Note: Descriptions are shown in the official language in which they were submitted.
CA 02590028 2007-05-17
ELECTRIC HEATING DEVICE
FIELD OF THE INVENTION
[0001] This invention is related to electric heating devices.
BACKGROUND OF THE INVENTION
[0002] Various types of electric air heating devices using PTC ("positive
temperature coefficient") elements are known. A PTC element has a given
resistivity at any given temperature, and the resistivity of the PTC element
rises
or falls with its temperature. In particular, the PTC element's resistivity
rises
exponentially once its temperature is increased over a certain temperature.
Accordingly, once the PTC element's temperature is high enough, the
resistivity
of the PTC element becomes sufficiently high that the flow of current
therethrough is nearly stopped. Because of this property, PTC elements have
the beneficial characteristic of being self-limiting, thereby reducing the
risk that
an electric heater which includes a PTC element may cause a fire. However, in
the prior art, PTC elements have been used primarily as sensors, to severely
limit current when necessary for safety.
[0003] In addition, a heater with a heating element including one or more
PTC elements which produces a specified output for a specified airflow is
known.
However, this prior art device does not provide for proportionate (i.e.,
variable)
control of the heating element. Instead, this device produces a preselected
power output for a preselected airflow when activated, i.e., the control is
fixed
because the heater can only be activated or de-activated, and if activated,
only a
certain output is provided thereby. Repeatedly turning this prior art heater
on
and off in response to signals from a thermostat tends to create significant
changes in the ambient temperature, i.e., the typical thermostat does not
signal
for more heat until room temperature is relatively far below the setpoint
temperature. Also, the typical thermostat does not stop a heater from
operating
CA 02590028 2007-05-17
until the setpoint temperature is exceeded, generally to an extent which is
noticeable by those in the room.
[0004] There is therefore a need for an electric heating device which
overcomes or mitigates one or more of the defects of the prior art.
SUMMARY OF THE INVENTION
[0005] In its broad aspect, the invention provides an electric heating
device including a fan for moving a volume of air at a rate substantially
corresponding to a speed of rotation of the fan and a fan motor for rotating
the
fan over a range of speeds. The device also includes a heat generator with one
or more PTC elements for generating heat, and for transferring the heat to the
moving volume of air. Also, the device has a control subassembly adapted for
proportionate control of the fan motor based on a variable required heat
output
so that the rate of movement of the moving volume of air varies in proportion
to
changes in the required heat output.
[0006] In another of its aspects, the invention provides a method of
heating air having an ambient temperature. The method includes, first,
providing
a fan for moving a volume of air at a rate substantially corresponding to a
speed
of rotation of the fan, and subsequently, providing an electric fan motor for
rotating the fan over a range of speeds. Next, a heat generator is provided
which
includes one or more PTC elements for generating heat, and for transferring
the
heat to the moving volume of air. Finally, a control subassembly is provided
which is adapted for proportionate control of the motor based on a variable
required heat output so that the rate of movement of the volume of air varies
in
proportion to changes in the required heat output.
[0007] In yet another aspect, the invention provides an electric heating
device including a fan for moving a volume of air at a rate substantially
corresponding to a speed of rotation of the fan, and an electric fan motor for
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rotating the fan over a range of speeds. The device also includes a heat
generator having one or more PTC elements for generating heat and one or
more heat transfer elements for transferring the heat from the PTC element to
the moving volume of air. Also, the device includes a control subassembly
adapted for proportionate control of the fan motor based on a variable
required
heat output so that the rate of movement of the moving volume of air varies in
proportion to changes in the required heat output.
[0008] In yet another of its aspects, the invention provides an electric
heating device including a fan for moving a volume of air at a rate
substantially
corresponding to a speed of rotation of the fan, and a fan motor for rotating
the
fan over a range of speeds. The device also includes a circuit having one or
more heating resistors for generating heat and one or more PTC elements
electrically connected in series with the heating resistor for generating heat
and
for controlling current flowing through the circuit. Also, the device includes
one or
more heat transfer elements for transferring the heat from the heating
resistor
and the PTC element to the moving volume of air. In addition, the device has a
control subassembly adapted for proportionate control of the fan motor based
on
a variable required heat output so that the rate of movement of the moving
volume of air varies in proportion to changes in the required heat output.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention will be better understood with reference to the
drawings, in which:
[0010] Fig. 1 is a cross-section of an embodiment of the electric air heating
device of the invention;
[0011] Fig. 2 is a schematic circuit diagram of an embodiment of an
electric circuit for the heating device of Fig. 1;
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[0012] Fig. 3 is an exploded isometric view of the heating device of Fig. 1,
drawn at a smaller scale, showing a grille removed from a housing body;
[0013] Fig. 4 is a rear view of the device of Fig. 1 with part of the housing
body removed;
[0014] Fig. 5 is a top view of the device of Fig. 1 with a portion of the
housing body removed;
[0015] Fig. 6 is a front view of an embodiment of a heat generator of the
invention, drawn at a larger scale; and
[0016] Fig. 7 is a graph of airflow versus power based on data resulting
from testing of an embodiment of the device of Fig. 1.
DETAILED DESCRIPTION
[0017] Reference is first made to Figs. 1 - 5 to describe an embodiment of
an electric heating device in accordance with the invention indicated
generally by
the numeral 20. Preferably, the device 20 includes a fan 22 for moving a
volume
of air (indicated generally by the numeral 24) at a rate substantially
corresponding to a speed of rotation of the fan 22, and a fan motor 26 for
rotating
the fan 22 over a predetermined range of speeds. The device 20 preferably also
includes a heat generator 28 and a control subassembly 30. In one embodiment,
the heat generator 28 preferably includes one or more PTC elements 32 (Fig. 4)
for generating heat, and for transferring the heat to the moving volume of
air.
Preferably, the heat generator 28 also includes one or more heat transfer
elements 34 (Fig. 4) providing relative large exposed surface areas, for
effective
heat transfer from the PTC elements 32 to the air flowing through the heat
generator. The control subassembly 30 (Fig. 2) is adapted for proportionate
control of the fan motor 26 based on a variable required heat output so that
the
rate of movement of the moving volume of air (i.e., moving through the heat
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generator 28) varies in proportion to changes in the required heat output, as
will
be described.
[0018] In one embodiment, the fan motor 26 is adapted to rotate the fan 22
over a range of speeds in proportion to a range of voltages of electricity
supplied
to the fan motor 26. The control subassembly 30 preferably includes a triac
for
altering voltages of electricity supplied to the motor 26 in proportion to
variations
in measured differences between ambient temperature and a preselected set
temperature. The measured differences are determined by any suitable
temperature sensor.
[0019] The control subassembly 30 preferably is adapted for proportionate
control of the fan motor 26 based on measured differences between ambient
temperature and a preselected set temperature. Preferably, the proportionate
control is effected via a closed loop control system, i.e., a control system
in which
feedback is provided to the system which determines whether the fan motor is
activated. The feedback preferably is provided any suitable ambient
temperature-sensing means. For example, a suitable thermostat (e.g., including
a thermistor for sensing ambient temperature) could be used to provide
feedback. Because such feedback-providing devices and closed loop control
systems generally are well-known in the art, further description thereof is
not
needed.
[0020] As shown in Fig. 2, the heating device 20 preferably includes a
circuit 35 to which the control subassembly 30 is operatively connected, for
controlling the fan motor 26 based on settings input by a user via a control
device
36, and also based on input from a thermistor 38. The control device 36
permits
control of the set temperature, which is compared to information about ambient
temperature provided by the thermistor 38. The control circuit 30 controls the
speed of the motor 26 based on differences between the set temperature and
ambient temperature. As noted above, the control circuit 30 preferably
controls
the speed of the motor 26 by causing the triac included therein to vary the
CA 02590028 2007-05-17
voltage of the electricity supplied to the motor 26 based on differences
between
the set temperature and the ambient temperature.
[0021] As shown in Figs. 3 - 5, the volume of moving air preferably is
directed to the heat generator 28 by a channelling device 40. The channelling
device 40 preferably is positioned in a housing body 42 in a housing
subassembly 43. The housing subassembly 43 is made of any suitable material
and preferably includes a grille 44 with an inlet portion 46 and an outlet
portion
48. The channelling device 40 preferably includes two substantially parallel
side
portions 41 (Fig. 4) generally extending from the fan 22 to the heat generator
28,
and a floor portion 47. Preferably, the moving volume of air is generally
defined
by the space enclosed by the channelling device 40 and a curved portion 45 of
the housing body 42 (Fig. 1). The channelling device 40 preferably is made of
any suitable material, e.g., light sheet metal.
[0022] Accordingly, the moving volume of air preferably is directed through
the heat generator 28 by the channelling device 40. The air thus directed
passes
through apertures 29 in the heat generator 28. As noted above, the heat
generator 28 preferably includes PTC elements 32 which generate heat when
current is passed therethrough, and heat transfer elements 34 configured for
transfer of heat from the PTC elements to the air moving through the apertures
29. In one embodiment, the heat transfer elements 34 are integrally formed
parts
of the PTC elements 32, shaped as appropriate for optimal heat transfer
characteristics. However, the heat transfer elements 34 may alternatively be
formed of a suitable heat-conducting material and suitably connected to the
PTC
elements 32, as will be described.
[0023] In use, the fan 22 is mounted in a bottom area 50 of the housing
42. The fan 22 is configured to draw air into the housing 42 through the inlet
portion 46, as indicated in Fig. 1 by arrow "A". Moving air is then directed
by the
fan 22 into the channelling device 40, as indicated by arrow "B". The
channelling
device 40 directs the moving air over (or through) the heat generator 28 and
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subsequently through the outlet portion 48 of the grille 44, as indicated by
arrow
"C". The positioning of the fan 22 below the heat generator 28, and also the
positioning of the outlet portion 48 above the inlet portion 46, are important
because they take advantage of the fact that a volume of warm air (i.e.,
relative
to air thereby surrounding) rises.
[0024] The control subassembly 30 controls the fan motor 26 based on a
required heat output. As noted above, the control subassembly 30 preferably
includes a triac which is adapted to alter the voltage supplied to the fan
motor in
proportion to the measured differences between ambient temperature and the
preselected set temperature. For instance, if the preselected set temperature
is
20 C and the ambient temperature is 18 C, the triac, which preferably is
operatively connected to a thermistor, adjusts the voltage of the electricity
supplied to the fan motor 26 accordingly. However, if the ambient temperature
were, for example, 17 C, then proportionately more voltage would be applied to
the fan motor 26. Increasing the voltage of the electricity supplied to the
fan
motor 26 results in a proportionate increase in the speed of rotation of the
fan 22.
[0025] It will be understood that an increase in the speed of the fan 22,
which results in a proportionate increase in the rate of movement of the
moving
air which moves over the heat generator, lowers the temperature of the PTC
element. Lowering the temperature of the PTC element results in more current
being allowed to pass through the PTC element (i.e., the heat generator).
[0026] As shown in Fig. 7, in one embodiment, the relationship between
airflow and power is nearly linear, although it is not exactly linear.
Instead, the
curve on the graph of airflow versus power shows that although the power
output
is at approximately 800 watts with an airflow of approximately 1.7 m/sec., at
approximately 1200 watts, the airflow is approximately 5.3 m/sec. The
relatively
flat profile of the curve shown in Fig. 7 indicates that the heater of the
invention
has a relatively high degree of operational stability, i.e., the relationship
is almost
linear.
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[0027] Another embodiment of the invention is disclosed in Fig. 6, in which
elements are numbered so as to correspond to like elements shown in Figs. 1 -
5. In an embodiment 120 of the heating device, a heat generator 128 includes
one or more PTC elements 132 for generating heat and one or more heat
transfer elements 134 for transferring heat from the PTC elements 132 to the
moving volume of air (Fig. 6).
[0028] Preferably, the heat transfer elements 134 are fins configured for
optimal heat transfer characteristics (i.e., for transfer of heat from the
elements
134 to the air moving past such elements), and suitably connected to the PTC
elements (Fig. 6) for maximum heat transfer from the PTC elements 132 to the
heat transfer elements 134. The heat transfer elements preferably are any
suitable heat-conducting material, such as aluminium.
[0029] In one embodiment, the heat generator 128 is approximately 9.5
inches long, approximately 0.5 inch wide, and approximately 3.3 inches high.
As
can be seen in Fig. 6, the heat generator 128 preferably includes a plurality
of
apertures 129, to provide a relatively large surface area, for effective heat
transfer. The solid volume is approximately 8.9 in.3 , and the surface area
therein
is approximately 187 in.2.
[0030] Any element in a claim that does not explicitly state "means for"
performing a specified function, or "step for" performing a specific function,
is not
to be interpreted as a "means" or "step" clause as specified in 35 U.S.C.
112,
paragraph 6.
[0031] It will be appreciated by those skilled in the art that the invention
can take many forms, and that such forms are within the scope of the invention
as claimed. The scope of the claims should not be limited by the preferred
embodiments set forth in the examples, but should be given the broadest
interpretation consistent with the description as a whole.
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