Abstrict
A forced gas electric heater has a serpentine heating element in
series with a preheat coil. Gas to be heated first flows through
the preheat coil before flowing through the serpentine heating element.
A thermistor is disposed between two adjacent turns of the preheat
coil. The thermistor controls the "on" time of a solid
state power control device which is in series with the preheat coil
and the serpentine heating element.
Claims
We claim:
1. A forced gas electric heater comprising: a serpentine heating
element, a preheat coil electrically in series with the serpentine
heating element, the preheat coil being disposed with respect to
the serpentine heating element so that at least part of the gas
being forced through the serpentine heating element in order to
be heated thereby must first flow through the preheat coil, a thermistor
disposed between two adjacent turns of the preheat coil so as to
be in close heat transfer relationship therewith, a solid state
power control device electrically in series with the preheat coil
and the serpentine heating element, means for varying the high-current
conducting state of the solid state power control device in accordance
with variations in the temperature of the thermistor, means for
introducing gas flow through the preheat coil and serpentine heating
element, and means for introducing electrical power to the forced
gas electric heater.
2. The electric heater of claim 1 wherein the serpentine heating
element is disposed within a cylindrical shell and the solid state
power control device is disposed within an enclosure, the cylindrical
shell being fastened to the enclosure.
3. The electric heater of claim 2 wherein the enclosure has a gas
inlet and an exit end, the relationship between the enclosure and
the preheat coil being such that gas flowing through the enclosure
passes through the preheat coil at about said exit end.
4. The electric heater of claim 3 wherein there is a heat sink
within the enclosure which is cooled by gas flowing through the
enclosure.
5. The electric heater of claim 4 wherein the solid state power
control device is mounted on the heat sink.
6. The electric heater of claim 5 wherein the enclosure contains
a potentiometer used to control outlet gas temperature.
7. The electric heater of claim 6 wherein the enclosure contains
a gas damper.
8. The electric heater of claim 1 wherein the preheat coil consists
of the first few turns of the serpentine heating element at the
cooler end thereof.
9. The electric heater of claim 2 wherein the serpentine heating
element is disposed within a quartz tube and wherein there is insulating
material between the quartz tube and the wall of the cylindrical
shell.
10. The electric heater of claim 2 wherein the electric heater
contains means for grounding the cylindrical shell.
11. A forced gas electric heater comprising: a serpentine heating
element, a preheat coil electrically in series with the serpentine
heating element, the preheat coil being disposed with respect to
the serpentine heating element so that gas being forced through
the serpentine heating element in order to be heated thereby must
first flow through the preheat coil, a thermistor disposed between
two adjacent turns of the preheat coil so as to be in close heat
transfer relationship therewith, a solid state power control device
electrically in series with the preheat coil and the serpentine
heating element, means for varying the high-current conducting state
of the solid state power control device in accordance with variations
in the temperature of the thermistor, means for introducing gas
flow through the preheat coil and serpentine heating element, means
for introducing electrical power to the forced gas electric heater,
the serpentine heating element being disposed within a cylindrical
shell, the solid state power control device being disposed within
an enclosure, the cylindrical shell being fastened to the enclosure,
the enclosure having a gas inlet and an exit end, the relationship
between the enclosure and the preheat coil being such that gas flowing
through the enclosure passes through the preheat coil at about said
exit end, there being a heat sink within the enclosure which is
cooled by gas flowing through the enclosure, the solid state power
control device being mounted on the heat sink.
12. The electric heater of claim 11 wherein the serpentine heating
element is disposed within a quartz or ceramic tube and wherein
there is insulating material between the quartz or ceramic tube
and the wall of the cylindrical shell.
Description BACKGROUND OF THE INVENTION
This invention concerns forced air or gas heaters. Examples thereof
are shown in U.S. Pat. Nos. 3,783,236, 3,654,431, 3,551,643 and
3,094,606.
This invention provides a forced gas heater that prevents premature
heater element burnout due to overheating.
A major problem with any process heat application involving forced
air or gas heaters is premature heater element burnout. Heater elements
oxidize with heat, the higher the temperature, the faster the oxidation,
and if the heater element gets hotter still, it will melt. Premature
burnout can occur if gas flow is too low and the heater element
gets hotter than it should. In this invention a built-in controller
keeps the heater element from burning out prematurely, even if gas
flow is completely stopped.
Because premature burnout is a common problem with gas heater use,
external controllers, which are sometimes expensive, have been used
in the past. These controllers normally operate by measuring exit
gas temperature from the heater, and, if the exit temperature is
too great, the controller will cut power to the heater. This works,
but it can take too long to react if the thermocouple used to measure
the gas is to massive or if it was moved somehow from the exit of
the heater. Also, if the gas flow is suddenly cut off, the thermocouple
will read an increasingly lower temperature, because the hot gas
will not pass over the junction. Therefore, the controller actually
puts more power to the element, when, in fact, the element itself
is too hot already. One way users have eliminated this problem is
by adding yet another component to the controller.
By using a flow-sensor/switch or a pressure-sensor/switch, the
controller will cut power when there is an interruption or reduction
in the gas flow. This must be used with the thermocouple temperature
control to make the system work. It can fail easily, if there is
too much pressure for the sensor/switch to take, for instance, if
there is an unintentional burst of flow, the sensor may be pushed
beyond its designed limits, and break. The user may also set these
controllers to the wrong settings, and end up burning out the element
because the controller was working, but within the wrong parameters.
Besides the added cost of all of this equipment to control, the
space requirement often becomes a problem. Many users need to fit
this equipment into small spaces, or into machines that they sell.
There are heaters available that will control in a small space,
but will not work when the gas flow is completely cut off.
SUMMARY OF THE INVENTION
A gas heater in accordance with this invention solves these problems.
The gas heater is small and self-contained, without the need of
external control devices and without the need of calibration. The
gas heater keeps controlling even when there is no gas flow. Control
is accomplished by means of a thermistor placed between two adjacent
turns of a coiled heater winding, referred to as a preheat coil,
at about the entrance end of the heater. Thus the thermistor is
not subjected to the high temperatures in the central part of the
coiled heater element or to the even higher temperatures at the
exit end of the coiled heater element. The heater element used is
that disclosed in U.S. Pat. No. 3,551,643, the disclosure of which
is incorporated herein by reference. All or part of the gas flowing
through the heater element flows through the preheat coil.
As disclosed in said patent, the heater element, hereinafter referred
to as a serpentine heating element, comprises a length of coiled
resistance wire, the individual turns of the coil having a substantially
polygonal shape and being radially displaced from adjacent turns.
As the temperature of the thermistor changes, its resistance changes.
This resistance change can be used to vary the "on" time,
that is to say, the high-current conducting state, of a solid state
power control device, such as a silicon-controlled rectifier or
a triac or a power transistor, which is in series with the serpentine
heating element. Thus the power to the serpentine heating element
can be controlled by the combination of the thermistor and the solid
state power control device, thereby controlling the maximum temperature
which the serpentine heating element can attain. Placing the thermistor
between two adjacent turns of a heater winding places the thermistor
in close heat transfer relationship therewith and makes the thermistor
react faster to changes in the temperature of the heater winding
than if the thermistor were located outside the turns or if the
thermistor were located downstream from a heater winding and relied
primarily on heat transfer by flowing gas. In the latter case, if
gas flow were accidentally shut off, reaction time of the thermistor
would be too slow to prevent the heater element from overheating.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective view of one example of a heater in accordance
with this invention while FIGS. 2 and 3 are exploded perspective
views.
FIG. 4 is a view of the thermistor and preheat coil.
FIG. 5 is an end view of the heater element portion of the electric
heater.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One example of a forced gas heater in accordance with this invention,
as shown in the drawings, comprised a cylindrical metal shell 2
fastened to an enclosure 3 made of electrically insulative material.
Enclosure 3 has a cover 30 fastened thereto by screws 31. A serpentine
heating element 4 was disposed within a quartz or ceramic tube 5
within metal shell 2. Insulation 32 was disposed between quartz
tube 5 and the cylindrical wall of metal shell 2. Forced gas, for
example, air, would enter inlet 6 of enclosure 3, pass therethrough
and out at exit 8, into inlet end 9 of metal shell 2, through serpentine
heating element 4 and out at outlet 10. Disposed within the exit
area of enclosure 3 was a short length of coiled heater winding,
preheat coil 11, consisting of four or five turns of resistance
wire. Preheat coil 11 was electrically in series with heating element
4. Disposed between two adjacent turns of preheat coil 11 was a
thermistor 12, which was electrically connected to an electrical
control circuit within enclosure 3, part of the electrical control
circuit being mounted on printed circuit board 13. The electrical
control circuit included a silicon-controlled rectifier 14, hereafter
SCR, as the solid state power control device, which was mounted
on heat sink 15. SCR 14 was electrically in series with preheat
coil 11 and heating element 4. SCR 14 controlled the time during
which heating current flowed through preheat coil 11 and heating
element 4. The high-current conducting state of SCR 14 was dependent
on the temperature of thermistor 12, and was such as to limit the
maximum temperature which preheat coil 11 could attain. Since serpentine
heating element 4 was in series with preheat coil 11, SCR 14 also
limited the maximum temperature which serpentine heating element
4 could attain.
A ceramic cylinder 16 extended through the axis of serpentine heating
element 4. A metal wire 17 extended through the axis of ceramic
cylinder 16, the purpose of which was to provide electrical connection
to the exit end of serpentine heating element 4. Wire 17 was connected
to metal rod 18. The other end of serpentine heating element 4,
the entry end, was connected to metal rod 20 by means of wire 19.
Preheat coil 11 was mounted on a threaded ceramic rod 21 and was
disposed between two ceramic blocks 22 which fit within exit 8 of
enclosure 3 and which were held together with bolts 26. Metal rod
18 was connected to the exit end of preheat coil 11 by means of
electrical connector 23. Metal rod 18 passed through a hole in thick
ceramic disk 28 which was disposed between exit 8 and serpentine
heating element 4.
Electrical current flow was as follows. Electrical power was supplied
through tube 24 by means of two lead-in wires (not shown). One lead-in
wire was connected to the entry end of preheat coil 11. Current
flowed from there through preheat coil 11, through electrical connector
23, through metal rod 18, through wire 17 to the exit end of serpentine
heating element 4, through serpentine heating element 4 to wire
19, to electrical connector 25, through a wire (not shown) to SCR
14 and from there, through a wire (not shown), to the other lead-in
wire entering tube 24. Thus SCR 14 was in series with preheat coil
11 and serpentine heating element 4.
Heat sink 15 was shaped and disposed so as to be cooled by the
gas entering inlet 6. The purpose of heat sink 15 was to dissipate
heat within enclosure 3. A gas damper 26 on enclosure 3 could be
used to vary gas flow. A potentiometer 27 in the electrical control
circuit could be used to vary the temperature of the gas exit outlet
10. Metal rod 29, which was electrically connected to metal shell
2, could be connected to ground of the power supply wires in order
to ground metal shell 2.
For purposes of this invention, it is not necessary that preheat
coil 11 be a winding separate from that of serpentine heating element
4. Preheat coil 11 could consist of the first few turns of serpentine
heating element 4 with thermistor 12 disposed between two adjacent
turns of said first few turns. |