GB2163248A - Insulation of electrical storage heaters - Google Patents

Abstract

An electrical storage heater comprises a heat store (10) and a heat insulating covering (13) partly enveloping the heat store (10), regulated electrical heating means for heating the heat store (10), and air flow passages (11) from which heated air flows to the surroundings, the heat insulating covering (13) being formed of a heat insulating filling (19) (e.g. microporous silica or silicious material) enclosed by a durable envelope (16, 17) of ceramic fibre (e.g. fine silica-alumina fibres) in which the fibres are held in place relatively to each other by being in interangled relation. The fibres may be in the form of a felted structure and/or be bonded by an inorganic substance. <IMAGE>

Description

SPECIFICATION Improvements relating to electrical storage heaters This invention relates to electrical storage heaters (hereinafter referred to as being of the kind specified) each comprising a heat store, in the form of one or more bodies of high specific heat material and a heat insulating covering partly enveloping such body or bodies, electrical heating means for heating the heat store, a circuit (herein called the charging circuit including charge control means for regulating the flow of current to the electrical heating means during an off peak charging period, means providing for flow of air or other heat distributing fluid (herein for convenience referred to as air) into contact with the body or bodies of the heat store and thereafter to a space or place to be heated, and flow control means for controlling the flow of said air.

The electrical heating means is ordinarily powered from an electrical supply source (herein called the off peak supply) which is operative, i.e. rendered live, by time controlled switch means not accessible to the user for one or more restricted periods (herein called off peak periods) during each basic twenty four hour period. One such off peak period which is frequently utilised extends from 2400 hours to 0700 hours and in some cases a second off peak period is provided extending from 1 300 hours to 1 500 hours.

At certain times during the basic twenty four hour period, the temperature of the body or bodies of the heat store means attains a high value typically of the order of 600"C and it is therefore important that the heat insulating covering shall at all times be effective not only to prevent unwanted dissipation of heat from the heat store body or bodies otherwise than under control of the flow control means but also in the interests of safety. The heat store means, including the heat insulating covering, is ordinarily enclosed in a casing which is usually of metal. Such casing could become dangerously hot if the heat insulating covering disintegrated or otherwise became ineffective at one or more positions over the surface of the heat storage body or bodies.

Further, it is necessary over a period of service for the casing or one or more panels thereof to be removed from time-to-time to permit of inspection of the enclosed parts, and if necessary cleaning or servicing thereof.

The present practice is that the heat insulating covering is formed of an envelope glass cloth containing a filling of heat insulating material. The glass fibres which form the cloth are initially held together to form the cloth by a starch bonding. The starch bonding deteriorates or is destroyed in service being unable to withstand temperatures in excess of 600"C to 650"C and in consequence of this the glass fibres tend to fragment leading to "thin" areas in the covering or even holes. This can expose the filling which is structurally weak or even non-coherent with a result that the insulation is no longer effective or even dangerous.

According to the present invention, an electrical storage heater of the kind specified has its heat storage body or bodies provided with a covering formed of a heat insulating filling enclosed by an envelope of ceramic fibre in which the fibres are held in place relatively to each other by being in interangled relation, e.g. as a result of needling or by being bonded by an inorganic substance or substances able to withstand the operating temperature.

In practice, the inorganic substance or substances may be selected to withstand an operating temperature in the range 700"C to 750"C which is significantly above the operating temperature to which the heat storage body or bodies are permitted to rise by the charge control means.

One embodiment of the invention is illustrated diagrammatically in the accompanying drawing, showing a fragmentary view in horizontal cross-section through a heater of the kind specified incorporating the invention.

A heat storage body 10 of the heater may be formed of any suitable material of high specific heat, for example, clay-bonded iron oxide brick, and is provided with passageways 11 extending upwardly from the lower end of the body 10 to its upper end for flow of heat distributing air at a rate determined by flow control means (not illustrated).

The body 10 is heated by an electrical heating element connected to a charging circuit which is in turn powered from an off peak supply; the construction and arrangement of the flow control means and charging circuit may be as disclosed in our co-pending Application 8207039 and/or our European Patent 0,017,476.

The body 10 is provided with a heat insulating covering comprising members 13, two only of which are shown at the sides of the body 10, although in practice members of similar construction will be provided in the spaces 1 4 at the ends of the body 10, the whole being enclosed by a casing 1 5 which may be of sheet metal.

Each of the members 1 3 as well as those provided in the spaces 14, comprises an envelope formed of relatively rigid plate-like members 1 6 and 1 7 of ceramic fibre such as fine silica-alumina fibres, the plates 16, 1 7 typically having a thickness, e.g. 2mm to 3mm. The fibres are held together. This may be done by a needling operation in consequence of which a felted structure is formed in which the fibres are interangled with each other. Alternatively or in addition, they may be bonded by an inorganic substance or inorganic substances able to withstand temperatures at least in the range 700"C to 750"C without significant loss of strength. For this purpose, a substance such as sodium silicate may be employed.

The space 1 8 between the envelope 1 6 and 1 7 is in each case filled with material selected to provide the requisite heat insulation, i.e.

one presenting a low coefficient of thermal conductivity and no unacceptable emissions at the operating temperature. For this purpose, material, namely a silica or silicious material, which is microporous in structure, may be employed. This is structurally weak and hitherto has been encased in starch bonded glass cloth which as indicated above is not satisfactory.

The ceramic fibre now provided has a heat durability up to 1200"C, therefore, with a covering of some thickness (say 3mm) greater than that of the earlier glass cloth, the microporous insulation contained therein is subjected to rather lower temperature as a consequence of the invention.

The merit derives from improved structural durability and strength of the members 1 3 allowing of less risk of damage during heater factory assembly.

Further, the envelope 1 6 and 1 7 remains rigid and sufficiently strong throughout the service life of the heater, and the casing 1 5 or panels thereof may be safely removed to have access to the interior of the heater for the purposes previously mentioned without the disturbance occasioned by service operations casuing disintegration of the envelope 1 6 and 1 7. The envelope 1 6 and 1 7 may be applied to the microporous filling material, of which, for convenience, only fragments are shown at 19, in a plastic or pliable state and spread on the surfaces of the coherent filling 1 9 to form layers of the required thickness. Junctions 20 are formed along and adjacent the exterior of the vertical edges of the material 1 9 while the envelope material is in a plastic or pliable state.

Alternatively, the plate-like members 16, 1 7 may be "flopped" wet onto receiving trays of a vacuum forming apparatus for shaping.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed results, may, separately or any combination of such features, be utilized for real is ing the invention in diverse forms thereof.

Claims (11)

1. An electrical storage heater of the kind specified having its heat storage body or bodies provided with a covering formed of a heat insulating filling enclosed by an envelope of ceramic fibre in which fibres are held in place relatively to each other by being in interangled relation.

2. A heater according to Claim 1 wherein the fibres are held together as a result of a needling operation.

3. A heater according to Claim 1 wherein the fibres are held together by being bonded by an inorganic substance or substances able to withstand the operating temperature.

4. A heater according to Claim 3 wherein the inorganic substance or substances are able to withstand temperatures at least in the range 700"C to 750"C without significant loss of strength.

5. A heater according to Claim 4 wherein sodium silicate is employed as such a substance.

6. A heater according to any one of Claims 1 to 5 wherein the envelope is formed of relatively rigid plate-like members formed from the ceramic fibre.

7. A heater according to Claim 6 wherein the plate-like members have a thickness of about 2mm to about 3 mm.

8. A heater according to any one of the preceding claims wherein the ceramic fibre has a heat durability up to about 1200"C.

9. A heater according to Claim 8 wherein the ceramic fibre comprises fine silica-alumina fibres.

1 0. A heater according to any one of the preceding claims wherein the heat insulating filling presents a low coefficient of thermal conductivity and no unacceptable emissions at the operating temperature.

11. A heater according to Claim 10 wherein a silica or silicious material, which is microporous in structure, is employed for the filling.

1 2. A heater according to any one of the preceding claims wherein the covering is formed by applying envelope material to the filling material while the envelope material is in a plastic or pliable state and spreading the envelope material on the surfaces of the filling material to form layers of the required thickness.

1 3. A heater according to Claim 1 2 wherein junctions are formed between said layers along edges of the filling while the envelope material is in a plastic or pliable state.

1 4. A heater according to any one of Claims 1 to 11 wherein members of the envelope are formed by being "flopped" wet onto receiving trays of a vacuum forming apparatus for shaping.

1 5. An electrical storage heater substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

1 6. A method of forming a heat insulating covering for a heat storage body or bodies of an electrical storage heater substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

1 7. Any novel feature or novel combination of features described herein and/or illustrated in the accompanying drawing.