NL2037444B1

NL2037444B1 - Induction heater and heat exchanger

Info

Publication number: NL2037444B1
Application number: NL2037444A
Authority: NL
Inventors: Józef Bukna Mariusz
Original assignee: Iskra Block B V
Priority date: 2024-04-12
Filing date: 2024-04-12
Publication date: 2025-11-03

Abstract

Title: Induction heater and heat exchanger An induction heater (1, 120) is provided with a heat exchanger unit (2, 60) comprising a fluid channel (7, 69, 70) extending through a heat exchanger body (5, 61, 62, 63). The heat exchanger body has a predetermined volume and an outer surface (35, 36, 96, 97). The channel (7, 69, 70) extends from an inlet (8, 93) to an outlet (9, 91) in the heat exchanger body (5, 61, 62, 63). A flux generating member (3,4, 33, 34, 102, 103, 104, 105) is placed at or near the outer surface (35, 36, 96, 97) for generating an alternating magnetic flux in the heat exchanger body (5, 61, 62, 63). At least 80 % of the volume of the heat exchanger body, preferably at least 90 %, is formed by a solid metal in which the fluid channel (7, 69,70) is formed. Fig. 1

Description

P136711NLOO

Title: Induction heater and heat exchanger

Technical Field

The disclosure relates to an induction heater comprising a heat exchanger unit and a flux generating member. The disclosure furthermore relates to a heating system comprising such an induction heater, in particular for domestic use such as in room heating and/or hot tap water supply.

Background

From CN 1085071768, an induction heater is known in which a metal heat exchanger pipe 1s transporting water along a number of parallel pipe sections that extend in an axial direction through the core of a coil. The heat exchanger pipe is inserted in a phase change body for the storage of heat. The coil windings are placed on a non-magnetic outer wall of a casing and excite the metal pipe and the phase change material by generating a changing magnetic field at frequencies between 10 kHz and 300 kHz. The magnetic field induces eddy currents in the metal pipe and in the phase change material that cause a rise in temperature that is passed to the water flowing through the pipe. Plate-type heat exchange fins are mounted on the straight-line heat pipe segments within the coil.

The known induction heater comprises a relatively complex array of heat pipes, fins and phase change material inside the windings of the induction coil. The large amount of open space inside the windings results in a relatively large volume of the heater and provides a relatively low heating capacity.

It is an object of the present invention to provide an induction heater that is of a relatively simple and robust construction. It is a further object to provide an induction heater that is of compact size and that has a high heating efficiency.

Summary

Hereto an induction heater according to the disclosure has a heat exchanger unit comprising a fluid channel extending through a heat exchanger body having a predetermined volume and an outer surface, the channel extending from an inlet to an outlet in the heat exchanger body and a flux generating member placed at or near the outer surface for generating an alternating magnetic flux in the heat exchanger body, at least 80 % of the volume of the heat exchanger body, preferably at least 90 %, being formed by a solid metal in which the fluid channel is formed.

By forming the fluid channels having a relatively small volume, directly in the metal of the heat exchanger body, either by drilling, machining, casting or additive manufacturing, a large volume of metal in the heat exchanger body is available for heat generation by induction, in close proximity to the fluid channels. The large amount of heat that is generated in the large mass of metal, which is much larger than the mass of the heating fluid in the channels, is effectively transferred to the fluid flowing through the channels that is rapidly heated at a relatively low energy input.

The solid heat exchanger body, which can be cuboid, cylindrical or have any other suitable shape, is of compact dimensions and can be easily incorporated in a small- size heating system for domestic use, including room heating and providing hot tap water.

The fluid channel may be composed of several interconnected straight-line sections or may extend along a spiral or other meandering path. The inlet and the outlet of the fluid channel may be situated on opposed sides of the heat exchanger body or may be positioned side by side.

The heat exchanger body may be delimited by an outer surface extending in a longitudinal direction L, opposed end faces of the body extending transversely to the outer surface, the channel comprising at least three longitudinal sections extending in the length direction between the end faces, a first and a second longitudinal section being connected via a transverse channel section extending in the transverse direction at one of the end faces, the second and third longitudinal sections being connected via a transverse channel section extending in the transverse direction at the other end face.

The end faces of the heat exchanger body may be covered by a sealing plate forming at least one side of a transverse channel section.

By covering and sealing of the transverse channel sections via the sealing plates, the interconnection of the longitudinal channel sections is simplified. The transverse channels can be formed in the end faces or in the sealing plates by machining and are covered by attaching the sealing plates to the end faces of the heat exchanger body, by a bolted connection, by welding or a combination thereof.

The fluid channel may comprise surface irregularities for creating turbulence in the fluid flowing through the channel.

The surface irregularities, which may be formed by grooves in the wall of the channel or by projections extending from the channel wall, cause turbulence in the fluid and improve heat transfer from the metal to the fluid at the channel wall.

The heating channel may comprise a metal insert having fins extending transversely to the channel wall.

For increasing the heat transfer surface, the channels may have a slit-shaped cross-section.

In an embodiment of an induction heater according to the disclosure, the heat exchanger unit comprises a first side body member, a central body member and a second side body member, the heat exchanger unit comprising a first side body member, a central body member and a second side body member, the side body members each forming a sidewall of the first and second longitudinal cannel sections and having side teeth extending transversely to the channel surface, the central body member being situated between the side body members and having a first surface and a second surface, central teeth projecting transversely to the first and second surfaces and meshing with the side teeth of the opposing side body members, forming sidewalls of the first and the second channel sections, the three body members near their perimeter comprising substantially flat interfaces, and being connected via bolts, welding or brazing.

This embodiment of the heat exchanger body can be easily manufactured and assembled. The flow resistance through the channel can be adjusted by changing the length and interspacing of the teeth.

The side teeth on the sidewalls may extend at an angle to a longitudinal center line, the central teeth on the central body member of the corresponding channel extending substantially in the direction of the side teeth at intermediate positions along the longitudinal center line.

The channel sections may have a width extending along at least one third of the dimension, preferably at least one half, in s width direction W of the heat exchanger body.

The relatively wide channels allow a high throughflow volume of heating fluid.

The heat exchanger body may be cuboid and comprises two opposed side faces, the flux generating member being tubular and forming a loop along the opposed side faces.

The windings of the tube lie in close proximity to the side faces and the alternating magnetic field induces a current in the metal of the heat exchanger body causing inductive heating.

A flux guiding body may comprise longitudinal channels being in contact with the top and bottom faces of the heat exchanger body, the tubular flux generating member being situated in the channels.

A high magnetic permeability of the flux guiding body, concentrates the magnetic field in the metal heat exchanger body. A ferrimagnetic material, such as a ferrite flux guiding body, couples a high magnetic permeability with a low conductivity such that little eddy currents are induced in the flux guiding body.

The tubular flux generating member may have a coolant inlet, the tubular flux generating member having a coolant inlet and a coolant outlet having a one-way valve in fluid connection with a water inlet of the heat exchanger unit.

By cooling the flux generating member, which may comprise for instance a copper tube, the resistance and structural integrity can be maintained within set operational limits that are optimal for the generation of the magnetic field and the induction of the heating currents in the heat exchanger body.

The outer surface and end surfaces may be covered by a ceramic heat insulating material (Al203). 5 The temperature of the heat exchanger body may rise to over 10009 C,

A heating system may be provided with an induction heater according to the disclosure, a tap water heat exchanger unit comprising a tap water heat exchanger unit being in heat exchanging contact with the outlet of the heat exchanger unit, a pump, and a control unit connected to the flux generating member.

The heat exchanger unit may In be in heat exchanging contact with a tap water supply. The heated fluid of the heat exchanger body may be circulated in a closed loop to provide room heating.

A thermostat may be included in the control unit of the heating system for comprising a thermostat for switching on and off the flux generating member, the heat exchanger unit being with an inflow duct and outflow duct connected to a heating circuit comprising one or more room heating elements such as radiators for convection heating.

Brief description of the drawings

Some embodiments of an induction heater according to the disclosure will, by way of non-limiting example, be explained in detail with reference to the accompanying drawings. In the drawings:

Fig. 1 shows a frontal view of an induction heater according to the disclosure,

Fig. 2 shows a bottom view of the induction heater of fig. 1,

Fig. 3 shows a frontal view of the heat exchanger unit of the induction heater of fig. 1,

Figs. 4a and 4b show a frontal view and a side view respectively of the heat exchanger body of fig. 3, having slit-shaped fluid channels,

Figs. 5a and 5b show a frontal view and a side view respectively of the end plates of fig. 3,

Figs. 6a and 6b show a frontal view and a side view respectively of a heat exchanger body, having cylindrical fluid channels,

Figs.7a and 7b show a frontal view and a side view respectively of the end plates for use with the heat exchanger body of figs 6a and 6b,

Fig. 8 shows a side view of an embodiment of three constituent parts of a heat exchanger body, having projecting teeth,

Fig. 9 shows the heat exchanger body of fig. 8 in the assembled state,

Figs. 10a-10c show a frontal view, a side view and a bottom view, respectively, of the heat exchanger body of fig. 9,

Figs. 11a-11c show a frontal view and a bottom and a top view of a flux generating member,

Figs. 12a-12¢ show a bottom view, a top view and a sideview respectively of an induction heater comprising a heat exchanger body according to figs. 10a-10c and a flux generating member according to figs. 11a-11c, and

Fig. 13 shows a heating system for room heating and tap water heating, comprising an induction heater according to the disclosure.

Detailed Description of the Invention

Fig. 1 shows a frontal view of an induction heater 1 comprising a heat exchanger unit 2 and tubular magnetic flux generating windings 3, 4 that extend along the outer surface of the unit 2. The induction heater 1 is particularly suited for the application in central heating systems. The heat exchanger unit 2 comprises a cuboid metal body 5, for instance formed of stainless steel, for instance SAE type 630 (17-4 PH). The body 5 is provided with a fluid channel 7 for transporting a heating agent, such as water, from an inlet 8 to an outlet 9. The fluid channel 7 comprises longitudinal sections 11, 12, 13, 14 extending in the length direction L parallel to side surfaces 10 and 15 of the body 5 and transverse sections 16, 17,

18, 19 extending in a transverse direction T. The transverse sections 16, 17, 18, 19 are formed in sealing plates 20, 21 that are sealingly engaged with end faces 22, 23 of the metal body 5, via bolts 27, 28.

The outer surface heat exchanger unit 2 is covered with a thermally insulating layer 29, for instance formed of aluminium oxide (Al:03) which has a melting point of about 2000°C and is encompassed hy an insulating layer 30 of epoxy.

A cooling duct 24 that may be formed of rubber, extends along side surface and is at one end connected to the tubular windings 3, 4. The duct 24 is at its other end connected to the cold-water supply 26, via a valve 25.

The longitudinal fluid channel sections 11-14 may be provided in the metal body 5 by drilling. Alternatively, the metal body 5 and the fluid channel sections may be formed by additive manufacturing methods, wherein the metal body 5 cluding 3D printing of the longitudinal channel sections 11-14 and the transverse channel sections 16-19.

The dimensions of the induction heater 1 of the present embodiment in the transverse direction T and in the Longitudinal direction L (including the length of the cold-water supply duct 26) amount to about 25 cm.

Figure 2 shows a side view of the induction heater 1 in the plane of the side surface 15. The longitudinal channel section 45 is slit-shaped shaped and extends between the upper and the lower transverse channel sections 43 and 44 in the end plates 20, 21. The windings 3, 4 on the front surface 35, and the windings 33, 34 on the rear surface 36 of the body 5 are formed of copper tubing with a coolant inlet 41 and a coolant outlet 42. The coolant inlet 41 is connected to the coolant supply duct 24 supplying coolant from the supply duct 26 via the valve 25. The coolant outlet 42 is connected to a coolant outlet duct 49 that is connected to a one-way valve 25’. Via a T-connection 46, the heated coolant flows back into the supply duet 26.

The windings 3, 4, 33, 34 are embedded in grooves 38, 39 in a flux concentrating member 40 which is of high resistivity and permeability, that may comprise a ferrimagnetic material such as Zinc-ferrite (Zn0.Fe203) or an amorphous metal,

and lie in close proximity to the front and rear surfaces 35, 36. At power terminals 47, 48, the copper tube is connectable to a power source, which may comprise a 48 V DC supply operating at a current of 20 A.

The dimension of the induction heater 1 of the present embodiment in the width direction W amount to about 12 cm.

Fig. 3 shows the heat exchanger unit 2, with the sealing plates 20, 21 being at their interface planes at end faces 22, 23 provided with silicone sealing gaskets that are resistant to temperatures of over 2509C. Copper or stainless-steel tubes 50, 51 are brazed or welded to recesses in the body 5. The walls of the longitudinal channel sections 11, 12, 13 and 14 are provided with projections 53, 54 that are placed at an angle to the side walls and that are facmg the fluid flow direction. For the flow direction that is indicated by the arrows at the inlet tubes 50 and the outlet tubes 51, the projections 53 in the channel section 12 are pointing in a downward direction, whereas the projections 54 mm channel section 11 are pointing in an upward direction. The projections 53, 54 cause a turbulent flow of the fluid flowing through the channel sections 11-14 and improve the heat transfer from the hot metal body 5 to the fluid flowing through the channels.

Figs. 4a and 4b show a planar view and a top view respectively, of the metal body 5 and show the slit-shape of the longitudinal channel sections 11-14. The slit- shape of the channel sections increases the heat exchanging surface. The teeth 53, 54 may be provided on a separate heat-conducting liner, such as a copper sheet, which is inserted into the channels 11-14.

Figs. 5a and 5b show a side view and a top view respectively of the end plate 20.

The transverse channel sections 16, 18 in the plate 20 interconnect each time two adjacent longitudinal channel sections 11, 12 and 13, 14. The lower sealing end plate 21 is of a similar construction.

In this embodiment, the cannel sections 16 and 18 have been provided by removal of metal from the sealing plate 20. It can be envisaged that a flat end plate 20 is used and that the channel sections 16, 18 are formed in the end face 22 of the metal body 5.

Fig 6a shows an embodiment wherein the longitudinal channel sections 11-14 are of cylindrical shape. Two parallel rows of channel sections 11, 11°, 12, 12’, 13, 13’and 14, 14’ are formed side by side in the metal body 5, as shown in fig. 6b.

Figs. 7a and 7b show a side view and a plan view respectively of the sealing end plate 20 and the transverse channel sections 16, 18. The transverse channel sections 16, 18 are dimensioned to cover each time two pairs of adjacent longitudinal channel sections 11, 117; 12, 12’ and 13, 13’; 14, 14’.

Fig. 8 Shows an embodiment of a heat exchanger unit 60 comprising a metal central part 61 and two metal side parts 62, 63. In the metal parts 61, 62, 63, a number of teeth 64, 65, 66, 67 is formed along the sidewalls of flow channels 69, 70. The parts 61, 62, 63 comprise planar peripheral sections 71, 72, 73, 74, 75 and 76 that are engaged in a fluid-tight manner through welding, brazing or a bolted connection. Silicone gaskets may be provided in grooves 77, 78, 81 and 82.

A connecting channel 84 and a degassing channel 85 are formed in the central metal part 61.

Fig. 9 shows the assembled central part 61 and side parts 63, 63 defining the upper heat exchanging flow channel 69 that is in fluid connection with outlet 91, and the lower heat exchanging flow channel 70 connected to an inlet 93. Fluid flows from the inlet 93, through the lower heat exchange channel 70 to the connecting channel 84. From there on the fluid flows through the upper heat exchange channel 69 to the outlet 91. The teeth 64, 67 of the upper and lower side parts 62, 63 mesh with the teeth 65, 66 of the central part 61, wherein the distance between the teeth 64, 65 and 66, 67 determines the flow resistance through the flow channels 69, 70 and is optimised to provide the required transfer of heat that is generated by induction in the metal parts 61, 62, 63 to the water flowing through the flow channels.

Figs.10a, 10b and 10c show a top view, a side view and an end view respectively of the heat exchanger unit 60, in which the parts 61 62, 63 are interconnected by bolts 86. The bolts 86 extend from the top surface 96 in the direction of the bottom surface 97, through the peripheral sections 71, 72. A degassing valve 87 1s connected to the channel 85 at the end face 95. Copper tubes 88, 89 are connected to the outlet 91 and to the inlet 93 of the heat exchange channels 69, 70, at the end face 94. Fig.10c shows a side view of the heat exchanger unit 60 in the flow direction with top surface 96, bottom surface 97 and side surfaces 98, 99.

Figs. 11a, 11b and 11c respectively show a side view, a frontal view and a rear view of a flux generating member 100 having a copper tube 101 forming windings 102, 103 and 104, 105. The tube 101 is connected with an inlet 106 to a cooling water supply, and with an outlet 107 to a cooling water outlet. The tube 101 is embedded in grooves 109, 110 of flux concentrating members 111, 112. In the flux concentrating members 111, 112, the flux density is concentrated so that the input power can be lowered, rapid and focused heating is achieved, and heat loss 1s prevented. At its ends, the tube 101 is provided with power terminals 115, 116 for attaching to an AC generator.

Figs. 124 and 12 b show the flux concentrating members 111, 112 and the windings 102, 104 of the flux generating member 100 being supported in close proximity to the top and bottom surfaces 96 and 97 of the heat exchanger unit 60. A housing 117 of thermally insulating material such as Al203, is placed around the heat exchanger unit 60. The heated-up fluid from the flux generator flows through outlet 107 via a one-way valve 118 of T-connection piece 119 into the inlet tube 89.

Fig. 13 shows an induction heater 120 according to the invention that is mcorporated in an induction central heating system. The system comprises a pump 121, a Zero Voltage Switching (ZVS) driver 122 for controlling the magnetic flux generated by the windings of the induction heater 120, a heat exchanger 123 for tap water heating, with cold water inlet 124 and warm water outlet 125 and control valves 128 for the heat exchanger 123. Cold water is supplied to the heater 120 through inlet 126. A hot water outlet 127 of the heater 120 is connected to a closed circuit with room heaters, such as radiators, which feed the cooled down water back to the inlet 126. The dimensions of the unit with the heater 120, the pump 121, the driver 122 and the heat exchanger 123 are about 40 by 40 cm.

In a heat exchanger according to fig. 1 -7h, two embodiments were tested at a temperature of the heat exchanger body of 120° C.

Embodiment 1

Volume of the heat exchanger body: 1080 cm?

Volume of the metal of the heat exchanger body: 90 %

Volume of the fluid channels: 10% pressure in heat exchanger body: about 10 bar

Flow rate: 5.6L/min

Embodiment 2

Volume of the heat exchanger body: 1080 cm?

Volume of the metal of the heat exchanger body: 82 %

Volume of the fluid channels: 18% pressure in heat exchanger body: about 1.5 bar

Flow rate: 5.3L/min

In a heat exchanger according to figs. 10a, 10b, a test was done at a temperature of the heat exchanger body of 120° C, with the following settings:

Volume of the heat exchanger body: 1080 cm?

Volume of metal of the heat exchanger body: 93 %

Volume of the fluid channels: 7% pressure in heat exchanger body: about 1.5 bar

Flow rate: 4.9L/min

Claims

Conclusions

1. An induction heater (1, 120) comprising - a heat exchanger (2, 60) having a fluid channel (7, 69, 70) passing through a heat exchanger body (5, 61, 62, 63) of predetermined volume and an outer surface (35, 36, 96, 97), the channel (7, 69, 70) passing from a inlet (8, 93) to an outlet (9, 91) in the heat exchanger body (5, 61, 62, 63), and - a flux generating means (3, 4, 33, 34, 102, 103, 104, 105) disposed on or near the outer surface (35, 36, 96, 97) for generating an alternating magnetic flux in the heat exchanger body (5, 61, 62, 63), at least 80 % of the volume of the heat exchanger body, preferably at least 90 %, being formed by a solid metal in which the fluid channel (7, 69, 70) is formed.

An induction heater (1, 120) as claimed in claim 1, wherein the outer surface (85, 36) extends in the longitudinal direction L, with opposite end faces (22, 23) of the body (5) extending transversely to the outer surface (35, 36), the channel (7) comprising at least three longitudinal portions (11, 12, 13, 14) extending longitudinally between the end faces (22, 23), first and second longitudinal portions (11, 12) being connected via a transverse channel portion (16) extending transversely to one of the end faces (22), and the second and third longitudinal portions (12, 13) being connected via a transverse channel portion (17) extending transversely to the other end face (28).

An induction heater (1, 120) as claimed in claim 2, wherein each end (22, 23) is covered by a sealing plate (20, 21) forming at least one side of a transverse channel portion (17, 18, 19).

An induction heater (1, 120) as claimed in claim 3, wherein a transverse channel portion (17, 18, 19) is formed in the sealing plate (20, 21), the end faces (22, 23) at the location of the transverse channel portion (17, 18, 19) being planar.

An induction heater (1, 120) as claimed in claim 3, wherein a transverse channel portion is formed in an end face (22, 23), the sealing plate (20, 21) having a flat surface along the transverse channel portion (17, 18, 19).

Induction heater (1, 120) according to claim 3, 4 or 5, wherein the sealing plates (20, 21) are interconnected via bolts (27, 28) which extend through the heat exchanger body (5) in the longitudinal direction L.

An induction heater (1, 120) as claimed in any preceding claim, wherein the fluid channel (7, 69, 70) includes irregularities in the surface (53, 54, 64, 65, 66, 67) to create turbulence in the fluid flowing through the channel.

An induction heater (1, 120) as claimed in claim 7, wherein the fluid channel (7) comprises a metal insert with projections extending perpendicular to the channel wall.

9. An induction heater (1, 120) according to any preceding claim, wherein the channel portions (11, 12, 13, 14) have a slit-shaped cross-section.

An induction heater (1, 120) as claimed in claim 1, wherein the heat exchanger (2) comprises a first lateral body part (62), a central body part (61) and a second lateral body part (63), the lateral body parts (62, 63) each forming a side wall of the first and second longitudinal channel parts (69, 70) and having side teeth (64, 67) extending transversely from the channel surface, the central body part (61) being between the lateral body parts (62, 63) and having first and second surfaces, with central teeth (65, 66) extending transversely from the first and second surfaces and engaging the side teeth (64, 67) of the opposing lateral body parts (62, 63), and forming side walls of the first and second channel parts (69, 70), the three body parts (61, 62, 63) have predominantly flat contact surfaces (71, 72, 73, 74, 75, 76) near their periphery and are connected by bolts (86), welding or soldering.

An induction heater (1, 120) as claimed in claim 10, wherein the side teeth (64, 67) project at an angle relative to a longitudinal centreline, the central teeth (65, 66) on the central body part (61) of the corresponding channel projecting at intermediate positions along the centreline towards the side teeth.

An induction heater (1, 120) according to claim 10 or 11, wherein the longitudinal channel parts (69, 70) have a width extending over at least one third of the dimension, preferably at least half, in the width direction W of the heat exchanger body.

An induction heater (1, 120) as claimed in any preceding claim, wherein the body (5, 61, 62, 63) is beam-shaped, having two opposing side surfaces (35, 36, 96, 97), the flux generating member (3, 4, 33, 34, 102, 103, 104, 105) being tubular and forming a loop along the opposing side surfaces.

An induction heater (1, 120) as claimed in claim 13, including a flux conducting body (40, 111, 112) having longitudinal channels (38, 39, 109, 110) in contact with the top and bottom surfaces (35, 36, 96, 97) of the heat exchanger body (5, 61, 62, 63), the tubular flux generating member (3, 4, 33, 34, 102, 103, 104, 105) being disposed within the channels (38, 39, 109, 110).

An induction heater (1, 120) as claimed in claim 14, wherein the flux conducting body (40, 111, 112) comprises a ferrimagnetic material.

An induction heater (1, 120) as claimed in claim 18, 14 or 15, wherein the tubular flux generating member (3, 4, 33, 34, 102, 103, 104, 105) has a coolant inlet (106) and a coolant outlet (107) with a one-way valve (118) in fluid communication with a water inlet (89) of the heat exchanger (2, 60).

An induction heater (1, 120) according to any preceding claim, wherein the outer surface (35, 36, 96, 97) is covered with a ceramic heat insulating material (A1208).

A heating system with an induction heater (1, 120) according to any one of claims 1 to 17, comprising a heat exchanger for tap water (123) in heat-exchanging contact with the outlet (127) of the heat exchanger, a pump (121) and a control unit (122) connected to the flux generating member (3, 4, 33, 34, 102, 103, 104, 105).

Heating system according to claim 18, wherein the control unit (122) comprises a thermostat for switching the flux generating element (3, 4, 33, 34, 102, 103, 104, 105) on and off, wherein the heat exchanger (2, 60) with an inflow channel and an outflow channel is connected to a heating circuit with one or more space heating elements such as radiators for convection heating.

20. A heat exchanger (2, 60) for use in an induction heater (1, 120), the heat exchanger comprising a fluid channel (7, 69, 70) extending through a heat exchanger body (5, 61, 62, 63) of predetermined volume, the channel (7, 69, 70) and an outer surface (35, 36, 96, 97), the channel (7, 69, 70) extending from an inlet (8, 93) to an outlet (9, 91) in the heat exchanger body, and a flux generating means (3, 4, 33, 34, 102, 103, 104, 105) disposed on or near the outer surface (35, 36, 96, 97) for generating an alternating magnetic flux in the heat exchanger body (5, 61, 62, 63), at least 80 % of the volume of the heat exchanger body, preferably at least 90 %, being formed by a solid metal in which the liquid channel (7, 69, 70) is formed.

A heat exchanger (2) according to claim 20, wherein the outer surface (35, 36) extends in the longitudinal direction L, opposite end faces (22, 28) of the body (5) project transversely of the outer surface (35, 36), the channel (7) comprises at least three longitudinal sections (11, 12, 13, 14) extending longitudinally between the end faces (22, 23), a first and a second longitudinal section (11, 12) being connected to each other via a transverse channel section (16) extending transversely at one of the end faces (22), and the second and third longitudinal sections (12, 13) being connected to each other via a transverse channel section (17) extending transversely at the other end face (23).

A heat exchanger (60) as claimed in claim 20 comprising a first lateral body part (62), a central body part (61) and a second lateral body part (63), the lateral body parts (62, 63) each forming a side wall of the first and second longitudinal channel portions (69, 70) and having side teeth (64, 67) extending perpendicular to the channel surface, the central body part (61) being located between the lateral body parts (62, 63) and having a first surface and a second surface, with central teeth (65, 66) extending perpendicular to the first and second surfaces and engaging the side teeth (64, 67) of the opposing lateral body parts (62, 63) and forming side walls of the first and second channel portions (69, 70), the three body parts (61, 62, 63) having substantially planar surfaces near their periphery have interfaces (71, 72, 73, 74, 75, 76) and are connected by bolts (86), welding or soldering.

23. A heat exchanger (2, 60) unit according to claim 20, 21 or 22 for use in an induction heater (1, 120) according to any one of claims 1 to 17.