Note: Descriptions are shown in the official language in which they were submitted.
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A VERTICAL ELECTRICALLY HEATED OVEN FOR BAKING COATED PARTS
The present invention relates to a vertical electrically heated oven for
baking coated parts.
BACKGROUND OF THE INVENTION
Various products require a coating applied thereto to be baked to cure
the coating so that it is fixed to the part. One example is that of circuit
boards where
a coating is applied and then baked. Another example relates to the powder
coating
of metal parts which is a relatively recent method for protecting and
beautifying
products. Powder coating makes a product highly chip and scratch resistant and
highly chemical, petroleum and salt resistant. Powder coating has the ability
to
apply a thick, even coat without drips or sags and has premium insulating
qualities,
both electrical and thermal and also removes the cold feeling from metals. It
gives a
decorative finish to a product along with a protective finish which is
resistant to
fading. Powder coating has the ability to encapsulate products, wrap around
corners, help eliminate sharp edges achieve with one high quality coat with no
primer necessary and can resist abuse in low and high temperature variances.
Powder coating has virtually unlimited number of colours and a wide variety of
finishes.
Generally powder coating is applied to many metal products powder
coatings are 100% solids coatings applied as a dry powder mix of resin and
pigment
and subsequently formed into a film with heat. The solid resin binder melts
upon
heating, binds the pigment, and results in a pigment coating upon cooling. The
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powder is applied either by an electrostatic spray or by passing the part over
a
fluidised bed of powder. In both cases the parts undergo subsequent oven
heating to
provide a smooth continuous film.
Powder coated materials have to be heated in an oven or the like, an
example of a powder coat oven is shown in U.S. Patent No. 5,155,335 issued
October 1982 of Habaki et al wherein the oven has an inlet air shield chamber
connected to a horizontal heating chamber. The oven of Habaki has a generally
horizontal conveyor which is limited to one item at a time in the oven on the
conveyor which is adequate for large items which need to be relatively spaced
out
but is not very effective for small items such as jewellery and the like.
U.S. Patent No. 4,009,301 issued February 1977 of Heckman et al
discloses a method for powder coating materials. A large apparatus is used to
carry
items on a conveyor through a series of steps wherein the item is powder
coated
and baked.
U.S. Patent No. 5,000,985 issued March 1991 of Salisbury shows a
similar method of powder coating wherein materials are carried on a conveyor
through the processes.
The above patents are not effective for small objects and are relatively
inefficient, it will be appreciated that the parts which are relatively small
are generally
suspended on the carriage from a hook or the like so that each is separate as
it
passes through the oven.
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In US Patent 6,194,688 issued February 27th 2001 of the present
inventor is shown a novel construction of oven which comprises a housing
having
side walls defining a closed top end and an open bottom end, with a pair of
opposed
heating elements in the housing at the side walls. A carrying arrangement in
the
housing carries a batch of parts from the bottom end where the parts are
loaded on
to the carrying arrangement to the top end where the parts are baked by the
heat
rising from the heating element. The carrying arrangement is mounted on a
guide
which has a track located on respective side walls in the housing, a pair of
wheel
arrangements on the carrying arrangement engage each track and a cable is
supported by a series of pulleys and is connected to a cable winch which is
controlled by the control station for driving the carrying arrangement in the
housing.
The carrying arrangement has a removable rack comprising a series of hooks in
which the parts are attached. The bottom end has an opening such that the
carrying
arrangement is accessible. The housing has insulated oven panels. The control
station has an up control, a pause control, a down control, a temperature
control
meter and an upper holding time control. In this patent is disclosed an air
blower for
circulating the heated air in the oven mixing the air and making the
temperature of
the heat uniform.
This oven shows significant improvements in energy efficiency and
convenience of operation. However some structural improvements are required
for
effective operation.
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SUMMARY OF THE INVENTION
It is one object of the present invention to provide a baking oven.
According to one aspect of the invention there is provided an
apparatus for baking one or more parts in a batch comprising:
a housing having side walls and a top wall defining a substantially
closed container with a substantially closed top end and an opening at a
bottom end;
at least one heating assembly within the housing at a position therein
below the top end for generating heat in the housing such that heat rises in
the
housing from said at least one heating assembly and causes the top end above
said
a lifting and guide arrangement for guiding movement of the carrying
arrangement upwardly in an upward path from a first position at the bottom end
of
the housing past said at least one heating assembly to a second position at
the top
the housing, the lifting and guide arrangement and the carrying
arrangement being arranged such that, in the first position, the carrying
arrangement
is exposed at the bottom end for loading the batch for baking and for
unloading of
and a control system for controlling said at least one heating assembly
and the lifting and guide arrangement such that, in the second position, the
carrying
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arrangement is held stationary for a period of time for baking of the batch;
said at least one heating assembly comprising a generally planar
heating array arranged along one side wall inwardly of the side wall and such
that
the carrying arrangement passes said at least one heating assembly as it moves
5 from the first position to the second position;
said at least one heating assembly comprising a shield arrangement
located in front of the heating array relative to the carrying arrangement so
as to
block at least part of the heat from the heating array from directly radiating
onto the
batch of parts on the carrying arrangement as the carrying arrangement passes
the
heating array to said second position above said at least one heating
assembly;
wherein the shield arrangement is shaped and arranged relative to the
side wall so as to define at least one inlet opening below the shield
arrangement for
entry of air behind the shield arrangement and an outlet opening above the
shield
arrangement for exit of air from behind the shield arrangement such that the
heat
from said heating array generates a flow of air upwardly behind the shield
arrangement to carry the heat in the airflow to said second position above
said at
least one heating assembly.
Preferably the shield arrangement is shaped and arranged to block
sufficient of the heating array to prevent overheating of the batch as the
carrying
arrangement is lowered to the first position.
There may be only a single heating array but preferably there are two
heating assemblies each comprising a generally planar heating array arranged
along
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a respective side wall inwardly of the side wall with the heating arrays
opposed to
face one another and such that the carrying arrangement passes between said
heating assemblies as it moves from the first position to the second position.
In this case there are two shields each located in front of a respective
the heating array relative to the carrying arrangement so as to block at least
part of
the heat from the heating array from directly radiating onto the batch of
parts on the
carrying arrangement as the carrying arrangement passes the heating array.
Preferably the heating assembly comprises an array of side by side
heating elements.
In one arrangement the shield arrangement comprises a single shield
plate covering the whole heating assembly. However it will be appreciated that
the
shield may be formed as a number of shield portions in an array.
Preferably the shield is planar and generally parallel to the heating
assembly.
Preferably the heating array comprises a plurality of upstanding
heating elements each having a bottom electrical connection, an upstanding
connection portion and an upper heating section.
In another arrangement, the shield arrangement comprises a series of
separate shield portions each associated with a respective one of the heating
elements.
In this case the shield portions can be in front of the heating elements
or more preferably defined by a series of tubes each surrounding a respective
one of
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the heating elements.
Preferably the shield is shaped and arranged to block substantially all
of the heating sections of the heating elements.
The objective of the shield is to preferably block sufficient of the
heating sections of the heating elements to prevent overheating of the batch
as the
carrying arrangement is lowered to the first position.
Where the heating array comprises a plurality of upstanding heating
elements each having a bottom electrical connection, an upstanding connection
portion and an upper heating section, preferably there is provided an
electrical
connection box having an upper wall with the electrical connection of each of
the
elements mounted in the upper wall and the connection portion and the heating
section stand upwardly from the upper wall and electrical connector wires of
the
elements are located in the box, and there is provided an insulated heat
shield plate
above the upper wall and generally parallel thereto with each of the elements
projecting through a respective hole in the heat shield plate so that the
upper heating
section is above the heat shield plate.
In this arrangement preferably there is provided a collar surrounding
the connection portion between the upper wall and the heat shield plate, the
collar
having openings therein to allow entry of air into the collar and escape of
air from a
top of the collar through the heat shield plate.
Preferably the top of the collar is coextensive with the respective hole
in the heat shield plate.
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The housing may be fully closed at the top but more preferably there is
provided a ventilation outlet at the top of the housing to extract a low air
flow to
extract fumes with the outlet being located in the top wall or more preferably
in a
side wall adjacent the top.
Preferably the housing has reflective inner walls to reflect infrared rays
emitted by the heating assembly.
According to a second aspect of the invention there is provided an
apparatus for baking one or more parts in a batch comprising:
a housing having side walls and a top wall defining a substantially
closed container with a substantially closed top end and an opening at a
bottom end;
at least one heating assembly within the housing at a position therein
below the top end for generating heat in the housing such that heat rises in
the
housing from said at least one heating assembly and causes the top end above
said
at least one heating assembly to reach an oven temperature for baking the
parts;
a carrying arrangement in the housing for carrying the batch of parts;
a lifting and guide arrangement for guiding movement of the carrying
arrangement upwardly in an upward path from a first position at the bottom end
of
the housing past said at least.one heating assembly to a second position at
the top
end above said at least one heating assembly and in a downward path back to
the
first position;
the housing, the lifting and guide arrangement and the carrying
arrangement being arranged such that, in the first position, the carrying
arrangement
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is exposed at the bottom end for loading the batch for baking and for
unloading of
the batch when baked;
and a control system for controlling said at least one heating assembly
and the lifting and guide arrangement such that, in the second position, the
carrying
arrangement is held stationary for a period of time for baking of the batch;
said at least one heating assembly being arranged along one side wall
inwardly of the side wall and such that the carrying arrangement passes said
at least
one heating assembly as it moves from the first position to the second
position;
wherein the heating assembly includes is located in at least one duct
within the housing with an inlet opening at a bottom of the heating assembly
and a
discharge opening at a top of the heating assembly so as to generate a flow of
air
upwardly within said at least one duct to carry the heat in the airflow to the
top of the
housing.
In this arrangement, the heating assembly preferably comprises a
plurality of side by side elements and each of the elements is located in a
respective
one of a plurality of separate ducts.
Alternatively the single duct or plurality of ducts includes or is partly
defined by a single shield plate located in front of all of the heating
elements or a
series of separate shield plates each covering one or more of the heating
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the invention will now be described in conjunction
with the accompanying drawings in which:
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Figure 1 is an isometric view of one embodiment of the oven according
to the present invention with the cover panels removed to show the interior
construction.
Figure 2 is a vertical transverse cross sectional view of the oven of
5 figure 1.
Figure 3 is a cross sectional view of one heating assembly of the oven
of Figures 1 and 2.
Figure 4 is an isometric view showing schematically an alternative
construction of heating assembly for use in an oven of the type generally
shown in
10 Figures 1 and 2.
Figure 5 is a cross sectional view of the heating assembly of Figure 4.
In the drawings like characters of reference indicate corresponding
parts in the different figures.
DETAILED DESCRIPTION
As shown in Figures 1 and 2, a baking oven, generally indicated as 1,
is arranged to bake parts 3 carried on a frame 2, which parts have been coated
with
a suitable coating material such as powder coat.
In some arrangements the frame may carry a large number of smaller
parts such as individual castings or the frame may carry a smaller number of
larger
parts, even to the extent of carrying a single part such as a large panel. The
dimension of the oven and the frame can be selected in accordance with
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requirements to receive the number and arrangement of parts in the batch which
are
necessary or suitable for the process concerned. However in each case the
parts or
part form a batch which is processed in the oven by loading at the bottom,
moving
to the baking location and moving back to the loading station for unloading.
This is
different from a continuous or line process where the parts move along a
processing
line.
Powder coatings are 100% solids coatings applied as a dry powder
mix of resin and pigment and subsequently formed into a film with heat. The
solid
resin binder melts upon heating, binds the pigment, and results in a pigment
coating
upon cooling. The powder is applied either by an electrostatic spray or by
passing
the heated object over a fluidized bed of powder with subsequent oven heating
to
provide a smooth continuous film.
The oven has a housing 5 and a frame 7 for supporting the housing. In
Figure 1, the panels covering the frame 7 and forming the housing are omitted
for
convenience of illustration. The frame has two elongate front vertical posts 9
and
two elongate rear vertical posts 11 arranged parallel to each other and the
two front
posts and the two rear posts are spaced equidistantly apart such that the
frame is
rectangular shaped. The dimensions can vary but in general the frame is higher
than it is wide. A plurality of cross beams 13 connect the vertical posts such
that the
frame has a hollow interior 15. A first set of cross beams connect the front
posts
wherein a first beam 19 is located generally at the bottom of the posts, a
second
beam 21 is located approximately at the middle of the posts and a third beam
23 is
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located at the top end of the posts. A second set of cross beams connect the
rear
posts wherein a first beam 19A is located generally at the bottom of the posts
at the
same height and parallel to the first beam 19 of the first set. A second beam
21A
located approximately at the middle of the posts at the same height and
parallel to
the second beam 21 of the first set. A third beam 23A is located at the
further most
top end of the posts at the same height and parallel to the third beam 23 of
the first
set. The first set and second set of beams are parallel and of the same
length.
A set of side beams 29 connect the front posts to the rear posts which
are perpendicular to and at the same height as each of the second set of beams
connecting a respective front post to a respective rear post. The side beams
are
shorter in length than the first and second set of beams.
A carrying arrangement 31 is arranged to carry the parts upwardly and
downwardly on a rack 33 in the hollow interior 15 of the frame. The carrying
arrangement is mounted on tracks 35 which extend upwardly from the bottom end
along each side of the frame on the side beams parallel to the posts. Each of
the
tracks is arranged to support a respective wheeled carriage 37. The wheels of
the
carriages 37 engage the track 35 such that the carrying arrangement is movable
upwardly and downwardly through the hollow interior along the track. The
carrying
arrangement is moved by a motor 41 mounted on the frame on a cross beam
adjacent to the middle beam which drives a chain at the adjacent track and
drives
though a cross shaft 44 a corresponding chain at the opposite track. The
chains run
over top and bottom sprockets 51 and 49 which are located within the housing
at the
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top and bottom respectively of the tracks 35. The carrying arrangement is thus
moved directly vertically upwardly and downwardly in the oven, as mentioned
above
by the two chains which are maintained in synchronism by the cross shaft 44.
Thus
the carrying arrangement follows the same vertical path in its movement with
no side
to side or horizontal movement which could cause shaking or jiggling of the
parts.
Also the cross-shaft 44 ensures that the movement of the two sides is
simultaneous
to prevent any tilting or binding of the parts or the carrying frame.
A heating arrangement 57 in the oven is arranged to heat the parts
when raised by the carrying arrangement, as described in more detail
hereinafter. A
main control panel (not shown) is provided which has a main breaker and
controls
the motors up and down drive, pauses the motor and controls the temperature
and
has a holding timer for holding the parts in the carrying arrangement at the
top end
of the oven for baking.
As shown in Figures 1 and 4 there are two pyrometer thermocouples A
and B located at the top end in the oven for measuring the temperature of the
oven
so that the oven can be heated at a required predetermined temperature. The
temperature sensor A is located at the top of the heating zone where the parts
are
located during the baking process and this is spaced downwardly from the top
wall
of the housing. The temperature sensor B is located at the bottom of the
heating
zone and this is spaced upwardly from the top of the heating assembly.
The housing includes insulated oven panels 69 arranged such that the
oven is fully closed at the side walls and top 74 and an opening 71 is located
only at
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the bottom defining a loading and unloading zone 73 below the open bottom 71
of
the oven. A ventilation outlet 78 is located at the op in one side wall
closely adjacent
the top wall. The ventilation outlet is arranged to remove fumes from inside
the oven
and carry the air through a pipe so that the air is released outside the
building.
Inner walls 81 in the oven are made of reflective material, such as
stainless steel or galvanized zinc, so that the infrared rays from the heating
elements
are reflected onto the parts for baking.
in operation, after powder coating, the parts are placed onto the rack
which is then placed onto the carrying arrangement. The control panel is
activated
such that the carrying arrangement is raised vertically into the oven. The
carrying
arrangement is held in the oven at the top above the heating elements for a
predetermined amount of time so that the parts are baked and the carrying
arrangement is lowered so that the rack can be removed and a second set of
parts
on a second rack can be placed onto the carrying arrangement for baking. When
the rack reaches the closed top of the oven, the baking is carried out for a
set baking
period as set by the control system.
The vertical orientation of the oven creates an efficient unit with low
energy costs and requires a very small floor space in a building. The oven
allows
easy loading and unloading of the rack of parts and the parts are maintained
stationary on their hanging rack during movement by the direct vertical
movement
along the path of the tracks.
The heating assembly 57 comprises two separate heating members 51
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and 52. These are symmetrical and arranged at opposite side walls so as to
face
one another across the width of the oven. The heating members 51 and 52 are
arranged on the opposite side walls from the tracks so that the frame 33
carried on
the tracks passes between the two heating members on its way to the top of the
5 oven at a position spaced above the two heating members. Thus the heating
action
at the top of the oven is generated by air rising from the heating members
rather
than by direct irradiation of the parts within the oven at the top of the
oven.
One of the heating members is shown in cross section in Figure 3 and
it will be appreciated that the other of the heating members is symmetrical
and
10 opposite to this and faces the heating member shown across the housing.
Thus the housing comprises a housing wall 69 as previously described
which is supported on the frame members including the bottom frame member 19
which is shown. The wall panels are formed of an insulated material of a
nature
which can be selected by a person skilled in the art.
15 The heating member shown in Figure 3 and also visible in Figure 1
includes an array of heating elements 53 at spaced positions across the width
of the
heating member. Each heating element 53 includes a bottom mounting bracket 54
to which is attached a tubular container 55 standing upwardly from the bracket
with a
tubular container having an upper loop section 56 at which it turns downwardly
to
form a downwardly projecting portion terminating at a lower end 58. Between
the
lower end 58 and the bracket 54 is a zone 59 of the heating element which is
unheated. Thus the heating effect generated by the resistance heating member
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within the tube is concentrated in the area above the lower end 58 in both of
the legs
of the tube in that area. Heating elements of this type are of course well
known and
commercially available. In this embodiment the length of the section 59 which
is
unheated and acts to merely transport the electricity within cables within the
heating
element to the resistance heating section above the lower end 58. The section
59 is
therefore relatively cool and is heated only by conduction and radiation from
the
heated section.
The heating elements are supported by the bracket 54 at the lower end
so that they are freestanding upwardly from the bracket in a vertical array in
a
common plane along the sidewall with each heating element being spaced from
the
next and standing vertically parallel to the next.
The heating elements are located between an inwardly facing shield
member 60 and an outwardly facing shield member 61. These shield members are
supported in parallel spaced relation by a series of transverse posts 62. The
space
in between the shield members is arranged such that the heating element is
spaced
from both of the shield members and is generally equidistantly located between
those two shield members. The height of the shield members is arranged such
that
the upper end of each of the shield members is above the upper end 56 of the
heating element and the lower end of the shield members is below the lower end
58
of the heating element.
The shield members are formed of a suitable heat resistant material
generally steel which can be stainless or galvanized steel. The shield members
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each formed by a single flat sheet of the shield material so that there is a
channel
defined between the shield members with an open top 63 of the channel and an
open bottom 64 of the channel. This forms an air passage between the two
shield
members in that air can enter between the two shield members through the
opening
64 of the lower end of the channel and can pass between the shield members to
the
upper open end 63 of the channel. The upper open end 63 is located below the
baking zone at the top of the oven. The presence of the heating elements
within the
channel generates significant heat in this area which causes strong convention
current within the channel acting to carry air from the opening 64 at the
bottom
through the opening 63 at the top so that air extends into the baking zone
above the
heating elements. The strong convection currents generated by this channel
avoid
the necessity for any form of fan or other air current assist system. The
heating
system is therefore a simple construction with no moving parts and yet carries
the
heated air to the top of the oven to the baking zone.
The inner shield member 60 covers the radiant heating section of the
heating elements so that radiant heat directly from the heating elements is
prevented
from passing directly to the parts as they pass the heating elements on the
carriage
moving to or from the baking zone at the top of the oven.
The presence of the inner shield 60 is particularly important when the
heated and baked parts are moved from the upper oven zone past the heating
elements to the loading position 73 at the open bottom of the oven. It has
been
found that there is a tendency for the parts to be overheated as they pass the
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heating elements on the way down after the baking has been completed. Rather
than turn off the heating elements as the parts pass, it is preferred to
provide the
shield which prevents the direct radiation impinging upon those parts.
The spacing between the shield members is arranged so that it allows
air to pass between the shield members to generate an effective convection
current
of a significant volume of air to provide the heating at the oven zone at the
top of the
oven. The inner shield members 60 are spaced by a sufficient distance to allow
the
parts to pass between the shield members from the loading position to the
baking
position.
The channel defined between the shield members is a continuous
channel along the length of the heating member spaced only by the sufficient
numbers of the spacing posts 62 to hold the shield members properly vertical
and
parallel.
The outer shield member 61 inhibits the penetration of the heat from
the heating elements to the panel 69. The shield member 61 thus acts to
confine
the heat between the two shield members for transportation by the air stream
to the
top of the housing.
The heating member defined by the heating elements and the shield
members is mounted on the frame by a lower portion 82 of the outer shield
member
61. Thus the lower portion 82 extends below the end of the inner shield member
60
and extends downwardly therefrom to the bottom member 19. At the bottom frame
member 19 is provided a horizontal shielding plate 83 which is attached to the
lower
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19
end portion 82 of the outer shield member 61. This shield plate 83 has an
outer
portion extending under the frame member 19 and an inner portion extending
inwardly to a position beyond a plane parallel to the inner shield member 60
to an
inner edge 84 at which is provided a downturned flange 85. The shield plate 83
thus
provides a horizontal shield against downwardly radiated heat energy. A layer
of
.insulation material 86 is applied on top of the plate 83 extends along the
full length of
the heating member so that it is coextensive with the inner and outer shield
members. This therefore provides generally an enclosure for the heating
elements
which is open only at the opening 64 between the lower end of the inner shield
member and the plate 83. The shield plate 83 has a series of holes 87 at
spaced
positions along its length each for receiving a respective one of the heating
elements
59. Thus each heating element extends from its respective bracket at the lower
end
through the hole 87 in the plate 83 into the channel between the inner and
outer
shield members. The layer 86 of the insulation material acts to inhibit heat
from
passing downwardly to a position below the shield plate 83 where it can act to
heat
the bracket 54. Attached to the underside of the shield plate 83 at each of
the holes
87 is provided a sleeve 88 which is welded at 89 around the hole 87 and
extends
downwardly therefrom to a lower end 90. The lower end of the sleeve 88 is
threaded as indicated at 91 to receive a threaded section of the bracket 54 so
that a
male thread of the bracket 54 screws into the female thread of the sleeve 88.
A
rectangular box 92 is attached to the lower end of the sleeves 88 by a flange
93
which is screwed up against the underside of the top plate 94 of the box so
that the
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top plate 94 is held clamped against the bottom of the sleeve by the flange 94
and
by the threaded bracket 54. The sleeve 88 has a series of holes around its
periphery to allow the entry of air through each of the holes 95 to flow into
the area
above the bracket 54 and surrounding the lower end of the element 59 so that
the air
5 can
pass upwardly through the holes 95 and outwardly through hole 87 to cause a
flow of cooling air around the lower most end of the heating element 59.
The box 92 has depending side walls 96 and 98 attached to the edges
of the top wall 94 together with a bottom wall 97 which can act as an opening
to
allow access to the interior of the box. The box 92 extends along the full
length of
10 the heating member and acts to contain the electrical wires and electrical
connections to the brackets 54 of each of the heating elements.
Thus each heating element passes through its respective hole 87 in
the plate 83 and then passes through its respective sleeve 88 to its bracket
underneath the top wall 94 of the box 92.
15 The box
92 and the electrical connections contained therein are
therefore maintained cool by the air flow through the sleeve 88 and by the
shield
plate 83 and its insulated covering layer 86.
Heat therefore from the heating element is confined so that it generally
passes upwardly through the channel and little of the heat communicates
20
downwardly to the electrical components within the box 92. This maintains
electrical
components at a cooled temperature sufficiently low to avoid damage to those
electrical connections and to the bracket 54 itself.
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Turning now to Figures 4 and 5 there is shown an alternate heating
unit assembly. The difference with this unit is that each of the heating
elements 53
is encased in a respective one of a plurality of stainless steel heat shield
tubes 99.
These tubes form a series of parallel ducts through which the air passes as
previously described to flow to the top of the housing. Each tube has a cut
out
portion on the front face in the area below the heating section of the element
to allow
the air to enter that cut out. Each tube has its lower end welded to a heat
shield
tube holder 100. This holder is in turn threaded into the sleeve 88. The heat
shield
tube 99 is open at the top 63 to allow the hot air to rise and is cut open on
half the
tube at the lower end 64 to allow cool air to enter the tube. All other items
are
maintained as in the initial design.
The heating zone between the temperature sensors A and B is spaced
downwardly from the top of the housing and below the discharge opening 78
which
is itself spaced downwardly from the top wall. Thus when the discharge opening
draws air out from the housing it does so above the heating zone leaving a
layer or
cushion or volume of heated air above the heating zone which can act as a heat
buffer. The discharge opening is controlled by a venturi which acts to
generate a
forced flow in the discharge direction to a suitable discharge area to carry
any
contaminants away from the oven and the parts being baked. The venturi can be
automatically controlled based on detection of escaping contaminants or on
temperature changes at the oven heating zone.
The two thermocouples or temperature sensors are arranged to
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maintain the temperature at the sensor within a temperature of a predetermined
range by activating selected ones of the heating elements 53. In Figure 4 it
will be
noted that the temperature sensors provide a control signal to a control unit
to the
heating assemblies. In the embodiment shown the control unit drives two
separate
power supplies to separate ones of the heating elements in dependence on the
temperature as detected by the respective sensors. Thus each heating assembly
comprises an array of side by side heating elements and the heating elements
are
separated into said first and second parts at spaced alternate positions
across the
heating assembly. In particular the elements are separated into pairs so that
the first
two are connected to the first supply and the second two are connected to the
second supply and so on across the width of the heating assembly. Thus as each
temperature sensor drops below its set value, it acts to turn on one half of
the
heating assemblies in a spaced pattern across the width to add additional
heat. This
arrangement has been surprisingly found to maintain the whole heating zone
within
a predetermined temperature differing from the set value by no more than 5
degrees
Celsius. This close control ensures that all of the parts of the batch see the
same
baking temperature with consistent baking of the parts.
