TECHNICAL FIELD

This invention relates to a system for installation of expandable pipes in heat exchangers especially aluminium or aluminium alloy expandable pipes. The invention also relates to expandable aluminium or aluminium alloy pipes suitable to be installed in such a system, and an expansion bullet suitable for use to install expandable pipes, especially expandable aluminium or aluminium alloy pipes, in such a system.

BACKGROUND

Heat exchangers of various kinds depend of the ability to transfer heat from one medium to another, so as to be able to lead away heat, or to provide heat from and to various locations and devices. Heat exchangers may be manufactured in various shapes, types and forms. Metal, having high heat conductivity, and being a commonly used construction material, is often used for manufacturing of heat exchangers. To provide a reliable heat transfer, a fluid is often provided to a pipe or similar, wherein said pipe is in contact with flanges and/or heat sinks, providing an indirect coupling of the fluid in said pipe and a medium and/or device coupled to said flanges and/or heat sinks. By pumping said fluid within said pipe, the basic principle of a heat exchanger is provided.

To achieve a good heat transfer between two metal objects, a firm contact of two such objects is of uttermost importance. A method to achieve said contact is to provide an expandable pipe into an opening of a plurality of metal fins, and then expanding the pipe to push it towards said fins. The metal fins may then lead heat to or from the pipe, which pipe houses a flow of a suitable heat conductive fluid.

Such expandable pipe is often paired with a so called expansion bullet, which bullet is pushed through the expandable pipe, wherein said pipe is expanded to an increased diameter that engage the surrounding metal fins firmly. Such pipes are furthermore often provided with inner heat transfer protrusions, arranged to engage the expansion bullet when in mechanical contact thereto, and transfer the mechanical energy from the expansion bullet to the wall of the pipe and push said wall outwards, making it expand. Such heat transfer protrusions may, for example, be provided as inwardly directed protrusions from the wall of the pipe, projecting and protruding inwards towards the hollow interior of the pipe where they will be engaged by the tapered expansion bullet when being installed.

Document JP2011208823 A discloses a system comprising expandable pipes and expansion bullets.

SUMMARY OF THE INVENTION

However, inwardly directed projections in known expandable pipes may also be deformed when engaged by the expansion bullet, leading to some extent of the mechanical energy provided by means of the expansion bullet to be lost, relative to the intended object of expanding the pipe. Furthermore, the more such projections get deformed, the lower their surface area will be, which in turn also lowers the heat transfer capabilities of the pipe, as such projections also will be exposed to the heating or cooling fluid within the pipe when the heat exchanger is used.

The disclosure herein aims to combat such drawback of prior art, and provide a system for installation of expandable aluminium or aluminium alloy pipes in heat exchangers, wherein such pipes will achieve an improved expansion and in turn a higher heat transfer within said heat exchangers.

According to a first aspect, a system for installation of expandable aluminium material or aluminium alloy pipes in a heat exchanger is provided. Said system comprises at least one aluminium or aluminium alloy expandable pipe (hereafter generally denoted "expandable pipe" or simply "pipe" unless otherwise specified) and at least one expansion bullet, said at least one expansion bullet comprising an elongated body with a tapered front portion and a curved portion having a curved shape. The at least one expansion bullet is arranged to be inserted in the at least one expandable pipe, which is performed during installations of the expandable pipe in a heat exchanger configured for such an expandable pipe. The at least one expandable pipe has a first outer diameter when in a first state, and achieves a second outer diameter when expanded by means of the expansion bullet. Said expansion is achieved after said expansion bullet has been inserted and forced through a hollow interior of said expandable pipe. The at least one expandable pipe further comprises elongated inner protrusions, arranged at an inner wall of said pipe, extending along a length of the pipe, and protruding inwards towards a centre line of the pipe. The protrusions are configured to transfer an outwards directed force from the expansion bullet for expanding the expandable pipe to the second diameter, wherein the number of protrusions is between 30 and 49. In an embodiment the number of protrusions is between 35 and 45. In the present context the term "protrusion" should be understood to include "fin".

Such a system comprising at least one expansion bullet together with at least one expandable pipe having a number of protrusions between 30 and 49 will result in an installation of expandable pipes in a heat exchanger with an increased heat transfer coefficient of the expandable pipes. Thus, an effective heat exchanger is achieved both for evaporation and condensation.

The number of protrusions between 30 and 49 will result in an increased heat transfer coefficient of the pipe, since a significant volume of the fluid, flowing in the pipe will flow in the space (may also be denoted grooves) created between the protrusions. When the number of protrusions is between 30 and 49 the space between the protrusions will have a shape and a volume, which result in a large heat transfer coefficient of the expanded pipes. When the number of protrusions is between 30 and 49 and the outer diameter of the expandable pipe before expansion is between 7 mm to 7.5 mm, a large heat transfer coefficient of the expanded pipes is achieved. From 35 to 45 protrusions and grooves has been found to perform especially well. Especially, in an embodiment wherein the number of protrusions is 35 to 45 and the outer diameter of the expandable pipe before expansion is in the range of 7 mm to 7.1 mm, a large heat transfer coefficient of the expanded pipes is achieved.

When the number of protrusions is between 30 and 49, such as between 35 and 45, a large enough number of protrusions are provided to split the expanding force between them with regards to contact points, which results in a uniformly expanded pipe, a large enough number of protrusions to make each individual protrusion small enough to not exhibit a too strong structural integrity with regards to be able to be slightly deformed when reaching maximum expansion. Further, a reduced number of protrusions may decrease the pressure drop within the pipes and result in an increased heat transfer coefficient of the pipe and thus a more effective heat exchanger.

The outwards directed force during expansion of the expandable pipe is achieved by means of the curved portion of the expansion bullet, wherein the angled surface of said tapered front and curved portion engages the protrusions, and then pushes them outwards when they slide along said angled surface until they reach an outer wall of the body of the expansion bullet. The surface of the expansion bullet may comprise a coating in order to reduce friction between the bullet and the elongated inner protrusions. The coating may be a DLC, a Diamond Like Coating. Further, a lubricant may be applied between the expansion bullet and the expandable pipe during the expansion of the expandable pipe. The expansion bullet may have an elongated solid body or be provided with an axial bore for connecting the expansion bullet to a rod.

A diameter of the outer wall of the body of the expansion bullet is configured to be larger than a diameter of a virtual circle enclosed by tips of the inner protrusions.

In an embodiment, the radius of curvature of the curved shape of the curved portion is between 5 mm and 20 mm.

Such a system comprising an expansion bullet with a radius of curvature of the curved shape of the curved portion between 5 mm and 20 mm together with an expandable pipe result in an installation of expandable pipes in a heat exchanger with an increased heat transfer coefficient of the expandable pipes. Thus, an effective heat exchanger is achieved both for evaporation and condensation.

The large amount of force results in a strong outward directed force and thus a solid contact with outer exterior elements of a heat exchanger in which such a pipe is installed. The lower amount of deformation further results in a larger intact surface area of said protrusions, wherein heat transfer is increased further. However, a small deformation of each protrusion may be acceptable and still achieve large heat transfer coefficient characteristics of the expandable pipe. Arranging the protrusions in a pattern provides an increased efficiency as, a uniform distribution of the protrusions distributes the outwardly directed forces uniformly around the circumference of the pipe. A helically arranged shape of the orientation of the protrusions will result in an angled directed deformation rather than a pure compression deformation, leaving a larger surface area of such deformed protrusions within the pipe.

The expansion bullet is arranged to be inserted in the pipe and thereafter passing through the pipe for expanding the outer wall of the pipe. The outer wall of the entire length of the pipe may be expanded by passing the expansion bullet through the pipe. The bullet is inserted in one end opening of the pipe and exits the pipe from another end opening or returned to the first end opening of the pipe by pulling after expansion. The expansion bullet is passed through the pipe by a force acting on the expansion bullet. The force acting on the expansion coincide with the extension of a centre line of the expansion bullet.

According to an aspect, the radius of curvature of the curved shape of the curved portion is between 5 mm and 10 mm.

This range and specific radius of curvature of the curved shape of the curved portion may engage the protrusions of the expandable pipe such that a low amount of deformation of said protrusions is achieved during expansion of the pipe.

In a specific embodiment, the curved shape of the curved portion of the expansion bullet may have the specific radius of curvature of 7 mm.

This specific radius of curvature of the curved shape of the curved portion may engage the protrusions of the expandable pipe such that a low amount of deformation of said protrusions is achieved during expansion of the pipe.

According to an aspect, a taper angle of the tapered front portion is between 9.5° and 12.5°. This has the advantage that the expansion bullet engages the inner protrusions with a low angle, providing a low deformation at the initial engagement, which in turn easily allows the expansion bullet to slide in within the expandable pipe. The inner protrusions may then gradually be subjected to more force, forcing the pipe to expand in a smooth transition between its two diameters.

According to an aspect, the curved portion has a maximal outer diameter at one point along the length of the expansion bullet.

This has the advantage that said one point provides the maximum expansion of the pipe, but as said one point provides said maximal outer diameter, and the remainder of the bullet is of lower diameter, the amount of losses in force due to friction is lowered. An efficient use of forces is thus achieved.

According to an aspect, the point of maximal outer diameter is located at a distance from a front end of the expansion bullet and along a centre line of the expansion bullet ,the distance being in the range of 50% to 90% of the entire length of the expansion bullet.

This location of the point of maximal outer diameter may engage the protrusions of the expandable pipe such that a low amount of deformation of said protrusions is achieved during expansion of the pipe.

According to an aspect, the distance is located between 60% and 75% of the entire length of the expansion bullet.

This has the advantage that the forward moving bullet will become stable during use, as the point of contact is located substantially towards a back portion of the expansion bullet.

When subjected to a force directed through the bullet in its forward direction, wobbling and tilting relative the centre line is minimized. In a specific embodiment, the distance may be located at about 67% of the entire length of the expansion bullet. Such location will result in an exceptional improvement of the stabilization of the expansion bullet during the forward movement of the expansion bullet in the expandable pipe. According to an aspect, each inner protrusion of the expandable pipe comprises a flat or curved abutment surface, arranged at a tip portion of said protrusion, wherein said abutment surface is configured to engage an outer surface of the expansion bullet.

This has the advantage that such flat or curved abutment surfaces may easily conform to and engage the expansion bullet with a large surface in total, but divided over a large number of inner protrusions, which will allow for an effective energy transfer with little to no deformation of said abutment surfaces. The width of the tip portion of the protrusions should be between 0.08 - 0.2 mm.

According to an aspect, each inner protrusion comprises a substantially rectangular cross section, said cross section being taken perpendicular to the centre line of the at least one pipe.

This has the advantage that a simple geometric form, which is easy to manufacture, may be utilized to achieve the desired properties of the system.

According to an aspect, each inner protrusion comprises a substantially tapered cross section, said cross section being taken in a plane perpendicular to the centre line of the at least one pipe, and wherein each inner protrusion tapering in the direction of the centre line with a taper angle between 15°- 40°, such as between 20°- 30°.

Such tapered configuration of each inner protrusion creates a space (groove) between the protrusions which have a shape and a volume, which result in a large heat transfer coefficient of the expanded pipes. Further, a tapered configuration of each inner protrusion will result in a large resistance against deformation by the influence of the expanding force from the expansion bullet. Each inner protrusion may taper in the direction of the centre line with a taper angle between 15°- 40°, such as 20°-30°. Protrusions having said taper angle may have an extraordinary large resistance against deformation by the influence of the expanding force from the expansion bullet. According to an aspect, each inner protrusion has a root width between 0.2 mm - 0.3 mm.

Such root width of each inner protrusions results in a large resistance against deformation by the influence of the expanding force from the expansion bullet.

According to an aspect, the angular distance between two protrusions arranged adjacent to each other is between 8°-10°, such as 9°.

Such configuration of each inner protrusion will create a space between the protrusions with a shape and a volume, which result in a large heat transfer coefficient of the expanded pipes.

According to an aspect, each protrusion is configured to be deformed less than 15% from the initial shape of the protrusion, after the pipe has been expanded to the second diameter.

Such a low amount of deformation will result in a large intact surface area of said protrusions and grooves resulting in a large heat transfer coefficient of the expanded pipes.

According to an aspect, the inner protrusions are arranged in a pattern at the inner wall. The protrusions may be uniformly arranged parallel to the centre line of the pipe. In another embodiment the pattern comprises a uniform distribution of helically arranged protrusions, relative to the centre line of the at least one pipe.

A helical pattern has the advantage that forces from the expansion bullet acting on the inner protrusions will be directed in a direction separate from the extending direction of the inner protrusions. Such a directed force will, when reaching deformation with regards to the inner protrusions, bend the inner protrusions rather than compressing them in parallel with their extending directions. Slightly bend protrusions will thus retain more of their original shape, and provide an improved heat transfer within the pipe due to providing a larger interacting surface area between the inner protrusions and a fluid within the pipe. Further, the helical pattern will create annular flow with centrifugal forces driving the liquid phase of the fluid to the bottom of the space between the inner protrusions, hence augment the heat transfer.

According to an aspect, the helix angle of the helical pattern is 1° - 50°, preferably 10° - 40°.

This has been experimentally observed to provide very good results and characteristics to the end product of a fully assembled heat exchanger. The helix angle of the helical pattern may be 15°, 18°, 28° or 40°. For each of these specific helix angles, exceptional good results and characteristics to the end product of a fully assembled heat exchanger is achieved.

The expandable pipe is made of aluminium material or aluminium alloy, which is a very suitable material, with regards to mechanical and heat transfer properties. Aluminium and aluminium alloys are malleable as a construction material, making it ideal to use in an expanding pipe, where sufficient expansion may be reached without the risk of cracking. Aluminium and aluminium alloys further have the beneficial material property of passively oxidising, wherein microscopic cracks and similar will create an oxide film passively, which oxide film prevents and/or slows failure mechanics such as crack corrosion and similar processes. Even further, aluminium and aluminium alloys have suitable heat transfer properties to be used as material in a heat exchanger.

According to a second aspect, the disclosure also relates to an expandable aluminium material or aluminium alloy pipe suitable to be installed in a heat exchanger. The expandable aluminium or aluminium alloy pipe may be arranged to be installed in a system according to the first aspect and the embodiments of the system.

According to an embodiment, the expandable pipe has a first outer diameter when in a first state and achieves a second outer diameter when expanded by means of an expansion bullet being inserted and forced through the hollow interior of the pipe. The expandable pipe comprises elongated inner protrusions, arranged at the inner wall of the expandable pipe. The elongated inner protrusions extend along a length of the expandable pipe, and protrude inwardly towards a centre line of the expandable pipe. The protrusions are configured to transfer an outwardly directed force from the expansion bullet for expanding the pipe to the second diameter. The number of said protrusions is between 30 and 49. In an embodiment the number of protrusions is between 35 and 45.

In an embodiment, the diameter of the expandable pipe, before expansion, is between 7 mm and 7.5 mm, such as between 7 mm and 7.1 mm.

According to an embodiment, each of the inner protrusions comprise a flat or curved abutment surface, arranged at a tip portion of said protrusion, which are configured to engage an outer surface of the expansion bullet being forced therethrough. The width of the tip portion of the protrusions should be between 0.08 mm and 0.2 mm.

In an embodiment, each inner protrusion comprises a substantially rectangular cross section, said cross section being taken perpendicular to the centre line of the pipe.

In another embodiment, each inner protrusion comprises a substantially tapered cross section, said cross section being taken in a plane perpendicular to the centre line of the pipe, and where each inner protrusion is tapering in the direction of the centre line with a taper angle (l) between 15°- 40°, preferably between 20°- 30°.

According to an embodiment, each inner protrusion has a root width (d) between 0.2 mm - 0.4 mm.

According to an embodiment, the angular distance (d) between two protrusions arranged adjacent to each other is between 8°- 10°.

In an embodiment, each protrusion is configured to be deformed less than 15% from the initial shape of the protrusion, after the pipe has been expanded to the second diameter.

In an embodiment, the inner protrusions are arranged in a pattern at the inner wall, which pattern comprises a uniform distribution. The protrusions can be arranged in a parallel pattern or helical pattern, relative to the centre line of the pipe. The helix angle (a) of the helical pattern may be 1° - 50°, preferably 10° - 40°. The helix angle (a) of the helical pattern may be 15°, 18°, 28° or 40°.

According to a third aspect, the disclosure also relates to an expansion bullet suitable for use in the system according to the first aspect and embodiments thereof. The expansion bullet comprises an elongated body with a tapered front portion and a curved portion having a curved shape. The expansion bullet may comprise an elongated tubular body with a tapered front portion and a curved portion having a curved shape.

In an embodiment, the radius of curvature of the curved shape of the curved portion is between 5 mm and 20 mm, such as between 6 mm and 10 mm.

According to an embodiment, a taper angle (b) of the tapered front portion is between 9.5° and 12.5°.

In an embodiment, the curved portion has a maximal outer diameter at one point along the length of the expansion bullet. The point of maximal outer diameter may be located at a distance from a front end of the expansion bullet and along a centre line of the expansion bullet, where the distance may be in the range of 50% to 90% of the entire length of the expansion bullet. In an embodiment, the distance is located between 60% and 75% of the entire length of the expansion bullet.

According to a further embodiment, an outer surface of the expansion bullet may comprise a friction reducing coating. The surface of the expansion bullet may comprise the coating in order to reduce friction between the bullet and the elongated inner protrusions. The coating may be a DLC, a Diamond Like Coating.

BRIEF DESCRIPTION OF THE DRAWINGS

Below is a description of examples, with reference to the enclosed drawings, in which:

Fig. 1 shows a schematic depiction of a heat exchanger in a perspective view, assembled by means of the system according to the disclosure, Fig. 2a shows schematic depiction of a cross-sectional view of an expandable pipe, according to an example of the disclosure,

Fig. 2b shows a detail view of the cross-sectional view of the expandable pipe in fig. 2a,

Figs. 3a and 3b show diagrams of the heat transfer coefficient in evaporation condition and condensation condition, respectively, for different pipes at different mass flux,

Figs. 4a and 4b show schematic depictions of longitudinal cross-sectional views of portions of an expandable pipe, according to examples of the disclosure, and

Fig. 5 shows a schematic depiction of a longitudinal cross-sectional view of an expansion bullet configured for use in the system, according to an example.

DETAILED DESCRIPTION

The description of the various features, and modifications thereof, according to the disclosure will herein be described in more detail with reference to the accompanied drawings. It is thus to be understood that examples comprising any of the described feature or a combination of features may be assembled in accordance with the description herein. The specific details described in the context of the various example embodiments and with reference to the attached drawings are not intended to be construed as limitations. Rather, the scope of the invention is defined in the appended claims.

Fig. 1 shows a schematic depiction of a heat exchanger 1 in a perspective view. The heat exchanger 1 is to be perceived as being assembled by means of the system 2 described throughout the disclosure herein, and thus comprising parts defined by any variation of the various examples as described herein. The heat exchanger 1 comprises a plurality of plate elements, hereafter called fins 4. Said fins 4 are, as depicted in fig. 1, provided with three through holes 6, through which an expandable pipe 8 may be inserted. In fig. 1 three such pipes 8 are to be perceived as already being installed into the fins 4. It should also be mentioned that additional elements such as pre-formed half circle hairpin shaped pipes 8 may be connected to the end portions of the pipes 8 as depicted, so as to achieve a single fluid circuit utilizing all three pipes 8 herein, or each pipe 8 may be used as an individual circuit, depending on the design and desired properties of the assembled heat exchanger 1.

The system 2 is configured for installation of the expandable pipes 8 in the heat exchanger 1. Said system 2 comprises at least one expandable pipe 8 and an expansion bullet 10, comprising an elongated body 12 with a tapered front portion 14 and a radius of curvature, arranged to be inserted in the at least one pipe 8. Such an expansion bullet 10 is described with reference to fig. 5.

The at least one expandable pipe 8 has a first outer diameter D1 when in a first state, and achieves a second outer diameter D2 when expanded by means of the bullet 10, after said expansion bullet 10 has been inserted and forced through a hollow interior of said pipe 8. The heat exchanger 1 depicted in fig. 1 is to be understood as already installed, wherein the pipes 8 are to be perceived as being in their expanded state, and thus sitting flush with the holes 6 in the fins 4 of the heat exchanger 1, wherein the pipes 8 herein having their second outer diameters D2.

The at least one expandable pipe 8 further comprises elongated inner protrusions 16, arranged at an inner wall 17 of said pipe 8. Said protrusions 16 extend along a length of the pipes 8, and protrude inwards towards a centre line Cl of the pipes 8. The protrusions 16 are configured to transfer an outwards directed force from the expansion bullet 10 for expanding the pipe 8 to the second diameter. Such a force is achieved when the inner protrusions 16 engage the tapered front portion 14 of the expansion bullet 10, and then slide along said tapered front portion 14 until they reach the elongated body 12, which body comprises an outer diameter that forces the inner protrusions 16 to move outwards and thus expand the pipe 8 until it fits firmly inside the holes 6 of the fins 4.

It may also be seen in fig. 1 that the inner protrusions 16 are arranged in a pattern 13 at the inner wall 17, which pattern 13 comprises a uniformly distribution of helically arranged protrusions 16, relative the centre line Cl of the at least one pipe 8. It should thus be obvious that the tapered front portion 14 of the expansion bullet 10 will act on the inner protrusions 16 in a direction that is separate from the extending direction of the inner protrusions 16. This means that when the expansion of the pipe 8 is stopped by means of the variable diameter of the pipe 8 reaching the fins 4, any further movement of the expansion bullet 10 within the pipe 8 will result in deformation of the inner protrusions 16 instead of further moving them outwards. The deformation on the inner protrusions 16 may be continuous during the expansion after the bullet surface has engaged the protrusions. In a specific example, when a pipe 8 provided with an outer diameter of 7 mm is expanded, the through holes 6 of fin may be provided with a diameter of 7.3 mm. The expected outer diameter of the pipe 8 after expansion may be about 7.42 mm. Thus, when the outer surface of the pipe 8 has touched the fin through holes 6, the pipe outer diameter further increases. With the force of the expansion bullet 10 acting on the inner protrusions 16 being in a direction separate from the extending direction of the inner protrusions 16, such deformation will cause a compression of the inner protrusions 16.

Fig. 2a shows schematic depiction of a cross-sectional view of an expandable pipe, according to an example of the disclosure, Fig. 2a depicts a cross-sectional view taken perpendicular to the centre line Cl of the pipe 8, more clearly showing the inner protrusions 16 with regards to their distribution and shape. Each inner protrusion 16 may, as depicted in fig. 2a, comprise a substantially flat or curved abutment surface 18, arranged at a tip portion 15 of said protrusion 16. Such an abutment surface 18 is configured to receive an outer surface 20 of the expansion bullet 10.

The example of the expandable pipe 8 as depicted in fig. 2a comprises a total of 40 inner protrusions 16, spaced uniformly around the circumference of the expandable pipe 8. The example shown in fig. 2a having 40 protrusions and an outer diameter (before expansion) of 7mm, each inner protrusion may have a nominal value of the root width of 0.254 mm.

The shape and size of each protrusion 16 may of course be modified depending on the number of protrusions 16 comprised in the pipe 8, and other total amounts of protrusions 16 is possible to utilize without deviating from the scope of protection defined by the disclosure herein. Other examples of the expandable pipe 8 may have a number of inner protrusions 16 in the range of about 30 to 49. Further examples of the expandable pipe 8 may have a number of inner protrusions 16 in the range of about 35 to 45. Such inner protrusions 16 may thus be altered with regards to their shape and size, so as to achieve a large enough accumulative abutment surface 18 area, and to withstand the forces applied from the expansion bullet 10 when in use. The first outer diameter D1 of the expandable pipe may be in the range of 7 mm to 7.1 mm before expansion.

Each inner protrusion 16 may, as seen in fig. 2a, comprise a substantially rectangular cross section, said cross section being taken perpendicular to the centre line Cl of the at least one pipe 8. A slight tapering to sidewalls 22 of said protrusions 16 may be present. Such slight tapering may provide benefits to the manufacturing of such a pipe 8, as the pipe 8 is round and thus the inner wall 17 to which the inner protrusions 16 are connected to is concave with regards to the extending direction of the inner protrusions 16.

Such a shape as described here above is structurally rigid and may transfer a large enough force to the outer wall 19 of the expansion pipe 8 without deforming too much. By means of not using a highly tapered structure, and/or a more complex shape, for example, material usage may be reduced and manufacturing may be simplified, providing a cost effective product.

Fig. 2b shows a detail view of the cross-sectional view of the expandable pipe in fig. 2a. The detail view in fig. 2b represents the details within the small circle in fig. 2a. The inner protrusion 16 comprises a substantially tapered cross section, said cross section being taken perpendicular to the centre line Cl of the at least one pipe 8, and wherein each inner protrusion 16 tapering in the direction of the centre line Cl with a taper angle l, which may be between 15°- 40°. The height h of the inner protrusion 16 may be in the range 0.1 mm - 0.4 mm. The space between the protrusions 16 will have a shape and a volume, which result in a large heat transfer coefficient of the expanded pipes 8.

Figs. 3a and 3b show diagrams of the heat transfer coefficient HTC for different pipes in evaporation condition and condensation condition, respectively, at different mass flux. Test results of the heat transfer coefficient at different mass flux for different expandable pipes at evaporation are shown in fig. 3a. The expandable pipe 8 in aluminium with 40 inner protrusions has a larger heat transfer coefficient for all different mass flux comparing to expandable pipes with 50 inner protrusions of aluminium and copper. Further, the expandable pipe in aluminium with a smooth inner surface and without inner protrusions has lowest heat transfer coefficient. All the expandable pipes had an outer diameter of 7 mm. The evaporation test in fig. 3a was performed at 10° C. The fluid flowing within the pipes was a refrigerant. In fig. 3b test results of the heat transfer coefficient HTC at different mass flux for different expandable pipes at condensation are shown. The expandable pipe 8 in aluminium with 40 inner protrusions has a larger heat transfer coefficient for different mass flux at lower values comparing to expandable pipes with 50 inner protrusions of aluminium and copper. At higher mass flux the expandable pipe 8 in aluminium with 40 inner protrusions had essentially the same heat transfer coefficient as the expandable pipe with 50 inner protrusions of copper, but still had a larger heat transfer coefficient comparing to expandable pipes with 50 inner protrusions of aluminium. Further, the expandable pipe in aluminium with a smooth inner surface and without inner protrusions has lowest heat transfer coefficient. All the expandable pipes had an outer diameter in the range of 7 mm to 7.1 mm. The condensation test in fig. 3b was performed at 40° C. The fluid flowing within the pipes was a refrigerant.

Fig. 4a depicts a schematic longitudinal cross-sectional view of a portion of a pipe 8, taken along the centre line Cl of said pipe 8 when the helix angle a is 0°. For the sake of simplicity, only a few inner protrusions 16 are shown herein. The helix angle a of the helical pattern 13 may preferably be in the range of about 1° to about 50°, which is illustrated in fig. 4b by the helix angle a therein, wherein may be perceived to be shifted within said defined range.

In fig. 4a the protrusions extend in a parallel with the centre line Cl of the pipe. In fig. 4b the helix angle a of the helical pattern 13 is 18°, as an example.

Fig. 5 shows a schematic depiction of a longitudinal cross-sectional view of an expansion bullet 10 configured for use in the system 2. The expansion bullet 10 comprising an elongated tubular body 12 with a tapered front portion 14 and a curved portion 30 having a curved shape 32. The expansion bullet 10 is arranged to be inserted in the pipe 8 and thereafter passing through the pipe 8 for expanding the outer wall 19 of the pipe 8. The outer wall 19 of the entire length of the pipe 8 may be expanded by passing the expansion bullet 10 through the pipe 8. The bullet 10 is inserted in one end opening of the pipe 8, and exit the pipe 8 in another end opening or returned to the first end opening of the pipe by pulling after expansion. The expansion bullet 10 is passed through the pipe 8 by a force acting on the expansion bullet 10. The force acting on the expansion bullet 10 coincide with the extension of a centre line C2 of the expansion bullet 10.

A first part 24 of the expansion bullet 10 has the shape of a truncated cone 26. A base 28 of the truncated cone 26 is arranged adjacent to a second curved part 30 of the expansion bullet 10. The second part 30 of the expansion bullet 10 has curved shape 32, provided with a maximal outer diameter D3 at one point 34 along the length LI of the expansion bullet 10. The point 34 of maximal outer diameter D3 at a distance L2 from a front end 36 of the expansion bullet 10 and along the centre line C2 may be located in the range of 50% to 90% of the entire length of the expansion bullet 10 based on the radius of curvature. In fig. 5 this point of maximal outer diameter D3 at the distance D2 from the front end 36 of the expansion bullet 10 and along the centre line C2 is located 67% of the entire length of the expansion bullet 10.

The radius R of curvature of the curved shape 32 of the curved portion 30 is between 5 and 20 mm. There may be a relationship between the taper angle of the tapered front portion and the radius of curvature of the curved shape of the curved portion. The taper angle may increase as the radius of curvature increases. The tapered front portion may be eliminated at a radius at 9.8 mm. The tapered front portion may decrease as the radius of curvature decreases. According to an example, the radius R of curvature of the curved shape 32 of the curved portion 30 is 7 mm and a taper angle b of the tapered front portion 14 is 11.9°.

At this specific taper angle, the expansion bullet engages the inner protrusions with a low angle, providing a low deformation at the initial engagement, which in turn easily allows the expansion bullet to slide in within the expandable pipe. The inner protrusions may then gradually be subjected to more force, forcing the pipe to expand in a smooth transition between its two diameters.

The outwards directed expansion force from the expansion bullet 10 acting on the protrusions 16 in the pipe 8 is achieved by means of the tapered front portion 14 of the expansion bullet 10, which has the shape of a truncated cone 26. The outer surface 20 of said truncated cone 20 engages the protrusions 16, which are pushed outwards when sliding along said outer surface 20 until they slide up on the curved shaped 32 second part 30 of the expansion bullet 10.

The surface 20 of the expansion bullet 10 may comprise a coating 38 in order to reduce friction between the bullet 10 and the elongated inner protrusions 16. The coating 38 may be a DLC, a Diamond Like Coating. The foregoing description of the embodiments has been furnished for illustrative and descriptive purposes. It is not intended to be exhaustive, or to limit the embodiments to the variations described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order to best explicate principles and practical applications, and to thereby enable one skilled in the arts to understand the invention in terms of its various embodiments and with the various modifications that are applicable to its intended use. The components and features specified above may, within the framework of the disclosure, be combined between different embodiments specified.