U.S. patent application number 17/616653 was filed with the patent office on 2022-07-21 for heat exchanger, method for producing a heat exchanger and power plant comprising such a heat exchanger.
This patent application is currently assigned to Siemens Energy Global GmbH & Co. KG. The applicant listed for this patent is Siemens Energy Global GmbH & Co. KG. Invention is credited to Paul Girbig, Olaf Michelsson, Olaf Schmidt, Jurgen Voss.
Application Number | 20220228809 17/616653 |
Document ID | / |
Family ID | |
Filed Date | 2022-07-21 |
United States Patent
Application |
20220228809 |
Kind Code |
A1 |
Girbig; Paul ; et
al. |
July 21, 2022 |
HEAT EXCHANGER, METHOD FOR PRODUCING A HEAT EXCHANGER AND POWER
PLANT COMPRISING SUCH A HEAT EXCHANGER
Abstract
A heat exchanger and method for producing such a heat exchanger
which during operation in a flow direction is flown through by a
medium to be cooled and by two different cooling media. A power
plant has a generator cooled by means of a generator cooling gas
and a heat exchanger cooling the generator cooling gas.
Inventors: |
Girbig; Paul; (Uttenreuth,
DE) ; Michelsson; Olaf; (Arnstadt, DE) ;
Schmidt; Olaf; (Arnstadt, DE) ; Voss; Jurgen;
(Berlin, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens Energy Global GmbH & Co. KG |
Munich, Bayern |
|
DE |
|
|
Assignee: |
Siemens Energy Global GmbH &
Co. KG
Munich, Bayern
DE
|
Appl. No.: |
17/616653 |
Filed: |
May 12, 2020 |
PCT Filed: |
May 12, 2020 |
PCT NO: |
PCT/EP2020/063124 |
371 Date: |
December 5, 2021 |
International
Class: |
F28D 7/08 20060101
F28D007/08; F01K 13/00 20060101 F01K013/00; F28F 1/32 20060101
F28F001/32; F28F 21/08 20060101 F28F021/08; F28F 9/00 20060101
F28F009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 13, 2019 |
DE |
10 2019 208 619.5 |
Claims
1. A heat exchanger, through which a medium to be cooled flows in a
flow direction during operation thereof, comprising: a first stack
of fins having a multitude of first fins stacked in a stacking
direction that extends transverse to the flow direction, wherein
the first fins are each provided with a multitude of first passage
holes that are flush with one another in stacking direction, at
least one further stack of fins arranged adjacent to the first
stack of fins in flow direction, and having a multitude of second
fins stacked in the stacking direction, wherein the second fins are
each provided with a multitude of second passage holes that are
flush with one another in stacking direction, first pipe conduits
that extend through the first passage holes of the first fins of
the first stack of fins and are press-fitted with the first fins,
second pipe conduits that extend through the second passage holes
of the second fins of the at least one further stack of fins and
are press-fitted with the second fins, wherein the first pipe
conduits and the second pipe conduits are not connected to one
another for flow purposes and are provided for passage of a first
cooling medium and a second cooling medium, wherein the first and
second cooling media are different from one another, and at least
one cover that connects the first stack of fins and the at least
one further stack of fins to one another, which is placed atop an
outer first fin of the first stack of fins and atop the adjacent
outer second fin of the at least one further stack of fins and
covers these fins, wherein the at least one cover has been provided
with first passage holes arranged and formed so as to correspond to
the first passage holes of the first fins of the first stack of
fins, through which the first pipe conduits are guided, and has
been provided with second passage holes arranged and formed so as
to correspond to the second passage holes of the second fins of the
at least one further stack of fins, through which the second pipe
conduits are guided.
2. The heat exchanger as claimed in claim 1, wherein the first fins
have a greater area than the second fins.
3. The heat exchanger as claimed in claim 1, wherein the design of
the surface of the first fins is different than the design of the
surface of the second fins.
4. The heat exchanger as claimed in claim 1, wherein the first fins
and the second fins are produced from sheet material.
5. The heat exchanger as claimed in claim 1, wherein a distance
between the first fins in stacking direction is different than the
distance between the second fins in stacking direction, preferably
greater.
6. The heat exchanger as claimed in claim 1, wherein the first fins
and the second fins have been produced from a sheet material having
a coating on one or both sides.
7. The heat exchanger as claimed in claim 1, wherein the first pipe
conduits are each connected to one another via U-shaped connecting
conduits, and the first cooling medium flows through them
successively, and/or wherein the second pipe conduits are each
connected to one another by U-shaped connecting conduits, and the
second cooling medium flows through them successively.
8. The heat exchanger as claimed in claim 1, wherein the flow cross
section of the first pipe conduits is different than the flow cross
section of the second pipe conduits.
9. The heat exchanger as claimed in claim 1, wherein the first pipe
conduits and the second pipe conduits have been manufactured from a
metallic material.
10. The heat exchanger as claimed in claim 1, wherein the inner
faces of the first pipe conduits and/or the inner faces of the
second pipe conduits are structured.
11. The heat exchanger as claimed in claim 1, wherein an
arrangement pattern of the first passage holes is different than
the arrangement pattern of the second passage holes.
12. The heat exchanger as claimed in claim 1, wherein the at least
one cover has been produced from a metallic material, and/or from a
metal sheet.
13. The heat exchanger as claimed in claim 1, wherein the at least
one cover encompasses the first stack of fins and the at least one
further stack of fins on opposite sides.
14. The heat exchanger as claimed in claim 1, wherein the first
stack of fins and the at least one further stack of fins are
connected to one another by at least one strut.
15. A process for producing a heat exchanger as claimed in claim 1,
wherein the first fins and the second fins are produced
simultaneously in a single fin compression device.
16. The process as claimed in claim 15, wherein the first pipe
conduits and the second pipe conduits are expanded simultaneously
in a pipe conduit expansion machine.
17. A power plant comprising: a generator cooled by means of a
generator cooling gas, and a heat exchanger as claimed in claim 1
that cools the generator cooling gas.
18. The power plant as claimed in claim 17, wherein the first
cooling medium that flows through the heat exchanger is cooling
water and the second cooling medium that flows through the heat
exchanger is a coolant.
19. The heat exchanger as claimed in claim 5, wherein the distance
between the first fins in stacking direction is greater than the
distance between the second fins in stacking direction.
20. The heat exchanger as claimed in claim 8, wherein the flow
cross section of the first pipe conduits is greater than the flow
cross section of the second pipe conduits.
21. The heat exchanger as claimed in claim 9, wherein the metallic
material comprises copper.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2020/063124 filed 12 May 2020, and claims the
benefit thereof. The International Application claims the benefit
of German Application No. DE 10 2019 208 619.5 filed 13 Jun. 2019.
All of the applications are incorporated by reference herein in
their entirety.
FIELD OF INVENTION
[0002] The present invention relates to a heat exchanger through
which a medium to be cooled flows in a flow direction during
operation thereof. The present invention further relates to a
process for producing such a heat exchanger. The invention
additionally relates to a power plant having a generator cooled by
means of a generator cooling gas and a heat exchanger that cools
the generator cooling gas.
BACKGROUND OF INVENTION
[0003] Power plants, for example gas turbine power plants, steam
turbine power plants, combined gas and steam turbine power plants,
solar power plants or the like, comprise a multitude of components
that require cooling, in order firstly to remove the waste heat
that arises and secondly to increase the output of the power plant.
This is also true of the generator used for power generation, which
is generally cooled with generator cooling gas recooled by means of
a heat exchanger. The heat exchanger is usually connected to a
closed cooling water system of the power plant, via which further
heat exchangers are also supplied with cooling water for example
those for lubricant oil and/or sealing oil cooling, for cooling of
pumps or the like. The cooling water of the cooling water circuit
can be re-cooled in various ways, for example by means of fresh
water flow cooling, circulation cooling using a cooling tower or
air-cooled coolers, etc.
[0004] A possible achievable electrical power in the generator
depends on the cold gas temperature of the generator cooling gas
defined for cooling of the generator windings, i.e. the generator
cooling gas temperature on entry into the generator. The lower the
cold gas temperature, the more mechanical energy can be converted
to electrical energy in the generator. The generator cooling gas is
recooled as described above within a heat exchanger, through which
cooling water from the cooling system of the power plant flows.
Thus, the cold gas temperature of the generator cooling gas is
coupled to the cooling water temperature of the cooling water
flowing through the heat exchanger. The cooling water temperature
in turn is dependent on the recooling of the cooling water and
consequently cannot be lowered at will. There are thus limits to
the electrical power achievable in the generator.
[0005] If power-increasing measures on the turbine of the power
plant result in a rise in mechanical power at the generator shaft,
it would be desirable to provide improved cooling for the generator
in order to be able to convert more power to electrical energy
therewith.
SUMMARY OF INVENTION
[0006] Proceeding from this prior art, it is an object of the
present invention to provide improved cooling, especially improved
generator cooling.
[0007] This object is achieved by the present invention by
providing a heat exchanger comprising a first stack of fins having
a multitude of first fins stacked in a stacking direction that
extends transverse to the flow direction, wherein the first fins
are each provided with a multitude of first passage holes that are
flush with one another in stacking direction, at least one further
stack of fins arranged adjacent to the first stack of fins in flow
direction, and having a multitude of second fins stacked in the
stacking direction, wherein the second fins are each provided with
a multitude of second passage holes that are flush with one another
in stacking direction, first pipe conduits that extend through the
first passage holes of the first fins of the first stack of fins
and are press-fitted with the first fins, second pipe conduits that
extend through the second passage holes of the second fins of the
at least one further stack of fins and are press-fitted with the
second fins, wherein the first pipe conduits and the second pipe
conduits are not connected to one another for flow purposes and are
provided for passage of a first cooling medium and a second cooling
medium, wherein the cooling media are different from one another,
and at least one cover that connects the first stack of fins and
the at least one further stack of fins to one another, which is
placed atop an outer first fin of the first stack of fins and atop
the adjacent outer second fin of the at least one further stack of
fins and covers these fins, wherein the at least one cover has been
provided with first passage holes arranged and formed so as to
correspond to the first passage holes of the first fins of the
first stack of fins, through which the first pipe conduits are
guided, and has been provided with second passage holes arranged
and formed so as to correspond to the second passage holes of the
second fins of the at least one further stack of fins, through
which the second pipe conduits are guided.
[0008] Such a heat exchanger is advantageous in that it can be
operated with two different cooling media. The first cooling medium
may be cooling water, for example. The second cooling medium used
may, for example, be a coolant which is recooled in a cooling unit.
If the medium to be cooled is generator cooling gas, the cold gas
temperature thereof on entry into the generator is not limited by
the degree of recooling of the cooling water of the cooling water
system of the power plant. Instead, the cold gas temperature of the
generator cooling gas can be lowered further via heat exchange with
the coolant that flows through the heat exchanger, such that it can
be adapted flexibly to the cooling demand of the generator if, for
example, power-increasing measures are undertaken on the turbine. A
further advantage of the heat exchanger of the invention is that
the mechanical coupling of the stacks of fins through which the
different cooling media flow via the at least one cover imparts
very good mechanical stiffness to both stacks of fins with a
simultaneously very inexpensive construction of low volume, even if
one of the stacks of fins in itself should have only very low
intrinsic stiffness, for example, on account of low construction
depth. This is important especially when an existing heat exchanger
in which the generator cooling gas has to date been recooled by
means of cooling water only is to be replaced by a heat exchanger
of the invention in order to lower the cold gas temperature of the
generator cooling gas by additional cooling by means of a coolant.
In such cases, the construction space available is defined by the
dimensions of the old heat exchanger and is very limited.
Correspondingly, barely any space is available for a stack of fins
through which a second cooling medium flows, and therefore this
second stack of fins can frequently be executed only with a very
low construction depth, which leads to low intrinsic stiffness.
[0009] In one configuration of the heat exchanger of the invention,
the first fins have a greater area than the second fins. In other
words, the dimensions of the fins of the respective stacks of fins
are matched to the respective cooling medium.
[0010] The design of the surface of the first fins is
advantageously different than the design of the surface of the
second fins. In this way too, it is possible to achieve adaptation
of the stacks of fins to the respective cooling medium.
[0011] Advantageously, the first fins and the second fins have been
produced from a sheet material, for example from aluminum, in order
to achieve good thermal conductivity.
[0012] In one configuration of the heat exchanger of the invention,
a distance between the first fins in stacking direction is
different than the distance between the second fins in stacking
direction, advantageously greater.
[0013] According to the invention, the first fins and the second
fins may have been produced from a sheet material having a coating
on one or both sides.
[0014] The first pipe conduits are each connected to one another
via U-shaped connecting conduits, and the first cooling medium
flows through them successively, and/or the second pipe conduits
are each connected to one another by U-shaped connecting conduits,
and the second cooling medium flows through them successively.
[0015] The flow cross section of the first pipe conduits is
advantageously different than the flow cross section of the second
pipe conduits, advantageously greater.
[0016] In one configuration of the present invention, the first
pipe conduits and the second pipe conduits have been produced from
a metallic material, advantageously from copper, which achieves
good thermal conductivity.
[0017] Advantageously, the inner faces of the first pipe conduits
and/or the inner faces of the second pipe conduits are structured
in order to increase their size, which is conducive to better heat
transfer.
[0018] An arrangement pattern of the first passage holes
advantageously differs from the arrangement pattern of the second
passage holes.
[0019] The at least one cover has advantageously been produced from
a metallic material, advantageously from a metal sheet. This leads
to a simple and inexpensive construction of the at least one
cover.
[0020] Advantageously, the at least one cover encompasses the first
stack of fins and the at least one further stack of fins on
opposite sides, which further increases the mechanical stiffness of
the construction.
[0021] In one configuration of the present invention, the first
stack of fins and the at least one further stack of fins are joined
to one another via at least one side section. Such a side section
is also very conducive to the mechanical stiffness of the
construction.
[0022] The present invention further provides a process for
producing a heat exchanger designed in accordance with the
invention, in which the first fins and the second fins are produced
simultaneously in a single fin compression device, for example
using a fin compression mold that defines both features of the
first fins and features of the second fins. In this way, very
effective manufacture of the heat exchanger of the invention is
achieved.
[0023] The first pipe conduits and the second pipe conduits are
advantageously expanded simultaneously in a pipe conduit expansion
machine. Such simultaneous expansion is also very conducive to
effective manufacture of the heat exchanger of the invention.
[0024] The present invention additionally provides a power plant
having a generator cooled by means of a generator cooling gas and a
heat exchanger of the invention that cools the generator cooling
gas.
[0025] Advantageously, the first cooling medium that flows through
the heat exchanger is cooling water, and the second cooling medium
that flows through the heat exchanger is a coolant, for example
tetrafluoromethane (R-134a) or carbon dioxide.
[0026] Further features and advantages of the present invention
become clear from the description that follows with reference to
the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The figures show:
[0028] FIG. 1 a schematic view of a power plant in one embodiment
of the present invention;
[0029] FIG. 2 a schematic view of one embodiment of a heat
exchanger of the invention in the power plant shown in FIG. 1;
[0030] FIG. 3 a schematic side view in the direction of the arrow
III in FIG. 2, showing a first stack of fins and a second stack of
fins of the heat exchanger, omitting struts and an upper and lower
cover for illustration purposes;
[0031] FIG. 4 a top view of a first fin of a first stack of fins of
the heat exchanger shown in FIG. 2;
[0032] FIG. 5 a top view of a second fin of a further stack of fins
of the heat exchanger shown in FIG. 2;
[0033] FIG. 6 a schematic view of a fin production machine for
production of the fins shown in FIGS. 4 and 5;
[0034] FIG. 7 a schematic perspective view of a fin compression
mold of the fin production machine shown in FIG. 6; and
[0035] FIG. 8 a schematic view of pipe conduit expansion tools of a
pipe conduit expansion machine.
DETAILED DESCRIPTION OF INVENTION
[0036] FIG. 1 shows a power plant 1 in one embodiment of the
present invention. The power plant 1 in the present case is a gas
turbine power plant, which may in principle likewise be any type of
power plant. The power plant 1 comprises an air compressor 2, a gas
turbine 3, a generator 4 and a transformer 5. During the operation
of the power plant 1, air compressed by the air compressor 2 is
mixed with fuel in a known manner, and the air-fuel mixture is
ignited. The resultant combustion gas is supplied to the gas
turbine 3, where it is expanded to drive a gas turbine rotor 6. The
gas turbine rotor 6 drives the rotor 46 of the generator 4, which
converts the kinetic energy to electrical energy. The transformer 5
transforms the electrical energy in such a way that it can be fed
to a power supply grid. The generator 4 is supplied by DC power in
operation via contact rings or a brushless exciter 47.
[0037] The generator 4 is cooled using generator cooling gas, which
is circulated through a generator cooling circuit 7 by means that
are not shown in detail. The generator cooling gas is recooled by
provision of a heat exchanger 8 in one embodiment of the present
invention. In the heat exchanger 8, the generator cooling gas is
cooled firstly using cooling water that circulates in a cooling
water circuit 9, and secondly by means of a coolant that circulates
in a coolant circuit 10. The cooling water circuit 9 in the present
case is what is called the intermediate cooling water circuit of
the power plant 1, to which further heat exchangers are also
connected, by means of which lubricant oil, sealing oil, pumps
and/or other components of the power plant 1, for example, are
cooled. The coolant circuit 10 through which the coolant flows
comprises a cooling unit for recooling of the coolant. The coolant
used in the present context is tetrafluoroethane (R-134a).
Alternatively, it is also possible to use another coolant such as
carbon dioxide, to give just one example.
[0038] During the operation of the power plant 1, the generator
cooling gas removes heat from the generator 4, is recooled in the
heat exchanger 8 and then is guided back into the generator 4. In
the heat exchanger 8, the heat withdrawn from the generator cooling
gas is transferred firstly to the cooling water that flows through
the cooling water circuit 9 and secondly to the coolant that flows
through the coolant circuit 10.
[0039] A significant advantage of the power plant 1 shown in FIG. 1
is that the generator cooling gas that flows through the generator
cooling gas circuit 7 is recooled not solely by means of cooling
water but additionally by means of a coolant. In this way, the cold
gas temperature of the generator cooling gas on entry into the
generator 4 is adjustable or controllable very flexibly and as
required. A further advantage is that the generator cooling gas is
recooled by the cooling water and by the coolant in a single heat
exchanger 8, since the use of a single heat exchanger 8 saves
construction space. This is of particular importance especially
when an existing heat exchanger of a power plant in which the
recooling is effected solely using cooling water is to be replaced
by a heat exchanger of the invention, since the construction space
that is then available is defined by the dimensions of the old heat
exchanger and is correspondingly limited.
[0040] FIG. 2 shows one possible design of a heat exchanger 8 of
the invention. The heat exchanger 8 through which a generator
cooling gas flows in a flow direction indicated by the arrows 11
comprises a first stack of fins 12 having a multitude of first fins
14 stacked in a stacking direction that extends transverse to the
flow direction indicated by the arrow 13. The first fins 14, as
shown in FIG. 4, are each provided with a multitude of first
passage holes 15 that are flush with one another in stacking
direction. The heat exchanger 8 further comprises at least one
further stack of fins 16 arranged adjacent to the first stack of
fins 12 in flow direction and having a multitude of second fins 17
stacked in the stacking direction, wherein the second fins 17 are
each provided with a multitude of second passage holes 18 that are
flush with one another in stacking direction.
[0041] The first fins 14 and the second fins 17 are each produced
from sheet material, in the present case from aluminum, wherein the
first fins 14 and/or the second fins 17 may be provided with a
coating on one or both sides. The first fins 14 differ from the
second fins 17 firstly in that they have a greater area. Secondly,
the surfaces of the first fins 14, apart from the first passage
holes 15, in the present case are smooth, whereas the surfaces of
the second fins 17 are structured. The structuring in the working
example presented is defined by elevated regions 19 that are
slotted at the side and are provided in the upward direction, which
increases the surface areas of the second fins 17 and influences
the flow of the generator cooling gas through the further stack of
fins 16. However, it should be pointed out that the design of the
surfaces both of the first fins 14 and of the second fins 17 may in
principle vary as required. A further difference is that a distance
a.sub.1 between the first fins 14 in stacking direction is greater
than a distance a.sub.2 between the second fins 17 in stacking
direction. Furthermore, the arrangement patterns of the first
passage holes 15 differ from the arrangement patterns of the second
passage holes 18, as apparent from FIGS. 3 and 4.
[0042] The heat exchanger 8 further comprises first pipe conduits
20 that extend through the first passage holes 15 of the first fins
14 of the first stack of fins 12 and are press-fitted with the
first fins 14, and second passage holes 21 that extend through the
second passage holes 18 of the second fins of the at least one
further stack of fins 16 and are press-fitted with the second fins
17. The first pipe conduits are each connected to one another via
U-shaped connecting conduits 22, and the cooling water flows
through them successively, entering the first stack of fins 12 in
the direction of the arrow 23 and exiting therefrom in the
direction of the arrow 24. The second pipe conduits 21 are each
connected to one another by U-shaped connecting conduits 25, and
the coolant flows through them successively, entering the further
stack of fins 16 in the direction of the arrow 26 and exiting
therefrom in the direction of the arrow 27. The first pipe conduits
20 and the second pipe conduits 21 have each been produced from a
metallic material, from copper in the present case, where the flow
cross section of the first pipe conduits 20 is greater than the
flow cross section of the second pipe conduits 21. The inner faces
of the first pipe conduits 20 and/or the inner faces of the second
pipe conduits 21 may be structured in order to increase their
surface area.
[0043] The heat exchanger 8 additionally comprises an upper cover
and lower cover 28, each of which connect the first stack of fins
12 and the further stack of fins 16 to one another. The covers 28
are respectively placed onto the outer first fins 14 of the first
stack of fins 12 and onto the adjacent outer second fins 17 of the
further stack of fins 16 in stacking direction from the bottom and
from the top, and cover these fins 14 and 17. The covers 28 have
been provided with first passage openings 29 that are arranged and
formed so as to correspond to the first passage holes 15 of the
first fins 14 of the first stack of fins 12, through which the
first pipe conduits 20 are conducted, and with second passage
openings 30 that are arranged and formed so as to correspond to the
second passage holes 18 of the second fins 17 of the further stack
of fins 16, through which the second pipe conduits 21 are
conducted. The covers 28 have been produced from a metallic
material, in the present case each from a metal sheet of aluminum.
They firstly have chamfers 31 that point in the direction of the
stacks of fins 12 and 16, which encompass these opposite sides, and
secondly chamfers 32 that point outward, which serve to protect the
pipe conduits 20, 21 or the connecting conduits 22, 25 that connect
these to one another. The covers 28 are connected to one another
via struts 33 in the present case, which impart good mechanical
stiffness to the heat exchanger.
[0044] FIG. 6 shows a schematic of a fin production machine 34 with
a sheet metal roll accommodation device 36 that accommodates a roll
of sheet metal 35, a sheet metal conveying device 37, a fin
pressing device 38 having an upper fin press mold 39 and a lower
fin press mold 40, a fin transport device 41 and a fin stacking
device 42.
[0045] During the operation of the fin production machine 34, sheet
metal from which the first fins 14 and the second fins 17 are to be
manufactured is unwound by means of the sheet metal conveying
device 37 from the roll of sheet metal 35 that is held by the sheet
metal roll accommodation device 36 and fed to the fin pressing
device 38. Both the first fins 14 and the second fins 17 are formed
therein by movement of the fin press molds 39 and 40 together and
movement thereof away from one another. As shown in FIG. 7, the fin
press molds 39 and 40 have different regions A1, A2, A3, A4, B1,
B2, B3 and B4, which form features of the fins 14 and 17. The
regions identified by A form features of the first fin 14, and the
regions identified by B form features of the second fin 17. The
regions identified by number 1 form slots in the sheet metal; the
regions identified by number 2 expand slotted regions; the regions
identified by number 3 perform deep drawing of the sheet metal. The
first fins 14 and second fins 17 manufactured in this way in the
fin pressing device 38 are then moved using the fin transport
device 41 to the fin stacking device 42, where the first fins 14
and the second fins 17 are respectively stacked one on top of
another.
[0046] The stacked fins 14 and 17 are then moved to a pipe conduit
expansion machine 43. The pipe conduits 20 and 21 that have been
introduced in the meantime into the corresponding passage holes 15,
18 of the stacked fins 14, 17 are expanded simultaneously therein
using suitably shaped pipe conduit expansion tools 44 by pushing
the pipe conduit expansion tools 44 through the pipe conduits 20,
21 from the top in the direction of the arrows 45.
[0047] In further steps, the covers 28, the struts 33 and the
connecting conduits 22, 25 are mounted.
[0048] Even though the invention has been further illustrated and
described in detail by the working example, the invention is not
limited by the examples disclosed, and other variations may be
derived therefrom by the person skilled in the art without
departing from the scope of protection of the invention.
* * * * *