SE2050529A1 - Procedure for manufacturing a waste heat exchanger - Google Patents

Abstract

The invention relates to a method for manufacturing a heat exchanger (1) for recovering heat energy from waste water, which comprises the steps of: - providing an inner tube (2), - providing an outer tube (3), - placing the inner tube (2) inside the outer tube (3 ), and, at each of two end sections (5a-b) of the outer tube (3): - Arranging a sealing member (6b) of an elastic material between the inner tube (2) and the end section (5b) of the outer tube (3), and - Compress the outer tube (3) against the inner tube (2), so that the sealing member (6b) located therebetween is compressed and forms a tight closure between the outer tube (3) and the inner tube (2). The invention also relates to a heat exchanger (1) manufactured by such a method.

Description

Method for manufacturing a waste heat exchanger and waste heat exchanger FIELD OF THE INVENTION The present invention relates to a method for manufacturing a heat exchanger according to the preamble to patent claim 1, and a heat exchanger according to the preamble to patent claim 12.

Such heat exchangers as referred to are used to recover heat energy from heated waste water, which is often called waste water. Waste water can come from households as well as industry, car washes and other businesses. Heat exchangers of the type referred to are used primarily in households, which is why a heat exchanger for use in households will be produced as a typical example without the invention being in any way limited thereby.

In Swedish water systems, large amounts of heated water flow away from the house, such as waste water from showers/baths, cooking, dishes and laundry. A lot of energy is used to heat this water, which is wasted if the heated water flows down the drain and directly away from the house after use. In order to reuse this energy, the hot waste water can be used to heat heating water that flows into the house's water heater for heating. A few degrees of heating of incoming cold water results in a lower total energy consumption and thus lower household costs and a more sustainable way of life.

PRIOR ART A common type of heat exchanger for use of this kind consists of two copper tubes, namely an inner tube and an outer tube. The outer tube is made of larger diameter and encloses the inner tube so that a gap is provided between the tubes.

The inner pipe is connected to an existing sewer pipe in the house, so that heated waste water flows through it. Incoming cold water to the house is led into the space between the pipes of the heat exchanger, where it is preheated by the inner pipe heated by the waste water, before it is led further into the water heater.

Heat exchangers of this type are described, for example, in the documents SE538978 C2, NL 1033043 C1 and US 2013/0306290 A1. Although such heat exchangers offer a fully satisfactory function with regard to energy extraction, they have not made a breakthrough on the market. This as relatively low energy extraction in combination with costly manufacturing methods results in a significant payback period and thus low interest in this type of product.

A central problem from a cost point of view in the manufacture of heat exchangers of this kind is to seal the ends of the outer tube against the inner tube, to create a tight chamber in between. In addition to keeping tight, this closure must withstand a certain pressurization in the chamber, which makes simple solutions to the problem difficult.

This closure is achieved today by welding together the ends of the outer tube with the inner tube, or by clamping devices arranged at the ends of the outer tube and sealingly connecting them to the inner tube, to name a few examples. These previously known methods are either associated with a substantial labor effort or advanced and/or expensive components, either of which contribute to a high manufacturing cost and thus an expensive price to the consumer for these heat exchangers.

PURPOSE OF THE INVENTION The purpose of the present invention is to provide a method for manufacturing a heat exchanger of the type described in the introduction improved in at least some respect in relation to such previously known methods. In particular, the aim is to provide such a method which, in relation to previously known methods, offers a more cost-effective manufacture of heat exchangers of this type, while maintaining a satisfactory performance of the finished product.

SUMMARY OF THE INVENTION According to the present invention, said object is achieved by means of such a method in which step d) includes the substeps of: m) Placing the inner tube inside the outer tube, and, at each of two opposite end sections of the outer tube: d2) ds) Providing a sealing means of an elastic material all around the outer circumferential surface of said inner tube, the intermediate inner tube and the end section of the outer tube, and compressing the outer tube in the direction of the inner tube, so that said closed space is formed by compressing the sealing means located between the outer tube and the inner tube and forming a tight seal between the respective end section of the outer tube and the outer circumferential surface of the inner tube.

With the inventive method, it is possible to manufacture a heat exchanger in a short time and without the need for expensive components, which results in a low manufacturing cost for the heat exchanger. At the same time, heat exchangers manufactured through this method have shown good properties in terms of both durability and function. The method thus offers cost-effective manufacturing of a heat exchanger with good performance.

According to one embodiment of the invention, the method further comprises the step of, prior to step d): C) wherein in Expanding each end section of the outer tube, to provide an end section bulging from a central main part of the outer tube, with an inner diameter greater than inner diameter of said main part, on each end of the outer tube, step d2) said sealing means is arranged inside said bulging end section, and wherein in step d3) said bulging end section of the outer tube is pressed together in the direction of the inner tube.

Expansion of the respective end section of the outer tube enables the use of a sealing member with a thickness which gives it an outer circumference substantially greater than the diameter of the inner circumferential surface of said main part of the outer tube, so that each end of said main part forms a stop for the sealing member, whereby positioning of the sealing means in a suitable place is made possible.

According to another embodiment of the invention, step c) includes the sub-step that, in order to expand an end section of the outer tube: c2) Linearly press an enlargement mandrel into a pipe opening in said end section to be expanded of the outer tube, which enlargement mandrel exhibits an external shape that includes a first circular section with an outer diameter that is smaller than an inner diameter of the outer tube, and a second circular section with an outer diameter that is larger than an inner diameter of the outer tube, whereby the enlargement mandrel is inserted with said first section primarily into said tube opening and upon continued insertion of the enlargement mandrel and thus of said second section of the tube opening, said end section of the outer tube is expanded, by being pressed out by said second section of the enlargement mandrel, to have an inner diameter corresponding to said outer diameter of the second section of the enlargement mandrel .

According to another embodiment of the invention, it is an enlargement mandrel with circular-cylindrical first and second sections and a third intermediate section, which connects said first and second sections to each other and exhibits the shape of a truncated cone with an outer diameter that increases steplessly from the first section to the the second section, which is pressed into the tube opening in step c2), so that when the entire first section of the enlargement mandrel is brought into the tube opening of the outer tube, the one tube opening defining edge is brought into contact with the third section and guided along it to said second section upon further insertion of the enlargement mandrel through the pipe opening of said end section of the outer pipe while enlarging the diameter of said end section.

According to another embodiment of the invention, step c) also includes the sub-step of, in order to expand an end section of the outer tube, before step c2): c1) arranging a pad around said end section to be expanded of the outer tube, which pad has an inner shaped to limit a space for maximum expansion of the end section of the outer tube and thereby define the shape of said bulging end section to be formed at the outer tube, whereby said outer shape of the enlargement mandrel corresponds appropriately to said inner shape of the pad.

By using such a pad, in combination with the said enlargement mandrel, expansion of the end sections of the outer tube is made possible while meeting prevailing requirements for tolerances in the technical area.

According to another embodiment of the invention, at each end section of the outer tube in step c) a first part extending away from said main part of the outer tube is formed, and in step d3) a second part extending at an outer end of said first part connecting back to the inner tube is formed, so that each end section is formed between itself and the outer circumferential surface of the inner tube to provide a channel of substantially V-shaped cross-section which runs around the outer circumferential surface of the inner tube and houses said sealing means.

Such formation of a V-shaped channel containing sealing means at each end section has been found to be a simple and cost-effective, but at the same time functional, solution for sealing the ends of the outer tube against the outer circumferential surface of the inner tube.

According to another embodiment of the invention, in step d3) the outer tube is pressed together in the direction of the inner tube by linearly applying a reduction mandrel over the respective end section of the outer tube, which reduction mandrel comprises a conical cavity defined by an inner circumferential surface that tapers in diameter from an opening to said cavity in a first end of the reduction mandrel towards a second end of the reduction mandrel, whereby the reduction mandrel is linearly applied over the end section of the outer tube so that an edge defining the tube opening in this end section is introduced into said opening of the reduction mandrel into contact with said conical inner circumferential surface and is guided along this ,when continuing to apply the reduction mandrel over said end section and thereby inserting the end section into the reduction mandrel, converging in the direction of the inner tube while compressing the sealing member located between this end section and the inner tube through the end section which is guided along the said inner circumferential surface of the reduction mandrel towards the inner tube, until said sealing means compressed, preferably at least 12%, more preferably at least 15%, to form a tight seal therebetween.

The inventive solution of compressing the outer tube in the direction of the inner tube by linearly applying force and using a mentioned reduction mandrel ensures high precision in the sealing between the tubes and minimizes the risk of any of them being damaged or shaped inappropriately during the pressing process.

According to another embodiment of the invention, the method also includes the step of: e) applying an insulating layer over an outer surface of the outer tube.

Such an insulation layer increases the thermal insulation and thus the possible energy recovery of the finished heat exchanger. This helps to maintain a satisfactory effect of the cost-effectively manufactured heat exchanger.

According to another embodiment of the invention, it is a nominal sealing device in the form of a rubber O-ring that is arranged around the inner tube in step d2).

According to another embodiment of the invention, it is an inner tube of copper that is provided in step a) and an outer tube of stainless steel that is provided in step b).

The inventive method enables the use of tubes of different materials such as inner tubes and outer tubes, and thus a steel tube can be used as the outer tube of the heat exchanger. Steel is significantly cheaper than the normally used material copper, which means that the manufacturing cost of a heat exchanger manufactured by this method can be greatly reduced.

According to another embodiment of the invention, it is a liquid medium in the form of water that is to be heated in said space.

The invention also relates to a heat exchanger according to the preamble to patent claim 12, which is manufactured by the method according to the invention according to an embodiment described above.

Further advantages and advantageous features of the invention will be apparent from the subsequent description of the embodiment of the method and the heat exchanger according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS An exemplary embodiment of the invention is described below with reference to the attached drawings, on which: Fig. 1 is a partially cut side view showing one step of a method according to an embodiment of the invention, to expand an end section of the outer tube, Fig. 2 is a partially cut side view showing another step of said method, to compress a end section of the outer tube in the direction of the inner tube, and Fig. 3 is a partially cut side view of a finished heat exchanger manufactured by said method.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION A method for manufacturing a heat exchanger 1 and a heat exchanger 1 manufactured by a method according to an embodiment of the invention is shown in the attached figures 1-3 and will now be described while references will be made to all these figures.

The heat exchanger 1 is intended to be installed in a sewage system in, for example, a villa, in order to recover part of the heat energy found in heated waste water from e.g. bath, shower and laundry in the house-pit.

The heat exchanger 1 comprises an inner tube 2 of copper designed to form part of a drain pipe in the house in use, to be flowed through by waste water from bath, shower etc. which is led away from it. The heat exchanger further comprises an outer tube 3 of a stainless material, preferably stainless steel, which has an inner diameter that is larger than an outer diameter of said inner tube 2, and a length that is less than the length of said inner tube. The outer tube is therefore shorter and wider than the inner tube.

Length and diameter dimensions of the pipes are not in themselves important for the invention, but for exemplary purposes it can be mentioned that the inner pipe 2 can have a length of 1-5 m (usually 1-2.5 m) and the outer pipe a length of approximately 10-20 cm less than the length of the inner tube. The inner tube typically has an outer diameter of 75-110 mm and the outer tube an inner diameter that is 2-10 mm larger than an outer diameter of the inner tube.

In the finished heat exchanger 1 (see Fig. 3), the outer tube 3 is arranged on the inner tube 2, in a position surrounding the inner tube, that is, the outer tube is stepped over the inner tube and placed essentially centrally on it, so that the opposite ends of the inner tube protrude on each side of the outer tube . The opposite ends of the outer tube are sealingly connected to an outer circumferential surface 4 of the inner tube. This sealing connection is provided by end sections 5a-b of the outer tube 3, at the opposite ends thereof, being pressed together against the outer peripheral surface 4 of the inner tube 2 and thus a sealing means of an elastic material, in the form of a rubber O-ring 6a-b , located between the inner tube and the respective end section 5a-b of the outer tube 3, compressed and forms a tight seal between the respective end section of the outer tube and the outer circumferential surface of the inner tube.

A closed space 7 is thereby provided between said outer circumferential surface 4 of the inner tube and an inner circumferential surface 8 of the outer tube. The outer tube 3 has an inlet 9 and an outlet 10 located at a distance from the inlet through the jacket of the outer tube. These consist of a respective pipe cut 11a-b welded onto the outer pipe over a respective borehole through this and are conveniently located in the vicinity of a respective opposite end of the outer pipe. The inlet 9 is in use, i.e. when the heat exchanger 1 is mounted in a house, via its pipe section 11b connected to a water pipe for cold water entering the house, which is to go to a water heater for heating. The outlet 10 is connected via its pipe 11a to a water line leading to the water heater. Incoming cold water to be heated in the water heater is led into said space 7 between the pipes 2, 3 via the inlet 9, in which it is preheated by absorbing heat energy from the waste water heated inner pipe 2, and further out through the outlet 10 and to the water heater. The heat exchanger is also provided with an insulation layer 12 covering the entire outer surface of the outer tube 3, which limits heat flow out through the outer tube and thereby increases the efficiency of the heat exchanger 1.

In order to manufacture the described heat exchanger 1 by a method according to an embodiment of the invention, the inner tube 2 and outer tube 3 are first provided. For example, tubes of copper and stainless steel are manufactured or procured and cut to suitable lengths, as described above . Tubes 11a-b are welded onto the outer tube, near one opposite end thereof, after which a hole is drilled through the outer tube's jacket as a continuation of an inner channel of each tube, to provide an inlet 9 and an outlet 10 through the outer tube.

In a next step, each end section 5a-b (e.g. an area of approx. 2-3 cm in the longitudinal direction from each end) of the outer tube 3 must be widened, to create an end section bulging from a central main part 5c of the outer tube, with a inner diameter which is greater than an inner diameter of said main part, on each end of the outer tube (see Fig 2). In Fig 1, it is shown how this expansion is carried out with the help of an enlarging mandrel 13 and a matching pad 14.

The enlarging mandrel 13 has an external shape which includes a first circular, more specifically circular-cylindrical, section 13a with an outer diameter that is slightly smaller than an inner diameter of the outer tube 3, and a second circular, more specifically circular-cylindrical, section 13b with an outer diameter that is larger than an inner diameter of the outer tube. The outer shape of the enlargement mandrel also includes a third intermediate section 13c, which connects said first 13a and second 13b sections to each other and exhibits a shape of a truncated cone with an outer diameter that increases steplessly from the first section to the second section. The third section thus exhibits on a first end connecting to the first section an outer diameter corresponding to the outer diameter of the first section, and on a second end connecting to the second section an outer diameter corresponding to the outer diameter of the second section .

The pad 14 consists of a body 15 showing a cavity extending through the body with an internal shape that corresponds to the external shape of the enlarging mandrel 13, i.e. with a first circular cylindrical section 14a corresponding to the first section 13a of the enlarging mandrel, a second circular cylindrical section 14b corresponding to the second section 13b of the enlargement mandrel and a third intermediate section 14c corresponding to the third section 13c of the enlargement mandrel and connecting said first 14a and second 14b sections of the pad 14 to each other. The fact that two sections are "contiguous" with each other means here that they have corresponding dimensions, i.e. length and diameter, but that the diameter of the respective section of the pad is somewhat larger than that of the corresponding section of the enlarging mandrel, so that the enlarging mandrel 13 fits perfectly of the pad 14 (like a puzzle piece), with a certain clearance between the respective surfaces of their connected sections, corresponding to the material thickness of the outer tube 3.

To extend an end section 5a-b of the outer tube 3, the first pad 14 is arranged around this end section (5a in Fig. 1) by threading the pad over the end section of the outer tube so that it pushes through the hollowness of the pad. As can be seen in Fig. 1, the inner diameter of the first section 14a of the pad corresponds to the outer diameter of the outer tube, the pad being arranged on the outer tube with the surface of the first section 14a adjacent to (or at least very close to) this, and with the surface of the third section 14c extending away from the outer tube and the surface of the second section 14b spaced from the end section 5a of the outer tube, so that the second 14b and third 14c sections of the pad 14 define a space 16 for maximum expansion of the end section 5a of the outer tube, in order to thus define the shape of said bulging end section to be formed at the outer tube. The pad can be attached in this position around the outer tube in a variety of ways. For example, both the pad and the outer tube can be attached by a tool or a machine, or the pad can be attached to the outer tube, e.g. be designed in two halves which are connected to each other and thereby clamped around the outer tube.

Once the pad 14 has been arranged around the end section 5a, the enlarging mandrel 13 is pressed into a tube opening 17 in this end section, and more precisely the enlarging mandrel is introduced with its first section 13 from the front into said tube opening, which first section 13a is offset along the inner circumferential surface 8 of the outer tube 3 and thus ensures that the enlargement mandrel is pressed linearly into the outer tube, ie with a center axis of the enlargement mandrel constantly aligned with a center axis of the outer tube 3. The pressing direction of the enlargement mandrel 13 is shown by arrow "A" in Fig 1. When the entire first section 13a of the enlargement mandrel is introduced into the tube opening 17 of the outer tube 3, an edge 18 defining the tube opening is brought into contact with the third section 13c of the enlargement mandrel and upon continued insertion of the enlargement mandrel into the tube opening, said edge 18 is guided along the third section 13c towards the second section the ion 13b of the enlargement mandrel while enlarging the end section 5a, i.e. enlarging the inner diameter, of the outer tube.

On continued insertion of the enlargement mandrel 13 into the end section 5a of the outer tube 3, this end section 5a is successively pressed out, first the third section 13c and then by the second section 13b of the enlargement mandrel, until the entire enlargement mandrel is brought into the pipe opening 17 of the end section 5a of the outer tube and presses this end section against the inner shape of the pad 14, so that the end section 5a is shaped internally according to the outer shape of the enlargement mandrel 13 and externally according to the inner shape of the pad 14.

The enlargement mandrel 13 is then pulled back out of the tube opening 17 and the pad 14 is removed from the expanded end section 5a. The same process is then carried out on the opposite end section 5b of the outer tube 3, in the event that both of these are not expanded at the same time.

This step is carried out in a suitable press machine, e.g. a hydraulic press, which in themselves are well known in various technical fields and thus are not described in further detail here.

In a subsequent step, the outer tube 3 is arranged on the inner tube 2, in a position surrounding the inner tube. This is done by first placing the inner tube inside the outer tube (see Fig 2). At each end section (5b shown in Fig. 2) of the outer tube, a sealing device, in the form of a rubber O-ring 6b, is then threaded over the inner tube 2 and arranged inside the bulging end section 5b, i.e. extending around the entire outer circumferential surface 4 of the inner tube, between the inner tube and the bulging end section 5b of the outer tube 3.

Each bulging end section 5a-b is then pressed together in the direction of the inner tube 2 by means of a reduction mandrel 19. The reduction mandrel comprises a conical cavity 20 defined by an inner circumferential surface 21 which tapers in diameter from an opening 22 to said cavity in a first end of the reduction mandrel opposite other end of the reduction mandrel. This cavity 20 could also have a tapered diameter giving it a shape other than a cone, as will be understood from the description of the cavity function below. For example, the inner peripheral surface 21 which defines the cavity could be arc-shaped from the first towards the second end of the reduction mandrel 19 and thus the cavity hemispherical.

In order to compress an end section (5b shown in Fig. 2) of the outer tube 3, the reduction mandrel 19 is applied linearly towards and over the end section, i.e. with a center axis of the reduction mandrel constantly aligned with a center axis of the outer tube 3. The pressing direction of the reduction mandrel 19 is shown by arrow "B" in Fig 2. This allows the tube opening 17 in this end section 5b defining edge 18 to be brought into the opening 22 of the reduction mandrel 19 into contact with said conical inner circumferential surface 21. Upon continued application of the reduction mandrel over said end section, and thereby introducing of the end section in the reduction mandrel, the pressing edge 18 of the tube opening 17 against said peripheral surface 21 and is guided along this converging in the direction of the inner tube. The end section 5b is thus continuously compressed in the direction against the inner tube 2 by being guided against the inner peripheral surface 21 of the reduction mandrel 19 and in turn is pressed together the O-ring 6b located between this end section 5b and the inner tube until the O-ring is compressed to the extent that it forms a tight seal between the outer circumferential surface 4 of the inner tube 2 and the inner circumferential surface 8 of the outer tube 3 (see Fig 3).

Fundamental to achieving said tight seal between the respective end sections 5a-b and the outer circumferential surface 4 of the inner tube 2 is that the respective O-rings 6a-b are compressed enough to completely close tightly between these parts. In order to ensure that a completely tight seal is achieved with a heat exchanger of this type, it is required that the O-rings 6a-b are compressed at least about 10%, preferably at least 12% and more preferably at least 15%. However, the further preference is that in the compression step described above, the end sections are pressed together until the outermost edge 18 defining the pipe opening 17 comes into contact with and is pressed against the outer surface 4 of the inner tube, in order to create an extra seal between edge 18 and inner tube 2. in addition to the one between O-ring 6a-b and inner tube 2.

When each of the opposite ends of the outer tube 3, and more specifically end sections 5a-b, have been sealingly connected to the outer circumferential surface 4 of the inner tube 2 in the manner described above, said closed space 7 is formed between the outer circumferential surface of the inner the tube and the inner peripheral surface 8 of the outer tube 3. This pressing step is also carried out in a suitable pressing machine, e.g. the same as used in pressing to expand the end sections 5a-b. Naturally, this pressing step can also be carried out in two sub-steps, ie the two opposite end sections 5a-b of the outer tube 3 are pressed together one after another, or in a single step in which both said end sections are pressed together simultaneously.

As can be discerned in Fig. 3, each end section 5a-b is in this stage shaped to provide between itself and the outer circumferential surface 4 of the inner tube 2 a channel 23a-b with a substantially V-shaped cross-section which runs around the entire said outer circumferential surface and contains respectively O-ring 6a-b. This by dividing each end section into a first part 24a-b extending away from said main part 5c of the outer tube 3, formed by pressing between said third sections 13c, 14c of the enlargement mandrel 13 in the expansion step described above, and one at an outer end of said first part 24a -b connecting back to the inner tube 2 extending second part 25a-b, formed by compression of the end section by means of the reduction mandrel 19 in the compression step described above. As shown in Fig. 3, said channel 23a-bi in this example has a cross-sectional shape of a blunt This could nevertheless be formed with a cross-sectional shape of a pointed or with a more round bend between said first 24a-b and second 25a-b part.

Finally, the space 7 is test-pressured and then the insulation layer 12 is applied over the outer surface of the outer tube 3. The heat exchanger 1 is then ready to be packaged and delivered to the customer.

The manufacturing process according to the invention thus makes it possible to manufacture a heat exchanger with full functionality at a very low cost and a heat exchanger according to the invention, manufactured using this method, can be offered to the market at a low price, which contributes to a shortened payback period and greater cost savings for this in relation to previously known such heat exchangers.

The invention is, of course, in no way limited to the above-described embodiment, but a number of possibilities for modifications thereof should be obvious to a person skilled in the field without thereby deviating from the framework of the invention as defined in the attached patent claims.

Of course, a heat exchanger manufactured according to the inventive method could also be used to cool a hot medium in said space, by means of a cooler medium flowing in the inner pipe.

The drawings show, as previously mentioned, only an example of an embodiment of the invention and have the purpose of visualizing the function of the invention. For this reason, there may be proportions of some shown components or parts thereof that do not agree between different figures, or with reality.

Claims (12)

1. Method for manufacturing a heat exchanger (1) for recovering heat energy from waste water, which includes the steps of: a) Providing an inner pipe (2), which is designed to divert waste water, b) Providing an outer pipe (3), which has an inner diameter greater than an outer diameter of said inner tube (2), and a length less than the length of said inner tube (2), and d) Arrange the outer tube (3) on the inner tube (2) , in a position surrounding the inner tube (2), by sealingly connecting the opposite ends of the outer tube (3) to an outer peripheral surface (4) of said inner tube (2) to form a closed space (7) between said outer peripheral surface (4) of the inner tube (2) and an inner peripheral surface (8) of the outer tube (3), wherein said outer tube (3) additionally has an inlet (9) and an outlet (10) located at a distance from the inlet (9) through the jacket of the outer tube (3), for inflow and outflow of a liquid medium to be heated to and from said space (7), characterized in that step d) includes the substeps of: d1) Place the inner tube (2) inside the outer tube (3), and, at each of two opposite end sections (5a-b) of the outer tube (3): d2) Arrange a sealing means (6a-b) of an elastic material around the entire outer circumferential surface (4) of said inner tube (2), between the inner tube (2) and the end section (5a-b) of the outer tube (3), and d3) Press the outer tube (3) together in the direction of the inner tube (2), so that said closed space (7) is formed by the sealing member (6a-b) located between the outer tube ( 3) and the inner tube (2) is compressed and forms a tight seal between the respective end section (5a-b) of the outer tube (3) and the outer circumferential surface (4) of the inner tube (2). 2. Method according to claim 1, characterized in that the method additionally comprises the step of, before step d): c) Expanding each end section (5a-b) of the outer tube (3), to achieve one from a central main part (5c) of the outer tube (3) bulging end section (5a-b), with an inner diameter greater than an inner diameter of said main part (5c), on each end of the outer tube (3), whereby in step d2 ) said sealing means (6a-b) is arranged inside said bulging end section (5a-b), and whereby in step d3) said bulging end section (5a-b) of the outer tube (3) is pressed together in the direction of the inner tube (2). 3. Method according to claim 2, characterized in that step c) includes the substep of, in order to expand an end section (5a-b) of the outer tube (3): c2) linearly press an enlargement mandrel (13) into a tube opening ( 17) in said end section (5a) to be expanded at the outer tube (3), which enlargement mandrel (13) has an outer shape comprising a first circular section (13a) with an outer diameter smaller than an inner diameter of the outer tube (3), and a second circular section (13b) with an outer diameter that is larger than the inner diameter of the outer tube (3), whereby the enlargement mandrel (13) is inserted with said first section (13a) primarily into said tube opening (17) and upon further insertion of the enlargement mandrel (13) and thus of said second section (13b) thereof in the tube opening (17), said end section (5a) of the outer tube (3) is expanded, by being pressed out of said second section (13b) of the enlarging mandrel (13), to get an inner diameter corresponding to said outer diameter of the second section (13b) of the enlargement mandrel (13). 4. Method according to claim 3, characterized in that it is an enlarging mandrel (13) with circular cylindrical first (13a) and second (13b) sections and a third intermediate section (13c), which connects said first (13a) and second ( 13b) section with each other and presents a shape of a truncated cone with an outer diameter that increases steplessly from the first section (13a) to the second section (13b), which is pressed into the pipe opening (17) in step c2), so that when the entire first section (13a) of the enlargement mandrel (13) is inserted into the tube opening (17) of the outer tube (3), an edge (18) defining the tube opening (17) is brought into contact with the third section (13c) and guided along it to said second section (13b) upon further insertion of the enlargement mandrel (13) through the pipe opening (17) of said end section (5a) of the outer pipe (3) while enlarging the diameter of this end section (5a). 5. Method according to claim 3 or 4, characterized in that step c) additionally includes the substep of, in order to expand the end section (5a) of the outer tube (3), prior to step c2):c1) arranging a pad (14) around said end section (5a) to be expanded at the outer tube (3), which pad (14) has an internal shape designed to limit a space (16) for maximum expansion of the end section (5a) of the outer tube (3) and thus define the shape of said bulging end section (5a) to be formed at the outer tube (3), whereby said outer shape of the enlargement mandrel (13) correspondingly corresponds to said inner shape of the pad (14). 6. Method according to one of claims 2-5, characterized in that for each end section (5a-b) of the outer tube (3) a first part (24a) extending away from said main part (5c) of the outer tube (3) is formed in step c) -b) and in step d3) a second part (25a-b) extending at an outer end of said first part (24a-b) connecting back to the opposite inner tube (2), so that each end section (5a-b) is shaped to between itself and the outer peripheral surface (4) of the inner tube (2) providing a channel (23a-b) with a substantially V-shaped cross-section running around the outer peripheral surface (4) of the inner tube (2) and housing said sealing means (6a-b) . 7. Method according to one of the preceding claims, characterized in that in step d3) the outer tube (3) is pressed together in the direction of the inner tube (2) by linearly placing a reduction mandrel (19) over the respective end section ( 5b) of the outer tube (3), which reduction mandrel (19) comprises a conformal cavity (20) defined by an inner peripheral surface (21) which tapers in diameter from an opening (22) to said cavity (20) in a first end of the reduction mandrel (19) against a second end of the reduction mandrel (19), whereby the reduction mandrel (19) is linearly applied over the end section (5b) of the outer tube (3) so that a tube opening (17) in this end section (5b) defining edge ( 18) is brought into said opening (22) of the reduction mandrel (19) into contact with said conical inner peripheral surface (21) and is guided along it, during continued application of the reduction mandrel (19) over said end section (5b) and thereby introducing end the section (5b) in the reduction mandrel (19), converging towards the inner tube (2) while compressing the sealing member (6b) located between this end section (5b) and the inner tube (2) through the end section (5b) which is guided along said inner circumferential surface (21) of the reduction mandrel (19) against the inner tube (2), until said sealing means (6b) is compressed, preferably at least 12%, more preferably at least 15%, to form a tight seal therebetween. 8. Method according to one of the preceding claims, characterized in that the method also includes the step of: e) applying an insulating layer (12) over an outer surface of the outer tube (3). 9. Method according to one of the preceding claims, wherein it is a mentioned sealing means (6a-b) in the form of a rubber O-ring which is arranged around the inner tube (2) in step d2)_ 10.A method according to any of the preceding claims, wherein there is an inner tube (2) of copper provided in step a) and an outer tube (3) of stainless steel provided in step b). 11. Method according to any of the preceding claims, wherein it is a liquid medium in the form of water that is to be heated in said space (7). 12. Heat exchanger (1) for recovery of heat energy from waste water, which comprises: - an inner pipe (2), which is designed to conduct waste water, - an outer pipe (3), which has an inner diameter that is larger than an outer diameter of said inner tube (2), and a length that is smaller than the length of said inner tube (2), whereby the outer tube (3) is arranged on the inner tube (2), in a position surrounding the inner tube (2), and the opposite ends of the outer tube (3) are sealingly connected to an outer circumferential surface (4) of said inner tube (2) such that a closed space (7) is provided between said outer circumferential surface (4) of the inner tube (2) and an inner circumferential surface (8) of the outer tube (3), and wherein said outer tube (3) also has an inlet (9) and an outlet (10) located at a distance from the inlet (9) through the jacket of the outer tube (3), for the inflow and outflow of a liquid medium to and from said space (7), characterized in that the heat exchanger (1) is manufactured by the method according to any of claims 1-11.