CA1190579A

CA1190579A - Electric heater

Info

Publication number: CA1190579A
Application number: CA000424673A
Authority: CA
Inventors: Kosta Pelonis
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-03-28
Filing date: 1983-03-28
Publication date: 1985-07-16

Abstract

ROGERS, BERESKIN & PARR

INVENTOR: KOSTA PELONIS
TITLE: ELECTRIC HEATER

ABSTRACT OF THE DISCLOSURE

A new type of electric heater is described. The heater includes a housing in which is mounted a generally planar core of semi-conductor material formed with a multiplicity of apertures. Conductive coating are formed on opposing faces of the core to permit a heating current flow to be generated in the core by applying a voltage difference to the conductive coatings. A fan mounted within the housing directs an air flow through the core apertures to draw heat from the core. The semi-conductor material is selected to have a high positive resistance-temperature coefficient which pre-vents the core from heating to above 200°C when conven-tional line voltages are applied to the core and which permits marked variation in the quantity of heat delivered by variation of fan speed alone.

Description

LD O~ l'HE INVENTION
The invention relates to an electrically powered heating device.

BACKGROUNG OF THE INVENTION
_ At present, electrically powered forced-air heaters incorporate a heating element which is a resist-ance coil or wixe. A voltage is applied to the heating element to raise its temperature, and an air flow is - directed over the heating element to remove heat to the adjoining environment.
A problem with such prior art devices is that the heating element lS normally in a red-hot state during operation. Such heating elements can ignite flammable materials upon contact, and additionally pose a threat of fire or explosion if volatile, flammable matericls are nearby. What is presently required is a , novel type of electric heater whose heating element can provide large quantities of heat without necessarily reauiring a high operatin~ temperature. Additionally, it is ?referable that such â heating device have rela,ively simple construction and be relatively inexpensive.

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BRIEF SUMMARY OF T~E INVENTION

The invention provides an electric heater including a heating element which is a generally planar core of a semi-conductor material fo~med with a multiplicity of apertures. ~eans are provided for connecting the heating element to a power source to produce a current flow in the core thereby raising its temperature. Means are also provided for directing an air flow through the apertures of the core to remove heat generated in the core to the adjoining environment.

The core material is preferably selected to have a high positive resistance-temperature co-efficient so that the temperature of the heater core .remains below a predetermined temperature ~preferably below 200C) even when standard line voltages are applied to the core and no air flow is directed to the core apertures. With no air flow through the apertures, the temperature and resistance of the core-will tend to rise thereby simultaneously increasing the heat conducted to the environment and decreasing the power consumed by the core. With a sufficiently high temperature co-efficient an e~uilibriu~ can be achieved at a relatively low temperature.

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BRIEF D~SCRIPTION O~ THE DRAWINGS
The invention will be better understood with reference in drawings, in which:
~ ig. 1 is a front perspective view of a heater embodying the invention;
Fig. 2 is an exploded perspective view of the heater;
Fig. 3 is a schematic representation of the heater illustrating the manner in which the heater is controlled.

DESCRIPTION OF PREFERRED EMBODIMENT
Reference is made to figs. 1 and 2 which - illustrate a heater 10 which is a preferred embodiment o' the invention. Much of the construction of the heater 10 is conventional and conse~uently will not De described in detail in order to highlight more inventive aspects.
The heater 10 includes a housing 12 of generally rectangular shape having open front 2nd back s~r aces. A protective metal grill la is affixed to circumferential flange 16 to cover the front of the housing 12. ~n electrically powered heating element 18 is mounted within the housing 12, and a conventional lcn 20 mounted within the housing serves to direct an air flow through the heating element 18. A generallv , rectangular sheet 22 of porous filter material is positioned behind the fan 20 to filter air drawn into the housing 12 and is secured against a protective metal grill 23 by a rectangu-lar retaining element which press fits into the back of the housing 12. Suitable construction and assembly of the various components (with the exception of the heating element 18) will be readily apparent to one skilled in the art.
The heating element 18 is a generally planar core 26 of a semi~conductor material formed with 2 : 10 multiplicity of apertures. Conductive silver coatings 28, 30 (which may be relatively thin) are deposited on opposing faces of the core 26. The coatings 28, 30 serve as terminals to which a voltage difference can be applied to produce a current flow between the opposing faces of the core 26.
Electrical contact with the coatings 28, 30 czn be made in any appropriate manner. ~or example, as illustrated, a pair of flat, rectangular metal rings 32, 3a can be abutted against the coatings 28, 30, one ring extending about the periphery of each coating. The r~ings 32, 34 and core 26 are formed with apertures that can be placed in registration, such as the apertures 36, through which threaded fasteners are inserted to secure these me.~bers to one another and additionally into fixed relation-ship with the fan 20. One such fastener is illustra.edzna consists oi a bolt 40,first and second flanged 7 ~ ~

insulating shoulder washers 42, 44, a first nut ~6, a tubular spacer 48 and a second nut 50. The shoul~er washers 42, 44 are inserted into the aligne~ apertures 36, the bolt 40 extended through the shoulder washers 42, 44, and the nut 46 tightened on the bolt 40 to draw the rings 32, 34 into engagement with the core 26. The spacer 48 is mounted on ~he shaft of the bolt 40 which is extended . through aligned apertures 52 in the housing of the fan 20 and which is then securecl with the nut 52. Electrical contact between a power cord 54 and the rings 32, 34 is affected during assembly by securing electrical connectors of the cord 5~ beneath the flanges of the shoulder washers 42,44, in an obvious manner. During this assembly stage, the fan 20 is also coupled to a fan control 58 which is in turn connected to the power cord 54.
When the heating element 18 has been fixed to the fan housing and all wiring effected, the resultant structure is wrapped in a rectangular glass-fille~ strip 56 sold by Dupont Corporation under the trade mark Nomek 4lD
an~ essentially press-fit into the housing 12. The strip 56 serves three functions, namel~, to electrically isolate an~ thermally insulate the heating element 18 lrom the housing 12, to permit location of the fan 20 and heating element 18 in the housing 12 without requiring elaborate attachment means, and to seal about the periphery of the element 18 to ensure that air is constrained to flow through .he apertures of the core 2..

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Alternative met~ods of electrically couplins .he core 26 to 2 power source and general assembl~ of such a heater will be readily apparent to one skilled in the art.
The core material is preferably a mixture oi lezd titanate and bariun titanate although a variety of other semi-conductor materials can be selected.
These materials provide the core 26 with 2 relatively high resistance-temperature co-efficient which is 10 nominally 7.65 ohms/C (between the opposing faces of the cores). The resistance as measured between the o?posing faces of the core is about 8 ohms at 165C and - 161 o~s at 185C. Thus, with a line voltage of llOv.
R~'S applied to the coatings 28, 30, the core 26 will have a nominal power consunption of 1500 W at an operating tem?erature of 165~C and ;75 W at an operating temper~ture o_ 185'7C. It will be readily apparent that the power consumption of the core 26 drops markedly as the operating temperature of the core rises. In most circumstances, 7~i th no air flow through the core 26 to draw heat rom it, it is expected that the core 26 will reach an e-u-librium temperature below 200~C. The specific2tio~.s prGvided are exemplary only: the dimensions and m2ter'als o the core can be selected in a manner ~7hich will now be o~vious to one skille~ in the art to accommodate v2rious power, supply voltage, and operating temperature reauirements.

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The inventor has also discovered a novel manner of con.rolling an electric heater embodying the invention, which can significantly reduce the cost of manufacture.
This will be explained with respect to the schematic representation of the heater 10 of fig. 3.
In prior art electric heaters, the quantity of heat delivered has been continuously controlled by varyins the amount of electric power available to the associated heating element. This could be done efficiently by using a silicon controlled rectifier (or some semi-conductor equivalent thereof) with a large current-handling capability and relatively low internal power dissipation.
Un~ortunately, such devices can be relatively expensive if they are required to h~ndle large currents. The control system illustrated in fig. 3 reduces the current demand on switching circuits needed to vary the ~uanti.y of heat deli.vered bv the core 26.
From fig. 3, it will be apparent that the heat delivered by the core 26 is varied solely by varying f2n speed. The fan control 58 preferably includes as a prir,ary switching element a bi-directional silicon con,rollec rectifier or.an equivalent switching device with low inter-l21 power consumption, that permits substantially continuous variation of lan speed. Suitable control circuitry incorporating such devices are well known. By lncreasing fan speed, the temperature of the core 26 is dropped, and the resultant increase in conductivity of the core 26 causes a very marked increase in power consumption. Thus, S without affecting the l1ne voltage applied to the core 26, the quantity of heat delivered by the core 26 can be varied. sucb operation has not been practical with con-ventional electrically powered heating devices. As apparent from fig. 3, the fan control ~8 also includes switches 64 (activated by rotation of a control knob 66 which also serves to vary fan speedj which serve to simultaneously de-activate the fan 20 and discontinue the application of power to the core 26.
Another method of controlling the operation of 1~ the heater 10 involves provision of a detector 68 which can be a thermostatic control responsive to temperature or a photodetector. Instead of coupling the sensing device 68 directly to the line supply of the heating element 18, the sensing device 68 is coupled to the fan 20. Thus, where the sensing device 68 is a thermostatic control, fc?. speed can be made to vary inversel~ wi,h the temperatu-e o the environment to be heated, either continuously, -ncrementally, or simply in an off-on fashion with fan o?eration commencing as a monitored temperature drops below a

2~ first predetermined level and discontinued when the 5~d~

temperature rises above a ~econd predetermined level.
The heater 10 can be controlled in a si~ilar ~ashion if a photo detector is selected for the sensing device 68. For example, the fan control 58 can be adapted to respond to the presence or absence of light, either natural or artificial, in a room to regulate heat delivery. The condition re~uired to activate the heater 10 can either be a low light level or a high light level depending which light state indicates likely use of the - 10 room. In a bedroom, the setting of the sun might be an indicator that the room will soon be in use and that a heater should be activated; whereas, in a living room, absençe of light after sunset might be an indicator that the room will no longer be in use. In all cases, however, control of heat delivery should be affected by regulation of fan speed to avoid costly switching circuitry ~nd the like.
~ ~lthough the invention has been described with res?ect to a portable-type heater, the present invention 2C can be embodied, for e~ample, in the ductwork of the hea~ing system of a house or the like, the duc,wor~
serving as a housing for the heating element and as means for directing air flows through the heating element. Such applications are considered to be part of the present invention and to file within the ambit of the appended claims.

Claims

THE EMBODIMENTS OF AN INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electric heater comprising:
a housing;
a heating element mounted in the housing, the heating element including a generally planar core of semi-conductor material formed with a multiplicity of apertures;
means for electrically connecting the core to an electrical power source to produce a current flow in the core; and, means attached to the housing for producing an air flow through the apertures of the core, the housing being apertured to permit the air flow to escape from the housing.

2. An electric heater as claimed in claim 1 in which the heating element includes conductive coatings formed on opposing faces of the core and the electrical connect-ing means permit a voltage difference to be formed between the conductive coatings.

3. An electric heater as claimed in claim 2 in which the core has a positive resistance-temperature co-efficient which is sufficiently large that the temperature of the heater core is below about 200°C in normal operation and remains below about 200°C when no air flow is produced through the apertures of the core,

4. An electric heater as claimed in claim 2 including control means for varying the power consumption of the core, said control means consisting of means for varying the air flow rate of the air flow producing means.

5. An electric heater as claimed in claim 4 in which the control means include an optical sensing device responsive to the light level of an environment, said control means being adapted to vary the air flow rate with the light level.

6. An electric heater as claimed in claim 2 including control means for sensing the temperature of an environment and for varying the air flow rate of the air flow producing means inversely with the temperature of the environment.

7. An electric heater as claimed in claim 6 in which the control means are adapted for on-off operation, the flow of air being substantially discontinued when the temperature of the environment rises above a first predetermined level and being recommenced when the temperature of the environment drops below a second predetermined level.

8. An electric heater as claimed in claim 3 including control means for varying the power consumption of the core, said control means consisting of means for varying the air flow rate of the air flow producing means.

9. An electric heater as claimed in claim 8 in which the control means including an optical sensing device responsive to the light level of an environment said control means being adapted to vary the air flow rate with the light level.

10. An electric heater as claimed in claim 3 including control means for sensing the temperature of an environment and for varying the air flow rate of the air flow produc-ing means inversely with the temperature of the environment.

11. An electric heater as claimed in claim 10 in which the control means are adapted for on-off operation, the flow of air being substantially discontinued when the temperature of the environment rises above a first predetermined level and being recommenced when the temperature of the environment drops below a second predetermined level.