GB2204220A

GB2204220A - Improved electric heater

Info

Publication number: GB2204220A
Application number: GB08807883A
Authority: GB
Inventors: Thomas Michael Deszberg
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-04-07
Filing date: 1988-04-05
Publication date: 1988-11-02
Also published as: GB8807883D0; GB8708325D0

Abstract

An improved electric heater comprises two layers of thin perforated metal sheet 4 between which a length of resistance wire 2 (optionally coated with an insulating layer) is contained in an arrangement evenly disposed over the length and width of the heater. Above the metal sheets 4 layers of thermally conductive material 5 act to diffuse the heat created by passage of an electric current through the wire 2. Layers 5 may be expanded sheet metal or random strips or wire. Below the metal sheets 4 further layers are provided, the thermal conductivity of which can be adjusted by suitable use of thermally conductive and insulating material. The layers 5 are contained by expanded metal sheet layers 6 and 8 which form upper and lower heat transfer surfaces, the combined layers 4, 5, 6 and 8 being retained in thermal contact with one another by a U-shaped frame 1. The heater combined with a thermostat and a flexible insulating enclosure find application in the protection of water storage installations against frost. <IMAGE>

Description

IMPROVED ELECTRIC HEATER The present invention relates to electric heaters and is particularly, but not exclusively concerned with space heating.

Electric heaters take a variety of forms, usually consisting of spirally wound resistance wire, which is open to air, for the purpose of heating air, or encased in a metal tube filled with ceramic for the purpose of heating liquids or air.

Commonly the surface temperature of these heaters is in excess of 2000C and,in the case of the air heater, require convected or force air currents to keep them sufficiently cool to prevent early failure.

A simple safe form of electric air heater is desirable for many applications where heater temperatures are not neces sarily high but where an even temperature distribution is preferred. It is also sometimes desirable for the heater to allow passage of air through its thickness so as to promote air flow by convection.

According to the present- invention there is provided an electric heater compr-ising a housing having a heat transfer surface via which heat is transferred from the heater, the housing containing a resistance element connected to electrical terminals outside the frame and thermally conductive heat diffusing material disposed between the resistance element and the-~heat transfer surface.

Preferably the arrangement is such that air can flow through the heater. A heater frame will usually have the heat transfer surface as one outer layer and will additionally have a second outer layer the other side of the resistance element from the first layer. The space between the resistance element and the second outer layer may be filled either with heat conductive material or insulating materia-l or a combination of the two..

Advantageously the heater is used with a thermostat to control the temperature of a space, for example to prevent water tanks suffering from frost. To effect this most sensitively the heater, thermostat and water tank are preferably housed in.a.removeable enclosure.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows in perspective an electric heater according to the invention in assembled form, and Figure 2 shows a cross section transversely through the heater of Figure 1 to illustrate the several layers of which it is composed.

Referring to the drawings, the electric heater is typically rectangular in shape having a frame l,which may be of thin aluminium or tin plate,having a U-shaped cross section capable of housing a plurality of layers of material in sheet form to give a rigid panel when assembled. Similar methods of construction are well known in the production and assembly of low-cost filters.

Within the body of the heater, loops of spirally wound or flat resistance wire 2 are laid uniformly so as evenly to cover the plan area of the panel, terminating in heat resistance power supply leads which may pass through the frame 1 or alternatively be connected to other electrical components within the frame. Ultimately they are electrically connected to terminals 3 outside the frame.

The resistance wires 2 may be encased in an insulating coating or layer (not shown) capable of withstanding high temperatures and providing protection from leakage of electric current. A suitable insulating coating system and method is described in U.K,.Patent Specification No 1,410,947 On each side of these resistance wires 2 a 'respective layer of thermally conductive sheet.material such as aluminium or copper sheet 4 may be placed in intimate contact with the insulation coating or layer and fixed so as to maintain contact therewith; or in the absence of the coating with the wires 2.Advantageously the layers of sheet 4 are perforated two permit the passage of air perpendicular to the plane of the heater.Above the layer' 4 are placed one or more layers of aluminium or other thermally conductive thin metal foil 5 which may be, for example, of expanded sheet metal or in the form of random strip or wire.

An outermost retaining layer 6 is composed of expanded metal sheet witch is sufficently strong to contain the layers 5 and maintain physical contact between the layers 4 and 5. The layer 6 is held in position by the frame 1 with the edges of the layer 6 engaging under inwardly directed flanges 10 of the frame. The outer layer 6 should also protect the layers 5 and other components within the panel from damage in use and may advantageously carry a decorative face.The layer 6 acts as a heat transfer surface.

An identical arrangement of thermally conductive layers 7 and a second expanded metal outer layer 8 may be positioned on the unders-ide of the resistance wire 2 and sheet 4 within the frame. This layer 8 will be held by the frame 1 in the same way as is the layer~6.

Where the heater is required to lie on a surface which has limited resistance to elevated temperature an alternative embodiment of the heater is- appropriate. In this second embodiment the layers of thermally conductive material 5 are replaced by one or more layers of insulating material such as glass-fibre, mineral wool, insulating board, or felt two provide thermal insulation of the resistance wire from the second panel layer 8. Advantageously this insulation allows the free passage of air therethrough. The layers 5 may also comprise a mixture of thermally insulating and thermally conductive material where an intermediate temperature transfer is required.

9 Additional insulationlmay usefully be applied to the inside of the frame 1 so that it does not get toohot.

In operation voltage is applied to the heater through the conductor terminals 11 causing electric current to flow through the resistance wire 2. The heating effect produced in the resistance wire 2 is transferred to the conductive layers 4 and in turn to the layers 5 and 7 above and below.

Finally some heat is tranferred to the frame 1 and the retaining layers 6 and 8.

The random distribution of foil layers 5 and 7 permits effective heat transfer to the air over a large surface area of the foil and good heat transfer characteristics are obtained when passing air through the heater, without high temperatures being required.

A particular application of the electric heater will now be described with reference to frost protection of a water storage tank and pipework as commonly found in a domestic loft and in commercial or industrial buildings.

In such situations, it is desirable and economical to avoid the possibility of water freezing by the application of heat only when frost conditions are encountered. Thermal insulation is of limited value for frost protection as it can only delay freezing of water in unheated conditions. Such conditions are frequently found where loft insulation has been applied or where the dwelling remains unheated for an extended period.

The source of heat is preferably of low temperature, so as to avoid the risk of fire which could arise from contamination by dust or debris depositing on.the electric heater. The heater should liberate heat over a reasonably large area so as to provide effective protection against freezing. In the domestic application the electric heater may have physical dimensions enabling it to be placed between the ceiling joists or other supports immediately beneath- the storage tank, with its upper surface parallel to the underside of the tank.

for example. The widespread use of plastics material for the manufacture of cold water systems places a further limitation on the maximum temperature which can safely be used for frost protection.

Advantageously the heater may be long in relation to its width so that it extends at least to the edges of the tank.

A remote frost thermostat 12, typically of bi-metal const- ruction and normally off, may be mounted above the tank or at some convenient location in the vicinity of the tank and/or pipework to sense the air or surface temperature in the space. The thermostat 12 is electrically connected to the heater conductor 11, preferably inside the frame 1, and set to -switch the heater on at a specific temperature which is likely to be in the range of -5 0C tOT+50C inclusive.

On-sensing the low-limit temperature, the thermostat switches on the electricity supply to the wire 2 and causes the panel to increase in temperature. The heat thus liberated serves to warm the storage tank and associated pipework by convection of warm air rising from the heater.

On reaching -a higher preset temperature of, for example 80C the thermostat 12 senses that the air temperature is at a safe level and switches off the electricity supply to the heater.

Thus the required frost protection is achieved and the thermostat is ready tokdetect the next fall in temperature to freezing levels, s-hould it occur. Advantageously a second thermostat having a different set temperature operating range may be wired in series with the thermostat 12 to reduce the temperature at which the heater is switched off.

In the example of the domestic storage tank in a large ventilated loft, the rate of air change. and therefore heat loss from the space can be high which may prevent the air temperature from attaining the thermostat cut-out temperature for a very long time after the water in the tank is at a safe temperature. In this example it is possible to reduce this time lag by covering the storage tank and associated pipework in an enclosure (not shown) made from an inexpensive material such as thin polythene sheeting. preferably this material w-ould have an aluminised or reflective surface on one side to reflect the heat, or be of bubble construction to provide a measure of insulation, and the enclosure may be removeable.

By mounting both the thermostat and the heater within the enclosure, the air temperature can be more closely controlled by the cooperation of the heater and the thermostat. Small openings in the enclosure may be desirable to provide adequate ventilation to the water storage system and improve the speed of response of the thermostat 12, by promoting air flow past the thermostat.

An additional advantage of this method of heat containment is the reduction of dust and other air-borne pollution or contamination of the water in,the storage tank whilst reducing the energy requirement to maintain frost-free conditions. In the interests of safety, the electric heater panel may under these conditions have a maximum surface temperature of aboutlOO0C on its underside by the use of an insulating layer as described hereinbefore and may also have fitted an electric switch 13 wired to override' the thermostat 12 to switch on the electric heater manually.

This would enable the heater to be checked each winter.

A simple warning lamp 14Zmay be mounted on the frame 1, or remote from the frame,- to indicate whether the heater is on or off by sensing the voltage across the heater.

Where remote pipework is present beyond the enclosure an electric trace-heating element may be connected to the heater at connection 15. This element is advantageously controlled by the thermostat 12.

Thus is provided -a simple and low cost means of frost protection which can be installed by any competent electrician or D-I-Y.enthusiast.

The panel heater construction herein described has wide versatility in so far as it results in a low weight 'heater which is quick to warm up, is easily controlled, and can readily be made in a variety of different sizes and different heat outputs.

It is possible to mount a thermostat on the side of the frame 1 to provide high-limit protection.

Other applications for this form of electric heater are envisaged which include comfort air heating in domestic, commercial, industrial and automotive situations.

The use of aluminium and/or tin plate allows the heater to be resistant to corrosion, aesthetically pleasing and exceptionally light in weight. By painting one or both surfaces of the heater with black high-emissivity paint the heater can be used for radiant heating applications instead of convective heating. Here it is envisaged that heater panels may be wall or ceiling mounted at strategic intervals. Again, air may be made to pass through the heater by means of a powered fan to provide the addition of convected heat.

The enclosure provides additional insulation if it is in the form of a plurality of layers of flexible sheeting incorporating air pockets therebetween.

In situations where it is not desirable for the heater to be in too close proximity to a wall or floor spacer legs 16 may be provided, one at each corner of the heater.

The legs are preferably of channel shaped construction, one side of which is secured under the bottom flange 10 of the frame 1. The pressure between the frame and the contained layers may enable the legs 16 to be held by friction alone. This way the position of the legs can be adjusted if required.

Claims

1. An electric heater comprising a housing having a heat transfer surface via which heat is transferred from the he-ater, the housing containing a resistance element electrically connected to electric terminals outside the housing and thermally conductive heat diffusing material disposed betwe n the resistance element and the heat transfer surface.

2 A heater according to claim 1 wherein the arrangement is such that air can flow through the thermally conductive heat diffusing material and out through or: adjacent the heat transfer surface.

3. A heater according to claim 1 or 2 wherein the heat diffusing material is thin metal foil in the form of any of the following, singly or in combination: i) expanded metal sheet ii) random strips iii)random wire.

4. A heater according to any of the preceding claims wherein the resistance element is coated with an insulating layer.

5. A heater according to any of the preceding claims including thermally conductive sheeting between the resistance element and the thermally conductive heat diffusing material.

6. A heater according to any of the preceding claims wherein the housing includes a second surface on the side of the heater opposed to the first, heat transfer surface, the heater additionally comprising one or more layers of material disposed between the resistance element and the second surface, the material being in the form of one or more of the following i) thermally conductive ii) thermally insulating,

7 A heater accordi-ng to claim 6 wherein the material is selected from the following list: expanded meta &

sheet; random strips of metal foil; random wire; glass fibre; mineral wool; felt; insulating board.

8. A heater according to any of the preceding claims including any one or more of the following electrically connected to the resistance wire: i) a thermostat ii) a switch; iii) a warning light

9. A heater according to any of claims 1 to 7 including a plastics material enclosure and a thermostat, the arrangement being such that the enclosure encloses a volume of space, the thermostat is sensitive to the temperature in the space and the electric heater,- when switched on by the thermostat heats the space.

10. A heater according to any of the preceding claims including spacer members attached to the housing to space the heater from the surface on which it is to be mounted or stand.

11. An electric heater substantially as herein described with reference to the accompanying drawings.

12. Apparatus for maintaining a predefined minimum temperature in a defined space, the apparatus comprising an insulating enclosure means defining the said space, an electric hea-ter and a thermostat the arrangement being such that the thermostat reads the temperature in the said space and the electric heater, when switched on by the thermostat, heats the space.

13. Apparatus according to claim 12 wherein the enclosure means is in the form of a sheet of plastics material.

14. . Apparatus according to claim 13 wherein the sheet has a reflective surface on one side thereof.

15. Apparatus according to any of claims 12 to 14 wherein the enclosure is apertured to allow airflow therethrough.

16. Apparatus according to claim 12 wherein the enclosure is in the form of flexible sheeting comprising a plurality of layers incorporating air pockets therebetween.

17. A method of protecting a water storage installation from frost damage comprising .apparatus according to any of claims 12 to 16 the heater being positioned in close proximity to the water storage tank, and the storage tank being at least partially housed in the enclosure.

18. S method according to claim 17 wherein the heater is in accordance'with any of claims 1 to 8.

19 A method of protecting a water storage installation from frost damage substantially as herein described, with reference to the accompanying drawings.