GB2586638A

GB2586638A - Flow-through heaters

Info

Publication number: GB2586638A
Application number: GB1912504.6A
Authority: GB
Inventors: Johannes Kloppers Gradus; Smith David; Hunt Andrew
Original assignee: FERRO TECH BV; Ferro Techniek BV
Current assignee: FERRO TECH BV; Ferro Techniek BV
Priority date: 2019-08-30
Filing date: 2019-08-30
Publication date: 2021-03-03
Anticipated expiration: 2039-08-30
Also published as: CN211953231U; GB2586638B; GB201912504D0

Abstract

An electrically powered liquid flow-through heater comprises a heating element with a heating surface, e.g. stainless steel element plate 5 heated by thick film heater tracks 7. The heater further comprises a housing 1 defining a space through which liquid flows in thermal contact with the heating surface and one or more components, e.g. fins or baffles 9, for disrupting liquid flow within the space. In a first aspect the components are fixed to the surface by a thermally conductive adhesive (12, figure 4a) such as epoxy resin or silicon resin having a thermal conductivity in the range of 0.5 – 5 W/mK. In a second aspect the components are thermally insulated from the surface, e.g. by being spaced from the surface and either attached to or forming part of the housing (figure 7). Alternatively, the components may be attached to but thermally isolated from the surface. In a third aspect the components are made of a similar material to the heating surface and fixed thereto by a braze (13, figure 4b). Ideally the heater is used to heat coolant of a rechargeable battery of an electric vehicle. A method of manufacture is also claimed.

Description

Flow-through Heaters

Field of the Invention

[0001] The present invention relates to electrically powered flow-through heaters for heating a continuous 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 novel 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. This approach is not well 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] Recently a requirement has arisen for flow though heaters 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 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 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 space, and means for disrupting the flow of liquid through the space.

[0007] Conventional flow-through heaters for liquids have one or more defined flow paths through which the liquid flows from one or more liquid inlets to one or more liquid outlets. In embodiments of the invention, the liquid flows across substantially the whole heating surface of the heating element, thus increasing the flow rate.

Disrupting the flow of liquid through the space 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 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.SW/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 space.

[0013] In a second 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 third embodiment, a component for disrupting the flow of liquid is not attached to the heating element but is located in the space 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.

Brief Description of the Drawings

10015] 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 an 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 of the embodiment, with an array of baffles attached thereto, in a 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 a second embodiment.

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

Detailed Description of the Embodiments

[0016] 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.

[0017] 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 space 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 space 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 space 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.

[0018] In this embodiment, the housing 1 is generally cuboid in shape with an upper surface providing an upper wall of the space 6, and sides providing side walls of the space 6. The lower face of the housing 1 is open, and the lower wall of the space 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.

[0019] Liquid may flow into a liquid inlet 3, through the space 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 space 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] The flow of liquid through the space 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 space 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.

[0024] 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.

[0025] 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 space 6. In this variant, the fins or baffles 9 may be made of aluminium, or other metal or thermally conductive material.

[0026] 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.

[0027] 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.

[0028] 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: Si - braze the fins 9 to the upper surface (i.e. facing the space 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 space 6) of the element plate 5.

S3 - Fire the thick film tracks 7 to form conductive tracks for the heating element.

54 - Attach the housing 1 to the element plate 5, to form space 6.

[0029] 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 space 6 and is held in place by abutment with the walls of the space 6 (i.e. the walls of the housing 1 and the heating surface of the element plate 5).

[0030] 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.

[0031] In the above embodiments, the apparatus may include an outer housing containing the housing 1 and element plate 5, connectors for electrical connection to the heater tracks 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 [0032] 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.

Claims

Claims 1. An electrically powered liquid flow-through heater comprising a heating element having a heating surface, a housing defining a space through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the space, the one or more components being fixed to the heating surface with a thermally conductive adhesive.
2. The heater of claim 1, wherein the adhesive comprises a resin.
3. The heater of claim 1, wherein the adhesive comprises an epoxy resin or a silicone resin.
4. The heater of any preceding claim, wherein the adhesive has a thermal conductivity in the range 0.5 -5 W/mK.
5. An electrically powered liquid flow-through heater comprising a heating element having a heating surface, a housing defining a space through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the space, the one or more components being thermally insulated from the heating surface.
6. The heater of claim 5, wherein the one or more components form part of the housing.
7. The heater of claim 5, wherein the one or more components are attached to the housing.
8. The heater of any one of claims 5 to 7, wherein the one or more components are spaced apart from the heating surface.
9. The heater of claim 5, wherein the one or more components are attached to but thermally isolated from the heating surface.
10. The heater of any preceding claim, wherein heating element comprises a thick film heating element.
11. An electrically powered liquid flow-through heater comprising a thick film heating element having a heating surface, a housing defining a space through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the space, the one or more components being made of similar material to the heating surface and being attached to the heating surface by a braze.
12. The heater of any preceding claim, wherein the one or more components comprise one or more fins or baffles.
13. The heater of any preceding claim, arranged as a coolant heater for a rechargeable battery.
14. 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 space through which liquid is able flow in thermal contact with the heating surface, and one or more components for disrupting liquid flow within the space, 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.
15. The method of claim 14, 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.
16. The method of claim 15, or the heater of claim 11, wherein the material is stainless steel.
17. The method of any one of claims 14 to 16, 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.