Note: Descriptions are shown in the official language in which they were submitted.
CA 02783126 2014-04-09
25861-94
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FIBRE-REINFORCED CERAMIC BODY
The present invention relates to a fibre-reinforced body, a
method for the production thereof and the use thereof as a pipe
or a pipe bottom in a heat exchanger.
Components made from ceramic material, such as silicon carbide
pipes, are often used in heat exchangers. Being of ceramic
materials, leak-proof silicon carbide pipes are prone to
brittle fracture. In the event of mechanical failure, the pipes
fracture catastrophically, i.e. into fractured sections. The
pipe loses its integrity. A heat exchanger that has been made
from pipes of this kind may be destroyed by a fracture of this
nature, as corrosive acids reach the heat exchanger's service
compartment, which is not protected against corrosion. In
addition, further damage may occur in the cooling system or the
heating system to which the heat exchanger is connected.
The problem addressed by the present invention is that of
providing a material that is immune to catastrophic brittle
fracture.
According to one aspect of the invention, there is provided a
body comprising a ceramic material which is suitable for use in
a heat exchanger and for conducting fluids, wherein the body is
a pipe-shaped body or a cover having a plurality of holes
extending in a longitudinal direction thereof, and wherein the
ceramic material is dense sintered silicon carbide and the
outer side of the body is at least partially encompassed by at
least two fibre bundles in either or both of the longitudinal
direction and the circumferential direction and is non-
positively connected thereto, wherein the fibre bundles are
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carbon fibre bundles, wherein the fibre bundles are pre-
tensioned and neighbouring sections of the fibre bundles are
arranged at a predetermined distance and form a network.
According to another aspect of the invention, there is provided
a method for the production of a body comprising the steps a)
providing a body which comprises a ceramic material and is
suitable for use in a heat exchanger and for conducting fluids,
wherein the ceramic material is dense sintered silicon carbide,
and b) encompassing at least some sections of the outer side of
the body by at least two fibre bundles under a predetermined
pre-tension forming a non-positive connection, wherein the
fibre bundles are carbon fibre bundles, wherein neighbouring
sections of the fibre bundle are arranged at a predetermined
distance, wherein the body is a pipe-shaped body or a cover
having a plurality of holes extending in a longitudinal
direction thereof.
The body according to the invention is a body comprising a
ceramic material which is suitable for use in a heat exchanger
and for conducting fluids. The outer side of the body is at
least partially encompassed by at least two fibre bundles in
the longitudinal direction and/or the circumferential direction
and non-positively connected thereto. The fibre bundles are
pre-tensioned. Neighbouring sections of the fibre bundles are
arranged at a predetermined distance. Reinforcing the body by
means of the fibre bundles means that it becomes more immune to
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brittle fracture and its pressure resistance and load-
bearing capacity are increased.
The fibre reinforcement improves the properties of bodies
as follows: Increase in bursting pressure, the body becomes
more immune to brittle fracture, steam hammering and
unpermitted exceeding of the operating pressure. Even if
fluids are conducted through the fibre-reinforced body
during routine operation and a longitudinal crack appears
therein, as a result of its age, for example, improper use
or overstress, this body does not exhibit any significant
leaks up to a predetermined differential pressure. The
pushing out or breaking out of fragments of the body is
intercepted to a certain extent due to the encompassing of
the body with fibre bundles, such that a piece pushing or
breaking out of the original form of the body is retained
in a predetermined form by the surrounding pre-tensioned
fibre bundle. The breaking out of pieces from the body and
therefore the emergence of large quantities or fluid are
prevented. The heat exchanger in which the body is used can
usually be further operated without interruption until
there is a planned shutdown. The body according to the
invention is therefore leak-proof to a certain extent, even
in a defective state, by comparison with the unreinforced
body.
The body is preferably a pipe-shaped body. Within the
meaning of the present invention, a pipe-shaped body is
particularly taken to mean a body which preferably has a
circular cross-section and is open at the ends of its
longitudinal extension, in order to be suitable for
conducting fluids. Alternatively, however, the pipe-shaped
body may also exhibit a square, oval or other shaped cross-
section. The longitudinal extension of the pipe-shaped body
is preferably greater than its cross-section. The pipe-
shaped body is preferably a pipe with a circular cross-
section.
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Alternatively, the body is a cover, wherein a plurality of
holes extends in the longitudinal direction of the cover. A
cover in the context of the present invention is taken to
mean a body with a preferably circular cross-section, which
does not exhibit a single cavity, but a plurality of
cavities. In order for it to be suitable for conducting
fluids, the cover has a plurality of holes, which extend in
the longitudinal direction of the cover and therefore
represent cavities. Within the meaning of the present
invention, the cover is regarded as a pipe-shaped body, the
length dimension of which is not crossed by a single
cavity, but by a plurality of cavities, which may lead into
a single cavity within the longitudinal g of the cover or
may continuously extend separately along the longitudinal
direction of the cover. The longitudinal extension of the
cover is preferably smaller than its cross-section. It may
be so small, for example, that the cover exhibits the shape
of a round disc or plate, which is crossed by holes
extending in the longitudinal direction. The entire cross-
section of the cover may exhibit a plurality of holes.
Alternatively, it is also conceivable that only at least a
partial section exhibits a plurality of holes.
In a preferred embodiment, the fibre bundles form a
network. This means, for example, that the at least two
fibre bundles are inclined towards one another, at + 80
for example, to the longitudinal axis of the body.
The density of the network depends on the nature of the
body's application, the load to which the body is exposed
and the strength and dimensions of the body. If it is
expected that the body will break into smaller fragments in
case of a fracture, a dense network of fibre bundles is
desirable. On the other hand, the greater use of materials
in fibre bundles also raises costs, so that the density of
networked fibre bundles used should be individually adapted
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to the desired effects with regard to the resulting
material costs.
In a preferred embodiment, the ratio of the distance
between neighbouring fibre bundles/diameter of the fibre
bundles is between 5 : 1 and 10 : 1. It is a function of
the body's mechanical load. The body's thermal resistance
in proportion to the ratio is essentially unchanged. In
each direction at an angle to the longitudinal extension of
the fibre bundles, comparatively thin fibre bundles and
uncovered strips with a broad surface area alternate on the
= outer side of the body.
The at least two fibre bundles may encompass or reinforce
the body partially or completely. A complete reinforcement
is desirable where bodies are subject to heavy loads.
Alternatively, it may also be expedient out of cost
considerations for only those parts of the body that are
subject to particularly heavy loads to be reinforced. In
the case of pipes, for example, end sections, in
particular, which are connected to other components, are
areas in an apparatus such as a heat exchanger which are
subject to a particular load or prone to fractures and may
require particular protection in the form of reinforcement.
If the body is a temperature-loaded component, it should
furthermore be taken into consideration that the body and
fibre bundle may have different thermal expansion
coefficients and the length and width of the fibre bundle
arrangement should be adapted accordingly. The fibre
bundles should therefore be arranged in such a manner on
the at least one outer side of the body, that the body's
thermal expansion can be compensated by the fibre bundles
or can be allowed without leading to the destruction of
said body.
The ceramic material is preferably dense sintered silicon
carbide. It is chosen for its outstanding properties, such
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as, for example, high thermal conductivity, high strength,
high corrosion resistance to acid and base media and high
load-bearing capacity. The silicon carbide is preferably
pressureless sintered silicon carbide, which exhibits
extremely high corrosion resistance to acid and base media,
which it can likewise withstand to very high temperatures,
high temperature change resistance, high thermal
conductivity, high wear resistance and a hardness
resembling that of a diamond. As a further alternative, the
silicon carbide may be a liquid phase-sintered silicon
carbide, which is produced from silicon carbide and
= different oxide ceramics and is characterised by its great
strength.
The silicon carbide may contain at least one ceramic or
mineral filler material, wherein the choice of filler
materials is adapted to the application. Examples or filler
materials are materials from the group of naturally
occurring flake graphite, artificially
produced
electrographite, soot or carbon, graphite or carbon fibres
or borocarbide. Furthermore, ceramic or mineral filler
materials may be used in grain, platelet or fibre form, as
silicates, carbonates, sulphates, oxides, glass or selected
mixtures thereof.
In a preferred embodiment, the fibre bundles are carbon
fibre bundles. A carbon fibre bundle has good tensile
strength, corrosion resistance and stiffness, low breaking
elongation and is resistant at the application temperatures
of loaded bodies. The specific performance of the carbon
fibre bundles means that the pre-tensioning of the
reinforcement is retained, even if the pipe is subject to
highly variable or dynamic loads. Due to the negative
thermal longitudinal expansion coefficient of carbon fibre
bundles, the reinforcement is further pre-tensioned in case
of a temperature rise, the bursting and leak-tight pressure
is greater at a higher temperature than at room
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temperature. The carbon fibre reinforcement improves the
properties of bodies, particularly in the case of silicon
carbide pipes, as follows: increase in bursting pressure,
the body becomes more immune to steam hammering and
unpermitted exceeding of the operating pressure, as the
body's bursting pressure at room temperature is 30 to 40 %
greater depending on the dimensions compared with the non-
reinforced body. Other examples of fibre bundles are glass
fibre bundles or aramid fibre bundles.
In a preferred embodiment, the non-positive connection
= between fibre bundles and the outer side of the body is an
adhesive system. It is used to fix the fibre bundles to the
body. The adhesive system is chosen from the group
comprising adhesives which are made up of phenolic resin,
epoxy resin or polysilazane-based resin. If necessary, the
adhesive system may contain a silicon or silicon carbide
filler material. It is also referred to as cement in the
present invention. The adhesive system may comprise one or
more of the adhesives mentioned earlier and/or cement. If
necessary, the adhesive or cement may further contain a
hardening catalyst and/or a plasticiser. Adhesives or
cements of this kind are usually oxidation-resistant. These
adhesives or cements also adhere well to both a ceramic
material, such as silicon carbide, and also to fibre
bundles, such as carbon fibre bundles, and are capable of
wetting a fibre effectively.
The adhesive system is preferably a phenol resin. More
preferably, the phenol resin is a resol. Alternatively, the
phenol resin may also be a novolac. Resin systems
containing bisphenol A-diglycidyl ether or bisphenol F-
diglycidyl ether are also suitable as epoxy resin. In
particular, resin systems which contain methyl
hexahydrophthalic acid anhydride, particularly in a
quantity of 25 to 50 % by weight, in addition to more than
50 % by weight bisphenol A-diglycidyl ether or bisphenol F-
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diglycidyl ether, based on the total weight in each case,
are suitable as epoxy resin systems. A polysilazane resin
system may also preferably be used as the adhesive system.
All the adhesives mentioned above may further contain
silicon or silicon carbide as the filler material. The
plasticity of the cement may be adjusted to the desired
adhesive bond by means of the proportion of resin in the
mixture or by adding plasticiser. The use of cement
.
containing silicon or silicon carbide alongside the resin
adhesive is particularly suitable when it is applied to the
. fibre bundle. Through impregnation of the fibre bundle with
the cement and subsequent burning, silicon with carbon
fibres can form silicon carbide or with silicon carbide as
the filler material the impregnated and burnt carbon fibre
exhibits silicon carbide.
The choice of adhesive system depends on the desired bond
and crucially on the nature of the application of body
according to the invention. When selecting an epoxy resin
as the adhesive system, which is applied to the body or
with which the fibre bundle is impregnated and hardened, a
greater reduction in tension is not possible, due for
example to the brittleness of the hardened layer, a rigid
connection is retained between the fibre bundles and the
body. By using plasticisers, this connection can be made
deformable, in order to intercept possible shear stresses
or different expansions of the fibre bundles and body
during temperature changes, for example.
The body and the fibre bundles may be fixed by means of an
adhesive system, wherein the adhesive system is either
applied to the body, the fibre bundles or both and then
hardened or burnt. Alternatively, the body and the fibre
bundles may each be provided with an adhesive system
independently of one another and fixed to one another. The
adhesive systems applied in this case may be identical or
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different. The choice depends on the adhesive power
required and may be appropriately chosen and adapted by the
person skilled in the art.
The adhesive system may be arranged at points or in
sections between the body and the fibre bundles, so that a
number of predetermined points on the fibre bundle are
fixed to the body. Alternatively, the fibre bundles may be
completely fixed to the body by means of adhesion. The
fibre bundles are preferably completely fixed to the body.
.
The fibre bundles may exist in the form of a yarn; this is
particularly true when the fibre bundles are wound onto
bodies and possibly fixed there. A yarn is taken to be a
fibre bundle made up of a plurality of filaments. The yarn
may exhibit sections running straight, diagonally and/or in
a curved fashion. In order to create a network, at least
one, preferably two, yarns intersect at predetermined
points at a desired angle, preferably + 80 O. Yarn sections
may also be intertwined, meshed or integrated in some other
way.
Otherwise, the fibre bundles may also be in the form of
braiding, laid webs, knitted fabric, woven fabrics or
interlaced yarns, preferably woven fabrics or interlaced
yarns, which are pulled onto the body in a pre-tensioned
state and fixed where necessary. Braiding is taken to mean
an area-measured fabric, which is produced through the
intersection of braid/thread systems running diagonally in
opposite directions, wherein the braided threads cross one
another at an adjustable angle to the fabric edge. A laid
web is regarded as an area-measured fabric made up of one
or more stretched, superimposed thread systems with
different orientation directions, with or without fixing of
the points of intersection. A knitted fabric is an area-
measured fabric, in which the meshes are formed
individually and consecutively from a horizontally laid
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thread, in addition further thread systems can also be
incorporated for reinforcement. An area-measured fabric
containing at least two thread systems usually crossing one
another at right angles is regarded as a woven fabric. An
interlaced yarn is an area-measured fabric, which is
produced from one or more threads through the simultaneous
formation of meshes in a longitudinal direction; further
threads may of course be incorporated for additional
reinforcement. At least one fibre bundle of a predetermined
length is regarded as the thread in this case. A thread
system is taken to mean several threads.
It is of course also possible when the fibre bundles are
arranged in the form of a woven fabric or interlaced yarn
for the woven fabric or interlaced yarn to be longer than
the body, so that where necessary the woven fabric or
interlaced yarn protects the connection of the body to a
further component through its arrangement on said body.
The body according to the invention can be produced using
the following method comprising the steps
a) providing a body which comprises a ceramic material
and is suitable for use in a heat exchanger and for
conducting fluids
and
b) encompassing at least sections of the outer side of
the body by at least two fibre bundles under a
predetermined pre-tension forming a non-positive
connection, wherein neighbouring sections of the fibre
bundle are arranged at a predetermined distance.
With this method, the body's pressure resistance usually
required in equipment production is achieved by reinforcing
the body with fibre bundles. The pre-tensioning used
according to the invention may be adjusted by the person
skilled in the art according to the fibre material and area
of application of the body.
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Step b) may preferably comprise encompassing at least
sections of the outer side of the body by at least two
fibre bundles, so that the fibre bundles are in the form of
a network. Alternatively, it is conceivable for the fibre
bundles to be pulled around the body in the form of an
area-measured fabric. Step b) is preferably carried out in
such a way that the ratio of the distance between
neighbouring fibre bundles/diameter of the fibre bundles is
between 5 : 1 and 10 : 1. The increase in the strength of
the body is thereby achieved with a relatively small
covering of the outer side of the body.
In a preferred embodiment, before step b) an adhesive
system is at least partially applied to the fibre bundle
and/or the body and then hardened or burned. The fibre
bundle arrangement is thereby fixed to the outer side of
the body. The adhesive system used for fixing is preferably
chosen from the group comprising adhesives, which are
formed from phenol resin, epoxy resin or polysilazane-based
resin and are possibly mixed with silicon and silicon
carbide filler material. Adhesive systems of this kind are
readily workable and can be adapted to the shape of the
body or are well-suited to the impregnation of a fibre,
they exhibit good adhesive strength to a ceramic material
such as silicon carbide and many types of fibres and, in
particular, to a carbon fibre, following thermal hardening
or burning.
The body and the fibre bundles may be fixed by means of an
adhesive system, wherein the adhesive system is applied
either to the body, the fibre bundle or to both and then
hardened or burned. An adhesive system which does not
contain silicon or silicon carbide as the filler material
is hardened, while an adhesive system containing silicon or
silicon carbide as the filler material is burnt. Hardening
is preferably carried out at temperatures between 120 and
180 C for one to up to two hours, in a pressureless
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environment or at pressures of between 0.5 and 1.5 bar. At
high temperatures, i.e. around 170 to 180 C, a hardening
time of up to 15 minutes is generally sufficient. The
higher the temperature is, the shorter the hardening time.
If the adhesive system contains a hardening catalyst, the
hardening may also take place at room temperature. Burning
is preferably carried out at temperatures of over 1500 C
for up to 2 hours, in a pressureless environment or at
pressures of 0.5 to 1.5 bar. Following the hardening of the
adhesive or burning of the cement, the fibre bundles are
arranged on the outer side of the body.
-*
The body and the fibre bundles may be each be provided with
an adhesive or cement independently of one another and then
fixed. The adhesives or cements applied may be identical or
different in this case. The person skilled in the art may
select suitable adhesives or cements, which adhere well to
one another.
In a preferred embodiment of the method according to the
invention, the fibre bundles are impregnated with an
adhesive or cement, after which they are hardened or burned
and finally arranged on the body.
The adhesive system may be arranged between the body and
fibre bundles at points or in sections, so that a number of
predetermined points of the fibre bundles are fixed to the
body. Alternatively, the fibre bundles may be completely
fixed to the body by means of adhesive or cement. The fibre
bundles are preferably completely fixed to the body.
The body used in the method according to the invention is
preferably a pipe-shaped body or cover, wherein a plurality
of holes extends in the longitudinal direction of the
cover.
The ceramic material used in the method according to the
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invention is preferably silicon carbide, which optionally
contains at least one ceramic or mineral filler material.
In the method according to the invention, the fibre bundles
are preferably carbon fibre bundles. The carbon fibre
bundles may be wound around the body in a predetermined
pre-tensioned state in the form of a yarn. Alternatively,
the carbon fibre bundles exist in the form of braiding,
laid webs, knitted fabric, woven fabric or interlaced
yarns, preferably woven fabric or interlaced yarns, and are
drawn over the at least one outer side of the body possibly
provided with a hardened adhesive or burnt cement. On the
.-
other hand, it is conceivable for the carbon fibre bundle
to be used in the method according to the invention as
fibre bundles provided with hardened adhesive or burnt
cement. Particularly in the case of carbon fibre bundles,
the use of cement with silicon as the filler material is
suitable, as silicon can react with the carbon fibre during
the burning process to produce silicon carbide and a firmer
bond between the carbon fibre and the cement can thereby be
achieved.
The body according to the invention is particularly
suitable for use as a pipe, for example for heat exchangers
where there is increased mechanical stress and/or extremely
corrosive media and solvents, and also for all other
components subject to pressure and temperature loads. It is
a particularly ideal material for the construction of heat
exchangers, because it is highly thermally conductive,
pressure-resistant and immune to brittle fracture. The body
according to the invention is particularly preferably used
as the pipe in a heat exchanger, because it is erosion-
resistant and permits high flow velocities and a self-
cleaning effect of the pipe is therefore achievable through
fast-flowing media, which may be charged with particles. In
addition or alternatively, the body according to the
invention is preferably used as a pipe base in a heat
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exchanger. In assembled form, several pipe-shaped bodies
and covers according to the invention are used as a pipe
bundle heat exchanger.
A heat exchanger comprising a body according to the
invention exhibits the following structure in accordance
with DE 197 14 423, for example. The heat exchanger
comprises a casing, a base with supports, a spacer to
create a distributor space, a distributor base with the
inner and outer pipe bases and pipes arranged in the bores
of the pipe bases and sealed therein by means of a sealant.
The base and casing are customarily screw-fastened, wherein
the spacer is inserted in between to create the distributor
space. The inner pipe base of the distributor base is
smaller in diameter than the inner casing diameter. The
outer pipe base is greater in diameter and therefore
assumes the sealing function between the casing and the
distributor space. The pipes represent the body according
to the invention in the form of a pipe made from
pressureless sintered silicon carbide, the outer side of
which is encompassed by pre-tensioned carbon fibre bundles.
If there is a temperature increase, the pre-tensioning of
the reinforcement is advantageously increased by the
negative thermal expansion coefficient of the carbon fibre.
The heat exchanger then works more reliably and safely. In
addition or alternatively, the outer and/or inner pipe base
may further comprise pressureless sintered silicon carbide,
which is encompassed by pre-tensioned carbon fibre bundles.
Alternatively, in addition to the silicon carbide pipe and
the network of carbon fibre bundles, the pipes further
exhibit one of the adhesive systems described above for
fixing the two elements. If the adhesive system is
oxidation-resistant, oxidation media may also be used for
cooling or heating in the service space of the heat
exchanger constructed using this.
Further features and advantages of the invention are now
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explained with reference to the following figures, without
being limited to these.
In the figures:
Figure 1 shows a schematic side view of a body according to
the invention;
Figure 2 shows a further schematic side view of the body
according to the invention shown in Figure 1, in which a
= partial cross-section is shown;
Figure 3 shows an enlarged section from Figure 2, which is
circled in Figure 2 using a dot-dash line and marked as
and
Figure 4 shows a cross-section through a partial area of a
further body according to the invention.
A schematic side view of a body 1 according to the
invention is shown in Figure 1. Body 1 comprises a smooth-
walled pipe 3 made from pressureless sintered silicon
carbide. The pipe 3 has an opening at both of its two ends
5, 7, so that it is suitable for conducting fluids. The
pipe 3 has yarns 9 made from carbon fibre bundles wound
round it, said bundles being highly pre-tensioned and
acting as reinforcement for the pipe 3. The yarns 9 exhibit
a phenol resin layer (not shown), which acts as an adhesive
layer. The yarns 9 are wound around the pipe 3 in such a
manner that they cross at predetermined points, so that
they form a network.
A further schematic side view of the body 1 according to
the invention shown in Figure 1 is depicted in Figure 2. In
Figure 2 the same reference numbers are used for the same
elements as in Figure 1. In Figure 2 the smooth-walled pipe
3 is likewise shown with the pipe ends 5, 7, said pipe
having yarns 9 made from pre-tensioned carbon fibre bundles
with a phenol resin layer wound round it. The part of the
cross-sectional view further shows a pipe wall 13 of the
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pipe 3, which exhibits an inner side 14 and an outer side
15. The inner side 14 delimits the hollow cavity 11 of the
pipe 3, which is unrestricted in the longitudinal direction
and ends in an opening at each of the pipe ends 5, 7. A
fluid may be conducted through the cavity 11 limited by the
inner side 14. The yarns 9 are arranged on the outer side
15 of the pipe wall 13.
Figure 3 shows an enlarged section from Figure 2, which is
circled in Figure 2 by a dot-dash line and marked as 111-
1
III. In Figure 3 the same reference numbers are used for
the same elements as in Figure 2. It can be seen from the
enlarged view that the yarns 9 are arranged on the outer
side 15 of the pipe wall 13, while the cavity 11 is formed
by the inner side 14 of the pipe wall 13.
Figure 4 shows a cross-section through a partial section of
a further body 41 according to the invention. The body 41
according to the invention is a smooth-walled pipe 43 made
from pressureless sintered silicon carbide. The pipe 43
exhibits a pipe wall 413, which has an inner side 414 and
an outer side 415. An adhesive 417 made from phenol resin
is disposed on the outer side 415 of the pipe wall 413, on
which yarns 49 made from carbon fibres are arranged. The
adhesive 417 is only located in those areas of the outer
side 415 of the pipe 413 in which the yarns 49 are
arranged. The adhesive 417 is used to fix the yarns 49 to
the outside 415 of the pipe wall 413. The pipe has a cavity
411, which is limited by the inner side 414 of the pipe
wall 413 of the pipe 43.
