ELECTRIC LIQUID HEATING VESSELS

Abstract: There is disclosed a method of preventing corrosion of the liquid contacting surface of a stainless steel base part of an electric liquid heating vessel. According to the method the surface is provided with a layer of glass, glass ceramic or ceramic. The surface is preferably pre-treated by grit blasting with alumina grit having a nominal size of 180-220 mesh. The grit blasting technique may be used to treat other steel surfaces which are to receive a glass layer.

Description: Electric Liquid Heating Vessels The present invention relates to electric liquid heating vessels and in particular to such vessels having a base part with a stainless steel liquid-contacting surface. Liquid heating vessels with stainless steel base parts are well known and may be of a construction such that the entire vessel, the base or only a portion of the base is made of stainless steel, the rest of the vessel being of another material such as plastics. Stainless steel is used in such vessels as it has a relatively low coefficient of thermal conductivity and can therefore be utilised to effectively enclose the heat from a heating element, for example to prevent damage to plastics components of vessels, such as the side walls, due to excessive temperature in the event of the element overheating. In use, the internal surfaces of such vessels are in contact with the liquid to be heated and in particular the base of the vessel will always be covered to some extent by the liquid to be heated. Thus, the stainless steel inner surfaces of the vessel are subjected to the corrosive effects of the liquid in the vessel, which effects are exacerbated by the temperatures at which the vessel operates and the consequent heating and cooling of the stainless steel. Such corrosion is particularly problematic in the case of the stainless steel surfaces of electric water heating vessels, as chloride ions in the water can enter pits or imperfections in the stainless steel surface and facilitate a galvanic reaction between the constituent elements of the stainless steel, known as pitting corrosion. In stainless steel such as BS 340, chloride ions also accelerate the stress corrosion to which the steel is susceptible. Furthermore, deposits of limescale from boiled water may lead to crevice corrosion of some stainless steels. In the case of electric liquid heating vessels in which the heating element is below the stainless steel base of the vessel, the base is constantly heated by the element in order to heat the liquid in the vessel. The consequent high temperature at the base further exacerbates the above-described corrosion problems. In order to combat the problems of corrosion, it is known to polish or grind the internal stainless steel surfaces of liquid heating vessels in order to remove oxide build-up and imperfections at the surface which result in pitting corrosion. However, the grinding or polishing process requires high-speed machines which are both large and expensive, and even after polishing, the corrosion of the stainless steel will not be entirely prevented. Thus, according to the present invention there is provided a method of preventing corrosion of a liquid- contacting stainless surface of a base part of an electric liquid heating vessel, wherein the stainless steel surface is provided with a layer of glass, glass ceramic or ceramic (hereinafter collectively termed 'glass' ) . By providing a layer of glass over the surface of the base part of the vessel, contact between the liquid in the vessel and the stainless steel is entirely prevented and thus corrosion cannot occur. The glass layer also provides a hard, abrasion resistant coating that resists limescale formation, but which allows any scale deposits to be removed by chemical or mechanically abrasive means. The glass layer will also prevent the deposition of metal ions from the base part into the liquid in the container. The base part may be integrally formed with the liquid heating vessel, in which case the other internal stainless steel surfaces of the vessel may also be provided with a layer of glass to prevent corrosion. However, the base part may also be a discrete stainless steel component, for example a plate, forming the bottom internal surface of the vessel, or only a part thereof. In this case, the rest of the vessel may be constructed of non-metallic material such as plastics. The base part may be positioned below the heating element in the assembled liquid heating vessel, but the method of the invention is of particular advantage when the base part is provided with a heating element on its underside. Such an underfloor heating element may be, for example, a sheathed heating element, preferably provided with a heat diffuser, or a thick film printed heating element. An example of this kind of plate heater is disclosed in WO 96/18331 which describes a heated metallic plate forming at least a part of the base of a liquid heating vessel having a peripheral channel grippingly receiving a depending portion of the vessel wall. In general, thick film printed heating elements on the underside of such a base part are provided on a glass (as herein defined) layer between the heating element and the lower stainless steel surface of the base part, such that the stainless steel base part is electrically insulated from the heating element. With this arrangement the present invention has the additional advantage that the glass layer on the upper side of the base part acts to balance out stresses in the base part due to the different rates of thermal expansion of the stainless steel compared to the glass layer which can cause distortion of the base part. The balancing effect of the layers of glass on both sides of the base part strengthens the structure of the base part sufficiently that peripheral strengthening grooves such as are described in WO 96/18331 are not required and the base part may be substantially flat. Thus, according to a further aspect of the invention there is provided a thick film printed heater comprising a substantially flat steel substrate provided on both opposed surfaces with a layer of glass as herein defined. In this way a thin heater of simple, flat design is obtained, while maintaining the mechanical strength of the heater. The glasses used on each face of the substrate may be the same. Typically, the layer of glass that will receive the resistive heating track will have a thickness of the order of 100-150 μm. Even more advantageously, the two layers of glass may be applied to the base part at the same point in the manufacturing process, and then may be fired simultaneously. In the case of an underfloor heater, heat travels from the element to the liquid through the base part and thus the base part is itself heated by the element which would result in accelerated corrosion. However, it is important to select an appropriate thickness for the glass layer, such that corrosion will be effectively prevented by ensuring a continuous pinhole free layer, but heat transfer from the base part to the liquid will not be greatly reduced. A suitable thickness generally would be not greater than 15 μm and preferably less than 10 μm. It is believed that the minimum achievable thickness of a continuous pinhole-free glass layer will be between 5 and 7 μm, and this is therefore the most preferable thickness. Of course, the necessary thickness of the layer will depend on the glass used and a layer greater than 15 μm may be necessary. However, such a layer will generally be less than 1 mm in thickness. The glass layer may be opaque or transparent. Such opaque material may be coloured or even patterned to increase the aesthetic appeal of the base. Equally, in the case of transparent material, the stainless steel of the base part may be polished or provided with a pattern, again to increase aesthetic appeal. The glass layer may be of any suitable material, for example soda lime glass such as EPSOM GLASS INDUSTRIES 23169/150 glass powder. The glass will generally be applied in powder form by, for example screen printing, transfer printing, electrophoretic deposition, decal transfer, tape lamination or electrostatic spraying, before being fired. The invention also extends to a stainless steel base part for an electric liquid heating vessel provided with a layer of glass on its upper surface. The invention further extends to an electric liquid heating vessel incorporating such a base part. There are also disclosed herein improvements relating to the manufacture of heating elements and resistors, and in particular to the manufacture of so- called thick film printed elements and resistors. Such elements and resistors typically comprise a metallic substrate, typically of polished stainless steel, onto which is deposited a glass (as herein defined) insulating layer. A thick film resistive track is provided on the insulating layer. In order to obtain a satisfactory surface for the insulating layer to key onto, the metallic substrate is usually pre-oxidised, either through heat treatment or chemically. However, such processes are expensive, particularly where stainless steels are being used since it is then normal to use pre-polished steel, which is expensive, particularly since the polish is then removed by the oxidation process. Also, stainless steel surfaces are typically supplied with a protective plastic film (itself an additional cost) . This film is removed manually from each individual element, prior to further processing. This is an expensive and critical process. It is not desirable to scratch the polished steel surface during this operation, and not permitted to leave finger print marks on the polished steel surface. The existing methods are unreliable and inconsistent, and vulnerable to many factors which may adversely affect the bonding of the glass layer to the substrate. From a further aspect, the present invention seeks to provide a less expensive and more reliable method by which a surface may be pre-treated to receive an insulating layer. From this further aspect, the invention provides a method of pre-treating a steel surface for receiving an electrically insulating glass layer for an electrically resistive track wherein said surface is grit blasted using dry and clean grit. Thus in accordance with the invention, a steel surface, preferably a stainless steel surface, is grit blasted prior to receiving the insulating layer. This is substantially less expensive than heat or chemical treatment but will provide a surface which is chemically clean and which will provide a suitable key for the insulating layer. It also means that an unpolished stainless steel may be used as a starting material, which is considerably less expensive than a polished one. The method provides a consistent surface for deposition of the glass layer leading to a more consistent bond. It is extremely important that the grit used is clean and dry. If it is not, impurities such as oil may be deposited on the surface which may, when the insulating layer is being fired, carbonize to cause incomplete bonding of the layer to the substrate locally. Furthermore, if the grit is not dry, moisture may be deposited on the surface leading to the formation of corrosion products which, again, will lead to an unsatisfactory bond being formed between the insulating layer and the substrate. The blasting process effectively removes the natural protective surface of the steel, which means that corrosion products will form on the surface of the steel within seconds, even in stainless steels, if impurities or moisture are present. The grit may be cleaned and dried using known and commercially available apparatus. For example, the grit may be cleaned inertially to remove dust and other particles therefrom. Furthermore the air or gas supply to the blaster may be dried and de-oiled in known manner, for example using commercially available equipment. This is important, since any oil deposited on the surface will lead to an unsatisfactory bond. It has been found that particularly satisfactory results can be obtained using alumina as the blasting grit. This is thought to be because alumina is compatible with the glass insulating materials usually used in thick film heaters, so that if residues of grit remain on the surface after treatment, they will not be detrimental to the glass layer. It has also been found that, as an alternative to grit blasting with alumina grit, a satisfactory treatment can also be obtained by abrading the surface with alumina in other ways, for example by abrading with alumina paper. One such method is to mount the paper on a lathe and bring the steel surface into contact with the rotating paper. From yet a further broad aspect, therefore, the invention provides a method of pre- treating a steel surface for receiving an electrically insulating glass layer for an electrically resistive track, wherein said surface is abraded by alumina particles. The steel substrate on which the glass insulating layer is deposited is preferably stainless steel, and is preferably in the form of a plate. In an effort to reduce material costs, it has been proposed in WO 96/18331 to reduce the thickness of such a plate to around .5 mm. Usually in a grit blasting process, the grit impinging on a surface expands the surface by cratering. In a thin plate, this may cause deformation of the plate, leading to difficulties in successfully depositing the glass insulating layer. It has been found, however, that this distortion can be minimised by limiting the maximum size of the grit particles used. Furthermore, if too large a grit size is used, the resultant surface may also be too rough to allow easy deposition of the glass layer. It has been found that a satisfactory result can be obtained by limiting the maximum nominal particle size to 100 mesh size (152 μm diameter) . From a yet further aspect, therefore, the invention provides a method of pre-treating a steel surface, preferably a steel plate surface for receiving an electrically insulating glass layer for an electrically resistive heating track, wherein said surface is grit blasted with a grit having a nominal maximum particle size of about 100 mesh. Preferably a grit having a nominal size of 120-150 mesh (having a nominal maximum particle size of 100 mesh) , more preferably 180-220 mesh (having a maximum nominal particle size of 150 mesh (104 μm) ) is used. In general terms the smaller the grit size, the better is the finish. Using a 120-150 mesh grit has been found to produce a surface having a roughness of 5 μm peak to peak height, and 180-220 mesh a surface having a 2 μm peak to peak height. Although it would be possible to treat only that surface of the plate intended to receive the resistive track, it may be desirable to treat both sides of the plate. This will act to even out the stresses induced by the blasting process and thereby reduce distortion. This process would be particularly preferred where the other side of the plate is itself to be provided with a glass layer, for electrical insulation or for protective reasons, such as is described earlier in this application. From a yet further aspect, therefore, the invention provides a method of treating both sides of a steel plate to receive respective glass layers, by grit blasting. Of course, if it is only required to apply a glass layer to the liquid contacting surface of a base part, the grit blasting technique may be used to prepare that surface only. Once the surface has been blasted, it may be cleared of excess grit and dust by any suitable, clean method, for example using a so-called 'air knife' which provides a clean, dry stream of air. It may also be cleaned with a solvent to ensure that no oil deposits are left on the surface. After this, the glass insulating layer may be applied to the treated surface in a desired manner. For example, the glass may be screen printed in a conventional manner, or alternatively it may be applied electrostatically as described in GB 2306873, or more preferably in the form of a pre-formed green ceramic member, as described in WO 97/01259. The invention also extends to a steel surface treated in accordance with the invention, to a heater comprising a surface treated in accordance with the invention, and to a liquid heating vessel having such a heater mounted or provided in a base thereof. The invention will now be described further in a non-limiting manner with reference to the following example. Example An unpolished .5 mm thick SS 306 stainless steel plate of the type shown in WO 96/18331, was grit blasted using Vacublast grade 'A' alumina grit of nominal mesh size 180-220 mesh in a Vacublast Ventus 90S blasting cabinet, which includes grit cleaning equipment. De- oiled and dried air was supplied to the cabinet from a 'Compair' compressor fitted with a proprietary air dessicator and oil filter, which dried the air to a dew point below -40°C. The surface of the plate was blasted at a pressure of about 80-90 psi for about 1 minute. After blasting, the surface was cleared of dust and other debris using an air knife, wiped with solvent and then once again cleared with an air knife. After cleaning, an insulating glass layer about 16μm thick was printed onto the treated surface in a conventional manner and fired. The resultant fired layer remained firmly attached to the plate even after the plate was bent through an angle of 45°. From the above it will be seen that in very broad terms, this aspect of the invention resides in grit blasting a metal surface prior to applying an electrically insulating glass layer thereto. The method of the invention provides a less expensive, but equally effective, method of treating a substrate than previously proposed methods. It also provides a more consistent method than the prior art methods discussed above as it creates, rather than tries to maintain and treat an existing surface. It will be clear to those skilled in the art that there exist various means and methods suitable for putting the invention into effect. Such means and methods, even if not specifically disclosed herein, are intended to be encompassed within the scope of the invention.

Claims: Claims 1. A method of preventing corrosion of the liquid contacting surface of a stainless steel base part of an electric liquid heating vessel, wherein the said surface is provided with a layer of glass as herein defined. 2. A stainless steel base part for an electric liquid heating vessel provided with a layer of glass on its upper surface. 3. An electric liquid heating vessel comprising a base part as claimed in claim 2. 4. The method, base part or vessel of any preceding claim, wherein the base part is a discrete component of a liquid heating vessel. 5. The method, base part or vessel of claim 4, wherein the base part forms part of a plastics liquid heating vessel. 6. The method, base part or vessel of any preceding claim, wherein the base part is provided on its underside with a heating element. 7. The method, base part or liquid heating vessel of claim 6, wherein the heating element is a thick-film printed heating element. 8. The method, base part or vessel of any preceding claim, wherein the glass layer has a thickness not greater than 15 μm. 9. The method, base part or vessel of claim 8, wherein the glass layer has a thickness not greater than 10 μm. 10. The method, base part or vessel of claim 9, wherein the glass layer has a thickness between 5 and 7 μm. 11. A method of pre-treating a steel surface for receiving an electrically insulating glass layer for an electrically resistive track wherein said surface is grit blasted using dry and clean grit. 12. A method as claimed in claim 11, wherein the steel surface is a stainless steel surface. 13. A method as claimed in claim 11 or 12, wherein the grit is alumina. 14. A method as claimed in any of claims 11 to 13, wherein the grit has a nominal maximum particle size of about 100 mesh. 15. A method as claimed in claim 14, wherein the grit has a nominal size of 120-150 mesh. 16. A method as claimed in claim 14, wherein the grit has a nominal size of 180-220 mesh. 17. A method of pre-treating a steel surface for receiving an electrically insulating glass layer for an electrically resistive track, wherein said surface is abraded by alumina particles. 18. A method of pre-treating a steel surface, preferably a steel plate surface for receiving an electrically insulating glass layer for an electrically resistive heating track, wherein said surface is grit blasted with a grit having a nominal maximum particle size of about 100 mesh. 19. A method of treating both sides of a steel plate to receive respective glass layers, by grit blasting. 20. A thick film printed heater comprising a substantially flat steel substrate provided on both opposed surfaces with a layer of glass as herein defined.

Inventors: DOYLE KEITH BARRIE, TAYLOR JOHN CRAWSHAW, DOYLE, KEITH, BARRIE, TAYLOR, JOHN, CRAWSHAW
Applicants: DOYLE KEITH BARRIE, STRIX LTD, TAYLOR JOHN C, DOYLE, KEITH, BARRIE, STRIX LIMITED, TAYLOR, JOHN, CRAWSHAW
Application year: 1997
Main classification: A47J27
IPC codes: A47J27, A47J36, H05B3