GB2316896A

GB2316896A - Electric heaters

Info

Publication number: GB2316896A
Application number: GB9618512A
Authority: GB
Inventors: John Crawshaw Taylor
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1996-09-05
Filing date: 1996-09-05
Publication date: 1998-03-11
Anticipated expiration: 2016-09-05
Also published as: GB2316896B; GB9618512D0

Abstract

A method of treating an exposed stainless steel surface of an electric heater having a support and resistive heating element applied or mounted to the support comprises projecting particles in a stream of gas at the stainless steel surface so as to produce a clean bright and corrosion resistant finish, after the insulating layer and track have been applied.

Description

Electric Heaters The present invention relates to electric heaters and, in particular, to heaters of the type having a stainless steel surface, for example, a heater comprising a thick film resistive heating track laid down on an electrically insulating substrate provided on a stainless steel support.

It has been proposed to use such thick film heaters in liquid heating vessels. In such applications, the heater forms part of the base of the vessel, or, as disclosed in WO 96/18331, is formed on a separate plate which can be mounted in the base of, for example, a plastics vessel. The insulating substrate and heater track are arranged on the dry side of the support with the other side of the support plate being visible inside the vessel.

In the manufacture of such heaters, prepolished stainless steel is commonly used. This is conventionally heat treated to form an oxide coating onto which the insulating substrate and track may be applied. The substrate and track are subsequently fired on the oxidised support. However, an oxide coating is inevitably produced on both sides of the support.

Further oxidation and discolouration of the visible side occurs during production due to the firing stages. This means that the side of the support which is visible in use within the liquid is discoloured and unsightly.

Up to now, the visible side of the support has been polished using extremely large and expensive customised polishing machines. The present invention seeks to avoid such processes, and from a first aspect provides a method of treating an exposed stainless steel surface of an electric heater comprising a support and resistive heating element applied or mounted to the support, the method comprising projecting particles in a stream of gas at the stainless steel surface so as to produce a clean bright and corrosion resistant finish, after the insulating layer and track have been applied.

It has been found that by using such a process, the oxide layer is removed from the surface of the support, and at the same time a corrosion resistant sheen can be produced on the surface. The finish is preferably smooth with an attractive "silk sheen" finish, rather than a mirror finish. The impact of the particles on the surface effectively peens the surface without cutting or pitting, and so reduces the susceptible to corrosion.

Furthermore, a mill finished steel rather than a highly polished steel can be used, thereby greatly reducing the cost of the heater.

This method may be applied to any heater where it is desirable to clean a stainless steel surface to which an element is applied or mounted. For example, a conventional metal clad element may be bonded to the bottom of a stainless steel support which forms the base of a vessel. However, the invention is particularly applicable to the thick-film heaters referred to above.

The particles may comprise any suitable, nonabrasive particles, which will not significantly cut, pit or scratch the surface. For example walnut shells may be used, but preferably the particles comprise glass beads, typically soda glass beads.

It has been found that the particle size should preferably be less than 150 microns in diameter. This is particularly preferred when a thick-film heater is used because the supports of such heaters tend to be thin. In such heaters, excessively large particles may cause the support to become distorted leading to a reduction in the breakdown voltage of the insulating layer applied. This applies to say a .5mm thick support, but obviously, the thicker the support, the less likelihood of deformation exists. The smaller the particle, the finer the finish, and beads having a nominal diameter of 44 microns have been found to give a very satisfactory finish.

The beads should be smooth, or the surface of the steel may be cut, reducing the attractive bright finish.

Commercially available beads, such as those supplied by Vacu-blast Limited under the trade name Vacubeads, have been found to be particularly suitable in this regard, being supplied with a maximum of 3 angular or broken beads. Accordingly, it is important that the quality of the beads is checked at regular intervals to ensure that the surface does not become abraded during the process.

It is also important that the beads and gas medium used are clean and dry. If they are not, impurities such as oil may be deposited on the surface which is undesirable and moisture may be deposited on the surface leading to the formation of corrosion products.

Although, in principle a variety of gasses could be used, in order to reduce costs it is preferred to use clean, dry compressed air.

The beads may be cleaned and dried using known and commercially available specialised apparatus. For example, the beads may be cleaned inertially to remove dust and other particles therefrom. Furthermore the air or gas supply should may be dried and de-oiled in known manner, for example using commercially available specialised equipment.

Only selected areas of the support need be treated if required. Accordingly, selected areas may be masked off during the cleaning process. For example, the track area formed on one side of the support may be masked.

Not only can the invention be used to treat the side of the support which is visible to the user, but it may also be used to remove oxides for other selected areas of the support, for example from seal areas.

The invention also extends to a heater treated in accordance with the invention, and to a liquid heating vessel comprising a heater in accordance with the invention. The heater may be a plate or the like for mounting in the base of the vessel, or actually form a part of the vessel base itself.

The invention will now be described further in a non-limiting manner with reference to the following example.

Example An .5 mm thick stainless steel plate of the type shown in WO 96/18331, was provided on one side with an insulating glass layer and thick film heating track in a conventional manner.

That side of the plate was then masked, and the other side of the plate was then cleaned using Vacubead soda glass beads of 10-50 micron diameter (nominal 44 micron diameter) in a Vacublast Ventus 90S apparatus, which includes bead cleaning equipment. De-oiled and dried air was supplied to the Vacublast cabinet from a 'Compair' compressor fitted with a proprietary air desiccator and oil filter, which dried the air to a dew point below -40 C.

The surface of the plate was subjected to a stream of particles at a pressure of about 80-90 psi for about 1-2 minutes. Afterwards, the surface was cleared of dust and other debris, to leave a sheen like finish resistant to corrosion.

From the above it will be seen that in very broad terms, the invention resides in using beads in a stream of gas to clean stainless steel surface to provide an oxide free and corrosion resistant surface. The method of the invention provides a less expensive, but equally effective, method of providing such a finish than previously proposed methods. Furthermore, the method of the invention does not introduce stresses into the support material, thereby helping to maintain the flatness of the heater.

Claims

1. A method of treating an exposed stainless steel surface of an electric heater comprising a support and resistive heating element applied or mounted to the support, the method comprising projecting particles in a stream of gas at the stainless steel surface so as to produce a clean bright and corrosion resistant finish, after the insulating layer and track have been applied.

2. A method as claimed in claim 1, wherein the particles are smooth and non abrasive.

3. A method as claimed in claim 1 or 2, wherein the heater element comprises a resistive track applied to an insulating substrate on the support.

4. A method as claimed in claim 1, 2 or 3, wherein the particles comprise glass beads.

5. A method as claimed in claim 4, wherein the beads are soda glass beads.

6. A method as claimed in any preceding claim wherein the particle size is less than 150 micron diameter.

7. A method as claimed in claim 6, wherein the particle size is about 44 micron diameter.

8. A heater treated by a method as claimed in any preceding claim.

9. A liquid heating vessel comprising a heater as claimed in claim 8, mounted in or providing the base thereof, the treated surface facing into the vessel.