WO2018065840A1

WO2018065840A1 - Assembly of turbines

Info

Publication number: WO2018065840A1
Application number: PCT/IB2017/055388
Authority: WO
Inventors: Dario RIZZI; Claudio SPADACINI
Original assignee: Exergy S.P.A.
Priority date: 2016-10-07
Filing date: 2017-09-07
Publication date: 2018-04-12
Also published as: IT201600101070A1

Abstract

The present invention relates to an assembly of turbines, comprising: a first counter rotating turbine (2) and a second counter rotating turbine (3); a first group of gears (13) and a second group of gears (14) arranged on opposite sides of the first and of the second counter rotating turbine (2, 3); a central shaft (33) configured to be connected to a generator (300); wherein the central shaft (33) is connected between the first and the second counter rotating turbine (2, 3) and is substantially parallel to the first and to the second turbine shafts (7, 8). The first group of gears (13) is operatively connected to the first turbine shaft (7) of the first counter rotating turbine (2), to the first turbine shaft (7) of the second counter rotating turbine (3) and to the central shaft (33) and is configured to transmit the rotary motion of the first turbine shafts (7) to the central shaft (33) according to a predefined rotation direction; The second group of gears (14) is operatively connected to the second turbine shaft (8) of the first counter rotating turbine (2), to the second turbine shaft (8) of the second counter rotating turbine (3) and to the central shaft (33) and is configured to transmit the rotary motion of the second turbine shafts (8) to the central shaft (33) according to the same predefined rotation direction.

Description

Title

"Assembly of turbines"

DESCRIPTION

Field of the invention

The present invention relates to an assembly of turbines.

The present invention falls within the field of plants for generating electrical energy (power plants such as, for example, thermoelectric or geo-thermoelectric or cogeneration power plants) and is intended for the expansion systems configured to transform the energy of a work fluid that expands into mechanical energy, making one or more turbine rotors rotate, and then into electrical energy. Preferably but not exclusively, the present invention relates to expansion turbines used in apparatuses for the production of energy through the Rankine water-steam cycle or the organic Rankine cycle (ORC).

The present invention relates in particular to the realisation of an assembly of turbines coupled by means of gear trains.

Background of the invention

Different public documents are known that illustrate assemblies of turbines connected to one another by means of gears.

For example, document GB108548 illustrates two turbines side by side with their shafts connected to an interposed central shaft. Each turbine is of the axial type and has two counter rotating concentric rotors.

Public documents are also known illustrating a single counter rotating turbine provided with two rotor discs and two shafts, in which the two shafts are connected to one another at one or more transmission shafts configured, for example, to be coupled to a generator.

On this point, document GB141948 and document GB136761 both illustrate a counter rotating radial turbine whose two shafts are connected, by means of toothed wheels, to a single transmission shaft.

Document US2624173 illustrates a counter rotating turbine provided with two rotors with axial blades supported by two counter rotating turbine shafts and a gear that transmits the motion of the two turbine shafts to two respective shafts, coaxial to one another and counter rotating.

Summary

In this field, the Applicant has observed that the known solutions relative to may be improved in terms of different aspects.

In particular, the Applicant has observed that the assemblies of counter rotating turbines, such as that illustrated in document GB108548, are structurally very complex and/or not very flexible. For example, in the assembly illustrated in document GB108548, the internal and external rotors of each axial turbine are assembled inside one another. Its construction and the maintenance operations are therefore lengthy and complex. Furthermore, the rigid connection between the counter rotating turbines of the prior art implies that the turbines to be coupled must be designed together considering the characteristics of both, so as to prevent the rotor dynamics of one having a negative influence on the other. This implies the use of a high number of engineering hours for designing the assembly for each new application.

Furthermore, the counter rotating turbines of documents GB141948, GB136761 and US2624173 are single counter rotating turbines. The gears with which they are provided have the sole purpose, as already highlighted above, of mechanically connecting their two shafts to one or more transmission shafts.

The Applicant has therefore perceived the need to propose a new assembly of two turbines that allows the production times and costs to be reduced while ensuring the use thereof in plants characterised by a wide power spectrum.

In particular, the Applicant has perceived the need to:

^■ obtain an effective solution for connecting two counter rotating turbines so as to realise a simple, reliable and relatively low cost assembly;

^■ obtain an assembly of turbines that is very flexible, i.e. that allows the suitable configuration for each plant to be identified by simply coupling predefined turbines and without the need for designing the entire turbine/group of turbines from scratch every time.

The Applicant has found that the objectives indicated above and others can be reached by an assembly of turbines formed by two turbines, each of the overhung, counter rotating type, that transmit motion to a single central shaft through two groups of gears located on opposite sides of the two turbines. Said two groups of gears are preferably connected to the ends of the four shafts of said two turbines through elastic joints.

More specifically, according to an independent aspect, the present invention relates to an assembly of turbines, comprising:

a first counter rotating turbine and a second counter rotating turbine; wherein each of said first and second turbines comprises: a case; a first and a second rotor disc housed in the case, facing one another and respectively supported by a first and by a second turbine shaft, wherein each turbine shaft is rotatably supported in the case, wherein each rotor disc is supported in an overhung fashion in the case; wherein the first and the second counter rotating turbine are side by side with their first and second turbine shafts substantially parallel;

a first group of gears and a second group of gears arranged on opposite sides of said first and second counter rotating turbine;

a central shaft configured to be connected to a generator; wherein the central shaft is located between the first and the second counter rotating turbine and is substantially parallel to the first and to the second turbine shafts;

wherein the first group of gears is operatively connected to the first turbine shaft of the first counter rotating turbine, to the first turbine shaft of the second counter rotating turbine and to the central shaft and is configured to transmit the rotary motion of said first turbine shafts to the central shaft according to a predefined rotation direction;

wherein the second group of gears is operatively connected to the second turbine shaft of the first counter rotating turbine, to the second turbine shaft of the second counter rotating turbine and to the central shaft and is configured to transmit the rotary motion of said second turbine shafts to said central shaft according to said predefined rotation direction.

Each of the two turbines is an overhung counter rotating turbine. An overhung counter rotating turbine means that each of the rotor discs is placed axially to the side of all the supporting elements, such as, for example, bearings, of the respective shaft. In other words, each rotor disc has a rear face, opposite the front face, from which the respective shaft extends and said rear face faces towards the elements that support the shaft in the respective case.

In the present description and the appended claims, the adjective "axial" intends to define a direction running parallel to an axis of rotation of the turbine. The adjective "radial" is meant to define a direction directed in radius-fashion orthogonally from the axis of rotation. The adjective "circumferential" means directions tangential to circumferences coaxial to axis of rotation.

The Applicant has verified that the present invention allows the two turbines to be connected very easily and a relatively simple, reliable and inexpensive assembly/module to be created.

The Applicant has also verified that the present invention allows the assembly to be designed, adapted for a determined plant for generating energy, simply by choosing and coupling two already predefined counter rotating turbines without the need to design from scratch each turbine for every new plant.

Furthermore, the Applicant has also verified that the assembly/module according to the present invention is suited to a connection of numerous assemblies and therefore for use in plants for generating high powers without having to design ad hoc a dedicated turbine of large dimensions.

In one aspect, the assembly comprises a connecting element interposed between each of the first and second turbine shafts and the respective first or second group of gears.

In a second aspect according to the previous aspect, said connecting element comprises at least one elastic joint configured to minimise the reciprocal rotor dynamic influence between the first counter rotating turbine and the second counter rotating turbine.

In one aspect, said at least one elastic joint has elasticity in bending and/or traction/compression.

The elastic, or flexible, joint is a device used to connect two shafts/rotors together with the aim of transmitting the torque even when the two shafts are slightly misaligned. The elastic, or flexible, joint further allows the absorption of torsional vibrations. The connecting element is able to absorb any slight misalignments of the main axes of the shafts/rotors. "Slight misalignments" means that the axes are inclined to one another by a few degrees (e.g. from about 0.2° to about 3°) and/or that said axes are radially staggered with a staggering of a few millimetres (e.g. from about 2 mm to about 10 mm).

The Applicant has verified that the elastic joint that connects the two turbines allows the rotor dynamics of the individual turbines to be uncoupled, which therefore have little influence on one another.

In one aspect, said connecting element comprises a transmission shaft substantially aligned with the first turbine shaft and with the second turbine shaft. In one aspect, the connecting element comprises two elastic joints, each placed at one end of the transmission shaft.

In one aspect, the first group of gears and the second group of gears each comprise a plurality of toothed wheels with axes of rotation that are parallel to one another and parallel to the first and the second turbine shafts.

In one aspect, the toothed wheels of each of the first group of gears and the second group of gears all lie in the same plane. In other words, the toothed wheels of each group are side by side and mesh into one another.

The groups of gears are therefore compact, as is the assembly comprising them. In one aspect, the first group of gears comprises: a first toothed wheel substantially coaxial and connected to the first turbine shaft of the first counter rotating turbine; a second toothed wheel substantially coaxial and connected to the first turbine shaft of the second counter rotating turbine; a first central toothed wheel connected to a first end of the central shaft; a first auxiliary toothed wheel operatively interposed between the first toothed wheel and the first central toothed wheel; a second auxiliary toothed wheel operatively interposed between the second toothed wheel and the first central toothed wheel.

Thanks to the first and second auxiliary toothed wheels, the rotation direction of the first central toothed wheel is concordant with the rotation direction of the first toothed wheel and of the second toothed wheel.

In one aspect, the second group of gears comprises: a third toothed wheel substantially coaxial and connected to the second turbine shaft of the first counter rotating turbine; a fourth toothed wheel substantially coaxial and connected to the second turbine shaft of the second counter rotating turbine; a second central toothed wheel connected to a second end of the central shaft. In one aspect, one of the connecting elements connects the first toothed wheel to the first turbine shaft of the first counter rotating turbine. In one aspect, one of the connecting elements connects the second toothed wheel to the first turbine shaft of the second counter rotating turbine. In one aspect, one of the connecting elements connects the third toothed wheel to the second turbine shaft of the first counter rotating turbine. In one aspect, one of the connecting elements connects the fourth toothed wheel to the second turbine shaft of the second counter rotating turbine.

In one aspect, the assembly comprises a first auxiliary elastic joint interposed between the first central toothed wheel and the first end of the central shaft and a second auxiliary elastic joint interposed between the second central toothed wheel and the second end of the central shaft.

In one aspect, the first auxiliary elastic joint and the second auxiliary elastic joint have elasticity in bending and/or traction/compression.

In one aspect, said first counter rotating turbine and second counter rotating turbine are of the radial, preferably outflow type.

In one aspect, the elastic joints and preferably also the auxiliary elastic joints are of the flexible disc type.

The present invention also relates to a plant for generating electrical energy (power plant, such as, for example, a thermoelectric or geo-thermoelectric or cogeneration power plant) comprising at least one circuit for a work fluid; at least one assembly of turbines of the type described in the previous aspects and/or in the appended claims connected to at least one electrical energy generator; wherein said at least one circuit is in fluid communication with said assembly of turbines.

Further characteristics and advantages of preferred, but not exclusive, embodiments of an assembly of turbines according to the present invention will appear more clearly from the detailed description. Description of the drawings

This description will be set out below with reference to the attached drawings, provided solely for indicative and therefore non-limiting purposes, in which: ^■ figure 1 schematically shows an assembly of turbines according to the present invention;

^■ figure 2A illustrates a basic diagram of a plant for producing energy through an Organic Rankine Cycle comprising an assembly like the one illustrated in figure 1 ;

^■ figure 2B illustrates a variant of the plant of figure 1 ;

^■ figure 3 illustrates a perspective view of an elastic joint belonging to the assembly of turbines of figure 1 .

Detailed description

With reference to the mentioned figures, reference number 1 denotes in its entirety an assembly of turbines according to the present invention. Such assembly 1 can be placed in a plant 100 for producing electrical energy through an Organic Rankine Cycle (ORC), such as the one illustrated schematically in figure 2A. Such plant 100, illustrated by way of non-limiting example, comprises a closed circuit in which an organic work fluid circulates. The organic work fluid may be, for example, selected from hydrocarbons, ketones, fluorocarbons and siloxanes. The plant 100 comprises a heat exchanger/evaporator 200 that exchanges heat between a high temperature source (e.g. a geothermal source) and the work fluid itself, so as to heat, evaporate and superheat said organic work fluid. The organic work fluid that leaves the heat exchanger/evaporator 200 in the vapour state is introduced into the expansion turbines of the assembly 1 . In said expansion turbines, the organic fluid in the vapour state expands causing the rotation of the rotors thereof so as to perform a conversion of the thermal energy present in the fluid into mechanical energy using a Rankine type cycle. A generator 300 is operatively connected to the expansion turbines of the assembly 1 in the way that will be detailed below. The generator 300 converts the mechanical energy produced by the expansion turbines into electrical energy. Downstream of the assembly 1 , a condenser 400 is located, in which the work fluid leaving the expansion turbines is condensed and then sent to a pump 500 which feeds it again to the heat exchanger 200.

Figure 1 shows an embodiment of the assembly of turbines 1 according to the present invention. The assembly 1 shown comprises a first and a second counter rotating turbine 2, 3 which are substantially identical to one another. In this embodiment, each of the two turbines 2, 3 is of the radial outflow, counter rotating, type in which a work fluid (such as steam or, in the example shown, organic fluid) moves prevalently along radial directions outwards, i.e. from a centre of the turbine towards the peripheral edges of its own rotor discs.

The first counter rotating turbine 2 comprises a case 4 which houses inside it a first rotor disc 5 and a second rotor disc 6. The rotor discs 5, 6 can rotate freely, each independently from the other, in the fixed case 4 about a respective first axis of rotation and second axis of rotation "X" which coincide. For that purpose, the first rotor disc 5 is solidly constrained to a respective first turbine shaft 7 mounted in the case 4 through bearings, un-numbered. The second rotor disc 6 is solidly constrained to a respective second turbine shaft 8 mounted in the case 4 through respective bearings, un-numbered. Each of the rotor discs 5, 6 is supported in an overhung fashion, i.e. each rotor disc 5, 6 is placed axially to the side of all the supporting elements, such as, for example the bearings, of the respective turbine shaft 7, 8. In other words, each rotor disc 5, 6 has a rear face, opposite the front face, from which the respective turbine shaft 7, 8 extends and said rear face faces towards the elements that support the turbine shaft 7, 8 in the respective case 4. The first rotor disc 5 is provided with a plurality of first rotor blades 9 arranged in series of concentric rings on a respective front face of said first rotor disc 5. The second rotor disc 6 is provided with a plurality of second rotor blades 10 arranged in series of concentric rings on a respective front face of said second rotor disc 6. The front face of the first rotor disc 5 is placed opposite the front face of the second rotor disc 6 and the first rotor blades 9 of the first disc 5 are radially alternating with the second rotor blades 10 of the second disc 6. The first rotor blades 9 of the first rotor disc 5 end in proximity to the front face of the second rotor disc 6 and the second rotor blades 10 of the second rotor disc 6 end in proximity to the front face of the first rotor disc 5. The rotor blades 9, 10 are placed in a work volume interposed between the two rotor discs 5, 6 through which the work fluid moves as it expands.

The first turbine shaft 7 has a proximal end on which the first rotor disc 5 is mounted and a distal end 1 1 , opposite the proximal one, accessible from the outside of the case 4. Likewise, the second turbine shaft 8 has a proximal end on which the second rotor disc 6 is mounted and a distal end 12, opposite the proximal one, accessible from the outside of the case 4. The two distal ends of the first and of the second turbine shaft 7, 8 are accessible on opposite sides of the case 4 of the first turbine 2.

The second counter rotating turbine 3 of the example illustrated in figure 1 is identical to the first one 2 therefore it will not be described again in detail. Figure 1 further presents the same reference numbers for the corresponding elements of the two turbines 2, 3. Only the axes of rotation of the second turbine 3 are indicated clearly with the different letter Ύ".

The first and the second turbine 2, 3 are side by side in the sense that the first axis of rotation and the second axis of rotation "X" of the first counter rotating turbine 2 are parallel and distanced from the first axis of rotation and from the second axis of rotation Ύ" of the second counter rotating turbine 3.

The assembly of turbines 1 further comprises a first group of gears 13 and a second group of gears 14 arranged on opposite sides of said first and second counter rotating turbine 2, 3. The first group of gears 13 faces the distal ends 1 1 of the two first turbine shafts 7 and the second group of gears 14 faces the distal ends 12 of the two second turbine shafts 8.

The first group of gears 13 comprises a plurality of toothed wheels meshing one another and having axes of rotation parallel to one another and parallel to the first axis of rotation and to the second axis of rotation "X", Ύ" of the first and of the second turbine 2, 3.

In detail, the first group of gears 13 comprises a first toothed wheel 15 substantially coaxial and connected to the distal end 1 1 of the first turbine shaft 7 of the first counter rotating turbine 2 and a second toothed wheel 16 substantially coaxial and connected to the distal end 1 1 of the first turbine shaft 7 of the second counter rotating turbine 3.

A first central toothed wheel 17 is interposed between the first toothed wheel 15 and the second toothed wheel 16 and it is mechanically engaged to them through a first auxiliary toothed wheel 18 operatively interposed between (and meshing with) the first toothed wheel 15 and the first central toothed wheel 17 and through a second auxiliary toothed wheel 19 operatively interposed between (and meshing with) the second toothed wheel 16 and the first central toothed wheel 17. Thanks to the first and second auxiliary toothed wheel 18, 19, the rotation direction of the first central toothed wheel 17 is concordant with the rotation direction of the first toothed wheel 15 and of the second toothed wheel 16.

The second group of gears 14 comprises a third toothed wheel 20 substantially coaxial and connected to the second turbine shaft 8 of the first counter rotating turbine 2 and a fourth toothed wheel 21 substantially coaxial and connected to the second turbine shaft 8 of the second counter rotating turbine 3.

A second central toothed wheel 22 is interposed between the third toothed wheel 20 and the fourth toothed wheel 21 and is mechanically and directly engaged with (and meshing with) them. In this way, the rotation direction of the second central toothed wheel 22 is opposite the rotation direction of the third toothed wheel 20 and of the fourth toothed wheel 21 .

The first toothed wheel 15 is connected to the distal end 1 1 of the first turbine shaft 7 by means of a connecting element 23 substantially aligned with the first turbine shaft 7 and with the second turbine shaft 8 of the first counter rotating turbine 2. The connecting element 23 comprises a transmission shaft 24 jointed respectively at the distal end 1 1 of the first turbine shaft 7 and at the first toothed wheel 15 through respective elastic joints 25, each therefore placed at an end of the transmission shaft 24.

Each elastic joint 25 may be of the known type. In the embodiment illustrated in figure 3, each elastic joint 25 is of the disc or flexible disc type. The elastic joint 25 illustrated in figure 3 is the one that connects the transmission shaft 24 to the first toothed wheel 15 and comprises a flange 26 connected rigidly, for example by means of bolts, to said first toothed wheel 15. The flange 26 carries a tubular body 27 ending with a first annular edge 28. A second annular edge 29 is rigidly connected to or is part of each end of the transmission shaft 24. One or more flexible discs 30 of a substantially annular shape are placed between the first annular edge 28 and the second annular edge 29. The surface of the flexible discs 30 is substantially orthogonal to the axes of rotation "X" and to the axis of the transmission shaft 24. The first annular edge 28 is connected through first bolts 31 to the flexible disc(s) 30 and the second annular edge 29 is connected through second bolts 32 to the same flexible discs 30. The first bolts 31 circumferentially alternate with the second bolts 32. The flexible discs 30 of each of the two elastic joints 25 allow limited relative movements between the transmission shaft 24 and the respective flange 26. Such relative movements are for example: translation along three axes orthogonal to one another and/or rotation about a plurality of axes other than the axes of rotation "X". The rotation about the axes of rotation "X" (torsion) is instead preferably prevented so to allow the correct transmission of the torque. In other words, with respect to a lying plane of the flexible discs in their flat and non-deformed configuration, said discs bend/are deformed outside said plane. Therefore, the elastic joint is yielding and has elasticity in bending and traction/compression but not in torsion. The bending is such as to allow an inclination between the rigid parts that said joint connects, for example of 0.2°. Such traction/compression is such as to allow a distancing/nearing between the rigid parts that said joint connects of about +/- 5mm.

Respective connecting elements 23 identical to that described above further connect: the third toothed wheel 20 to the distal end 12 of the second turbine shaft 8 of the first counter rotating turbine 2, the second toothed wheel 16 to the distal end 12 of the first turbine shaft 7 of the second counter rotating turbine 2 and the fourth toothed wheel 21 to the distal end 12 of the second turbine shaft 8 of the second counter rotating turbine 2.

The assembly of turbines 1 further comprises a central shaft 33 configured to be connected to the generator 300. The central shaft 33 is connected between the first and the second counter rotating turbine 2, 3 and is substantially parallel to the first and to the second turbine shafts 7, 8. According to what is illustrated in figure 1 , the central shaft 33 has a first end 34 connected to the first central toothed wheel 17 and a second end 35, opposite the first 34, connected to the second central toothed wheel 22. A first auxiliary elastic joint 36, for example completely identical or similar to the elastic joint 25 described above, is interposed between the first central toothed wheel 17 and the first end 34 of the central shaft 33 and, likewise, a second auxiliary elastic joint 37 is interposed between the second central toothed wheel 22 and the second end 35 of the central shaft 33. In the embodiment illustrated, a shaft of the generator 300 is connected to the second central toothed wheel 22 on the opposite side with respect to the central shaft 33. In operation, the organic fluid in the vapour state that expands in the first and in the second turbine 2, 3 determines the rotation of the first rotor discs 5 and of the first turbine shafts 7 according to a first common rotation direction and determines the rotation of the second rotor discs 6 and of the second turbine shafts 8 according to a second rotation direction opposite the first rotation direction. The first toothed wheel 15 and the second toothed wheel 16 through the first auxiliary toothed wheel 18 and through the second auxiliary toothed wheel 19 impart the rotation to the first central toothed wheel 17 in accordance with the first rotation direction. The third toothed wheel 20 and the fourth toothed wheel 21 impart the rotation to the second central toothed wheel 22 in accordance with the same rotation direction. Such rotation is transmitted to the shaft of the generator 300. In the example embodiment of the plant illustrated in figure 2A, from the point of view of the path of the work fluid, the first and the second counter rotating turbines 2, 3 are connected in parallel. The work fluid in the vapour state that leaves the heat exchanger/evaporator 200 is split into two flows and each of said flows enters into one of the counter rotating turbines 2, 3. The expanded work fluid leaving from each of the counter rotating turbines 2, 3 is then conveyed into a single conduit that enters the condenser 400.

In a different embodiment, illustrated in figure 2B, from the point of view of the path of the work fluid, the first and the second counter rotating turbines 2, 3 are connected in series. The work fluid in the vapour state that leaves the heat exchanger/evaporator 200 enters into the first counter rotating turbine 2 and, leaving this partially expanded, enters into the second counter rotating turbine 3 to then flow into the condenser 400.

List of elements

1 assembly of turbines

2 first counter rotating turbine

3 second counter rotating turbine

4 case

5 first rotor disc

6 second rotor disc

7 first turbine shaft 8 second turbine shaft

9 first rotor blades

10 second rotor blades

1 1 distal end of the first turbine shaft

12 distal end of the second turbine shaft

13 first group of gears

14 second group of gears

15 first toothed wheel

16 second toothed wheel

17 central toothed wheel

18 first auxiliary toothed wheel

19 second auxiliary toothed wheel

20 third toothed wheel

21 fourth toothed wheel

22 second central toothed wheel

23 connecting element

24 transmission shaft

25 elastic joint

26 flange

27 tubular body

28 first annular edge

29 second annular edge

30 flexible discs

31 first bolts

32 second bolts

33 central shaft

34 central shaft first end

35 central shaft second end

36 first auxiliary elastic joint

37 second auxiliary elastic joint

100 ORC plant

200 heat exchanger/evaporator

300 generator condenser pump axes of rotation

Claims

1 . An assembly of turbines, comprising:

a first counter rotating turbine (2) and a second counter rotating turbine (3); wherein each of said first and second counter rotating turbines (2, 3) comprises: a case (4); a first and a second rotor disc (5, 6) housed in the case (4), facing one another and respectively supported by a first and by a second turbine shaft (7, 8), wherein each turbine shaft (7, 8) is rotatably supported in the case (4), wherein each rotor disc (5, 6) is supported in an overhung fashion in the case (4); wherein the first and the second counter rotating turbine (2, 3) are side by side with their first and second turbine shafts (7, 8) substantially parallel;

a first group of gears (13) and a second group of gears (14) arranged on opposite sides of said first and second counter rotating turbine (2, 3);

a central shaft (33) configured to be connected to a generator (300); wherein the central shaft (33) is located between the first and the second counter rotating turbine (2, 3) and is substantially parallel to the first and to the second turbine shafts (7, 8);

wherein the first group of gears (13) is operatively connected to the first turbine shaft (7) of the first counter rotating turbine (2), to the first turbine shaft (7) of the second counter rotating turbine (3) and to the central shaft (33) and is configured to transmit the rotary motion of said first turbine shafts (7) to the central shaft (33) according to a predefined rotation direction;

wherein the second group of gears (14) is operatively connected to the second turbine shaft (8) of the first counter rotating turbine (2), to the second turbine shaft (8) of the second counter rotating turbine (3) and to the central shaft (33) and is configured to transmit the rotary motion of said second turbine shafts (8) to said central shaft (33) according to said predefined rotation direction.

2. The assembly of turbines according to claim 1 , comprising a connecting element (23) interposed between each of the first and second turbine shafts (7, 8) and the respective first or second group of gears (13, 14), wherein said connecting element (23) comprises at least one elastic joint (25) configured to minimise the reciprocal rotor dynamic influence between the first counter rotating turbine (2) and the second counter rotating turbine (3).

3. The assembly of turbines according to claim 2, wherein the connecting element (23) comprises a transmission shaft (24) substantially aligned with the first turbine shaft (7) and with the second turbine shaft (8).

4. The assembly of turbines according to claim 3, wherein the connecting element (23) comprises two elastic joints (25), each placed at an end of the transmission shaft (24).

5. The assembly of turbines according to one of claims 2 to 4, wherein said at least one elastic joint (25) has elasticity in bending and/or traction/compression.

6. The assembly of turbines according to one of claims 2 to 5, wherein the connecting element (23) is able to absorb misalignments of main axes of the first and second turbine shafts (7, 8), wherein the main axes are inclined to one another by 0.2° to 3° and/or wherein said main axes are radially staggered with a staggering from 2 mm to 10 mm.

7. The assembly of turbines according to one of the preceding claims, wherein the first group of gears (13) and the second group of gears (14) each comprise a plurality of toothed wheels (15, 16, 17, 18, 19, 20, 21 , 22) with axes of rotation that are parallel to one another and parallel to the first and the second turbine shafts (7, 8).

8. The assembly of turbines according to one of the preceding claims, wherein the first group of gears (13) comprises: a first toothed wheel (15) substantially coaxial and connected to the first turbine shaft (7) of the first counter rotating turbine (2); a second toothed wheel (16) substantially coaxial and connected to the first turbine shaft (7) of the second counter rotating turbine (3); a first central toothed wheel (17) connected to a first end (34) of the central shaft (33); a first auxiliary toothed wheel (18) operatively interposed between the first toothed wheel (15) and the first central toothed wheel (17); a second auxiliary toothed wheel (19) operatively interposed between the second toothed wheel (16) and the first central toothed wheel (17).

9. The assembly of turbines according to the preceding claim, wherein the second group of gears (14) comprises: a third toothed wheel (20) substantially coaxial and connected to the second turbine shaft (8) of the first counter rotating turbine 82); a fourth toothed wheel (21 ) substantially coaxial and connected to the second turbine shaft (8) of the second counter rotating turbine (3); a second central toothed wheel (22) connected to a second end (35) of the central shaft (33).

10. The assembly of turbines according to the preceding claim when dependent on one of claims 2 to 6, wherein one of the connecting elements (23) connects the first toothed wheel (15) to the first turbine shaft (7) of the first counter rotating turbine (2), one of the connecting elements (23) connects the second toothed wheel (16) to the first turbine shaft (7) of the second counter rotating turbine (3), one of the connecting elements (23) connects the third toothed wheel (20) to the second turbine shaft (8) of the first counter rotating turbine (2), one of the connecting elements (23) connects the fourth toothed wheel (21 ) to the second turbine shaft (8) of the second counter rotating turbine (3).

1 1 . The assembly of turbines according to one of claims 8, 9 or 10, comprising a first auxiliary elastic joint (36) interposed between the first central toothed wheel (17) and the first end (34) of the central shaft (33) and a second auxiliary elastic joint (37) interposed between the second central toothed wheel (22) and the second end (35) of the central shaft (33).

12. The assembly of turbines according to the preceding claim, wherein the first auxiliary elastic joint (36) and the second auxiliary elastic joint (37) have elasticity in bending and/or traction/compression.

13. The assembly of turbines according to at least one of the preceding claims, wherein said first counter rotating turbine (2) and said second counter rotating turbine (3) are of the radial type.

14. The assembly of turbines according to the preceding claim, wherein said first counter rotating turbine (2) and said second counter rotating turbine (3) are radial outflow turbines.