GB2218310A

GB2218310A - Electric heating element

Info

Publication number: GB2218310A
Application number: GB8909384A
Authority: GB
Inventors: Asko Lampikari
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-05-02
Filing date: 1989-04-25
Publication date: 1989-11-08
Also published as: SE8901549L; DE3912756A1; SE8901549D0; GB8909384D0; FI882036A0; FI882036A

Description

1 Electric heating element The present invention relates to an electric

heating element, intended particularly for heating of water coming into contact with the outer element surface, said element comprising a resistance wire and a protective housing inside which the resistance wire is closed.

2218310 The invention relates also to a method for manufacturing such an electric heating element.

This type of electric heating element can be used for heating an electric sauna stove, a hot-water boiler or for serving e.g. as a radiator.

In conventional water heaters, resistances heat the water directly and water temperature is controlled by means of thermostats which indirectly control and limit the resistances against overheating.

When the water in a heating boiler is heated by means of thermal resistances placed in a body of water and the water contains impurities, the problems include a short seryice life of thermal resistances and malfunctions in the heating system caused by materials getting loose from the thermal resistances.

In the invention, this problem has been resolved by means of.a novel electric heating element which is characterized in that the protective housing is made of ceramic material which surrounds a resistance wire with a very small clearance so that thermal expansion of a resistance wire is possible without damaging the ceramic material.

2 This type of solut ion, wherein a resistance wire is cast inside a ceramic material, prevents the corroding agents always present in water from reaching the surface of a metallic resistance wire. on the other hand, neither do the thermal resistance release in water any components which in time could build up blockages or the like malfunctions in a heating system. A ceramic material is wellknown for its purity and its chemical reactivity is virtually non-existent after properly treating the mass in connection with the casting operation.

An essential aspect in a manufacturing process of the invention is the treatment of a resistance wire prior to casting in a manner that a sufficient space remains for thermal expansion of the resistance and that this treatment is a simple and easy procedure without substantial drawbacks. This is achieved by a method of the invention in a manner that the outer resistance wire surface is provided with a layer of heat expanding and/ or evaporating material, the resistance wire is placed in a protective housing casting mould, a ceramic mass is poured into the mould to surround the resistance wire all over and, after the casting, the resistance wire is heated w#h electric current so as to expand and/or evaporate said coating layer.

A particularly preferred way of practicing a method of the invention is that said heat expanding and/or evaporating agent comprises waterglass in which the resistance wire element is dipped prior to casting.

A characteristic feature of waterglass is that, on heating, it first expands as the evaporating water tends to escape. When heating is continued at a high temperature, 0 3 most of the waterglass material disappears through evaporation.. This produces a clearance between resistance wire and ceramic material which is sufficient for the thermal expansion of a resistance wire. The small amount of non-evaporating waterglass ingredients remaining in the clearance does not produce a force that would break the ceramic material.

Another essential aspect of the invention is a postcasting treatment of the element for working the ceramic material into a structure which is strong and resistant to thermal fluctuation. Also, the positioning of a resistance temperature control sensor inside the ceramic material is a simple, technically easy and economically advantageous procedure which facilitates electronic control of the electric power fed into the heating element.

Materials other than waterglass can also be used for providing a clearance for the thermal expansion of a resistance wire. Examples of these include various fats and paraffin. A drawback with these is, however, that in later working steps they let great amounts of "smoke fumes" in the ai.r. When waterglass is used, this problem does not occur in the post-casting heat treatment.

The temperature adjustment and control of a heating ele-ment can be effected either electronically or by means of a capillary thermostat, as known in the art. In this invention, a sensor (e.g. a Chromel-aluminium wire) required for electronic control can be cast inside a ceramic material. Casting is effected by the process as in the case of a resistance wire.

4 Whenever the heating element is used for heating water, it is possible to employ conventional thermostats for preventing the resistances from overheating.

One embodiment of the invention will now be described with reference made to the accompanying drawing, in which Fig. 1 shows a longitudinal section of an electric heating element of the invention and Fig. 2 shows a cross-section of the same.

Fig. 3 shows in a larger scale a section along the line III-III in fig. 1.

In the illustrated embodiment, an Mshaped resistance wire 1 is cast inside a flat rectangular, ceramic protective housing 2. It is obvious that the configuration of resistance wire 1 as well as the appearance of protective housing 2 may vary as desired. Resistance wire 1 can be e.g. spirally shaped and the cross-section of housing 2 can be circular or elliptical.

one side of a ceramic protective housing 2 is defined by a sheet metal 4 provided with tapped holes for fastening resistance wire lead-in members 5. Between the sheet metal 4 and the inner surface of the wall 8 of a water tank is fitted a sealing 6 which surrounds a lead-in member 5. This sealing prevents the water to be heated from working its way inside housing 2 at said lead-in member 5. Another sealing 6 is fitted between a fastening screw 7 and the outer surface of said tank wall 8.

The element is manufactured in a manner that a resist- ill 1 ance wire 1 of any desired configuration is dipped in waterglass having a viscosity selected so that a suitable layer of waterglass adheres to the surface of resistance wire 1. The waterglass-coated resistance wire 1.is placed in an element casting mould. A ceramic mass, whose main component comprises e.g. aluminium trioxide, is poured into the mould around resistance wire 1. A cast element, along with its mould, is held for about 6 hours in a furnace having a temperature of 50 0 - 60 0 C. This is followed by removing the element from the mould and by placing it in a water basin in which the ceramic material is allowed to harden for about 30 hours. When the ceramic material is hardened, the element is carried into a furnace having a temperature of 50 0 - 60 0 C in which the element is dried for about 4 hour's. Thereafter, the element is subjected to a thermal treatment in which an electric current running through resistance wire 1 is used to increase the surface temperature of said element to about 700 0 800 0 C. The element is maintained at his temperature e.g. for 3 hours during which time the evaporating or volatile ingredients escape from the element.

Preliminary tests have shown that said deposit of waterglass is pufficient for producing a sufficient clearance 3 between resistance wire 1 and ceramic material 2, so that the thermal expansion of resistance wire 1 does not cause damage to ceramic material 2. At the same time, on a ceramic surface defining said clearance 3 there.is formed a dense, sort of glazed face which prevents moisture from penetrating into resistance wire 1.

The outer surface of ceramic protective housing 2 can also be glazed for effective protection against the penetration of moisture inside the ceramic material.

1 1 6 When using a ceramic mass with a suitable density, such glazing of the outer surface is nevertheless unnecessary.

A thermostat wire possibly cast inside the ceramic material can be passed through a lead-in 5 or through its own separate lead-in. A cast-in thermostat can be replaced by a thermostat sensing the surface temperature of said element and connected to a current limiter which can be an electronic controller.

The above describes the invention as a water heating element but it can also be used as a sauna stove element or for the direct heating of room air. What is essential about the invention is that a resistance wire has been covered with a ceramic material the casting of which has been possible directly around a resistance wire without the thermal expansion of resistance wire breaking the ceramic material.

i 7

Claims

1. An electric heating element, intended particularly for heating of water coming into contact with the outer element surface, said element comprising a resistance wire (1) and a protective housing (2) inside which the resistance wire is closed, c h a r a c t e r i z e d in that said protective housing (2) is made of a ceramic material which surrounds resistance wire (1) with a very small. clearance (3) so that thermal expansion of resistance wire (1) is possible without breaking the ceramic material.

2. An electric heating element as set forth in claim 1, c h a r a c t e r i z e d in that said protective housing (2) has a flat rectangular shape and said resistance wire (1) is M-shaped.

3. An electric heating element as set forth in claim 1 or 2, c h a r a c t e r i z e d in that the size of said clearance (3) is 1 - 10 times the change of the radius of resistance wire (1) as a result of a thermal expansion caused by the change of temperature to 800 a C.

An electric heating element as set forth in any of claims 1 3, c h a r a c t e r i z e d in that one side of ceramic protective housing (2) is defined by a sheet metal (4) to which are fastened lead-in members (5) for a resistance wire.

5. An electric heating element as set forth in claim c h a r a c t e r i z e d in that the outer surface of sheet metal (4) is provided with a sealing (6) which surrounds said resistance wire lead-in member (5) and which can be placed against the inner surface of the wall (8) of a water tank.

4, 8

6. An electric heating element as set forth in any of claims 1 - 5, character izedinthatthe outer surface of ceramic protective housing (2) is glazed.

7. A method for manufacturing an electric heating elementassetforth inclaimi, characterized in that the outer surface of a resistance wire (1) is provided with a layer of a heat expanding and/or evaporating agent, resistance wire (1) is placed in a casting mould for protective housing (2), into the mould is poured a ceramic mass which surrounds the resistance wire all over and, after the casting, said resistance wire (1) is heated with electric current for expanding and/or evaporating said coating layer.

8. A method as set forth in claim 8, c h a r a c t e r i z e d in that said heat expanding and/or evaporating agent comprises waterglass in which a resistance wire element is dipped prior to casting.

9. A method as set forth in claim 7 or 8, c h a r a c t e r i z e d in that the casting is followed by heating the electric heating element both before and after the harde.ning of ceramic mass.

10. A method as set forth in claim 9, c h a r a c t e r i z e d in that the casting is followed by heating the electric heating element in a furnace, the heat ing is followed by removing the element from a mould and by placing it in a water basin to harden, whereafter the element is dried in a furnace and finally subjected to a thermal treatment using an electric current running through resistance wire (1) for maintaining the element for several hours at a temperature whereat the surface 7 0 k 1 1 1 )i::, 9 temperature of the element is about 700 0 - 800 0 C.

Published 1989 CL The Patent Office, State House.66 71 Eligb Hol',,jrr London WC1R4TP. Purther copies rnky be obtained from The Patent office.

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