SE544039C2 - 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

FIELD OF THE INVENTION The present invention relates to a method of manufacturing a single heat exchanger according to the preamble of claim 1, and a heat exchanger according to the preamble of claim 12.

Such heat exchangers as intended are used to recover heat energy from heated wastewater, which is often called wastewater. Wastewater can come from households as well as industry, car washes and other businesses. Heat exchangers of the type in question are mainly used in households, so a heat exchanger for domestic use will be presented as a typical example without the invention being limited in any way.

In Swedish water systems, large amounts of heated water flow away from the house, such as wastewater from the shower / bath, cooking, washing dishes and washing. A lot of energy is used to heat this water, which is wasted in the event that 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 cold 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 reduced household costs and a more sustainable way of life.

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

The inner pipe is connected to a drain pipe in the house, to be flowed through by heated waste water. Cold water entering the house is led into the space between the pipes of the heat exchanger, so that 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 kind are described, for example, in the documents SE538978 C2, NL 1033043 C1 and US 2013/0306290 A1. Although such heat exchangers offer a satisfactory function with respect to energy recovery, they have not penetrated the market. This is because relatively low energy recovery in combination with costly manufacturing methods results in a significant payback time 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 close the ends of the outer tube against the inner tube, in order to create a tight chamber therebetween. In addition to holding tight, this closure must withstand a certain pressure in the chamber, which complicates simple solutions to the problem.

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

OBJECT OF THE INVENTION The object 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 object is to provide such a process which, in relation to previously known processes, 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) comprises the steps of: a) placing the inner tube inside the outer tube, and, at each of two opposite end sections of the outer tube: d2) ds) Arranging a sealing member of an elastic material surrounds the outer circumferential surface of said inner tube, the middle inner tube and the end section of the outer tube, and compresses the outer tube in the direction of the inner tube, so that said closed space is formed by the sealing member located between the outer tube and the inner tube compressing and forming a between the respective end section of the outer tube and the outer circumferential surface of the inner tube.

The method according to the invention enables the manufacture of a single 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 by this method have shown good properties in terms of both durability and function. of single heat exchangers with good performance.

The method according to the invention further comprises the step of, before step C) wherein d): Extending 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 which is larger than the inner diameter of said main part, on each end housing of the outer pipe, in step d2) said sealing means are arranged inside said bulging end section, and in step d3) said bulging end section of the outer pipe is compressed in direction towards the inner pipe.

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 larger than the diameter of the said inner part of the outer circumferential surface surface of the outer tube, so that each end of said main part forms a stop for the sealing member. the sealing member in a suitable place is made possible.

In addition, step c) comprises the sub-step of, in order to expand an end section of the outer tube: c2) Linearly pressing a magnifying mandrel into a tube opening said end section to be enlarged at the outer tube, which enlarging mandrel has an outer shape comprising a first circular section having an outer diameter less than an inner diameter of the outer tube, and a second circular section having an outer diameter greater than an inner diameter of the outer tube, the magnifying mandrel being inserted with said first section in front of said tube opening and upon further insertion of the magnifying mandrel and thus of said second section thereof the tube opening, said end section of the outer tube is expanded, by being forced out of said second section of the magnifying mandrel, to have an inner diameter corresponding to said outer diameter of the second section of the magnifying mandrel.

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

According to another embodiment of the invention, step c) further comprises 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 extended at 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 of the outer tube, said outer shape of the magnifying mandrel co-corresponding to said inner shape of the pad.

By using such a cushion, in combination with the said magnification mandrel, it is possible to expand the end sections of the outer pipe while meeting current requirements for tolerances in the field of technology.

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 the outer end of said first part connecting back to the inner tube, so that each end section is formed between itself and the outer circumferential surface of the inner tube provided with a channel with a 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 accommodating sealing means at each end section has been found to be a simple and cost-effective, but at the same time functional, solution for closing 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 compressed in the direction of the inner tube by linearly placing the reduction mandrel over the respective end section of the outer tube, which reduction mandrel comprises a conical cavity defined by an inner circumferential surface tapered in diameter from an opening. to said cavity in a first end of the reduction mandrel against a second end of the reduction mandrel, the reduction mandrel being linearly applied over the end section of the outer tube so that an edge defining the tube opening in this end section is inserted into said opening of the reduction mandrel and the convex , when continuing to apply the reduction mandrel over said end section and thereby inserting the end section into the reduction mandrel, converging towards 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 said the inner circumferential surface of the reduction mandrel against the inner tube, until said sealing means is compressed, preferably at least 12%, more preferably at least 15%, to form a tight seal therebetween.

The solution according to the invention to compress the outer tube in the direction of the inner tube by linear application of force and use of a said reduction mandrel ensures high precision in the closure 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 further comprises the step of: e) applying an insulating layer over an outer surface of the outer tube.

Such an insulating layer increases the thermal insulation and thus the possible energy recovery of the finished heat exchanger. This contributes to maintaining a satisfactory effect of the cost-effective heat exchanger.

According to another embodiment of the invention, it is a said sealing member in the form of an O-ring of rubber which is arranged around the inner inner tube in step d2).

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

The method according to the invention enables the use of pipes of different materials such as inner pipes and outer pipes, and thus a steel pipe can be used as the outer pipe 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 through this process can be greatly reduced.

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

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

Further advantages and advantageous features of the invention will appear from the following 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 accompanying drawings, in which: FIG. Fig. 1 is a partially cut-away side view showing a step of a method according to an embodiment of the invention, widening an end section of the outer tube; end section of the surface 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 of manufacturing a heat exchanger 1 and a heat exchanger 1 made by a method according to an embodiment of the invention are shown in the accompanying Figures 1-3 and will now be described while references will be made to all figures.

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

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

Length and diameter dimensions of the pipes are not in themselves important for the pre-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 length of about 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 which is 2-10 mm larger than an outer diameter of the inner tube.

The outer tube 3 of the finished heat exchanger 1 (see Fig. 3) is arranged on the inner tube 2, in a position enclosing the inner tube, i.e. the surface tube is threaded over the inner tube and placed substantially centrally thereon, so that the opposite ends of the inner tube project on the respective 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 against the outer circumferential surface 4 of the inner tube 2 and thereby a sealing member of an elastic material, in the form of an O-ring 6a-b of rubber , located between the inner tube and the respective end section 5a-b of the outer tube 3, compressed and forming a tight seal between the respective end section of the outer tube and the outer circumferential surface of the inner tube.

Thereby a closed space 7 is 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 pulley 11a-b welded to the outer pipe over a respective borehole through it and are suitably located in the vicinity of a respective opposite end of the outer pipe. the inlet 9 is in use, ie when the heat exchanger 1 is mounted in a housing, via its pipe pellet 11b connected to a water pipe leading to the housing incoming cold water, which is to go to a hot water heater for heating. The outlet 10 is connected via its pipe pellet 11a to a water pipe leading to the water heater. Incoming cold water to be heated in the water heater is thus guided 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 further provided with an entire insulating layer 12 covering the 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, first said inner tubes 2 and outer tubes 3 are provided. For example, tubes of copper and stainless steel are manufactured or procured and cut to suitable lengths, as described above . Pipe pellets 11a-b are welded to the surface pipe, in the vicinity of an opposite end thereof, after which a hole is drilled through the jacket of the outer pipe as a continuation of an inner channel of each pellet, to provide an inlet 9 and an outlet 10 through the outer pipe.

In a next step, each end section 5a-b (eg an area of about 2-3 cm in the longitudinal direction from the respective end) of the outer tube 3 shall be widened, to provide an end section bulging from a central main part 5c of the outer tube, with a inner diameter which is larger than an inner diameter of said main part, on each end housing of the outer tube (see Fig. 2). Fig. 1 shows how this expansion is performed by means of a magnifying mandrel 13 and a mating pad 14 therewith.

The magnifying mandrel 13 has an outer shape comprising a first circular, more precisely circular cylindrical, section 13a having an outer diameter slightly smaller than an inner diameter of the outer tube 3, and a second circular, more precisely circular cylindrical, section 13b having an outer diameter having an outer diameter than an inner diameter of the outer tube. The outer shape of the magnifying mandrel further comprises a third intermediate section 13c, which connects said first 13a and second 13b section to each other and has a shape of a truncated cone with an outer diameter which increases steplessly from the first section to the second section. The third section thus has on one first, connecting to the first section, an outer diameter corresponding to the outer diameter of the first section, and on another, 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 having a body-extending cavity with an inner shape which correspondingly corresponds to the outer shape of the magnifying mandrel 13, i.e. with a first circular cylindrical section 14a associated with the first section 13a of the enlarging mandrel, a second circular cylindrical section 14 associated with the second section 13b of the magnifying mandrel and a third intermediate section 14c associated with the third section 13c of the magnifying mandrel and connecting said first 14a and second 14b sections of the pad 14 to each other. The fact that two sections are "connected" to each other here means that they have corresponding dimension ions, ie length and diameter, but that the diameter of the respective section of the pad is slightly larger than that of the associated section of the magnifying mandrel, so that the magnifying mandrel 13 fits the cavity of the pad 14 (like a piece of a puzzle), with a certain clearance between the respective surfaces of their associated sections, corresponding to the material thickness of the outer tube 3.

To expand 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 the pad being threaded over the end section of the outer tube so that it projects through the cavity 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) it, 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, so as to define the shape of said bulging end section to be formed in the outer tube. The pad can be held in this position around the outer tube in a variety of different ways. For example, both the pad and the outer tube can be held by a tool or a machine, or the pad can be mounted firmly on the outer tube, e.g. be formed in two halves which are connected to each other and through this clamped around the outer tube.

Once the pad 14 is arranged around the end section 5a, the magnifying mandrel 13 is pressed into a pipe opening 17 in this end section, and more specifically, the enlarging mandrel is inserted with its first section 13 at the front of said pipe opening, which first section 13a is displaced along the inner circumferential surface 8 of the outer tube 3 and thus ensures that the magnifying mandrel is pressed linearly into the outer tube, i.e. with a center axis of the magnifying mandrel constantly aligned with a central axis of the outer tube 3. The pressing direction of the magnifying mandrel 13 is shown by the arrow "A" in Fig. of the magnifying mandrel is inserted into the tube opening 17 of the surface tube 3, a defining edge 18 of the tube opening is brought into contact with the third section 13c of the magnifying mandrel and upon continued insertion of the magnifying mandrel into the tube opening, said edge 18 is guided along the third section 13c towards the second section. ion 13b of the magnifying mandrel during expansion of the end section ion 5a, i.e. magnification of the inner dia meters, at the outer tube.

Upon further insertion of the magnifying mandrel 13 into the end section 5 of the outer tube 3, this end section 5a is successively pushed out, first the third section 13c and then of the second section 13b of the magnifying mandrel, until the entire magnifying mandrel is inserted into the tube opening 17 of the end section 5a of the outer end section. against the inner shape of the pad 14, so that the end section 5a is formed internally after the outer shape of the magnifying mandrel 13 and externally after the inner shape of the pad 14.

The magnifying mandrel 13 is then withdrawn from the tube opening 17 and the pad 14 is removed from the enlarged end section 5a. The same process is then performed on the opposite end section 5b of the outer tube 3, in which case both of these are not expanded simultaneously.

This step is performed 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 enclosing 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 member, in the form of an O-ring 6b of rubber, 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 compressed 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 other end of the reduction mandrel. This cavity 20 could also have a tapered diameter which gives it a shape other than a cone, as will be understood from the description of the cavity function below. For example, the inner circumferential surface 21 defining the cavity could be arcuate from the first counter to the other end of the reduction mandrel 19 and thus the cavity is hemispherical.

To compress an end section (5b shown in Fig. 2) of the outer tube 3, the reduction mandrel 19 is arranged 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 tube 19 is shown by the arrow Fig. 2. This allows the edging 18 defining the tube opening 17 in this end section 5b to be inserted into the opening 22 of the reduction mandrel 19 in contact with said conical inner circumferential surface 21. Upon further application of the reduction mandrel over said end section, thereby inserting of the end section of the reduction mandrel, the pressing edge 18 of the pipe opening 17 towards said circumferential surface 21 is guided along this converging in the direction of the inner pipe. the O-ring located between the end section 5b and the inner tube 6b 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).

Basic for achieving said tight closure between the respective end section 5a-b and the outer circumferential surface 4 of the inner tube 2 is that the respective O-ring 6a-b is compressed sufficiently to close completely between these parts. To ensure that a completely tight seal is achieved in a heat exchanger of this kind, the O-rings 6a-b are required to be compressed at least about 10%, preferably at least 12% and more preferably at least 15%. It is further preferred, however, that in the compression steps described above, the end sections be compressed until the outermost, defining the pipe opening 17, the edge 18 comes into contact with and is pressed against the outer surface 4 of the inner pipe, to provide an additional seal between edge 18 and inner pipe 2. in addition to that 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 tube the tube and the inner circumferential surface 8 of the outer tube 3. This pressing step is also performed in a suitable pressing machine, e.g. the same as used in pressing to extend the end sections 5a-b. Of course, this pressing step can also be performed in two sub-steps, i.e. the two opposite end sections 5a-b of the outer tube 3 are compressed one after the other, or in a single step in which both said end sections are compressed simultaneously.

As can be seen in Fig. 3, each end section 5a-bi of this bag is shaped to provide between it 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 accommodates respectively O-ring 6a-b. This in that each end section is divided 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 enlarging mandrel 13 in the expansion step described above, and a first part 24a mentioned at an outer end -b connecting back to the inner tube 2 extending second part 25a-b, formed by compressing the end section by means of the reduction mandrel 19 in the above-described pressing-up step. As shown in Fig. 3, said channel 23a-b in this example may have a cross-sectional shape of an obtuse This dock may 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 tested and then the insulating 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 method according to the invention thus makes it possible to manufacture a heat exchanger with satisfactory function at a very low cost and a heat exchanger according to the invention, manufactured by this method, can be offered to the market at a low price, which contributes to shortened payback time and greater cost savings. this in relation to previously known such heat exchangers.

The invention is of course not in any way limited to the above-described embodiment, but a number of possibilities for modifications thereof should be obvious to a person skilled in the art without departing from the scope of the invention as defined in the appended claims.

Of course, a heat exchanger manufactured according to the method according to the invention could also be used for cooling a hot medium in said space, by means of a cooler medium flowing in the inner tube.

The drawings show, as previously mentioned, only an example of a single embodiment of the invention and have for the purpose of visualizing the function of the invention. For this reason, proportions of certain components or parts thereof shown may not correspond 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 by step d) comprises 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 peripheral 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 means (6a-b) located between the outer tube (3) and the inner tube (2) being compressed and forming a tight seal between the respective end sections (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 wherein 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) also includes the substep of, in order to expand the end section (5a) of the outer tube (3), before 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 -section (5b) in the reduction mandrel (19), converging iri sealing against the inner tube (2) while compressing between this end section (5b) and the inner tube (2) located the sealing member (6b) through the end section (5b) which is guided along said inner circumferential surface (21) of the reduction mandrel (19) towards 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. Method according to any preceding claim, 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 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 requirements 1-11.