EXCHANGER

The invention relates generally to the technical field of heat exchangers and particularly, but not exclusively, to internal heat exchangers and more particularly those used for air- conditioning systems for automotive applications.

Air-conditioning systems of motor vehicles, for example, are frequently equipped with a so- called internal heat exchanger. Such heat exchangers may be used to increase the operating efficiency of the air-conditioning system by pre-heating the refrigerant supplied to a suction side of a compressor of the air-conditioning system and at the same time cooling the refrigerant (liquid side) being conveyed to an expansion device.

WO2010124871A2 discloses a heat exchanger bent into a U- shape because of space restrictions in an application where a certain heat exchange area is required. The heat exchanger is described prior to being bent into a U-shape to have two concentric tubes, an inner tube and an outer tube. The inner tube originally is cylindrical, i.e. has a circular cross section with a centre point and a radius. During production, a part of the inner tube is deformed to have an elliptical cross section (i.e. an oval cross section) or a cross section which is substantially elliptical (oval) but with flattened sides. This (substantially) elliptical cross section is applied to the inner tube by clamping portions of the inner tube sequentially. A first portion of the inner tube is clamped and given the (substantially) elliptical cross section. In doing so, a part of the wall of the inner tube will be closer to the centre point of the inner tube (corresponding to a short axis of the ellipse with a length smaller than the radius) and a part of the wall will be further away from the centre point of the inner tube (corresponding to a long axis of the ellipse with a length larger than the radius). Then the clamp is removed and the inner tube is advanced a distance along its longitudinal axis. The inner tube is also rotated by a fixed angle. Then it is clamped again to give a second portion the (substantially) elliptical cross section. By repeating this process over a part of the length of the inner tube, that part of the inner tube has a (substantially) helical shape. When fitted in the outer tube, there are two channels between the outer surface of the inner tube and the inner surface of the outer tube. The combination is then bent into shape.

During bending of the heat exchanger with the inner tube mounted in the outer tube, the inner tube and the outer tube tubes may bend differently, such that the total surface area of the cross sections of the two channels reduces. This for instance happens when the heat exchanger is bent around a bending axis along the long axis of the ellipse in the cross section. Before bending, the distance between the outer surface of the inner tube and the inner surface of the outer tube is maximal in the direction of the short axis of the ellipse. However, after bending the inner tube may make contact with the outer tube in the direction of the short axis.

Moreover, the reduction has a large production variance, which makes that the efficiency of the heat exchanger has a large variance as the total surface area of the cross section determines the flow velocity and the heat transfer coefficient.

It is desirable to provide a heat exchanger that at least reduces one or more of these problems.

The object of the invention is reached by a heat exchanger comprising an outer conduit having a longitudinal axis and an inner conduit having an outer surface and arranged inside and along the longitudinal axis of the outer conduit, the outer surface of the inner conduit and an inner surface of the outer conduit being arranged to form a first opposing wall and second opposing wall of a fluid flow channel, characterized by the first opposing wall comprising a plurality of discrete protrusions protruding towards the second opposing wall. As the protrusions are discrete, fluid may flow between the protrusions. Because the protrusions protrude from the first opposing wall to the second opposing wall, the protrusions function as spacers when bending the heat exchanger, keeping a distance between the inner conduit and the outer conduit. This prevents that the fluid flow channel (i.e. a channel for a gas or liquid) between the inner conduit and the outer conduit collapses at the bend which contributes to maintaining the area of the cross section of the fluid flow channel. If the cross section were to decrease, locally the fluid would face a large resistance which in turn means that a larger pressure drop would occur. To overcome this pressure drop, more energy would be needed to operate the internal heat exchanger to exchange the same amount of heat. In an embodiment of the invention, the first opposing wall is formed by the inner conduit.

When the first opposing wall is formed by the inner conduit, the protrusions increase the surface area of the outer surface of the inner conduit. Since, by the laws of physics, the heat transfer from the fluid in the fluid flow channel to the material of the inner conduit increases with an increased surface area, the presence of the protrusions also increases the heat transfer of the heat exchanger compared to the heat exchanger without the protrusions.

In an embodiment of the invention wherein the inner conduit comprises an inner surface forming a second fluid flow channel, the protrusions correspond to dents in an inner surface of the inner conduit.

The presence of dents in the inner surface of the inner conduit increases turbulence of a fluid flowing in the second fluid flow channel. Increased turbulence in the second fluid flow channel increases the heat transfer from the material of the inner conduit to the fluid in the second fluid flow channel or vice versa.

In an embodiment of the invention, the protrusions are arranged in a spiral configuration along the longitudinal axis.

Because the protrusions are arranged in a spiral configuration, the average length of path of the fluid flow in the fluid flow channel is increased, which increases the heat transfer efficiency of the heat exchanger. In an embodiment of the invention, there is provided a method of manufacturing a heat exchanger comprising an outer conduit having a longitudinal axis and an inner conduit having an outer surface, the inner conduit being arranged inside and along the longitudinal axis of the outer conduit, the outer surface of the inner conduit and the inner surface of the outer conduit being arranged to form a first opposing wall and second opposing wall of a fluid flow channel,

comprising providing an inner conduit with a first cross section area defined by the outer surface of the inner conduit; deforming the inner conduit by forming a plurality of discrete protrusions where the outer dimension of the inner conduit is increased and obtaining a second cross section area defined by the outer surface of the inner conduit that is larger than the first cross section area. Because the outer surface of the inner conduit forms one of the opposing walls, the protrusions, corresponding to locally increased outer dimensions of the inner conduit, protrude into the direction of the inner surface of the outer conduit.

As the protrusions are discrete, fluid may flow between the protrusions. Because the protrusions protrude from the first opposing wall to the second opposing wall, the protrusions function as spacers when bending the heat exchanger keeping a distance between the inner conduit and the outer conduit. This prevents that the fluid flow channel (i.e. a channel for a gas or liquid) between the inner conduit and the outer conduit collapses at the bend. Additionally, as both the outer dimension of the inner conduit increases there is a better heat exchange.

In an embodiment of the invention the inner conduit comprises an inner surface forming a second fluid flow channel, the step of deforming the inner conduit comprises increasing the inner dimension of the inner conduit at the protrusions.

As the inner dimension is increased at the protrusions, which are discrete, dents are formed in the inner surface of the inner conduit. The presence of dents in the inner surface of the inner conduit increases turbulence of a fluid flowing in the second fluid flow channel. The method therefore provides an internal heat exchanger whereby in use, the increased turbulence in the second fluid flow channel increases the heat transfer from the material of the inner conduit to the fluid in the second fluid flow channel or vice versa.

In an embodiment of the invention, the inner conduit is manufactured according to the steps of inserting a ball cage into the inner conduit, the ball cage being arranged to be used in a forward direction, the ball cage embedding a piston comprising at least one groove and at least one ball, each of the at least one grooves corresponding to one of the at least one balls, the at least one groove having a depth which increases in the forward direction of the ball cage, driving the piston back and forth in the forward direction such that at least one balls move along the corresponding grooves and deform the inner conduit thereby forming the plurality of discrete protrusions.

Because the depths of the grooves increase in the forward direction, at a forward motion of the piston, the depths of the grooves decreases at the locations where the balls are and the balls accordingly are driven radially outward. As they are driven radially outward, the balls form dents in the inner surface of the inner conduit and at the same time corresponding protrusions in the outer surface of the outer conduit. The protrusions can be discretely distributed along the inner conduit because the balls are in separate grooves and because at a backward motion of the piston, the balls can sink into the grooves again. The ball cage and piston can then be inserted further into the inner conduit, without the protrusion being extended in the longitudinal direction of the inner conduit. In an embodiment of the invention wherein the inner conduit comprises a further longitudinal axis, the method comprises placing a backup ring around the inner conduit, the backup ring having an inner ring surface corresponding to the outer surface of the inner conduit, the inner ring surface comprising at least one dent, the method further comprising arranging the dents to be at locations where the protrusions will be formed by the at least one ball.

As the dents are in the inner ring surface, they face the inner conduit once the backup ring is placed around the inner conduit. By arranging the dents to be at locations where the protrusions will be formed, they reinforce the material of the inner conduit and therefore together with the ball determine the shape of the protrusions.

Examples of embodiments the invention will now be described with reference to the accompanying schematic drawings. Corresponding reference symbols in the schematic drawings indicate corresponding parts. The schematic drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain the present invention. Further, the examples are not intended to be exhaustive or otherwise limit or restrict the invention to the precise configurations shown in the drawings and disclosed in the following detailed description. Figure 1 is a schematic diagram of an air conditioning system for an automotive application comprising an internal heat exchanger;

Figure 2 shows a schematic illustration of the internal heat exchanger shown in Fig. 1 in a U- shaped configuration;

Figure 3 a shows the internal heat converter;

Figure 3b shows a part of the internal heat converter;

Figure 3 c shows a cross section of the internal heat converter;

Figure 3d shows a part of the cross section of the internal heat converter; Figure 4 a ball cage with piston inserted in the inner conduit to create the protrusions during manufacturing.

An air conditioning system (1) for use in a motor vehicle comprises a compressor (2), which may be driven, for example, by the engine of the vehicle or by a separate electric motor or the like. The air conditioning system is shown in figure 1. The compressor (2) has an inlet (4), connected to a low-pressure line (21), via which the compressor (2) takes in refrigerant, or coolant, at low pressure. The compressor also has an outlet (3), via which pressurized refrigerant is output, into a high-pressure line (5). The high-pressure line (5) leads to a cooling device (6) where the compressed and thus heated refrigerant is cooled and / or condensed. Therefore, the cooling device (6) is also referred to as a condenser or a gascooler in transcritical systems. In this example, the refrigerant used is R-134a that works at low pressure. However, other refrigerants could also be used, like R744 which works at pressures up to 160 bar. At an outlet (7) of the cooling device, the refrigerant is discharged to another high- pressure line (8) that leads to a high-pressure inlet (9) of an internal heat exchanger (11). The internal heat exchanger (11) has a high-pressure outlet (12) that is in turn connected to an expansion device (15) via a high-pressure line (14). The expansion device (15) relaxes the refrigerant that is introduced into an evaporator (16). The refrigerant evaporates in the evaporator (16) and, as a result, absorbs thermal energy from the environment; in this example, cooling the air supplied to the interior of the motor vehicle. The resultant refrigerant vapour is then transported from the evaporator (16), via a low-pressure line (17), to the low- pressure inlet (18) of the internal heat exchanger (11). This refrigerant vapour flows through the internal heat exchanger (11) in a counter-current direction to the refrigerant that is being fed through the high-pressure inlet (9). In so doing, the refrigerant vapour cools the pressurized refrigerant, thus itself becoming heated. The refrigerant vapour is discharged, having been heated, at the low-pressure outlet (19) of the internal heat exchanger (11). It is then conducted, via a low-pressure line (21), to the inlet (4) of the compressor (2).

The internal heat exchanger (11) allows the temperature of the refrigerant flowing to the compressor (2) to be increased, which in turn increases the temperature of the refrigerant at the outlet (3) of the compressor. Therefore, the cooling device (6) releases a greater amount of thermal energy. At the same time, the internal heat exchanger (11) lowers the temperature of the refrigerant fed to the evaporator (16), thus providing an improved heat transfer between the evaporator (16) and ambient air. In this manner, the internal heat exchanger (11) may be used to increase the efficiency of the air conditioning system.

Figure 2 shows a further schematic illustration of the internal heat exchanger (11). In this example, it is shown as a U-shaped bent pipe (22). It will be appreciated that the exact shape of the heat exchanger will depend upon its application. However, in certain

applications, but not all, bending of the heat exchanger (11) is required. The bent pipe (22) has two legs (23,24), that are bent away from each other at their upper ends. The high-pressure inlet (9) and the low-pressure outlet (19) are in fluid connection with the remainder of the system (1) at position (26a). The low-pressure inlet (18) and the high-pressure outlet (12) of the internal heat exchanger (11) are in fluid connection with the remainder of the system (1) at position (26b). As can be seen from the figure, positions 26a and 26b are located at or relatively close to the terminations of the upper ends of the bent pipe (22).

Referring now to Figures 3a, 3b, 3c and 3d,the structure of the internal heat exchanger (11) will be described in more detail. Figure 3a shows the internal heat exchanger (11) according to the invention in its assembled state but prior to being bent into its final U-shaped configuration. As can be seen from the figure, the internal heat exchanger (11) includes an outer conduit (30), and an inner conduit (32) having end portions 32a and 32b. Both the outer conduit (30) and the inner conduit (32) are arranged to be refrigerant conduits. The inner conduit (32) is partly located inside and runs the entire length of the outer conduit (30). The inner conduit (32) has a first longitudinal axis (121) and the outer conduit (30) has a second longitudinal axis (120). The longitudinal axis (121) of the inner conduit and the longitudinal axis (120) of the out conduit (30) are equal. Between the outer surface (101) of the inner conduit (32) and the inner surface (102) of the outer conduit (30) there is a space, forming a fluid flow channel (103).

The high-pressure inlet (9) and the high-pressure outlet (12) are each connected to an opening in respectively an inlet chamber and an outlet chamber arranged at the ends of the outer conduit (30). This way, a fluid connection is formed between the high-pressure inlet (9) and the high-pressure outlet (12) via the outer conduit (30). The chambers may be machined, or otherwise manufactured using any conventional process. The outer conduit (30) may be used as a connection sleeve which allows the system costs to be reduced. The sealing surfaces (36) of the chambers are arranged to seal the connection with the inner conduit (32) to ensure that the seal is effective against leakage of the refrigerant. Again a conventional process may be used to arrange this joint; for example insertion of o-rings, crimping and or welding or brazing.

In the figure, the inner conduit (32) has end portions 32a and 32b that are circular. They are made of unmodified base conduit material. End portion 32a forms a low-pressure inlet (18) and end portion 32b forms a low-pressure outlet (19) of the internal heat exchanger (11). The low-pressure inlet (18) and the low-pressure outlet (19) are in fluid connection via the inner surface (104) of the inner conduit (30) thereby forming a further fluid flow channel (105). As the end portions 32a and 32b are of unmodified base conduit material, they may be configured to be the required lengths to provide the function of the low-pressure lines (17) between the evaporator (16) and the internal heat exchanger and the low-pressure line (21) between the internal heat exchanger and the compressor. This in turn means that no additional suction side connection tubes are needed, obviating the need for costly connection processes, such as welding and eliminating the risk of refrigerant leakage at such

connections. Between the end portions 32a and 32b of the inner conduit (32) is a central portion 32c. The central portion of the inner conduit is formed from a cylindrical tube (i.e. a tube having a circular cross section perpendicular to its longitudinal axis (121)) comprising a plurality of buckles or protrusions (100). The diameter (D1) of the inner surface of the inner conduit (32) is equal to the diameter of the inner surface at the end portions. The protrusions protrude into the direction of the inner surface (102) of the outer conduit (30). At the same time, at the inner surface (104) of the inner conduit (32) the protrusions correspond to dents.

Herein, angular positions relate to positions defined by the angle around the longitudinal axis (121). At a certain longitudinal position (i.e. a position on the longitudinal axis), say position z1, the inner surface (104) comprises dents (200) and angular positions where there are no dents. Where there are no dents, the inner surface (104) of the central portion (32c) of the inner conduit (32) has a first diameter (D1) (or first dimension) which is equal to the diameter of the inner surface at the end portions 32a and 32b of the inner conduit. However, where the dents (200) are formed in the inner surface (and protrusions in the outer surface), the dimension (D2) of the inner surface (104) is slightly larger than the first diameter D1. This means that the circumference along the inner surface (104) of the inner conduit (32) is larger than what it would have been if it was circular with a diameter D1 and had no dents. Correspondingly, at the outer surface of the inner conduit (32) the protrusions (100) correspond to a forth diameter (D4) (or forth dimension) which is larger than the third diameter (D3) corresponding to angular positions where there are no protrusions. This means that the circumference along the outer surface (101) of the inner conduit is larger than what it would have been without the protrusions. At can be seen in figure 3a and 3b, at other longitudinal positions (such as position z3), the protrusions are at other angular positions. In figure 3c, reference 110 refers to a protrusion at longitudinal position different from where the cross section is made, here longitudinal position z3. The protrusions (100) are partly formed as a spherical caps with an inner radius (R1) and an outer radius (R2). This is shown in figure 3d, which shows a magnified part of the cross section within the dashed circle in figure 3c. As the material of the inner conduit (32) is stretched at the protrusion (100) the thickness of the wall at the protrusion is less than where there is no protrusion. This is advantageous, as the smaller thickness allows for a better heat transfer. Because the spherical cap shape, the protrusions are relatively strong, i.e. will not easily deform. This means, that they function well to maintain space between the inner conduit (32) and the outer conduit (30), when the inner conduit (32) and outer conduit (30) are bent, for instance to obtain a U-shaped form of the internal heat exchanger (11).

The inner conduit (32) is deformed to have the protrusions using a ball cage (1001) (see figure 4). A piston (1003) is arranged inside the ball cage. The piston comprises a number of grooves (1007) (two of which are shown). In each groove there is a ball (1002). During manufacturing of the heat exchanger, the ball cage (1001) is inserted into the inner conduit to a first longitudinal position (corresponding to z1 in figure 3 a). The ball cage has a direction in which it is preferably moved later on during production. In figure 4, this direction is to the right. The piston (1003) is then driven in the forward direction by a piston rod (1006) while maintaining the inner conduit (32) in a fixed position. The grooves are deeper in the forward direction. As the piston is driven in the forward direction, the balls are driven in a radial outward direction and into the material of the inner conduit (32) at the longitudinal position z1. This causes the inner conduit (32) to locally deform whereby dents (200) are created on the inner surface (104) and corresponding protrusions (100) are formed on the outer surface (101). For the balls to be able to move outward, either the ball cage (1001) is from a material that can be stretched outwardly or has openings at the grooves with a dimension smaller than the radius of the balls so that the ball cannot get out of the openings completely.

After creating the protrusions with the ball cage (1001) at the first longitudinal position (z1), the piston (1003) is moved by the piston rod (1006) in the direction opposite to the forward direction of the ball cage (1001) while the ball cage and the inner conduit are maintained in a fixed position. As the piston is moved, the grooves get deeper at the locations where the balls are. At the end of the backward movement of the piston, the groove has a depth such that the ball does not stick out of the ball cage (1001) anymore. As a next step, the complete ball cage (1001) including the piston (1003) and balls is moved in the forward direction, whereas the inner conduit (32) maintains its position. In this example the ball cage (1001), and piston are also rotated around the longitudinal axis of the inner conduit (32). It will be understood that the ball cage (1001) and piston (1003) may not be rotated in other examples. Then the process is repeated to create additional protrusions (100) at longitudinal position z3.

In this example between longitudinal position z1 and longitudinal position z3 there is an intermediate longitudinal position z2 where there are no protrusions. In other examples there may be protrusions in any cross section of the inner conduit (32). At the intermediate longitudinal position, the inner diameter of the inner conduit is D 1 and the outer diameter of the inner conduit is D4. The inner diameter D1 is also the inner dimension at the longitudinal positions z1 and z3 at the angular positions where there is no dent. Similarly, the outer diameter D4 is also the outer dimension at the longitudinal positions z1 and z3 at the angular positions where there is no protrusion.

Optionally a backup ring (1004) is used to reinforce the material of the inner conduit (32) while forming the protrusions. The backup ring (1004) is positioned of the outer surface of the inner conduit (32) before forming the protrusions. Being so positioned, the backup ring (1004) has an inner ring surface (1041) that faces the outer surface of the inner conduit (32). The inner ring surface has moulding dents. The moulding dents are arranged in positions where the balls will form protrusions. When the piston is moved forward and the balls outward, the material of the inner conduit will be enclosed by the balls and the inner ring. As the moulding dents are arranged where the balls move outward, the material of the inner conduit is driven into the dents of the backup ring (1004).

Once the inner conduit (32) is provided with the protrusions as described above, it is assembled with the outer conduit (30), by inserting the inner conduit into the outer conduit. The fit between the inner conduit (32) and the outer conduit (30) now depends on the third diameter (D3) and the inner diameter of the outer conduit. Preferably, the fit is a loose fit or a slight interference fit. The inner conduit (32) may be inserted by hand or in an automated way. Then the chambers may be fitted and the sealing surfaces (36) sealed to the inner conduit. Finally the internal heat exchanger is bent to obtain the U-shaped form of figure 2.

It will be understood that the heat exchange characteristics required for different applications will vary. Accordingly, the heat transfer surface of the example may be varied. Clearly, the dimensions, such as length and diameter, of the internal heat exchanger may be varied (where space permits). Also the number of protrusions, the density of the protrusions, their shape, width and height may be varied.

It will also be understood that various changes may be made to the above described examples. For example, whilst the internal heat exchangers of the examples have been described such that the high and low-pressure fluid flow through the internal heat exchanger in opposite direction (counter current use), the internal heat exchanger could also be used in an embodiment wherein the high and low-pressure fluid flow in the same direction.

Furthermore, whilst the refrigerant used in the above described examples is R- 134a, other refrigerants could equally be used. For example, other low pressure refrigerants or refrigerants that work at high pressures, such as carbon dioxide could be used. Moreover, although the examples have been described in relation to automotive applications, it will be appreciated that the invention may be applied to a wide range of other application. These may include for example, busses, lorries, trains, aircraft and non-mobile applications.

Additionally, whilst the above described embodiments have been described as utilizing base tube material that is circular in cross section, other cross sections could be used, such as elliptical, square and rectangular cross sections.