GB2352603A - Electric liquid heating vessels - Google Patents

Abstract

An electric liquid heating vessel has a stainless steel planar base substsrate, with a heating element provided on its underside. The upper side of the base, which is exposed to liquid in use, is provided with a polished mirror surface finish by means of a mechanical or electropolishing process. Preferably the surface finish has a maximum <B>Ra</B> value of 0.1žm, and a low <B>Lo</B> value. The <B>Rsk</B> value is preferably positive or low negative ( & 0.2). In this way, the heater remains cooler in use as heat transfer from the heated base to liquid in the vessel is not impeded by the mirror surface. Scale deposition on the vessel base is diminished, and the propensity of the base to shed scale is increased.

Description

2352603 ELECTRIC LIQUID HEATING VESSELS The present invention relates to

electric liquid heating vessels, and in particular, but not exclusively to electric liquid heating vessels such as kettles and hot water jugs having a planar heater provided in the base of the vessel.

Traditionally in the United Kingdom, electric liquid heating vessels have used sheathed electric heating elements extending into the vessel through a side wall or base of the vessel. A potential disadvantage of such heaters is that in hard water areas they scale up with calcium deposits. This is unsightly and is difficult to clean.

On the Continent, and indeed more recently in the United Kingdom, such vessels are provided with planar heaters which are mounted -n, or form at least a part of, the base of the vessel. The heater comprises a generally planar substrate usually of stainless steel on the underside of which is provided a heating element either in the form a traditional sheathed element or, more recently, a thick film heating element. This arrangement has the advantage that the inside of the vessel is clear of a potentially unsightly elements and is also more easy to clean.

However, such planar heaters are still prone to deposition of scale and also corrosion. Several suggestions have been made as to how these problems may be alleviated. For example, PTFE or similar coatings have been applied to the water facing side of the heater.

The present invention seeks to provide a further way in which these problems may be alleviated.

The Applicant has recognised that the deposition and adherence of scale to especially steel heater surfaces is directly related to the number and relative stability of molecular binding sites at the surface.

Stable binding sites, where scale will adhere strongly, are associated with surface defects including steps and dislocations. In order to decrease the amount of scale adhering to the metal heater surface, the Applicant now proposes that the exposed surface of the heater is provided with a polished surface finish.

According to a first aspect of the invention therefore, there is provided an electric liquid heating vessel having a heated base wherein at least a portion of said base which is exposed to liquid in use is provided with a polished, mirror, surface finish.

In this way scale deposition on the vessel base is diminished, and the propensity of the base to shed scale may also be increased. Furthermore, since corrosion of an especially steel surface is also strongly associated with surface defect phenomena, a reduction in the rate of corrosion of the surface may also be obtained by this process.

Also, corrosion is actually promoted by the presence of scale on the heater surface. Scale deposition on the heater induces temperature variations across the surface which promotes the formation of actively corroding anodic regions in the hot zones under the scale relative to cooler areas on the heater. The present invention obviates or at leats substantially reduces this problem in that it inhibits scale formation on the heater surface.

An additional advantage of using a polished mirror finish is that it does not interfere with the heat transfer from the heated base to the liquid in the vessel, meaning that the heater itself will remain cooler during use. This is a particular advantage in the context of plastic walled vessels.

A yet further advantage is that the finish exhibits a low wetting angle with water so that there is little steam bubble generation leading to quieter operation of the vessel in use.

The heated base may be an integral part of the liquid heating vessel or it may form part of a separate heater mounted in an opening in the bottom of the vessel. The invention also extends to such a heater so from a further aspect, the invention provides a planar electric heater for forming at least a part of the base of a liquid heating vessel wherein a liquid facing surface of the heater is provided with a polished mirror surface finish.

It will also be appreciated that from a further aspect the invention provides a method of reducing scale deposition on a planar heater in the base of a liquid heating vessel comprising providing the liquid facing surface of the heater with a polished mirror finish.

From a yet further aspect the invention provides a method of treatment of a planar heater substrate comprising providing the liquid facing surface of the substrate with a polished mirror finish.

The heater or base preferably comprises a generally planar substrate, preferably of steel, more preferably stainless steel, having a sheathed heating element or a thick film heating element provided on its underside, with the upper side of the base being polished. Typical Stainless steel substrates which are particularly used in the production are 430 and 440 grade steels, more particularly 430S17 and 444.

The surface finish can be defined in a number of ways. The primary relevant parameter is Ra which is the arithmetic mean of the departures of the surface profile from the mean surface line and can be measured for example using a Talysurf traverse Unit. As the Ra value provides an indication of the height of the peaks and troughs at the surface, it should clearly be as low as possible. Preferably, therefore, the surface finish has a maximum Ra value of O.iAm and more preferably less than 0.05Am.

A second important parameter is Lo. This is a 4 - recognised parameter which provides information about the relative surface area of the metal as a result of the surface f inish. The higher the Lo value the greater the surface area available for scale adhesion, so that Lo values should preferably be as low as possible.

Using a 16mm traverse length on a Talysurf SOmm Traverse Unit, preferably the surface has a maximum Lo value of 15.24mm and more preferably a maximum of 15.21mm.

Ra and Lo are the primary parameters which indicate the suitability of the surface finish. However, other parameters are also useful in this regard. one such parameter is Rsk - skewness - which is a measure of the symmetry of the amplitude distribution curve about the surface mean line. Again this is measurable with a Talysurf Transverse Unit, as discussed above. a positive value for Rsk indicates that the amplitude distribution curve is skewed below the mean line indicating a surface with predominantly low regions with a few peaks. A negative value indicates a predominantly high surface with a few troughs, which provide sites for scale adhesion. a positive Rsk value should therefore be associated with the presence of fewer small troughs or pits in the surface and is thus desirable. However, low negative values are still acceptable, and preferably, therefore Rsk is positive or low negative (:0.2).

A yet further useful parameter is Rku - Kurtosis which is a measure of the shape or sharpness of the amplitude distribution curve. A low Rku value for the amplitude distribution curve indicates the surface has an even wave-like distribution of peaks and troughs, which, coupled with low Ra and Lo values, confirms the presence of wide shallow troughs and peaks. Preferably Rku is less than 3.5 and more preferably less than 3.0.

As stated above, the above parameters can be 3S measured using a Form Talysurf 50mm Traverse Unit. In a particular test procedure, the traverse length for each profile set at 16mm. a cut- off length of 0.8mm was chosen giving 20 cut-off lengths per assessment length.

The Roughness assessment mode was selected with Gaussian filter, low pass cut-off and associated Bandwidth 100:1.

An LS reference line was selected, and a least squares best fit line fitted to the data which is displayed relative to this line, with any slope removed. The instrument was calibrated before testing with a 3 Line Standard.

The polished mirror finish can be produced in any suitable manner, for example by mechanical polishing, electropolishing or other suitable polishing process. A typical mechanical polishing process is disclosed below.

Preferably the starting material is a stainless steel plate, onto the underside of which (as is conventional) has been mounted a sheathed heating element or on which has been provided a thick film heating element. Preferably the plate has a flatness value of <0.5mm across its upper face, and the finish of the base substrate is preferably a 2D or better according to EN 10088-2 1995 Stainless Steel standard.

Accordingly, it is inappropriate to use grit-blasted, chemically etched or peened surfaces as a starting point.

As a first step, contaminants or debris are removed from the surfaces to be polished, for example by air jets or brushing.

Then, the surface is coarse polished to remove all oxide coatings. Suitable polishing materials for this stage include a polishing mop of cotton, sisal or similar and a stainless steel polishing compound.

After the coarse. polishing stage, the surface is finely polished to give the desired finish. Suitable polishing materials for this stage include a cotton polishing mop an appropriate stainless steel polishing compound.

The process is then completed by degreasing, preferably with an alkaline wash or similar, to remove all polishing compounds and to leave an oil/grease free surface, and hot air jetted so as to remove any excess water.

Of course, suitable precautions should be taken during processing to scratch, bend or distort the heater plates.

It will be seen from the above that the process of the invention overcomes various disadvantages of other finishing techniques such as peening, bead blasting and grit blasting which do not significantly reduce the variation in surface profile.

It will be appreciated that the process of the invention can be applied in its various aspects to all types of underfloor heaters and even to traditional fully immersed sheathed elements. The preferred finish values given above are also applicable to such heaters. The underfloor elements may, for example be generally flat or domed. For example the invention may be applied to the Applicant's Suresealo heater plates. Also, it may be applied to all types of stainless steel substrates.

It is anticipated that it will find greatest application in the pretreatment of heaters for mounting in openings in the base of liquid heating vessels, having a body of metal, plastics, glass and so on. Thus the invention may find application in kettles, jugs, coffee makers, other beverage makers and so on.

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Claims (14)

Claims

1. An electric liquid heating vessel having a heated base wherein at least a portion of said base which is exposed to liquid in use is provided with a polished, mirror, surface finish.

2. A generally planar electric heater for forming at least a part of the base of a liquid heating vessel wherein a liquid facing surface of the heater is provided with a polished mirror surface finish.

3. A vessel or heater as claimed in claim 1 or 2 wherein the heater or base comprises a stainless steel substrate, having a sheathed heating element or a thick film heating element provided on its underside, the upper side of the substrate being polished.

4. A vessel or heater as claimed in any preceding claim wherein the finish has a maximum Ra value of 0.111m.

5. A vessel or heater as claimed in claim 4 wherein the finish has a maximum Ra of less than about 0.05Am.

2S G. A vessel or heater as claimed in any preceding claim wherein the finish has a low Lo value.

7. A vessel or heater as claimed in claim 6 wherein the Lo value is a maximum of 15.24mm and more preferably a maximum of 15. 21mm., 8. A vessel or heater as claimed in any preceding claim wherein the surface has a skewness, Rsk, which is positive or low negative i.e:0.2.

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9. A vessel or heater as claimed in any preceding claim wherein the kurtosis of the surface, Rku, is less than about 3.S and more preferably less than about 3.0.

10. A method of reducing scale deposition on a planar heater in the base of a liquid heating vessel comprising providing the liquid facing surface of the heater with a polished mirror finish.

11. A method of treatment of a planar heater substrate comprising providing the liquid facing surface of the heater with a polished mirror finish.

12. A method as claimed in claim 10 or 11 wherein the surface is polished mechanically.

13. A method as claimed in claim 12 wherein the surface is coarsely polished to remove surface oxides and then finish polished.

14. An electric heater having a surface exposed to liquid in use, said surface being provided with a polished, mirror, surface finish.