FLOW-THROUGH HEATERS

Abstract

 An electrically powered flow-through heater comprises a thick film heating element, a housing to which the heating element is sealingly fixed to create a chamber adjacent to the heated surface of the heating element, at least one liquid inlet and at least one liquid outlet for liquid flow within the chamber and means for guiding or disrupting the flow of liquid through the chamber. There may be provided two thick film heating elements on either side of the chamber. The means for guiding the liquid flow within the chamber may comprises a channel plate or corrugated channel guide defining parallel fluid channels on both sides thereof.

Description

Field of the Invention

[0001] The present invention relates to electrically powered flow-through heaters for heating a flow of liquid.

Background of the Invention

[0002] Electrically powered flow-through heaters are used in diverse applications such as instantaneous water heaters for showers, vending machines and coffee makers. There are a number of different constructions used in the manufacture of these products. Typically, heaters used in electric showers use conventional tubular sheathed elements housed in a small tank which is made from plastic or from copper or brass or a combination of these materials. The elements are often formed into helical shapes to save space. These heaters tend to have relatively high flow rates and low pressure drops. A typical shower uses mains water at a minimum water pressure of 1 bar and has flow rates in the region of 8 litres per minute. Heaters for coffee makers on the other hand have relatively low flow rates, typically less than 0.5 litres per minute. The water used in coffee makers is forced through the heater by a pump, so pressures in excess of 1 bar are available.

[0003] A flow-through heater suitable for coffee makers, including a thick film heating element, is disclosed in GB-A-2481265. This uses a planar thick film heating element to which a pressed metal channel plate is brazed. Good heat transfer between the heating tracks on the thick film heating element and the liquid is ensured by designing the channel plate so that the flow of liquid closely follows the path of the heating tracks on the heating element. Whilst very suitable for applications such as coffee makers, this approach is less suited to a flow-through heater with a high flow rate and low pressure drop because the track widths are in the region of 3 mm or less and lengths of 450 mm.

[0004] One potential application of flow-through heaters is to heat the rechargeable battery packs in electric vehicles and the like. The output of the lithium ion batteries used in these applications is reduced at low temperatures and it is possible to damage the batteries if they are charged at low temperatures. These flow-through heaters required higher flow rates and lower pressure drops than the type of flow-through heater described in GB-A-2481265. The channel plate disclosed in GB-A-2481265 reduces the flow rate for a given pressure drop or increases the pressure drop for a given flow rate.

[0005] An automotive application may provide particular stringent requirements in terms of ambient temperature and humidity, shock, vibration and/or reliability. In particular, an automotive application may require a small, lightweight and robust heater. In order to design a heater housed in a small space envelope whilst ensuring that the operating temperature of the heating element is low enough to provide long service life and reliability, good heat transfer between the heater and the liquid is required. It is also desirable to limit the temperature of the components which contact the liquid to avoid deterioration of the liquid, which in a battery pack heater could be water with additives such as ethylene glycol.

Statements of the Invention

[0006] According to one aspect of the present invention, there is provided an electrically powered flow-through heater comprising a thick film heating element, a housing to which the heating element is sealingly fixed to create a space or chamber adjacent to the heated surface of the heating element, at least one liquid inlet and at least one liquid outlet for liquid flow within the chamber, and means for guiding or disrupting the flow of liquid through the chamber.

[0007] In embodiments of the invention, the liquid flows through a chamber across substantially the whole heating surface of the heating element, thus increasing the flow rate. Disrupting the flow of liquid through the chamber tends to produce turbulent flow, which increases the thermal transfer from the heated surface to the liquid. The combination of the increased surface area and the turbulent flow significantly increases the heat transfer rate.

[0008] The means for disrupting the liquid flow may comprise one or more components, such as fins and/or baffles, placed in the path of the liquid. When thermally connected to the heating element, the components increase the surface area for heat exchange. The components may additionally or alternatively increase the rigidity of the flow-through heater.

[0009] It has been found that the flux supplied for brazing aluminium to aluminium in heat exchangers can be used to braze aluminium components, such as fins, to the steel substrates of thick film heating elements to produce flow-through heaters. However, the process is not ideal. It tends to produce brittle joints because of the formation of intermetallic compounds. The brazing process requires specialized equipment. A brazing furnace with a well-controlled nitrogen atmosphere is required, in which the atmosphere should have a dew point no higher than -40°C.

[0010] The manufacturing process for thick film heating elements starts by applying one or more layers of insulating glass or similar material to a stainless-steel substrate by screen printing or spraying, then drying and firing the layers. The firing temperature is higher than the temperatures encountered during the aluminium brazing process. Therefore, the brazing process must be performed after the manufacture of the thick film heating element. Nevertheless, the brazing process can be detrimental to the thick film heating element.

[0011] The following embodiments of the invention address at least some of the problems described above.

[0012] In a first variant of a first embodiment, a component for disrupting the liquid flow is attached to the heating element with thermally conductive adhesive. The adhesive may be an epoxy resin, a silicone compound or any other suitable material. The thermal conductivity of a typical silicone adhesive is 3.5W/mK. This may be regarded as low compared to the conductivity of aluminium brazing alloy, which can be as high as 380W/mK. However, when the thickness of the adhesive joint is taken into account it will be appreciated that the temperature drop across the adhesive joint is low, possibly less than 1 degree K. Moreover, the component still disrupts the flow of liquid through the chamber.

[0013] In a second variant of the first embodiment, a component for disrupting the flow is attached to the heating surface of the heating element and is made of a similar material to the heating surface of the heating element, such as stainless steel. The attachment may be made by brazing. This is advantageous in that a braze between similar materials avoids the formation of intermetallic compounds. The use of stainless steel is particularly advantageous because the two stainless steel components can be brazed at a high temperature, such as using a nickel-based alloy at a temperature in the region of 1000°C. This is higher than the highest firing temperature for the thick film heating element, so that the brazing process can be performed before the thick film element is applied to the stainless-steel substrate or element plate.

[0014] In a second embodiment, a component for disrupting the flow of liquid is not attached to the heating element but is located in the chamber between the heating element and the housing. In this embodiment, the component may be an aluminium or other metal component. Alternatively, the housing itself may be designed to disrupt the flow of liquid to induce turbulent flow, and there may be no separate component for disrupting the flow of liquid.

[0015] According to another aspect of the invention, there is provided an electrically powered flow-through heater in which a thick film heating element comprising one or more heater tracks is provided on both sides of a space or chamber through which liquid flows, thus increasing the power density of the heating and reducing the risk of thermal distortion.

[0016] The chamber may be divided into parallel liquid channels by a channel plate. This arrangement tends to encourage laminar rather than turbulent flow, and reduces the pressure drop through the heater. Liquid may flow on both sides of the channel plate, adjacent the respective thick film heating element. Instead of a channel plate, a corrugated sheet or other flow directing means may be provided within the chamber. There may be a channel guide positioned between the heaters, so that liquid flows along a plurality of parallel channels and is evenly heated. There may be a manifold arranged between an inlet or outlet and the parallel channels, so as to equalise pressure across the channels. There may be an area of restricted cross-section between the manifold and the channels, and the restricted cross-sectional area may vary between channels, so as to improve the equalisation of pressure and provide a substantially equal flow through each of the channels.

[0017] In some cases, it may not be possible to achieve a substantially equal flow through each of the channels. A heater track located adjacent an area of low flow could result in that heater track operating at an excessively high temperature. To overcome this problem, the power dissipated by the heater track(s) can be varied along the length of each heater track and/or between different heater tracks, where present. In one embodiment the width of a heater track varies along its length. A portion of the track with increased width has a lower resistance resulting in a lower voltage drop along that portion of the track and reducing the power dissipated by that portion of track. Additionally, the wider track results in a lower power density, expressed in W/cm2.

[0018] An alternative arrangement has multiple heater tracks, preferably arranged in parallel. Each track is associated with (e.g. arranged along/adjacent to) one of the channels formed by the channel guide. The heater track width may vary between the heater tracks according to variations in flow rate between the different channels. For example, for channels with a lower flow rate the resistance of the associated track can be increased by reducing the width of that track. In this case, the voltage drop along the track as a whole stays the same, so that reducing the width reduces the power dissipated by that track, thus reducing the temperature of the track.

[0019] The housing may be formed from a pair of plates that are fixed together in a watertight manner, for example by welding or using a gasket seal. Each plate may have a respective set of thick film tracks formed thereon. Preferably, a dielectric layer formed on one or both plates does not extend to the edge of the respective plate, so that welding at the edge of the plate does not disrupt the dielectric layer.

[0020] One or both plates may be shaped, for example by press-forming, so that the chamber is formed between the plates. In a preferred embodiment, both plates are shaped symmetrically so that the housing can be formed from a pair of substantially identical plates, thus reducing the number of different components that need to be manufactured. Fluid inlets and outlets may then be formed in the plates as required.

Brief Description of the Drawings

[0021] Specific embodiments of the present invention will now be described with reference to the accompanying drawings, as itemised below.

Figure 1 shows an exploded view of a flow-through heater, in a first or second embodiment of the invention.

Figure 2 shows a section view of the flow-through heater of Figure 1, with arrows showing the direction of liquid flow.

Figure 3 shows a thick film heating element with an array of baffles attached thereto, in the first embodiment.

Figure 4 shows a schematic cross-section through the plane A-A of Figure 3, including a housing.

Figures 4a is a close-up view of a part of Figure 4 shown in dotted outline, in a first variant of the first embodiment.

Figures 4b is a close-up view of a part of Figure 4 shown in dotted outline, in a second variant of the first embodiment.

Figure 5 is a flow chart of a method of construction of the second variant of the first embodiment.

Figure 6 shows a housing of the flow-through heater, in the second embodiment.

Figure 7 shows a schematic cross-section through the plane B-B of Figure 6, including a housing.

Figures 8a to 8c show respectively perspective, exploded and cross-sectional views of a flow-through heater in a third embodiment.

Figures 9a to 9c show respectively perspective, cross-sectional and exploded cross-sectional views of a flow-through heater in a fourth embodiment.

Figures 10a to 10c show respectively perspective, exploded and cross-sectional views of a flow-through heater in a fifth embodiment.

Figures 11a and 11b show respectively cross-sectional perspective views of a flow-through heater in a sixth embodiment.

Detailed Description of the Embodiments

[0022] In the description, the words 'upper' and 'lower' and related terms refer to the orientation shown in the respective drawing(s), and do not necessarily correspond to the orientation of the apparatus in use. Except where otherwise indicated, the drawings are schematic and not to scale. The thickness of layers may be exaggerated so that they can be seen clearly.

[0023] Functionally similar, though not necessarily identical parts may carry the same reference numeral between different embodiments. However, differences between the function of parts carrying the same reference numerals will be apparent from the description below.

First Embodiment

[0024] A flow-through heater according to an embodiment of the invention is shown in Figures 1 and 2, in which an element plate 5 is attached to an enclosure or housing 1 so as to form an enclosed chamber 6 between the inner wall of the housing 1 and a heating surface of the element plate 5. A seal 4 is mounted around the perimeter of the chamber 6, for example in a groove or rim of the housing 1 and is pressed against the surface of the element plate 5 to provide a seal around the chamber 6. The housing 1 has a peripheral flange 8 for mounting to the periphery of the surface of the element plate 5. The peripheral flange 8 may be attached by screws or bolts passing through holes in the element plate 5, or by another suitable fixing method such as spot welding or folding tabs that extend from the housing 1 around the edges of the element plate 5.

[0025] In this embodiment, the housing 1 is generally cuboid in shape with an upper surface providing an upper wall of the chamber 6, and sides providing side walls of the chamber 6. The lower face of the housing 1 is open, and the lower wall of the chamber 6 is provided by the element plate 5, which is planar and rectangular. However, alternative shapes may be used for the housing 1 and/or the element plate 5.

[0026] Liquid may flow into a liquid inlet 3, through the chamber 6, and out of a liquid outlet 2 as shown by the arrows in Figure 2. The direction of flow is preferably reversible i.e. so that the liquid inlet may act as an outlet and vice versa. In this embodiment, the liquid inlet 3 and outlet 2 are formed as short tubes at opposite ends of the upper wall of the housing 1. Alternatively, the inlet 3 and outlet 2 may be provided at the same end of the housing 1. In that case, an internal wall or baffle may be provided within the chamber 6, for example formed on the upper wall of the housing 1, so as to direct the flow of liquid and prevent the liquid from passing directly from the inlet 3 to the outlet 2.

[0027] Alternatively or additionally, the liquid inlet 3 and/or the outlet 2 may be provided in the element plate 5, for example by forming an aperture in the element plate 5 and attaching a tube to the lower surface of the element plate 5, around the aperture.

[0028] As shown in Figure 1, an electrical heater is provided on the underside of the element plate 5. The heater may comprise one or more thick film heating tracks 7 that are printed on the underside of the element plate 5 and fired during manufacture, to form a thick film heating element. The element plate 5 is preferably formed of stainless steel, with an insulating layer 10 provided between the element plate 5 and the thick film heating tracks 7. Alternatively, the element plate 5 may be of ceramic material, in which case no insulating layer 10 is required.

[0029] Heat is conducted from the thick film heating tracks 7 through the element plate 5 to the upper side of the element plate 5 which is in contact with the liquid to be heated. Hence, this upper side may be referred to as the heating surface of the element plate 5. In alternative terminology, the side that carries the heating tracks 7 is referred to as the dry side, and the side that contacts the liquid is referred to as the wet side.

[0030] As an alternative to the thick film heating track(s), the heater may comprise a sheathed heater, or the element plate 5 may be heated in some other way without a heater being attached, such as by induction.

[0031] The flow of liquid through the chamber 6 may be laminar. However, the heat transfer to the liquid may be greatly improved if the flow is turbulent. Hence, in at least some embodiments the flow of liquid through the chamber 6 is disrupted or diverted with a baffle or series of baffles or fins 9 to produce a flow over substantially the whole of the heated surface of the element plate 5. The fins 9 may comprise an array of hollow fins as shown in Figures 3 and 4, provided as a unitary array with contact surfaces for attachment to the element plate 5. The array may be formed of sheet material. The array may comprise rows of fins or baffles 9, each row extending in a perpendicular direction to the direction of flow, with successive rows arranged in the direction of flow and mutually offset in the perpendicular direction.

[0032] Unlike the channel plate of GB-A-2481265, the baffles 9 do not constrain the flow of liquid to a single flow path between the inlet and the outlet, nor even to multiple flow paths that converge only at the inlet and outlet. Instead, the liquid may flow through and around the baffles 9, thus allowing a high flow rate and low pressure drop. Effectively, the baffles 9 allow a large number of possible flow paths, which intersect at a plurality of intermediate points between the inlet 3 and outlet 2. As shown in Figure 4, the fins 9 may not extend completely to the inner surface of the housing 1, so that some liquid flows between the fins 9 and the inner surface of the housing 1.

[0033] In a first variant of the first embodiment as shown in Figure 4a, the fins or baffles 9 are attached to the upper surface of the element plate with a layer of thermally conductive adhesive 12, for example with a thermal conductivity in the range 0.5 - 5 W/mK. The adhesive may be an epoxy resin, a silicone compound or any other suitable material. The thermal conductivity of the silicone adhesive may be 3.5W/mK, for example. The layer of adhesive 12 is comparatively thin, so that heat is conducted from the element plate 5 through the adhesive 12 to the fins or baffles 9, and thence to the liquid flowing through the chamber 6. In this variant, the fins or baffles 9 may be made of aluminium, or other metal or thermally conductive material.

[0034] In an alternative, the fins or baffles 9 may be attached to the element plate 5 by a non-thermally conductive or thermally insulating attachment, solely for location purposes.

[0035] In a second variant of the first embodiment as shown in Figure 4b, the fins or baffles 9 are made of similar material to the surface of the element plate 5 to which they are attached, for example stainless steel. The attachment may be made by brazing, to form a braze 13. This is advantageous in that a braze between similar materials avoids the formation of intermetallic compounds.

[0036] The use of stainless steel is particularly advantageous because the two stainless steel components can be brazed at a high temperature, such as using a nickel-based alloy at a temperature in the region of 1000°C. This is higher than the highest firing temperature for the thick film heating tracks 7, so that the brazing process can be performed before the thick film element is applied to the element plate 5. This is shown in Figure 5, in which a method of manufacture of the flow-through heater comprises the following steps:

S1 -

braze the fins 9 to the upper surface (i.e. facing the chamber 6) of the element plate 5, to form braze 13.

S2 -

Print the thick film tracks 7 on the lower surface (i.e. outside the chamber 6) of the element plate 5.

S3 -

Fire the thick film tracks 7 to form conductive tracks for the heating element.

S4 -

Attach the housing 1 to the element plate 5, to form chamber 6.

Second Embodiment

[0037] In a second embodiment, as shown for example in Figures 6 and 7, the fins or baffles 9 are not attached to the element plate 5, but may be attached to the inner surface of the housing 1, may be integrally formed on the inner surface of the housing 1, or may be formed as an insert that is located within the chamber 6 and is held in place by abutment with the walls of the chamber 6 (i.e. the walls of the housing 1 and the heating surface of the element plate 5).

[0038] In the second embodiment, the fins or baffles 9 may be made of aluminium or other metal. The fins or baffles 9 may be spaced apart from the element plate 5, as shown in Figure 7, or may contact the element plate 5 at their lower ends. In the latter case, heat may be conducted from the element plate 5 into the fins or baffles 9.

Third Embodiment

[0039] In the third embodiment as shown in Figures 8a to 8c first and second element plates 5a, 5b, carrying respective first and second thick film tracks 7a, 7b deposited on corresponding insulating layers 10a, 10b, are provided on both sides (e.g. upper and lower sides) of the heater so as to increase the heating density of the flow-through heater. Moreover, because the area of the heater may be reduced, thermal distortion of the heater may also be reduced and the element plates 5a, 5b may be thin and therefore light.

[0040] A channel plate 15 is sealingly attached to the upper side of the first element plate 5a, for example by brazing. The heating surface of the second element plate 5b is attached to the upper side of the channel plate. An inlet 3 and an outlet 2 are provided in the channel plate.

[0041] The channel plate 15, when attached to the first element plate 5a, comprises an inlet manifold 16 connected to the inlet 3 and an outlet manifold 17 connected to the outlet 2. The channel plate 15 has a corrugated surface 18 defining a plurality of parallel channels extending between the inlet and outlet manifolds 16, 17, which open onto both sides of the corrugated surface 18 so that liquid passes on both sides thereof, in thermal contact with the first and second element plates 5a, 5b respectively.

[0042] The element plates 5a, 5b may be attached to the channel plate 15 by brazing, in which case the thick film heating tracks 7a, 7b are deposited and fired onto the respective dry sides of the element plates 5a, 5b after brazing, since the brazing temperature is higher than the firing temperature of the thick film tracks 7a, 7b.

[0043] The inlet 3 and outlet 2 may be attached to the channel plate 15 by forming respective apertures in the channel plate 15 and welding tubes thereto.

Fourth Embodiment

[0044] In the fourth embodiment as shown in Figures 9a to 9c an element plate 5, having a first set of heating tracks 7a on an insulating layer 10a on the underside thereof, is attached to a housing 1 having a second set of heating tracks 7b deposited on an insulating layer 10b on the upper side of the housing 1. This embodiment has similar advantages to the third embodiment by being heated from both sides, but may be more compact.

[0045] The housing 1 is concave so as to form a chamber 6 against the heating surface of the element plate 5, and has a substantially planar central area for deposition of the insulating layer 10b and heating tracks 7b. The housing 1 has an inlet depression 19 downstream of the inlet 3 and an outlet depression 20 upstream of the outlet 2. The housing 1 may be formed of sheet steel, such as ferritic steel suitable for deposition of the thick film heating tracks 7b, and is formed into the required shape by a process such as press forming.

[0046] The housing 1 may be sealingly attached, for example at a peripheral flange 8 thereof, to a peripheral area of the element plate 5 by welding, for example laser, TIG, micro TIG or electron beam welding. The insulating layer 10a on the underside of the element plate 5 preferably does not extend to this peripheral area, so that the insulating layer 10a is not damaged by the welding process. The insulating layers 10a, 10b and heating tracks 7a, 7b may be deposited respectively on the element plate 5 and the housing 1 before the two are welded together, since the welding process does not damage the insulating layers 10a, 10b and heating tracks 7a, 7b. Preferably, the inlet 3 and outlet 2 are attached to the housing after deposition of the insulating layer 10b and tracks 7b, so as not to interfere with the deposition process.

[0047] The portion of the chamber 6 between the inlet and outlet depressions 19, 20 contains a corrugated channel guide 21 defining a plurality of parallel channels. As shown in Figure 9c, the channel guide 21 may be inserted in the portion of the chamber 6 between the depressions 19, 20 prior to fixing the housing 1 to the element plate 5. The channel guide 21 is then held in that portion of the chamber by abutment between the housing 1 and the element plate 5 and need not be attached to either. Hence, the material of the channel guide is not constrained by the need to attach it to any other part. Longitudinal movement of the channel guide 21 may be constrained by the depressions 19, 20.

[0048] The portion of the chamber 6 between the inlet 3 and the inlet depression 19 forms an inlet manifold 16, the portion of the chamber 6 between the outlet depression 20 and the outlet 2 forms an outlet manifold 17, and the portion between the inlet and outlet depressions 19, 20 forms a plurality of parallel channels on both sides of the corrugated channel guide 21. The depressions 19, 20 restrict the cross-sectional area between the respective manifolds 16, 17 and the parallel channels, which helps to equalise the flow-through the parallel channels across the width of the heater.

[0049] Instead of channel guide 21, a flow disrupting component 9 comprising fins or baffles, as in the first or second embodiments, may be used.

Fifth Embodiment

[0050] The fifth embodiment as shown in Figures 10a to 10c is similar to the fourth embodiment, but comprises first and second concave housings 1a, 1b each similar to the housing 1 of the fourth embodiment and fixed together so as to form a chamber 6 therebetween. In other words, the planar element plate 5 of the fourth embodiment is replaced by another, similar housing. The first and second housings 1a, 1b are fixed together at their peripheries, with the corrugated channel guide 21 sandwiched therebetween.

[0051] Preferably, the peripheries (e.g. peripheral flanges 8a, 8b) of the first and second housings 1a, 1b are fixed together by welding. The respective insulating layers 10a, 10b do not extend to the peripheries of the first and second housings 1a, 1b, and are therefore not damaged by welding.

[0052] The first and second housings 1a, 1b may be formed in a similar way and with a mutually similar shape. This reduces the number of different components required, as there is no need to manufacture planar element plate 5.

[0053] The inlet 3 and outlet 2 may be formed in either or both of the housings 1a, 1b, depending on the configuration required.

[0054] Due to the reduced thermal distortion, the housings 1a, 1b may be manufactured from thinner material, for example 0.5 mm instead of 1 mm thickness, thus reducing the weight and cost of the heater. For an automotive application such as an electric vehicle, the reduced weight is a significant advantage.

[0055] If additional rigidity is required, the housings 1a, 1b may be formed with one or more corresponding depressions in their central regions, which are welded together. The corrugated channel guide 21 may have apertures through which these depressions pass.

[0056] Instead of welding, other means may be used for fastening the housings 1a, 1b together. For example, the peripheral flanges 8a, 8b may be clamped or bolted together with a gasket seal.

Sixth Embodiment

[0057] A sixth embodiment is a variant of the fifth embodiment in which the inlet and/or outlet depressions 19 increase in depth from the edges to the middle of the housings 1a, 1b. Figures 11a and 11b shown cross-sections through one of the depressions 19, respectively in a direction facing towards the inlet or outlet 2, 3 and towards the channel guide 21. The effect of this increase in depth is to equalise the flow rate as between the different channels, by constricting the flow more through the middle channels, which are closer to the inlet or outlet 2,3. In a variant where the inlet or outlet 2, 3 is closer to one side of the housings 1a, 1b, then the depression 19 would be deeper to that side. The depression 19 is a convenient means for restricting the flow, which can be formed as part of the housings 1a, 1b. Alternative means, such as a separate part inserted between the housings 1a, 1b, may be used.

[0058] In embodiments where the heating tracks 7a, 7b provide substantially even heating across the heated area of the housings 1a, 1b, then equalising the flow rate between the different channels helps to equalise the temperature of the heating tracks and prevent overheating.

[0059] In some embodiments, it may not be possible to equalize the flow rates between the different channels. Alternatively or additionally, the temperature of the heating tracks may be equalised by varying the power output of different portions of the heating tracks 7a, 7b, for example so that the heating tracks have a higher power output in areas of higher flow rate. One way to do this is to vary the width of the tracks 7a and 7b along their length. The portions of the tracks 7a, 7b with increased width have a lower resistance, resulting in a lower voltage drop along that portion of the track, reducing the power dissipated by that portion of the track 7a, 7b. Additionally, the wider track portion results in a lower power density, expressed in W/cm2.

[0060] An alternative arrangement of heater tracks, not shown in the figures, has multiple tracks arranged in parallel. Each track is associated with one channels formed by the corrugated channel guide 21. In channels with less flow, the resistance of the associated track can be increased by reducing the width of that track. This reduces the power dissipated by that track, thus reducing the temperature.

Combination of Features from Different Embodiments

[0061] Features may be combined between different embodiments. For example, the housing 1 in the first and second embodiments may have one or more a heater track(s) 7 deposited on the dry side thereof, so that the chamber 6 is heated on both sides. In another example, there may be provided an area of restricted cross-section between the inlet 3 and/or outlet 2 and the chamber 6 in the first and second embodiments, so as to even out the flow rate across the chamber 6. In the third embodiment, there may be provided an area of restricted cross-section between the inlet and/or outlet manifolds 16, 17 and the parallel channels defined by the corrugated surface 18.

Additional Features

[0062] In any of the above embodiments, the apparatus may include an outer housing, connectors for electrical connection to the heater track(s) 7, and/or fluid ports for connection to the inlet 3 and outlet 2. The apparatus may be used as a coolant heater for a rechargeable battery, for example for automotive applications.

Alternative Embodiments

[0063] The embodiment described above is illustrative of, rather than limiting to, the present invention. Alternative embodiments apparent on reading the above description may nevertheless fall within the scope of the invention.

Alternative Statements of invention

[0064] Alternative statements of invention are recited below as numbered clauses.

1. An electrically powered liquid flow-through heater comprising at least first and second thick film heating elements each comprising a substrate having one or more thick film heating tracks on one major face thereof such that an opposite major face thereof comprises a heating surface, the heater being arranged so that liquid flows through a chamber between the heating surfaces of the first and second thick film heaters, wherein one or more flow directing components are arranged within the chamber.

2. The heater of clause 1, wherein the one or more flow directing components define a plurality of parallel channels for fluid flow within the chamber.

3. The heater of clause 2, including a manifold connected to the parallel channels.

4. The heater of clause 3, including an area of restricted cross-section between the manifold and the parallel channels.

5. The heater of clause 4, wherein the restricted cross-section varies between the channels so as to equalise the flow rates of the channels.

6. The heater of any preceding clause, wherein one or more of the heating tracks is of varying width along its length.

7. The heater of any preceding clause, wherein there are a plurality of said thick film heating tracks, the width of which varies between the different heating tracks.

8. The heater of any one of clauses 2 to 5, wherein the one or more components comprise a channel plate.

9. The heater of any one of clauses 2 to 5, wherein the one or more components comprise a corrugated component.

10. The heater of clause 8 or clause 8, wherein the parallel channels are arranged on both sides of the channel plate or corrugated component.

11. The heater of any preceding clause, wherein at least one of the substrates is concave so as to form said chamber.

12. The heater of clause 11, wherein both of the substrates are concave.

13. The heater of clause 12, wherein the substrates are substantially identical in shape.

14. The heater of any one of clauses 12 to 13, wherein at least one of the substrates has a central planar portion on which the thick film heating tracks are formed.

15. The heater of any preceding clause, wherein the substrates are sealingly fixed together around their peripheries.

16. The heater of clause 15, wherein the substrates are sealingly fixed together by welding.

17. The heater of clause 15, wherein a gasket seal is provided between the substrates.

18. The heater of any one of clauses 1 to 13, wherein the substrates are spaced apart so as to form the chamber therebetween.

19. An electrically powered liquid flow-through heater comprising a heating element having a heating surface, a housing defining a chamber through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the chamber, the one or more components being fixed to the heating surface with a thermally conductive adhesive.

20. The heater of clause 19, wherein the adhesive comprises a resin.

21. The heater of clause 20, wherein the adhesive comprises an epoxy resin or a silicone resin.

22. The heater of any preceding clause, wherein the adhesive has a thermal conductivity in the range 0.5 - 5 W/mK.

23. An electrically powered liquid flow-through heater comprising a heating element having a heating surface, a housing defining a chamber through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the chamber, the one or more components being thermally insulated from the heating surface.

24. The heater of clause 23, wherein the one or more components form part of the housing.

25. The heater of clause 23, wherein the one or more components are attached to the housing.

26. The heater of any one of clauses 23 to 25, wherein the one or more components are spaced apart from the heating surface.

27. The heater of clause 23, wherein the one or more components are attached to but thermally isolated from the heating surface.

28. The heater of any one of clauses 19 to 27, wherein the or each heating element comprises a thick film heating element.

29. An electrically powered liquid flow-through heater comprising a thick film heating element having a heating surface, a housing defining a chamber through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the chamber, the one or more components being made of similar material to the heating surface and being attached to the heating surface by a braze.

30. The heater of clause 1 or any one of clauses 19 to 29, wherein the one or more components comprise one or more fins or baffles.

31. The heater of any preceding clause, arranged as a coolant heater for a rechargeable battery.

32. A method of manufacturing the electrically-powered liquid flow-through heater of any one of clauses 1 to 31, the method comprising forming the heating tracks on the respective substrates before fixing the substrates together.

33. A method of manufacturing an electrically-powered liquid flow-through heater comprising a thick film heating element having a heating surface, a housing defining a chamber through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the chamber, the thick film heating element comprising an element plate and one or more thick film heating tracks deposited on a side of the element plate opposite the heating surface, the method comprising:

a. brazing the one or more components to the heating surface of the element plate; and, subsequently:

b. depositing and firing the one or more thick film heating tracks.

34. The method of clause 33, wherein at least a surface of the one or more components for brazing to the heating surface is of a similar material to the heating surface.

35. The method of clause 33, or the heater of clause 29, wherein the material is stainless steel.

36. The method of any one of clauses 32 to 35, wherein an insulating layer is applied to the element plate after the step of brazing the one or more components to the heating surface of the element plate and before printing and firing the one or more thick film heating tracks.

Claims

1. An electrically powered liquid flow-through heater comprising at least one liquid inlet (3), at least one liquid outlet (2), and at least first and second thick film heating elements each comprising a substrate (1; 1a, 1b; 5) having one or more thick film heating tracks (7a, 7b) on one major face thereof such that an opposite major face thereof comprises a heating surface, the heater being arranged so that liquid flows between the at least one inlet (3) and the at least one outlet (2) through one or more flow directing components (9; 15; 21) between the heating surfaces of the first and second thick film heaters.

2. The heater of claim 1, wherein the one or more flow directing components (18; 21) define a plurality of parallel channels.

3. The heater of claim 2, including a manifold (16, 17) arranged between the at least one liquid inlet (3) or outlet (2) and the parallel channels.

4. The heater of any preceding claim, including an area of restricted cross-section (19) between the at least one inlet (3) and/or outlet (2) and the flow directing components (9; 15; 21).

5. The heater of claim 4, wherein the area of restricted cross-section (19) varies so as to equalise the rate of flow of the liquid through the flow directing components (9; 15; 21).

6. The heater of any preceding claim, wherein one or more of the heating tracks (7a, 7b) is/are of varying width along its/their length.

7. The heater of any preceding claim, wherein there are a plurality of said thick film heating tracks (7a, 7b), the width of which varies between the different heating tracks.

8. The heater of any one of claims 2 to 7, wherein the one or more flow directing components comprise a channel plate (15) or a corrugated component (21).

9. The heater of claim 8, wherein the parallel channels are arranged on both sides of the channel plate (15) or corrugated component (21).

10. The heater of any one of claims 1 or 4 to 7, wherein the one or more flow directing components comprise one or more fins or baffles (9).

11. The heater of any preceding claim, wherein the one or more flow directing components (9; 21) are arranged within a chamber (6) between the substrates (1; 1a, 1b; 5).

12. The heater of claim 11, wherein at least one of the substrates (1; 1a, 1b; 5) is concave so as to form said chamber (6), wherein optionally both of the substrates (1a, 1b) are concave and are preferably substantially identical in shape.

13. The heater of any preceding claim, wherein at least one of the substrates (1; 1a, 1b; 5) has a central planar portion on which the thick film heating tracks (7a, 7b) are formed.

14. The heater of any preceding claim, wherein the substrates (1; 1a, 1b; 5) are sealingly fixed together around their peripheries, for example by welding or by a gasket seal provided between the substrates (1; 1a, 1b; 5).

15. A method of manufacturing the electrically-powered liquid flow-through heater of any one of claims 1 to 14, the method comprising forming the heating tracks on the respective substrates before fixing the substrates together.

Drawing