TECHNICAL FIELD The present invention relates to a method of manufacturing a heat exchanger according to the preamble of claim 1.

Such heat exchangers as intended are used to recover heat energy from heated wastewater, which is often called sewage. Waste water 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 use in house holds will be presented as a typical example without limiting the invention thereto.

In Swedish water systems, large amounts of heated water run off 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. 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 lifestyle.

BACKGROUND ART

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

The inner tube is connected to a drain tube in the house, to be flowed through by heated waste water. Cold water entering the house is led into the space between the tubes of the heat ex changer, to be preheated there by the inner tube heated by the waste water, before it is led further into the water heater.

RECORD COPY TRANSLATION (Rule 12.4) Heat exchangers of this type are described, for example, in the documents SE 538978 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 products.

A central problem from a cost point of view in the manufacture of heat exchangers of this type is to close the ends of the outer tube towards the inner tube, in order to create a tight chamber there between. In addition to holding tight, this closure must withstand a certain amount of pressure in the chamber, which complicates simple solutions to the problem.

This closure is today achieved 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 ad vanced 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.

OBJECT OF THE INVENTION

The object of the present invention is to provide a method for ma- nufacturing a heat exchanger of the type described in the intro duction being improved in at least some aspect with respect to such methods already known. In particular, the object is to provide such a method which, in relation to methods already known, offers a more cost-effective manufacture of heat exchangers of this type, while maintaining a satisfactory performance of the finished pro duct. SUMMARY OF THE INVENTION

According to the present invention, said object is achieved by such a method in which step d) comprises the sub-steps of: d1 ) Placing 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) Arranging a sealing member (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) Compressing the outer tube (3) in the direction of the inner tube (2), so that said closed space (7) is formed by compressing the sealing member (6a-b) located between the outer tube (3) and the inner tube (2) so that it 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), and that the method further comprises the step of, before step d): c) Expanding each end section (5a-b) of the outer tube (3), to provide an end section (5a-b) bulging from a central main part (5c) of the outer tube (3), with an inner diame- ter greater than an inner diameter of said main part (5c), at each end of the outer tube (3), wherein in step d2) said sealing member (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 compressed in the direction of the inner tube (2), and that step c) comprises the sub-step of, in order to expand an end section (5a-b) of the outer tube (3): c2) linearly pressing an enlargement mandrel (13) into a tube opening (17) in said end section (5a) of the outer tube (3) to be enlarged, which enlargement mandrel (13) has an outer shape comprising a first circular section (13a) with an outer diameter which is smaller than an inner diameter of the outer tube (3), and a second circu- lar section (13b) having an outer diameter which is gre ater than an inner diameter of the outer tube (3), the en largement mandrel (13) being inserted with said first section (13a) first in said tube opening (17) and upon continued 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 said second section (13b) of the enlar gement mandrel (13), to get an inner diameter corre sponding to said outer diameter of the second section (13b) of the enlargement mandrel (13).

The method according to the invention makes it possible to manu facture 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 manu factured by this method have shown good properties in terms of both durability and function. The method thus offers cost-effective manufacture of a heat exchanger with good performance. 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 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 member in a suitable place is made possible.

According to an 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 which has a shape of a truncated cone with an outer diameter which continuously increases from the first section to 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 inserted into the tube opening of the outer tube, an edge defining the tube opening is brought into contact with the third section and guided along this to said second section by further inserting the enlargement mandrel through the tube opening of said end section of the outer tube while expanding the diameter of this 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 die (14) around said end section (5a) of the outer tube (3) to be expanded, which die (14) has an inner shape designed to limit a space (16) for maximum ex pansion of the end section (5a) of the outer tube (3) and thereby defining the shape of said bulging end section (5a) of the outer tube (3) to be formed, wherein said outer shape of the enlargement mandrel (13) matches with said inner shape of the die (14).

By the use of such a die, in combination with said enlargement mandrel, it is possible to expand the end sections of the outer tube while meeting the prevailing requirements for tolerances in the te chnological field.

According to another embodiment of the invention in each end section of the outer tube a first part extending away from said main part of the outer tube is formed in step c) and a second part con nected to an outer end of said first part and extending back to the inner tube is formed in step d3), so that each end section is formed to provide a channel, between itself and the outer circumferential surface of the inner tube, with a substantially V-shaped cross- section running around the outer circumferential surface of the in ner tube and accommodating said sealing member.

Such formation of a V-shaped channel accommodating the sealing member of 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 compressed in the direction of the inner tube by li nearly bringing 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 which 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, the reduction mandrel being linearly brought 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 into contact with said conical inner circumferential surface and guided along this, upon continued bringing of the reduction mandrel over said end section and thereby insertion of the end section into the reduction mandrel, convergingly in the direction 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 inner circumferential sur face of the reduction mandrel against the inner tube, until said se aling member is compressed, preferably at least 12%, more prefe rably 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 clo sure between the tubes and minimizes the risk of any of these being damaged or inappropriately shaped during the pressing pro cess.

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 helps to maintain a satisfactory effect of the cost-effective heat exchanger. According to another embodiment of the invention it is a said se aling member in the form of an O-ring of rubber which is arranged around the inner tube in step d2).

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

The method according to the invention enables the use of tubes of different materials 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 material normally used - copper, which means that the manufacturing cost of a heat exchanger ma nufactured by this method can be significantly 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.

Further advantages and advantageous features of the invention will become apparent from the following description of an embodi- ment of the method according to the invention and a heat ex changer made thereby.

BRIEF DESCRIPTION OF THE DRAWINGS Hereinafter an embodiment of the invention cited as an example is described with reference to the accompanying drawings, in which: Fig. 1 is a partially sectional lateral view showing a 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 sectional lateral view showing another step of said method, to compress an end section of the outer tube in the direction towards the inner tube, and

Fig. 3 is a partially sectional lateral view of a finished heat ex changer made by said method.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION A method of manufacturing a heat exchanger 1 and a heat ex changer 1 manufactured 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 these figures.

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

The heat exchanger 1 comprises an inner tube 2 of copper desig ned to form, during use, a part of a drain pipe in the house, to be flowed through by wastewater from baths, showers, etc. which is led away therefrom. The heat exchanger further comprises an ou- ter tube 3 of a stainless 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 outer tube is thus shorter and wider than the inner tube.

Length and diameter dimensions of the tubes are not important for the invention per se, but for exemplifying purposes it can be mentioned that the inner tube 2 may have a length of 1 -5 m (usu ally 1-2.5 m) and the outer tube a length of approx. 10-20 cm smal ler 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 dia meter 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 outer tube is threaded over the inner tube and placed sub stantially centrally thereon, so that the opposite ends of the inner tube project out at the respective side of the outer tube. The op posite ends of the outer tube are sealingly connected to an outer circumferential surface 4 of the inner tube. This sealing connection is provided in that end sections 5a-b of the outer tube 3, at the opposite ends thereof, are pressed against the outer circumfer ential surface 4 of the inner tube 2 and thus 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 circumfer ential surface of the inner tube.

Thereby a closed space 7 is provided between said outer circum ferential 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 part 11a-b wel ded to the outer tube over a respective borehole therethrough and are suitably located in the vicinity of a respective opposite end of the outer tube.

The inlet 9 is during use, i.e. when the heat exchanger 1 is mounted in a house, connected via its pipe part 11 b to a water line for cold water entering the house, which is to be led to a water heater for heating. The outlet 10 is connected via its pipe part 11a to a water pipe leading to the water heater. Incoming cold water to be heated in the water heater is thus led into said space 7 between the tubes 2, 3 via the inlet 9, in which it is preheated by absorbing heat energy from the inner tube 2 heated by wastewater, and furt her out through the outlet 10 and to the water heater. The heat exchanger is further provided with an insulating 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, first said in- ner 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 parts 11a-b are wel ded to the outer tube, in each case in the vicinity of opposite ends thereof, after which a hole is drilled through the jacket of the outer tube as a continuation of an inner channel of each part, 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 about 2-3 cm in the longitudinal direction from the respective end) of the ou- ter tube 3 should be expanded, to provide an end section bulging from a central main part 5c of the outer tube, with an inner diame ter greater than an inner diameter of said main part, at each end of the outer tube (see Fig. 2). Fig. 1 shows how this expansion is performed by means of an enlargement mandrel 13 and a die 14 mating therewith.

The enlargement mandrel 13 has an outer shape comprising a first circular, more specifically circular cylindrical, section 13a having an outer diameter slightly smaller than an inner diameter of the outer tube 3, and a second circular, more specifically circular cy lindrical, section 13b having an outer diameter which is larger than an inner diameter of the outer tube. The outer shape of the enlar gement mandrel further comprises a third intermediate section 13c, which connects said first 13a and second 13b section to each other and has the shape of a truncated cone with an outer diameter which increases continuously from the first section to the second section. The third section thus has on a first end, connecting to the first section, an outer diameter corresponding to the outer di ameter of the first section, and on a second end, connecting to the second section, an outer diameter corresponding to the outer dia meter of the second section.

The die 14 consists of a body 15 having a cavity extending through the body with an inner shape corresponding to the outer shape of the enlargement mandrel 13, i.e. with a first circular cylindrical section 14a associated with the first section 13a of the enlarge ment mandrel, a second circular cylindrical section 14b associated with the second section 13b of the enlargement mandrel and a third intermediate section 14c associated with the third section 13c of the enlargement mandrel and connecting said first 14a and se cond 14b sections of the die 14 to each other. The fact that two sections are "associated" with each other here means that they have corresponding dimensions, i.e. length and diameter, but that the diameter of each section of the die is slightly larger than that of the associated section of the enlargement mandrel, so that the enlargement mandrel 13 fits in the cavity of the die 14 (like a piece of a puzzle), with a certain clearance between 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 die 14 is firstly arranged around this end section (5a in Fig. 1 ) by threading the die over the end section of the outer tube so that it projects through the cavity of the die. As can be seen in Fig. 1 , the inner diameter of the first section 14a of the die corresponds to the outer diameter of the outer tube, the die being arranged on the outer tube with the surface of the first section 14a abutting (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 die 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 of the outer tube. The die may be held in this position around the outer tube in a variety of ways. For example, both the die and the outer tube can be held by a tool or a machine, or the die can be mounted firmly on the outer tube, e.g. be formed in two halves which are connected to each other and through this are clamped around the outer tube.

When the die 14 is arranged around the end section 5a, the enlar- gement mandrel 13 is pressed into a tube opening 17 in this end section, and more specifically, the enlarging mandrel is inserted with its first section 13a first in said tube opening, which first section 13a is slid 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, i.e. 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 the arrow "A" in Fig. 1 . When the entire first section 13a of the enlargement mandrel is inserted 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 further insertion of the enlargement mandrel into the tube opening, said edge 18 is guided along the the third section 13c towards the second section 13b of the enlargement mandrel while expanding the end section 5a, i.e. enlarging the in ner diameter, of the outer tube.

Upon further 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 by the third section 13c and then by the second section 13b of the enlargement mandrel, until the entire enlarge ment mandrel is inserted into the tube opening 17 of the end section 5a of the outer tube and presses this end section against the inner shape of the die 14, so that the end section 5a is inter- nally shaped after the outer shape of the enlargement mandrel 13 and externally after the inner shape of the die 14.

The enlargement mandrel 13 is then withdrawn from the tube opening 17 and the die 14 is removed from the expanded end section 5a. The same process is then performed on the opposite end section 5b of the outer tube 3, in case both of these are not expanded simultaneously. This step is performed in a suitable pressing machine, e.g. a hyd raulic press, which are well known in various fields of technology 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 mem ber, 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 re duction mandrel comprises a conical cavity 20 defined by an inner circumferential surface 21 which tapers in diameter from an opening 22 into said cavity in a first end of the reduction mandrel towards a second end of the reduction mandrel. This cavity 20 could also have a tapered diameter which gives it a different shape than a cone, as is understood from the description of the function of the cavity below. For example, the inner circumferential surface 21 defining the cavity could be arcuate from the first toward the second end of the reduction mandrel 19 and thus the cavity be hemispherical.

For compressing 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 mand rel constantly aligned with a center axis of the outer tube 3. The pressing direction of the reduction mandrel 19 is indicated by the arrow. ”B" in Fig. 2. This leads to the edge 18 defining the tube opening 17 in this end section 5b being inserted into the opening 22 of the reduction mandrel 19 into contact with said conical inner circumferential surface 21. Upon continued bringing of the reduct ion mandrel over said end section, and thereby inserting the end section into the reduction mandrel, the edge 18 of the tube opening 17 is pressed against said circumferential surface 21 and guided along this convergingly in the direction of the inner pipe. The end section 5b is thus continuously compressed in the direction of the inner tube 2 by being guided towards the inner circumferential sur face 21 of the reduction mandrel 19 and in turn compresses 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 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 compres sed sufficiently to completely close tightly between these parts. To ensure that a completely tight seal is achieved in a heat exchanger of this kind, 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%. It is further preferred, however, that in the compression steps described above, the end sections are compressed until the outermost edge 18, defining the tube opening 17, comes into con tact with and is pressed against the outer surface 4 of the inner tube, to provide an additional seal between edge 18 and inner tube 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 man ner described above, said closed space 7 is formed between the outer circumferential surface of the inner 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 the pressing to expand the end sections 5a-b. Of course, this pressing step may be performed in two sub-steps, i.e. the two opposite end sections 5a-b of the outer tube 3 are compressed one by one, 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-b is at this stage shaped to provide a channel 23a-b, between itself and the outer circumferential surface 4 of the inner tube 2, with a substantially V-shaped cross-section running around the entire said outer cir- cumferential surface and houses the respective O-ring 6a-b. This is realized by that each end section is divided into a first part 24a- b extending away from said main part 5c of the outer tube 3, which is formed by being pressed between said third sections 13c, 14c of the enlargement mandrel 13 in the expansion step described above, and a second part 25a-b connected to an outer end of said first part 24a-b extending back to the inner tube 2, which is formed by compressing the end section by means of the reduction mandrel 19 in the above-described compression step. As shown in Fig. 3, said channel 23a-b has in this example a cross-sectional shape of an obtuse "V". However, this could nevertheless be formed with a cross-sectional shape of a sharp "V", or with a more round bend between said first 24a-b and second 25a-b part.

Finally, the space 7 is pressure 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 manufactured by this method can be offered to the market at a low price, which contributes to shortened payback time and greater cost savings compared to such heat exchangers already known.

The invention is of course not in any way limited to the embodiment thereof described above, but many 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 appen- ded 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 an embodiment of the invention and are intended to visualize 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.