Note: Descriptions are shown in the official language in which they were submitted.
LD O~ l'HE INVENTION
The invention relates to an electrically
powered heating device.
BACKGROUNG OF THE INVENTION
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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
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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
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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.
