disk 5 is external with respect to the same; similarly, the bearings of the second shaft 3 are denoted by the numerals 10 and the second supporting disk 6 is external with respect to the same and faces the first supporting disk 5.

Numeral references 8 and 9 identify the seals.

This configuration allows partially disassembling the turbine 1 in a practical way, by taking out the shafts 2 and 3 respectively from the bearings 7 and 10 and Opening' the groups of stages El, E2, etc.

It has to be noted that the first shaft 2 is directly connected to a generator Gl of suitable size, for example having two poles, without the interposition of a reduction gear. In this example the first shaft 2 rotates at a speed of 3000 rpm. Also the second shaft 3 can be configured for the direct coupling with a corresponding generator G2, but in the example shown in the figures, the generator G2 is coupled with the shaft 2 through the interposition of the reduction gear 11. The second shaft 3 rotates at a speed of 1500 rpm.

Figure 4 is an enlargement of the flow inversion area in a second embodiment of the turbine 1. In this embodiment, the first supporting disk 5 is U- shaped, if it is considered in meridian section.

Figure 5 refers to still another solution free of the group El but comprising, at the inlet, a stator blade assembly 12 having a variable pitch angle. By the numeral reference 13 part of the mechanism for adjusting the pitch angle of the blades is indicated.

Figure 6 shows a turbine Γ similar to that one shown in figures 1-3, but different from the latter for being free of the group El of stages of first expansion. In practice, the operating fluid passes through the chamber C in radial centripetal direction, then passes through the channel C and expands through the group E2 of stages. Preferably, but not necessarily, also the groups E3 and E4 of stages are present. The Applicant reserves the right of filing a divisional patent Application for this variation.

This time the generator Gl is not interposed between the bearings of the turbine and is coupled with the shaft 2 by means of the reduction gear 1 Γ.

Claims

1. A turbine (1) for the expansion of a compressible operating fluid, for example gas or steam, comprising at least two groups (El, E2) of expansion stages, each defined by arrays of stator blades (SI, S2) and/or arrays of rotor blades (Rl, R2, R3, R4), wherein:

- a first group (El) of stages is defined as group of stages of first expansion, and

- a second group (E2) of stages is defined as group of stages of second expansion,

- the group (E2) of stages of second expansion is positioned downstream of the group (El) of stages of first expansion with respect to the expansion direction of the operating fluid,

- the arrays of rotor blades (Rl) of the group (El) of stages of first expansion are constrained to a first shaft (2), and

- some (R3, R5) of the arrays of rotor blades (R2, R3, R4, R5) of the group (E2) of stages of second expansion are constrained to the first shaft (2) and the others (R2, R4) to a second shaft (3),

- the first shaft (2) and the second shaft (3) are aligned on a common rotation axis X-X, rotate in way opposite one to another and the rotation speed of the first shaft (2) is higher than the rotation speed of the second shaft (3), and

- the group (El) of stages of first expansion extends in a radial centripetal direction with respect to the rotation axis X-X and the group (E2) of stages of second expansion extends in a radial centrifugal direction with respect to the same axis X-X, in order to achieve high efficiency also in the group (El) of stages of first expansion of the operating fluid.

2. Turbine (1) according to claim 1, wherein the rotation speed of the first shaft (2) is equal to 1.5 times the rotation speed of the second shaft (3), or higher, and preferably it is lower than four times the rotation speed of the second shaft (3).

3. Turbine (1) according to claim 1 or claim 2, wherein the arrays of rotor blades (Rl) of the group (El) of stages of first expansion are assembled on a first supporting disk (5) keyed to the first shaft (2) at an end thereof, anyway not in the area between the bearings (7), according to a so-called "cantilevered" configuration, and some (R3, R5) of the arrays of rotor blades of the group (E2) of stages of second expansion is assembled on said first supporting disk (5) and the remaining ones are assembled on a second supporting disk (6) keyed to the second shaft (3), in a cantilevered configuration.

4. Turbine (1) according to any one of preceding claims 1-3, wherein the first shaft (2) can be directly coupled, i.e. with no reduction gear, with an electric generator (Gl) preferably having two poles, and the second shaft (3) can be directly coupled with an electric generator (G2) that preferably has four poles.

5. Turbine (1) according to any one of preceding claims 1-4, wherein the group (El) of stages of first expansion is defined by arrays (SI, S2) of stator blades, fastened to a stationary portion of the turbine, for example a volute (4), and by arrays of rotor blades (Rl) alternated to the stator blades (SI, S2) constrained to the first shaft (2), and wherein the group (E2) of stages of second expansion is defined by rotor blades (R3, R5) constrained to the first shaft (5) and counter-rotating rotor blades (R2, R4) constrained to the second shaft (3).

6. Turbine (1) according to any one of preceding claims 1-5, wherein the arrays of rotor blades (Rl) of the group (El) of stages of first expansion and some arrays of rotor blades (R3, R5) of the group (E2) of stages of second expansion are assembled on a common first supporting disk (5) provided with a reversing blade assembly or channel (C) at which the operating fluid reverses its own way of expansion from radial centripetal to radial centrifugal one.

7. Turbine (1) according to claim 6, wherein the first supporting disk (5) has, in a meridian section, a U-profile and the reversing channel or blade assembly (C) is arranged in the most radially internal part of the U, i.e. the portion closest to the first shaft (2).

8. Turbine (1) according to any one of preceding claims 1-7, wherein at least one array (12) of stator blades of the group (El) of stages of first expansion is of the type having a variable pitch angle.

9. Turbine (1) according to any one of preceding claims 1-8, comprising, downstream the group (E2) of stages of second expansion, additional stages (E3, E4) of expansion of the operating fluid which extend, with respect to the axis X-X, in a radial centrifugal direction and/or axial direction, anyway at diameters greater than the last arrays of blades (R5) of the group (E2) of stages of second expansion.

10. Turbine (1) according to claim 9, comprising an array (AB) of rotor or stator blades, named angular blades, intermediate between the last stage (R4, R5) of the group (E2) of stages of second expansion and the first axial stage (R8, S5) where at the angular blades (AB), the expansion direction of the operating fluid switches from substantially radial centrifugal to substantially axial and tangential with respect to an observer integral with said angular blades (AB).

11. Turbine (1) according to claim 10, comprising at least one adduction or extraction duct or channel of the operating fluid, which opens upstream of the arrays of angular blades (AB).

12. Turbine (1) according to any one of preceding claims 10-11, wherein the fluid, through the arrays of angular blades (AB), is subjected to an enthalpy drop considerable with respect to the average enthalpy drop per stage in the turbine, for example equal to at least 10% of the average enthalpy drop per stage.

13. Turbine (1) according to any one of preceding claims 9-12, comprising a uniforming/diffusion chamber free of stages downstream of the group (E2) of stages of second expansion.

14. Turbine (1) according to any one of preceding claims 9-13, wherein at least one adduction or extraction duct (21) of the operating fluid opens onto said uniforming/diffusion chamber.

15. Turbine (1) according to any one of preceding claims 9-14, wherein k'₍i_S) is greater or equal to ten, where

k'(is) = Ah(i ot) / ( i²/2),

and where Ah₍i_{S; to}t₎ is the overall enthalpy drop performed in the groups (El, E2, E3) of radial stages, calculated as the difference between the overall enthalpy drop of the turbine (1) and the enthalpy drop performed in the groups (E4) of axial stages downstream of the radial stages, and where ui is the peripheral speed at the average diameter of the first axial stage (R8).

16. Turbine (1) according to any one of preceding claims 1-15, wherein the first shaft (2) and the second shaft (3) are one outside the other.

17. Turbine (1) according to any one of preceding claims 1-16, free of the group (El) of stages of first expansion, wherein at the beginning the operating fluid expands in the group (E2) of stages of second expansion.

18. A method for expanding a compressible operating fluid, for example a gas or steam, in a turbine (1), comprising the steps of:

- providing a turbine comprising a first shaft (2) and a group (El) of stages of first expansion defined by arrays of stator blades (SI, S2) constrained to a stationary part (4) of the turbine and arrays of rotor blades (Rl) assembled on a first supporting disk (5) fastened to said first shaft (2), and comprising a second shaft (3) and a group (E2) of stages of second expansion defined by arrays of rotor blades (R3, R5) assembled on said first supporting disk (5) and arrays of rotor blades (R2, R4) assembled on a second supporting disk (6) fastened to the second shaft (3);

- carrying out a first centripetal expansion of the operating fluid in the group (El) of stages of first expansion, in a radial direction towards the first shaft (2);

- inverting the way of the expansion direction of the operating fluid, and

- carrying out a second centrifugal expansion of the operating fluid in the group (E2) of stages of second expansion, in a radial direction away from the second shaft (3);

so that the first shaft (2) rotates in an opposite way with respect to the second shaft (3) and at a speed higher than the latter.

19. Method according to claim 18, further comprising one or more of the following steps:

- the operating fluid being equal, assigning the rotation speed of the shafts so that the first shaft (2) rotates at a speed equal to at least 1.5 times the rotation speed of the second shaft (3);

- carrying out additional expansions of the operating fluid downstream of the second expansion, in further groups (E3, E4) of radial and/or axial stages, which are anyway positioned at diameters greater than the group (E2) of stages of second expansion;

- downstream of the second expansion, switching the expansion direction of the operating fluid from radial to axial, at an array of blades named angular blades (AB);

- injecting or extracting a flow rate of operating fluid just downstream of the second expansion and/or just upstream of the expansion in the angular blades (AB), if they are present;

k'_(iS) = Ah_{(i ot)}/(ui²/2),

and where Ah₍i_{S; tot}) is the overall enthalpy drop performed in the groups (El, E2, E3) of radial stages, calculated as the difference between the overall enthalpy drop of the turbine and the enthalpy drop performed in the groups (E4) of axial stages downstream of the radial stages, and where ui is the average peripheral speed of the first axial stage (R8).

20. Method according to claim 19, wherein the enthalpy drop of the operating fluid expanding through the angular blades (AB) is considerable with respect to the overall enthalpy drop in the turbine, for example it is equal to at least 10% of the average enthalpy drop per stage.