Note: Descriptions are shown in the official language in which they were submitted.
CA 02743378 2016-03-02
"EXPLOSION PROOF FORCED AIR ELECTRIC HEATER"
FIELD OF THE INVENTION
The present invention relates to an improved explosion proof forced air
electric heater.
The heaters are primarily used for heating hazardous environments where the
incidences
of fire or explosion are increased due to the presence of flammable gases,
vapors, or
liquids; combustible dust particles, filings, or ignitable fibers.
Furthermore, the heaters
can similarly be used to heat non-hazardous environments.
BACKGROUND OF THE INVENTION
(Prior Art) Explosion proof forced air electric heaters have been on the
market since the
1970's under US patent: 4,117,308. The majority of the forced air electric
heaters
currently in use rely upon a liquid filled heat exchanger. (See US design
patent: D
356,367). These heat exchangers are generally comprised of three main
components: (1)
a steel bottom header; (2) steel tubes with roll formed aluminum fins; and (3)
a steel top
header, which houses a pressure relief valve. The bottom header contains the
electric
heating source, which is typically a tubular electric resistant element
submerged in a
glycol water mixture within the cavity of the bottom header. The prior art's
heat transfer
process is initiated by supplying the electric heating elements with
electricity. The
electricity is converted into heat, thereby increasing the temperature of the
glycol water
mixture to its boiling point, thus creating glycol steam, which rises through
the steel tubes
and into the top header. The heat is then conducted to the steel tubes and
transferred to
the roll formed aluminum fins where an air mover forces cool air over the fins
to
distribute the heat. The heating cycle is repeated when the glycol steam cools
and reverts
back to the bottom header in liquid form for reheating. The heat exchanger is
typically
vacuum charged to reduce the resistance exerted upon the glycol steam and to
allow for
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even heat distribution during the cycle. The prior art's safety mechanisms
include: the top
header's pressure relief valve, which protects the heat exchanger in the event
pressure
limits are exceeded; and the high limit temperature switch imbedded in the
bottom
header, which cuts power to the electric heating elements and air mover should
the
30 system overheat.
The prior art is typically controlled by a thermostatic switch which monitors
the ambient
or desired environmental temperature. A call for heat typically activates a
switch that
engages a mechanical contactor, thereby triggering the electric heating
elements and air
mover simultaneously. Once the demanded temperature is achieved, the switch
35 disengages the mechanical contactor and immediately turns off the
electric heating
elements and air mover.
The prior art's typical explosion proof forced air heaters have proven
themselves reliable,
however, the manner with which they transfer electric heat into the atmosphere
and the
lack of controllability thereof suggests several key short comings in the
current design.
SUMMARY OF THE INVENTION
The present invention is designed to improve the performance and
controllability of
explosion proof forced air electric heaters.
The present invention is a liquid free dry heat exchanger. The dry heat
exchanger is
comprised of a top header and bottom header with a heat exchanger coupled
between
the headers. The heat exchanger contains high heat conductive metal tubes with
press fit
high heat conductive metal fins. The heating source of the unit is assembled
by mounting
electric heating elements within the metal tubes. The bottom and top headers
are then
press fit on either end of the metal tubes to form an electrical enclosure on
either end of
the heating elements.
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The present invention is an electronically controlled heating system that
makes use of an
electronic control circuit to control and monitor the electric heating
elements, air mover,
operating temperature, internal enclosure temperature and hi limit switch. The
heating
cycle begins when there is a demand for heat which engages a thermostatic
switch,
55 activating the electronic control circuit. The electronic control
circuit then uses a solid
state relay to momentarily engage the air mover to ensure it is in working
condition
before engaging the heat source. A current transformer device assesses the air
mover's
functionality and sends a signal to the electronic control circuit to either
engage the
electric heating elements if the air mover is operational, or terminate the
startup cycle if
60 the air mover is not working. Upon confirmation the air mover is
operational, the
electronic control circuit switches a solid state relay to engage the electric
heating
elements. Once the elements heat the heat exchanger to a predetermined
temperature,
the electronic control circuit re-engages the air mover, drawing cool air
through the intake
opening, over the heated metal fins and out the exhaust opening into the
atmosphere. A
65 temperature monitoring device mounted in the heat exchanger continuously
monitors the
exchanger's temperature to ensure the air mover is not prematurely activated
before the
fins are sufficiently heated. The heating elements remain engaged until the
demand for
heat is met, at which point the thermostatic switch disengages and signals the
electronic
control circuit to disconnect power leading to the electric heating elements.
The air
70 mover remains engaged until the heat exchanger has sufficiently cooled,
at which time the
temperature monitoring device sends a signal to the electronic control circuit
disengaging
the air mover and completing the heating cycle. In the event the heater
overheats, a hi
limit monitoring device mounted in the heat exchanger signals the electronic
control
circuit to disconnect power leading to the electric heating elements and air
mover
75 simultaneously in order to safely shutdown the heat exchanger. Unlike
the present
invention, the prior art typically uses mechanical relays to control the
heater, and the
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electrical heating elements and air mover are engaged or disengaged
simultaneously
when there is a demand for heat or when the demanded temperature has been met.
The present invention heats the surrounding environment via two distinct heat
transfer
80 phases. Initially, the electric heating element is supplied with an
electrical current which
heats the heat exchanger that is comprised of heat conductive metal tubes and
fins. An
air mover then transfers and distributes this heat from the tubes and fins
into the
surrounding environment by forcing cool air over the heat exchanger. Unlike
the present
invention's dual transfer theory, the prior art employs a less efficient
design by requiring
85 five distinct heat transfer phases to occur. The prior art initially
transfers heat from (1)
the electric heating elements to a glycol and water mixture. (2) As the
mixture's
temperature increases it is converted from a liquid into steam. (3) The
steam's heat is
then transferred to the steel tubes. Finally, (4) the steel tubes transfer
their heat to the
aluminum fins, at which time (5) the heat is distributed from the fins into
the surrounding
90 environment by an air mover.
During the present invention's heating cycle the heat exchanger increases in
temperature
proportionately to the ambient air's entering temperature. As a result, the
ambient air
could reach an elevated temperature where the heat exchanger risks reaching
its hi limit
temperature before the demanded temperature has been met, triggering the hi
limit
95 safety mechanism and initiating premature shutdown of the unit. To
mitigate this
scenario, the present invention's electronic control circuit controls a
cycling event
whereby the electric heating elements are intermittently switched on and off
at
predetermined temperature ranges until the demanded temperature has been met,
completing the heating cycle.
100 The electronic control circuit of the present invention has several
fail safe safety
mechanisms which will shut the heater down by disconnecting the power leading
to the
electric heating elements and air mover. The scenarios that will elicit a safe
shut down
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include but are not limited to the electronic control circuit failing to
register a
predetermined current from the: air mover, electric heating elements,
operating
105 temperature monitoring device, internal enclosure hi temperature limit,
or the heat
transfer hi limit temperature switch. In the event the heat exchanger
overheats, the hi
limit temperature monitoring device sends a signal to the electronic control
circuit
indicating the preset hi limit temperature has been exceeded, forcing
immediate
shutdown. The prior art's failsafe safety mechanisms typically use a preset hi
limit
110 temperature switch placed in series with the thermostat switch,
therefore the mechanical
relay stays engaged until the hi limit temperature switch disengages,
disconnecting power
leading to the electric heating elements and the air mover. Certain systems
have also
provided a high ambient preset temperature device positioned in the explosion
proof
enclosure box. Apart from the previously mentioned safety devices, no other
form of
115 safety shut down and/or monitoring features have been applied to the
previous art.
The present invention also has an integral disconnect switch built directly
into the
explosion proof enclosure box. This disconnect switch is an added safety
feature which
allows the heater to be fully disconnected from the power source and locked
out, by
means of a pad lock, during routine maintenance. The previous art only
provides this
120 option on specialty models where a separate enclosure and added conduit
is required,
thereby increasing installation complexity and the total mass of the heating
unit.
Further objects and advantages of the invention will become apparent from the
following
description read together with the accompanying drawings.
125
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BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, aspects of the invention
130 demonstrating the concepts of the present invention are illustrated, by
way of example, in
the enclosed Figures in which:
Figure 1 is a schematic drawing showing a preferred three phase electronic
control system
of the present invention;
Figure 2 is a schematic drawing showing a preferred single phase electronic
control system
135 of the present invention;
Figure 3 illustrates a preferred heating cycle of the present invention in
graphic format;
Figure 4 is a front face view showing a preferred embodiment of the present
invention;
Figure 5 is a left hand side view thereof;
Figure 6 is a right hand side view taken generally on line 6-6 of Figure 4;
140 Figure 7 is a rear side view taken generally on line 7-7 of Figure 4;
Figure 8 is a top view taken generally on line 8-8 of Figure 4;
Figure 9 is a perspective view from the front side of a preferred embodiment
of the
present invention;
Figure 10 is a perspective view from the rear side of a preferred embodiment
of the
145 present invention;
Figure 11 is an illustration of a preferred embodiment of the heat exchanger
of the
present invention.
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Figure 12 represents the front and side views from the prior art heat
exchanger as per US
patent application D 356,367;
150 Figure 13 is a cross-sectional view taken generally on line 13-13 of
section A-A of Figure 8
of the present invention and,
Figure 14 is an illustration of a preferred alternate embodiment of the heat
exchanger
showing the front and side views of the present invention.
155 DESCRIPTION OF THE DRAWINGS AND OF THE PREFERRED EMBODIMENT
The present invention may be embodied in a number of different forms. The
specifications
and drawings that follow describe and disclose only some of the specific forms
of the
invention and are not intended to limit the scope of the invention as defined
in the claims
that follow herein.
160 With reference to Figures 1, 2, 3 and 13, an explosion proof forced air
electric heater
according to the present invention operates in the following preferable
operating
sequence.
The typical heat cycle is preferably started by a thermostatic switch 17
enabling the
electronic control circuit 9 to perform a preprogrammed heating cycle. The air
mover 3 is
165 preferably mounted to the electric motor 2a (three phase power), or 2b
(single phase
power) and is engaged by switching a preferred solid state relay 7
momentarily, at a
preferably set predetermined current value to determine the working condition
of the air
mover, by transmitting a signal from a preferred electrical motor current
transformer
device 8 to the electronic control circuit 9. The electronic control circuit 9
engages the
170 electric heating elements within the heat exchanger 4a (three phase
delta power), 4b
(three phase star power) or 4c (single power) by switching a preferred solid
state relay 6.
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The electric heating elements 74 within the heat exchanger 4a, 4b, 4c are
continuously
monitored preferably by a current transformer device 5 that transmits a signal
to the
electronic control circuit 9. A preferable temperature sensing device 10
monitors the
175 operating temperature of the electric heating elements within the heat
exchanger 4a, 4b,
4c. The heat exchanger 4a, 4b, 4c will heat to a predetermined temperature and
once the
temperature has been met, the electric motor 2a, 2b is engaged by switching
preferably a
solid state relay 7. Engagement of the electric motor 2a, 2b draws cooler air
through the
intake opening 72 and forces the cooler air over the electric heating elements
74 within
180 the heat exchanger 4a, 4b, 4c and pushes warm air through the exhaust
opening 73 into
the atmosphere. Once the demanded ambient temperature has been reached, the
thermostatic switch 17 disengages, followed by the electronic control circuit
9 disengaging
the solid state relay 6, which disconnects the power leading to the electrical
heating
elements 74 within the heat exchanger 4a, 4b, 4c. The electric motor 2a, 2b
stays engaged
185 and the air mover 3 operates until the heat exchanger 4a, 4b, 4c has
cooled to a
predetermined set temperature, as monitored by the temperature sensing device
10. The
cooling of the heat exchanger 4a, 4b, 4c completes the heating cycle and is
illustrated in
graphic format as per Figure 3: "Preferred Heating Cycle". During the heat
cycle, the
electric motor 2a, 2b is continuously monitored preferably by an electric
motor current
190 transformer device 8. The temperature monitoring device 11 is the hi
limit safety device
which commands the electronic control circuit 9 to disconnect the power
leading to the
electric heating elements 74 within the heat exchanger 4a, 4b, 4c and the
electric motor
2a, 2b should the heat exchanger 4a, 4b, 4c overheat. In the event there is a
demand for
heat within an environment where the ambient temperature is already elevated,
the heat
195 exchanger 4a, 4b, 4c could potentially reach the hi limit temperature
and initiate a forced
shutdown prior to reaching the demanded temperature. To avoid premature
shutdown
the electronic control circuit 9 automatically triggers an oscillating cycle,
whereby, the
heat exchanger 4a, 4b, 4c cycles between a predetermined upper and lower
temperature
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limit, as monitored by the temperature monitoring device 10. The oscillating
cycle
200 continues until the demanded temperature has been met and the heating
cycle is
complete. This oscillating concept enables the heater to maintain maximum
forced air
output levels at all times within the capabilities of the present invention.
The oscillating
cycle described above is illustrated in graphic form by Figure 3: "Preferred
Elevated
Ambient Heating Cycle," while the heating cycle of the prior art is similarly
included for
205 comparison purposes.
Preferably a switch 13 provides the option of controlling the electric motor
2a, 2b with an
air mover 3 operating in either a preferable "automatic on" selection mode or
"continuous on" selection mode. Under the "automatic on" selection mode the
electric
motor 2a, 2b operates according to the preprogrammed heating cycle.
Conversely, under
210 the "continuous on" selection mode the electric motor 2a, 2b runs
continuously providing
mere air movement as part of a cooling cycle.
A preferably explosion proof enclosure 19 houses potential explosion causing
devices and
the enclosure internal predetermined hi limit temperature monitoring device
16, which
monitors the internal temperature within the explosion proof enclosure 19.
215 Events eliciting the safety mechanisms of the preferred embodiment are
listed below in
paragraphs (a) through (h), but not limited to events (a) through (h). Should
either of
these events described below occur, the power leading to the electric heating
elements 74
within the heat exchanger 4a, 4b, 4c and the electric motor 2a, 2b will be
disconnected,
thereby, shutting down the heater. The heater shutdown will then preferably be
relayed
220 to the operator by preferably providing a predetermined sequenced
audible beep through
a preferably audible beeping device 12 or by preferably providing a
predetermined
sequenced blinking by a preferably illumination device 14. The heater will
shut down if:
a) the electronic control circuit 9 does not register a predetermined
electrical
current from the electric motor current transformer device 8 during the heat
exchanger
9
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225 4a, 4b, 4c warm up, due to failure of the: electric motor 2a, 2b;
electric motor current
transformer device 8; or solid state relay 7;
b) the electronic control circuit 9 does not register a predetermined
electrical
current from the electric motor current transformer device 8 during the normal
heat
cycle, due to failure of the: electric motor 2a, 2h; electric motor current
transformer
230 device 8; or solid state relay 7;
c) the electronic control circuit 9 does not register a predetermined
electrical
current from the electric heating element current transformer device 5 due to
failure of
the electric heating element within the heat exchanger 4a, 4b, 4c; electric
heating element
current transformer device 5; or solid state relay 6;
235 d) the electronic control circuit 9 does not register a signal from
the operating
temperature monitoring device 10 due to device failure;
e) the electronic control circuit 9 does not register a signal from the hi
limit safety
temperature monitoring device 11 due to device failure;
f) the electronic control circuit 9 registers a signal from the hi limit
safety
240 temperature monitoring device 11 indicating the heat exchanger 4a, 4b,
4c has reached
the predetermined hi limit temperature and is overheating;
g) the electronic control circuit 9 does not register a signal from the
enclosure
internal predetermined hi limit temperature monitoring device 16 due to device
failure;
h) the electronic control circuit 9 registers a signal from the enclosure
internal
245 predetermined hi limit temperature monitoring device 16 indicating the
explosion proof
enclosure 19 has reached the predetermined internal predetermined hi limit
temperature
and is overheating.
Communication port 15 provides an interface to the electric control circuit 9,
where the
250 operator can upload and test heating cycle software or remotely operate
the heater.
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A preferred built-in time monitoring device 18 provides feedback for the
operator
pertaining to the amount of time the electrical heating elements 74 within the
heat
exchanger 4a, 4b, 4c have been engaged.
In order to safely perform maintenance on the preferred embodiment of the
present
255 invention, an integral disconnect switch 1 can be disconnected and
locked out to eliminate
power leading to the entire unit. This switch also functions as a reset device
through its
disconnection and re-connection of power leading to the electronic circuit 9,
thereby
clearing the safety event.
The solid state relay 6, 7 can be substituted for an electrical mechanical
relay.
260 With reference to the drawings and in particular, Figures 1, 2, 4 - 10,
the preferred
embodiment of the present invention is comprised of a top and bottom panel 20
which is
bolted to the right side panel 21 and left side panel 22, as well as the
shroud panel 23.
Panel's 20, 21, 22 and 23 form the heater cabinet and are all preferably
manufactured of
sheet metal. A preferably explosion proof electric motor 24 is then preferably
bolted to
265 the motor mount 25, which is preferably bolted to the explosion proof
junction box mount
bracket 26 and the motor mount bracket 27. Similarly, the mount brackets 25,
26, 27 are
preferably formed of sheet metal and are all preferably bolted to the cabinet.
An axial air
mover 28 preferably constructed of non-sparking materials is secured to the
shaft of the
explosion proof electric motor 24, however, the axial air mover 28 can be
substituted with
270 any type of air mover. The axial air mover 28 is centered on the shroud
panel 23 and an
axial mover guard 29, preferably constructed of metal wire, is preferably
bolted to the
shroud panel 23. An explosion proof expansion conduit union "A" represented by
members 30 and 31, preferably manufactured from non-sparking metal, whereby
explosion proof expansion conduit union member 30 is threaded onto the
electric motor
275 connection 24a. The explosion proof expansion conduit union member 31
preferably axial
telescoping inside the explosion proof expansion conduit union member 30,
preferably
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manufactured from non-sparking metal, and is preferably threaded into the
explosion
proof junction box 32 which is preferably constructed of a non-sparking metal.
The
explosion proof junction box 32 is preferably bolted to the explosion proof
junction box
280 mount bracket 26 and has a explosion proof junction box cover 33,
preferably constructed
of a non-sparking metal, and preferably bolted to the explosion proof junction
box 32. The
explosion proof junction box 32 may hold a built in thermostatic switch 17 as
per Figure 1
and 2, as opposed to a preferred explosion proof junction box cover 33.
An expansion conduit union "B", represented by members 34 and 35, preferably
285 manufactured from a non-sparking metal, whereby explosion proof conduit
member 34 is
preferably threaded into the explosion proof junction box 32 and the explosion
proof
expansion conduit union member 35 is preferably axially telescoping over the
explosion
proof expansion conduit union member 34, preferably manufactured from a non-
sparking
metal and is preferably threaded into the explosion proof enclosure box 36,
preferably
290 constructed of a non-sparking metal.
The explosion proof enclosure box 36 may house, but is not limited to housing,
the
following components as per Figure 1 and 2: A disconnect switch 1, an
electronic control
circuit 9, with a built in time monitoring device 18, a solid state relay 6,
7, an electric
heating element current transformer device 5, an electric motor current
transformer
295 device 8, an audible device 12, a communication port 15, and an
enclosure internal
predetermined hi limit temperature monitoring device 16.
The explosion proof enclosure box 36 is covered off by an explosion proof
enclosure cover
37 preferably constructed of a non-sparking metal that is preferably bolted-on
and hinges
on a preferably bolted-on hinging device 38.
300 An explosion proof expansion conduit union "C" represented by members 39,
40, 41,
preferable manufactured from a non-sparking metal, whereby the explosion proof
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expansion conduit union member 39 preferably axially telescoping inside the
explosion
proof enclosure box 36 and is preferably threaded into explosion proof
expansion conduit
union member 40 The explosion proof expansion conduit union member 41
preferably
305 axially telescopes inside the explosion proof expansion conduit member
40 and preferably
threads into the heat exchanger 42.
The axially telescoping feature of the explosion proof expansion conduit
unions "A", "B"
and "C", is constructed preferably of an inner and outer conduit, preferably
telescoping on
an axial plane and preferably manufactured with a close tolerance fit forming
an explosion
310 proof conduit.
The explosion proof expansion conduit union members 30, 31, 34, 35, 39, 40, 41
simplify
the installation process of the explosion proof expansion conduit unions "A",
"B" and "C
and ensures the conduit unions can expand and retract in the event of
thermally induced
stresses in the conduits or the like thereof.
315 The explosion proof expansion conduit union members 30, 31, 34, 35 form
an explosion
proof protective conduit allowing electrical wires 75a, 75b as per Figure 1
and 2 from the
explosion proof electric motor 25, to pass through the explosion proof
junction box 32 and
be terminated in the explosion proof enclosure box 36. The explosion proof
expansion
conduit union members 39, 40, 41 form a protective conduit allowing electrical
wires 76a,
320 76b, 77, 78 as per Figure 1 and 2 from the heat exchanger 42 to pass
through and be
terminated in the explosion proof enclosure box 36.
An integral disconnect switch handle 43 preferably operating the integral
disconnect
switch 1 as per Figures 1 and 2. Switch handle 44 preferably operating switch
14 as per
Figures 1 and 2 providing the "automatic on" selection mode or "continuous on"
selection
325 mode for the explosion proof electric motor 24, and an illuminating
device 45 preferably
providing a predetermined sequenced blinking to relay the safety mechanisms
are
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preferably mounted on the outside of the explosion proof enclosure box 36. The
switching
mechanism of the integral disconnect switching handle 43 is preferably mounted
on the
inside of the explosion proof enclosure box 36. The switching mechanism of
switch handle
330 44 is preferably mounted on the inside of the explosion proof enclosure
box 36. The
illuminating device mechanism of the illumination device 45 is preferably
mounted on the
inside of the explosion proof enclosure box 36.
Entry 46 represents a preferable auxiliary entrance, while entry 47 represents
a preferable
primary power entrance.
335 A louver 48 preferably constructed of non-sparking material and
preferably fastened to
the right side panel 21 and left side panel 22, provides the ability to direct
the air exiting
through the exhaust opening 73 and into the atmosphere.
Figure 11 illustrates a preferable breakdown of the heat exchanger 42 used in
the current
invention. A fin 49, preferably formed from a heat conductive metal, is
preferably press fit
340 onto preferably manufactured similar heat conductive metal tubes 50
that preferably
have an electric heating element 74 as per Figure 1 and 2 inserted within the
body of the
tube. The electric heating element 74 as per Figure 1 and 2 is preferably made
according
to conventional construction and includes: a tubular metal sheath 51, a
resistor coil 52,
and a cold pin 53. The cold pin 53 is housed within the metal sheath and
compacted
345 granular refractory material 54 within the sheath, to electrically
insulate the coil from the
sheath and conduct heat from the coil to the sheath. A fastening terminal 55,
preferably
manufactured from an electric conductive metal, is preferably crimped onto the
cold pin
53 and provides a connection between the bus bar 56, preferably manufactured
from an
electric conductive metal, and the consecutive electric heating elements. A
header 57,
350 preferably formed of a heat conductive sheet metal, is preferably press
fit onto the metal
tubes 50. The header 57 is preferably enclosed by a header end plate 58 with a
preferable
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threaded fastener provision 58a, a header end plate 59 with a preferable
fastener
provision 59a, and a threaded electric conduit entrance provision 59b.
The components 50, 57, 58, 59, and header cover 60 preferably manufactured of
a heat
355 conductive metal are preferably fastened together with a threaded
fastener and form the
header assembly.
The header assembly forms an internal void 61, which is preferably filled with
a preferably
electric resistant and preferably heat conductive encapsulation compound. The
encapsulation compound is poured within the header assembly and hardens as the
360 encapsulation compound cures. Once cured, the encapsulation compound forms
a hard
and resilient substance that replaces the need for a bulky explosion proof
enclosure and
provides a protective hazardous area barrier. (For bulky explosion proof
enclosure
example see prior art US Patent 4,117,308, items 72, 73, 82 and 83) An
operating
temperature monitoring device 10 and temperature hi limit monitoring device 11
are
365 preferably mounted in one of the metal tubes 50 within a pre-
manufactured cavity 62.
Figure 12 illustrates prior art. For reference see US patent D 356,367. The
prior art
submerges electric heating elements 64 in a water glycol mixture within the
bottom
header 63. The metal tubes with spiral wound aluminum fins 65 form the heat
transfer
media and are connected to the bottom 64 and top header 66. The pressure
safety device
370 67 provides an overpressure safety feature.
Figure 13 illustrates the air flow characteristics of the present invention.
Cool air 68 is
drawn into the intake opening 72 and into the heater cabinet via a preferred
axial air
mover 28. This accelerated cool air 69 is forced through the pre-heated heat
exchanger
42. The heated fin 49 path causes the cool air 69 to heat up. As the air
heats, it expands,
375 creating a higher exit velocity 70. This acceleration event is caused
by particular and
specific fin spacing. If the fins spacing is too wide there is insufficient
heating surface,
CA 02743378 2016-03-02
which leads to less than sufficient air flow resistance to generate the forced
air to heat up,
thus failing to provide a sufficient exit temperature increase. If the fin
spacing is too
narrow the air flow is restricted, causing excessive air flow resistance. This
resistance
380 causes the cool forced air 69 to bulk up behind the heat exchanger 42,
thus causing hot air
to exit the heat exchanger 42 with substantially slower air velocity, thereby
defeating the
purpose of a forced air heater.
Figure 14 illustrates an added feature of the heat exchanger 42 used in the
present
invention. The axial air mover 28 creates an area of decreased air movement in
the center
385 of the heat exchanger 42. Because the center of the axial air mover is
incapable of
producing air movement, a hot spot could occur within the heat exchanger 42,
as
represented by the highlighted perimeter 71. To eliminate the potential for a
hot spot and
premature resistor coil 52 burnout, the resistor coil 52 mounted in the
electrical heating
element 74 as per Figure 1 and 2 within the highlighted perimeter 71 is
substituted with a
390 specific coil design that elicits a cooler heating area within the
electrical heating element
creating a cooler heat source. The cooler heating area eliminates the hotspot
scenario,
while simultaneously conserving energy by ensuring areas incapable of heat
dispersion
remain cool during the heating cycle.
Although the invention has been described in connection with a preferred
embodiment it
395 should be understood that various modifications, additions and
alterations may be made
to the invention by one skilled in the art without departing from the spirit
and scope of
the invention as defined in the appended claims.
400
16
