GB2326807A

GB2326807A - Electric water heating element with hydrophilic coating

Info

Publication number: GB2326807A
Application number: GB9813885A
Authority: GB
Inventors: Raymond Moore; Vernon James Donnelly
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1997-06-26
Filing date: 1998-06-26
Publication date: 1998-12-30
Also published as: GB9713561D0; GB9813885D0

Description

Electric Heaters The present invention relates to electric heaters, and in particular to coatings for immersed surfaces of electric heaters used in water heating vessels such as kettles and hot water jugs.

There has been a move recently, particularly in the United Kingdom, towards the more widespread use of underfloor heating elements in kettles and hot water jugs, rather than the use of the more traditional wholly immersed elements. Several underfloor heating arrangements have been proposed. In a first proposal, a sheathed heating element is bonded, typically via a thermal diffusion plate, to the underside of the base of the liquid heating vessel. A second proposal is to provide a so-called "thick film" element on the underside of the vessel base. In both these cases, the heating element may be provided on a vessel base which is formed integrally with the vessel wall or on a plate which is mounted to close an opening formed in the bottom of the vessel. Examples of such proposals are disclosed in WO 96/18331. Typically the vessel base or base plate is formed from stainless steel. A yet further proposal is to cast a sheathed heating element into an aluminium or aluminium alloy plate which closes the base of the vessel.

Particularly in the case of a cast heater, it is desirable to provide the liquid contacting surface of the heater with a coating which will prevent corrosion, reduce adhesion of scale to the heater and provide an attractive appearance. Furthermore, manufacturers are unwilling to use exposed aluminium in cookware or the like in view of the possible link between aluminium and Alzheimer's disease. The present invention is based on the realisation that while certain materials may, at first sight, appear suitable for coating purposes, they are not, and can in fact result in dangerous overheating of the heater. In particular it has been found that if the immersed surface of a heater is coated with say PTFE, a material commonly used to coat saucepans and the like, the heater seriously overheats as water in the vessel comes to the boil. This has been found to be due to the fact that PTFE is a hydrophobic material. When water in the vessel reaches boiling point, boiling begins by nucleation of water vapour bubbles on the heater surface. These bubbles get larger and detach themselves from the heater, rising through the liquid.

However, with a hydrophobic coating, it has been found that bubbles do not form and detach from the heater properly. Indeed, rather than detaching and rising through the liquid, the bubbles remain attached to the heater surface and spread laterally over the surface, leading to the formation of a layer of vapour between the liquid and the heater surface, which leads to severe overheating of the heater surface. Such "film boiling" is thus potentially extremely dangerous. The present invention seeks to overcome the above problem, and from a first aspect provides a water heating vessel comprising an electric heater having at least one generally planar heating surface immersed in water in use, said immersed surface being provided with a hydrophilic chemical coating.

The use of a hydrophilic rather than a hydrophobic coating will allow vapour bubbles to form and detach properly from the immersed heater surface and will thereby prevent serious overheating of the heater as the liquid in the vessel boils.

From a further aspect, therefore, the invention also provides a method of protecting the liquid contacting surface of a planar electric heater in a water heating vessel and preventing overheating of the heater as liquid in the vessel boils comprising applying a hydrophilic chemical coating to the said surface.

It will be appreciated that the invention also extends to electric water heaters, having at least one generally planar heating surface immersed in water in use, said immersed surface being provided with a hydrophilic chemical coating.

The coating material is preferably a polymeric material. Suitable examples are Fluoralon B5517 and Fluoralon E8588, and more preferably, a poly ether sulphone based resin.

Additives may be introduced to the coating material to, for example, increase scale resistance or thermal conductivity. Primarily, however, it is the hydrophilic properties of the coating material which are significant for the purposes of this invention.

The thickness of the coating need not be great.

Indeed, it is preferred if the layer is kept relatively thin, typically less than 20 ym and more preferably less than 15 ym, so that it does not create a significant thermal barrier between the heater and the water, which would lead to the heater running hotter during normal operation, which is undesirable.

The invention is particularly suited to cast heaters as discussed above. In one embodiment therefore, the heater comprises a cast body containing a sheathed heating element. Most preferably, the cast body has a generally planar upper surface which will be immersed in use and which is provided with the coating.

The cast body may be mounted by suitable means in an opening in the base of the vessel.

However, the invention is equally applicable to "thick film" heaters in which an electrical resistive heating track is applied to an insulating layer provided as a metallic e.g. stainless steel plate. The heating track is provided on the 'dry' side of the heater, and in accordance with the invention, the other side of the plate is provided with the coating.

Moreover, the invention is not limited to heaters mounted in an opening in the base of a vessel, or serving in use as the base of the vessel. It can equally be applied to heaters forming a wall of a vessel, or extending into the vessel through a side wall, in a manner similar to that of a traditional sheathed heating element. For example, a sheathed element may be contained in a cast body having one or more generally planar heating surface portions the cast body having a "head portion similar to that of traditional heaters.

The preferred coatings of the invention can easily be applied in spray form onto a pre-treated, e.g. grit blasted, surface and then oven dried e.g. at 4000C for 5-10 minutes in air.

It has been found in tests that compared to a heater having no coating on its water facing side, a 13 Hm thick hydrophobic coating of Fluoralon A6532 causes the heater temperature to rise by nearly 100"C, (i.e. to over 200"C) which in practice would mean that an overheat protection device for the heater would be activated before liquid properly boils. However using the same thickness of coating of Fluoralon B5517 or E8588, the temperature rise is less than 20"C, which is acceptable.

Whilst the invention has been described above in terms of heaters having only one side immersed in use, it may also apply to fully immersed heaters.

Furthermore it should be understood that the term generally planar as used herein is intended not only to cover flat surfaces but surfaces which are gently curved, and where the potential problem of film boiling might, without the use of the invention, still occur.

Claims

Claims:

1. A water heating vessel comprising an electric heater having at least one generally planar heating surface immersed in water in use, said immersed surface being provided with a hydrophilic chemical coating.

2. An electric water heater, having at least one generally planar heating surface immersed in water in use, said immersed surface being provided with a hydrophilic chemical coating.

3. Apparatus as claimed in claim 1 or 2 wherein said coating is polymeric.

4. Apparatus as claimed in claim 3 wherein said material is Fluoralon B5517 or Fluoralon E8588.

5. Apparatus as claimed in claim 3 wherein said material comprises a poly ether sulphone based resin.

6. Apparatus as claimed in any preceding claim wherein said heater closes or forms the base of a liquid heating vessel, and has a generally planar upper surface provided with said coating.

7. Apparatus as claimed in any preceding claim wherein said heater comprises a cast body incorporating a sheathed heating element.

8. Apparatus as claimed in any of claims 1 to 6 wherein said heater is a thick film heater, with said coating provided on the surface opposite that carrying the heating track of the heater.

9. A method of protecting the liquid contacting surface of a planar electric heater in a water heating vessel and preventing overheating of the heater as liquid in the vessel boils comprising applying a hydrophilic chemical coating to the said surface.