GB2484321A

GB2484321A - A thick film heater/ heat dissipater assembly associate with a flow heater flow channel.

Info

Publication number: GB2484321A
Application number: GB1016839.1A
Authority: GB
Inventors: Gradus Johannes Kloppers; David Andrew Smith; Johannes Gerardus Maria Gelinck
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2010-10-06
Filing date: 2010-10-06
Publication date: 2012-04-11
Also published as: GB201016839D0

Abstract

This invention includes improvements relating to flow through heaters for liquid heating appliances, an electrical heating element comprising a combination of a thick film resistor 6 mounted on a metallic substrate 4 via an insulating layer 5, the element being mounted by a braze/solder layer 7 to a heat dissipater 2. The dissipater may have channels 12 in the form of tubes 12 cast within the dissipater, which channels may also be arranged in layers or in an inclined row (figs 1b,c) and the element may be present on opposing surfaces of the dissipater Alternatively the channels may be formed in the surface of the dissipater onto which the element is secured. The thick film track may align with the channels or with the contact portions of the dissipater (fig 2b). Methods of assembly specifying temperature data are disclosed and sensors may be positioned within or adjacent the flow.

Description

Thick Film Heaters

Field of the Invention

[00011 This invention includes improvements relating to flow through heaters for liquid heating appliances and in particular an electrical heating element comprising a combination of a thick film resistor and a heat dissipater.

[0002] The invention concerns an assembly comprising of a combination of an electrical heating element and a heat dissipater to be heated by the electrical heating element, the heating element comprising a metal substrate, an insulating layer located on one side of the substrate and a thick film resistor located on the insulating layer, wherein the second side of the substrate is in contact with the heat dissipater.

Background of the Invention

[00031 Such combinations are generally known in the field of water heating kettles, flow heaters and heaters for food, like baking plates used in fast food restaurants or braising pans.

In these prior art combinations of heaters and dissipaters, the heating element is attached to the heat dissipater by bohs and nuts, external clamping means or a bonding layer between the heating element and the heat dissipater.

[0004] The proprietor's granted patent GB-A-2 351 894 discloses a brazed connection between a heater and a heat dispersion plate. However, in this prior art solution the surface area of the heating element is substantially smaller than that of the heat dissipater. Hence an additional heat dispersion layer is present between the heating element substrate and the heat dissipater. The size of the heat dispersion layer is larger than that of the heating element, so that the heat transfer takes place over only a part of the surface of the dispersion layer which will result in a slower heat up time and a temperature gradient across the heat dissipater which may be a problem, for example in flow through heater applications.

[0005J Prior art flow through heaters that rely upon magnesium filled heating elements are also well known and generally fall into two categories.

[0006] The first category relies upon a spiral tube and a magnesium-filled sheathed heating element being diecast or stamped into an alloy or aluminium casting which acts as a dissipater. Generally the power to mass ratio is very low, typically between 1.5 and 2 watts per gram. These heater assemblies are suitable to heat water close to boiling point providing the flow rate of the water to be heated is constant. However these heater types are slow to heat up and slow to react to changing conditions such as the flow rate of the water.

Additional tubular heaters may be incorporated, but these add to the cost and increase the size and mass of the assembly.

100071 The second type relies upon one or more magnesium filled sheathed heating elements being attached to a straight tube. This type does have higher power to mass ratio however the heat transfer is slow, which is satisfactory if run at lower temperatures, for example in a washing machine; however they tend to overheat and cause steam if temperatures closer to the boiling point are required.

[0008] Prior art flow through heaters are also known that incorporate thick film heaters as a heat source, for example, as described in the proprietor's patent publication WO-A- 2005/080885. However, these also suffer from problems related to the control of the water temperature and complexity of assembly.

[00091 In view of the recent trend to smaller and quicker reacting liquid heating devices, that do not rely upon the need to store heated liquid, it is advantageous to increase the power density of the heating means without the risk of the temperature overshooting to boiling point. When increasing the power density it is essential to ensure the integrity of the mechanical and thermal contact between the heating element and the heat dissipater. With prior art techniques in connecting the heating element with the heat dissipater, lowering the thermal mass may lead to warping of one or both of the components, which can result in a reduced contact area between the heating element and the dissipater reducing the optimal heat transfer.

[0010] This problem is in particular present in situations where in the overall power density is high, for instance higher that 8 W/cm2. It would be advantageous to provide a high power density combination wherein the transfer of heat from the heater to the heat dissipater is maintained, throughout the lifetime of the component.

Statement of the Invention

[00111 According to one aspect of the present invention, there is provided a flow-through heating apparatus according to claim 1. According to another aspect of the present invention, there is provided a method according to claim 27.

[0012] According to one preferred embodiment the dissipater is formed by a high mass heat dissipater which is understood may have a thermal mass of at least an order of three times greater than the thermal mass of the heating element. Hence the heat dissipater has a substantially larger thermal mass than the heating element itself so that the thermal

dynamics are different from the prior art.

[0013] It has been discovered that the larger the mass of the heat dissipater, the less likely that either the heat dissipater or the heater will distort at the point at which they are joined.

Furthermore, it has been found the strength of the joint may allow a reduction in the depth of substrate on the thick film heating element. Other issues, such as differences in the thermal coefficient of expansion of the heat dissipater and the heating element substrate must also be taken into consideration to prevent shearing forces developing.

100141 It is also possible to use a cast flow through heat exchanger as a heat dissipater. The combination of this flow through heat exchanger and the thick film heating element results in a flow through heater. These flow though heaters can be used in apparatus for preparing heated liquids, for example, domestic and commercial water heaters and brewing machines.

Herein the application of thick film heaters is particularly advantageous as it reduces the thermal mass of the combination leading to a short time for warming up the combination and hence for preparing a hot drink.

[00151 Heating the liquid as it is required is energy efficient providing the assembly brings the liquid to the required temperature quickly without causing the liquid, for example water, to overheat. Therefore it is necessary to provide a quick response for both the heating and temperature control processes.

[0016] The heating of water requires a select choice of materials for the channels in which the water is to be heated to conform to the requirements for hygiene, etc. The available materials are often not compatible with the materials best suited for casting a flow through heater. Hence a further embodiment provides the feature that the channel in the cast flow through heat exchanger is formed by a separate tube, for example copper or stainless steel.

[00171 To increase the thermal coupling between the heating element and the liquid to be heated, the channel or separate tube in the cast flow through heat exchanger may be located adjacent to the heating element.

[00181 In flow through heaters that rely on spiral tubes and traditional heating elements cast into aluminium, the power to mass ratio is approximately in the range of 1.5 to 2 watts per gram, which results in a slow heat up time. With the use of a thick film heating element it is expected to increase the power to mass ratio to around 6 watts per gram so that the heat up time is substantially reduced. More than one resistive track can be provided on the thick film clement so that, for example, additional heat can be added on initial start up and then reduced as the temperature of the dissipater increases.

Brief Description of the Drawings

100191 There now follows, by way of example only, a detailed description of preferred embodiments of the present invention, with reference to the figures identified below.

Figure 1 a is a cross sectional diagram of a first embodiment of the present invention.

Figure lb is a cross sectional diagram of a first variant of the first embodiment.

Figure 1 c is a cross sectional diagram of a second variant of the first embodiment.

Figure 2a is a cross sectional diagram of a second embodiment of the invention.

Figure 2b is a cross sectional diagram of a variant of the second embodiment.

Detailed Description of the Embodiments

First Embodiment 100201 The structure shown in Figure 1 is a flow through heater 10, suitable for example for applications in coffee makers and similar appliances for preparing hot drinks. In such devices it is important to heat a flow of liquid, usually water, to a temperature suitable to brew a hot drink, like coffee, which in some cases is as high as 95°C. In this embodiment, the heat dissipater 2 or diffuser comprises a block of high thermal mass, including a tube 13 that forms a flow through heat cxchanger. The tube 13 forms one or more channels 12 through the dissipater 2. The tube 13 may have a meandering or a spiral structure to allow the tube 13 to extend evenly over the heated surface. The tube 13 may be preformed into the required shape and subsequently incorporated into the dissipater 2, for example by casting [00211 The thermal properties of the assembly can be tuned by increasing or decreasing the mass of the dissipater 2 and increasing or decreasing the distance of the tubes 13 from the heating substrate 4. This tuning of the mass will be carried out in cooperation with the control mechanism so as to optimise the operating, safety and efficiency characteristics of the apparatus. The dissipater 2 preferably has a substantially larger thermal mass than the heating element 3, for example at least an order of three times larger.

[0022J The structure of Figure Ia includes a thick film element 3 comprising a substrate 4 of thermally well conducting material, such as a metal, and an electrically insulating or dielectric layer 5 applied on the lower side of the substrate. This electrically insulating layer should have reasonable or good thermal conducting properties, and may for example comprise vitreous enamel. On the lower side of the insulating layer 5 at least one resistor track 6 has been applied by the thick film technique, which is known from the prior art. The thick film track (s) 6 may be distributed substantially evenly across the substrate 4 or may be printed to match the shape and required load of the channels 12.

[0023] Figure lb shows a first variant of the first embodiment, in which the channel(s) are at a varying distance from the substrate 4 as the liquid flows through the channel(s) 12. The channels 12 may comprise a first channel at a first distance from the substrate 4 and a second channel at a second distance from the substrate; each of the first and second channels S may have a spiral, serpentine or meandering structure. The first and second channels may be connected in series or in parallel, or may be independent, as described below.

100241 The channel closer to the substrate 4 will generally have a better rate of heat transfer, so this arrangement allows the heating effect of the channel 12 to be tuned to the requirements of the particular application. Moreover, as shown in Figure ib, this arrangement may allow the channel 12 to be more densely packed within the dissipater 2.

As another alternative, the channel 12 may follow a generally spiral curve that alternately moves towards and away from the substrate 4, in the flow direction along the channel.

[00251 Figure 1 c shows a second variant of the first embodiment, in which the channel 12 progressively moves away from the substrate 4, in the direction of flow along the channel 12. This allows the heating of the channel 12 to be progressively increased or reduced, depending on which way the liquid flows along the channel 12.

Second Embodiment [0026] The embodiments depicted in Figures 2a and 2b differ from the embodiment of Figures la to ic by the positioning of the channels 12 and the absence of tubes surrounding these channels 12. The channels 12 are formed or incorporated in a surface of the heat dissipater 2, which surface is then joined to the substrate 4, which therefore forms a wall of the channel 12. Altematively or additionally, the channels 12 may be formed in the surface of the substrate 4.

[0027] The positioning of the channels 12 in contact with the substrate 4 increases the thermal coupling between the heating element 3 and the liquid flowing through the channels 12. This direct coupling shortens the warming up time of the liquid. Further, the second embodiment differs from the first embodiment by the rectangular shape of the channels 12, so that more of the liquid is in direct contact with the substrate 4. Altematively, channels 12 with a round or elliptical cross section could be used. Tubes 13 with a rectangular cross section could be used in the embodiment of Figures la and lb. [0028] It has been found that, by specifically designing the heat output of a thick film element to the specific load demands of liquid passing through the channel 12, it is possible to provide power to mass ratios of up to around 16 watts per gram. This can be achieved by: a. Matching the shape of the tracks substantially or exactly to the shape of the tube or channel in the heat dissipater and/or b. Applying a plurality of tracks that will increase and decrease the heat output at different stages of the heating cycle.

[00291 In the embodiment of Figure 2a, the thick film track(s) 6 are aligned with the channel(s) 12; this may increase the thermal transfer to liquid within the channel 12. In the variant of Figure 2b, the thick film track(s) 6 are aligned with the portions of the dissipater 2 between the channel(s) 12; this may provide more even heating of the channel(s) 12.

Multiple Tracks [0030] In the above embodiments, the thick film heater may include a plurality of independently switchable thick film heating tracks, which may be selectively switched on or off or connected together in series or parallel to achieve the desired heating output and/or profile. In the flow-through heater embodiments, this feature may be used to determine the heating at different points along the flow-through channel.

Multiple Channels [00311 In the above embodiments, there may be more than one channel 12 for the liquid to be heated: for example, there may be multiple channels either arranged in parallel, with a shared inlet and outlet, or independently, each with their own inlet and outlet, or there may be a single channel with one inlet and multiple outlets at different sections along the length of the channel, the flow of fluid from the outlets being controlled by one or more valves.

There may be arranged independently switchable thick film heating tracks aligned with the different channels or channel sections.

Brazing [00321 In the above embodiments, the heating element 3 may be joined with the dissipater 2 by brazing, resulting in an alloy layer 7 connected to both the dissipater 2 and the substrate 4. Figures 2a and 2b show the alloy layer 7 across the entire surface of the substrate 4; however in these embodiments the layer 7 could be selectively formed, so no alloy is present within the portions of the channel 12.

[00331 Brazing leads to a permanent connection between the heating element 3 and the dissipater 2, so as to minimise the tendency for warping caused by the heating and cooling cycles. The term brazing is understood to cover any connection method that relies on a heating process to provide an alloy based intermediate layer 7 between the substrate 4 and the dissipater 2, for example soldering.

[0034] When manufacturing the assembly the following typical temperatures must be considered: * Firing temperature range of electrically insulating layer -approximately -800-950 °C * Firing temperature range of thick film materials -approximately -625-750°C * Melting temperature range of aluminium or alloy heat dissipater -approximately - 490-650 °C [0035J In one embodiment the thick film element 3 is manufactured as a first stage and soldered or brazed to the heat dissipater 2 as a second stage.

[0036] Such a method requires the use of a soldering or brazing alloy having a melting temperature lower than the heat dissipater 2 and lower than the temperature which would affect the thick film track 6 or the electrically insulating glass or glass-ceramic or porcelain enamel layer 5 onto which the thick film track 6 is applied. In practice this requires a soldering or brazing alloy of which the melting temperature is higher than 25 0°C and lower than firing temperature of the thick film materials and/or the heat dissipater 2 (whichever is the lowest). The lower boundary is determined by the normal running temperature of the thick film element 3 in use during the life of the appliance. An example of such an alloy is zinc, which is preferably used with a suitable flux at a soldering or brazing temperature of 550°C.

[0037] Alternatively it is possible to make the soldering or brazing connection first and then apply insulating layer(s)5 and thick film track(s) 6. This leads to a method for producing a combination of an electrical heating element 3 and a heat dissipater 2 to be heated by the electrical heating element 3; the method comprising the steps of providing a heating element 3 comprising a metal substrate 4 and providing a heat dissipater 2 comprising a layer of metallic material, wherein the substrate 4 of the heater is made of metallic material; the substrate 4 is brazed over substantially its full surface to the metallic layer of the heat dissipater 2 and subsequently the insulating layer 5 and subsequently a thick film heating track 6 are provided, on the brazed substrate 4.

[0038] In each of the following specific embodiments the method of joining the substrate 4 to the dissipater 2 is described as brazing, but the inventors envisage that other joining methods may be applicable, including soldering, welding, laser welding, hot stamping, cold stamping, die-casting, gluing and induction or friction welding.

[00391 In a further embodiment it may be possible to apply the insulating layer 5 before the brazing or soldering, and the thick film track 6 after the brazing process.

[0040] As the soldering or brazing connection must not be affected by the later application of the thick film track 6, this method requires the use of a brazing alloy having a melting temperature higher than 900°C, for example a nickel based alloy.

Alternative Thick Film Heating Arrangements [00411 In a variant of the above embodiments, it is envisaged that the insulating layer 5 and thick film track(s) 6 may be printed directly onto one or more sides of the heat dissipater 2, so removing the need for a separate substrate 4, and the subsequent fixture of the separate substrate 4 to the dissipater 2. In that case it will be necessary to match the materials of the dissipater 2 and the thick film track(s) 6 so that the melting temperature of the dissipater material is greater than the processing temperatures of the insulating layer(s) 5 and associated heating tracks 6.

[00421 The heat dissipater 2 may have a substantially flat upper surface on which the further insulating layer 5 and thick film track(s) 6 may be printed. The upper and lower surfaces of the dissipater 2 may be substantially parallel, so that the thick film track(s) 6 may be printed on one surface and the dissipater 2 then turned over for printing further thick film track(s) 6 on the opposite surface. The insulating layer(s) 5 may have been formed previously on both surfaces, for example by a coating and firing process, or each insulating layer 5 may be formed immediately before the thick film track(s) 6 is printed thereon.

[0043] In another altemative embodiment, first and second thick film heaters 3 as described above, may be joined to the dissipater 2 so that their metallic substrates 4 are joined to respective opposite faces of the dissipater 2, using any of the joining techniques described above. In other words, the dissipater may be sandwiched between the metallic substrates 4 of the first and second thick film heaters 3.

Sensors [0044] Each of the above embodiments may include one or more sensors, for example an NTC sensor or thermocouple, to measure the temperature of the liquid as it is heated. With large mass water heaters, the sensor may be positioned within or adjacent the flow of the water so that rapid fluctuations in temperature can be sensed.

Alternative Embodiments [0045] It will be clear that numerous other variations can be applied on the embodiments discussed above within the scope of the invention as defined by the appending claims. In particular features of the different embodiments can be combined.

[0046] The embodiments described above are 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.

Claims

Claims 1. Flow-through heating apparatus comprising a thick film electrical heater (3; 6) in thermal contact with a heat dissipater (2), the apparatus having at least one channel (12) for flow of liquid therethrough.
2. Apparatus of claim 1, wherein the channel (12) is formed by a tube (13).
3. Apparatus of claim 2, wherein the tube (13) comprises a pre-formed component incorporated within the dissipater (2).
4. Apparatus of any preceding claim, wherein the channel (12) is located adjacent to the heating element (3).
5. Apparatus of any preceding claim, wherein the channel (12) is formed within the heat dissipater (12).
6. Apparatus of any preceding claim, wherein the channel (12) has a circular or elliptical cross-section.
7. Apparatus of any one of claims 1 to 4, wherein the channel (12) has a rectangular cross-section.
8. Apparatus of any preceding claim, wherein the channel (12) has a meandering or spiral form, extending substantially evenly over the surface of the element (3).
9. Apparatus of any preceding claim, wherein the channel (12) is at a varying distance from the heater (3; 6), along the length of the channel (12).
10. Apparatus of any preceding claim, comprising a plurality of said channels (12).
11. Apparatus of claim 10, wherein the plurality of channels (12) are connectable independently.
12. Apparatus of claim 10, wherein the plurality of channels (12) are connected in series.
13. Apparatus of claim 10, wherein the plurality of channels (12) are connected in parallel.
14. Apparatus of any preceding claim, wherein the heat dissipater (2) has a greater thermal mass than that of the heating element.
15. Apparatus of claim 14, wherein the heat dissipater (2) has a thermal mass at least three times greater than that of the heating element.
16. Apparatus of any preceding claim, having a power to mass ratio of approximately 6 Watts per gram or greater.
17. Apparatus of any preceding claim, wherein the heat dissipater (2) comprises aluminium or aluminium alloy.
18. Apparatus of any preceding claim, wherein the heater (3) comprises a metallic substrate (4) joined to the heat dissipater (2).
19. Apparatus of claim 18, wherein the heat dissipater (2) is joined to the substrate (4) by brazing.
20. Apparatus of claim 18, wherein the heat dissipater (2) is joined to the substrate (4) by soldering.
21. Apparatus of any one of claims 18 to 20, wherein the substrate (4) forms at least one wall of the channel (12).
22. Apparatus of any preceding claim, wherein the heater (3) comprises at least one thick film heating track (6) deposited on an electrically insulating surface (5) of the dissipater (2).
23. Apparatus of any preceding claim, wherein the melting point of the heat dissipater (2) is in the range of approximately 490 -650 °C.
24. Apparatus of any preceding claim, including at least one sensor arranged to sense the temperature of liquid within the channel (12).
25. Heating apparatus substantially as herein described with reference to and/or as shown in the accompanying drawings.
26. An appliance for preparing hot drinks, including the apparatus of any preceding claim.
27. A method of forming a flow-through heating apparatus, comprising placing a thick film electrical heater (3; 6) in thermal contact with a heat dissipater (2), the apparatus having a channel (12) for flow of liquid therethrough.
28. Method of claim 27, wherein the channel (12) is formed by a tube (13).
29. Method of claim 28, including incorporating the tube (13) as a pre-formed component within the dissipater (2).
30. Method of claim 29, including casting the tube (13) within the dissipater (2).
31. Method of any one of claims 27 to 30, wherein the heater (3) includes a metallic substrate (4).
32. Method of claim 31, wherein the heat dissipater (2) is joined to the substrate (4) by brazing.
33. Method of claim 31, wherein the heat dissipater (2) is joined to the substrate (4) by soldering.
34. Method of any one of claims 27 to 30, wherein the heater (3) is deposited as one or more thick film tracks (6) on an electrically insulating surface (5) of the dissipater (2).
35. Method of forming a flow-through heater, substantially as herein described with reference to the accompanying drawings.