Curing oven for enameled wire and control system therefor

The specification discloses a curing oven (10) and a control system (120) therefor. The oven (10) includes a heat exchanger (70) which preheats process air from the work chamber (36) before thermal oxidation. A separate recirculating fan (78), mixing box (84), and set of motorized dampers (92, 94) are provided for each zone (40, 42) of the work chamber (36) for controlling mixture of hot and cold air and delivery thereof to the work chamber for better temperature control. The motorized dampers (92, 94) are preferably controlled by microprocessors (124) responsive to temperature sensors (126, 128) located in the work chamber (36). Discharge of the exhaust fan (76) is adjusted by a motorized damper (96) through a controller (136) responsive to a pressure sensor (138) inside the oven (10).

TECHNICAL FIELD 
The present invention relates generally to an oven for curing coated wire 
or the like on a continuous basis. More particularly, this invention 
concerns a wire enameling oven and control system therefor adapted to 
accommodate continual sensing and feedback of various conditions in order 
to achieve improved temperature control and distribution in the zones of 
the work chamber while maintaining environmental control. 
BACKGROUND ART 
Magnet wire is produced by coating conductive lengths of wire with a 
suitable insulator in liquid form and then feeding the coated wire through 
a curing oven on a continuous basis to dry the enamel. The wire is usually 
copper or aluminum and the insulator is typically enamel. 
Traditionally, curing ovens have provided only elevated temperatures for 
proper curing of the enamel insulator; however, since such coatings give 
off volatile fumes during evaporation, the need soon became apparent for 
environmental control particularly with ovens having high production 
capacities. 
There have been basically two distinct approaches to the problem of 
handling the evaporated solvents, which typically comprise hydrocarbons, 
from coatings during drying and curing. One approach has been to circulate 
air containing the unoxidized or partially oxidized vapors from the liquid 
coating through a burner, catalyst, or combination thereof to effect 
oxidation of the vapors. The use of a suitable catalyst, however, is 
expensive not only in terms of the initial expense in constructing the 
curing oven but also in terms of the periodic maintenance expense during 
shutdown of the oven to service the catalyst and remove accumulated 
deposits therefrom. On the other hand, the use of a burner to thermal 
oxidize the vapors creates other difficulties in maintaining proper 
temperature control within the oven due to wide temperature difference 
between the thermal oxidizer and the oven work chamber. Both approaches 
have their particular drawbacks and none of the ovens available heretofore 
has been completely satisfactory. 
In addition, it will be appreciated that various parameters are involved 
including type and size of the wire, type and thickness of the coating 
material, rate of wire feed through the oven, etc., any one of which, if 
varied, could change the system balance sufficiently to require resetting, 
which is a manual process that can be time consuming and therefore 
expensive in terms of lost production. The set-up of a wire enameling oven 
for proper curing of the enamel and environmental control has been more of 
an art than a science. Different temperatures are required in zones of the 
work chamber to achieve evaporation then curing. 
U.S. Pat. No. 3,106,386 to Harris, which is assigned to Acrometal Products, 
Inc., the assignee hereof, discloses a curing oven incorporating both a 
catalyst and a burner to effect oxidation. Heated air from the work 
chamber is subjected to the flame of a burner which partially oxidizes the 
fumes released from the enamel and carried by the air to a combustion 
chamber. The heated air, still containing some unoxidized fumes, is then 
passed through a catalyst for oxidation of the remaining fumes. 
U.S. Pat. No. 3,810,736 to Dumas, which is also assigned to the assignee 
hereof, is an example of a curing oven which employs a burner alone to 
effect substantially complete thermal oxidation of the coating vapors. 
This oven comprises a casing with partitions therein dividing the interior 
into an elongated work chamber for receiving the coated wire, combustor 
inlet and outlet chambers interconnected by a combustor tube having a 
burner at the inlet end thereof, and gas passageways. A blower in the 
combustor outlet chamber moves air and heated gases through some 
passageways to opposite ends of the work chamber for passage through the 
curing and evaporating zones to pick up fumes evaporating from the enamel 
which are then carried back to the combustion inlet chamber for oxidation 
by the burner. In this oven, the heated air and gases from the combustor 
outlet chamber are mixed directly in the work chamber with the air 
inspirated through the open ends of the work chamber. Temperature control 
is achieved by setting manual dampers and by motorized volume control in 
order to throttle the amount of superheated air from the combustor outlet 
chamber with the "cold" outside air. Under conditions of reduced 
temperature requirements in the work chamber, proper temperature control 
and distribution becomes progressively more difficult to achieve due to 
the relatively small proportionate quantity of superheated air and 
combustion gases to be mixed with "cold" outside air. Moreover, any 
changes in the wire and coating parameters require manual readjustment of 
some dampers to rebalance the system. By reason of its control 
arrangement, the air flow in this oven could thus be characterized as 
constant temperature/variable volume. 
A need has been developed for an improved wire enameling oven and control 
system therefor which is adapted to provide better temperature control and 
temperature distribution across the work chamber, and which is adapted to 
provide greater operating economy. 
SUMMARY OF INVENTION 
The present invention comprises wire enameling oven and control system 
therefor which overcomes the foregoing and other difficulties associated 
with the prior art. In accordance with the invention, there is provided an 
oven comprised of an elongated casing with partitions therein dividing the 
interior into an elongate work chamber with evaporating and curing zones, 
combustor inlet and outlet chambers interconnected by a combustor tube 
with a burner in the inlet and thereof, an intermediate (residence) 
chamber, and gas passageways. Process air from the evaporating and curing 
zones of the work chamber enters the intermediate chamber and passes 
through a heat exchanger where it is preheated before entering the upper 
inlet combustor chamber for feeding to the burner in order to effect 
substantially complete oxidation of any volatile fumes carried thereby. 
The superheated combustion products and air from the burner in the 
combustor tube pass through the heat exchanger from the lower combustor 
outlet chamber to a hot air plenum and on into mixing boxes for mixture 
with "cold" outside air before directing the mixture by recirculation fans 
back through the work chamber. Mixing of the cold outside air and 
superheated combustion air is automatically controlled by a 
microprocessor-based controller responsive to thermocouples positioned in 
the work chamber. Separate mixing boxes, motorized damper assemblies, and 
recirculation fans are provided for the zones of the work chamber. Excess 
superheated combustion air from the hot air plenum is exhausted by an 
exhaust fan through a motorized damper which is controlled by a controller 
responsive to a pressure sensor, located in the intermediate chamber of 
the oven. Since each recirculation fan operates at substantially constant 
volume and speed in accordance with the temperature requirements of its 
associated zone, the air flow through the oven herein can be characterized 
as constant volume/variable temperature in contrast to the oven disclosed 
in the '736 patent to Dumas.

DETAILED DESCRIPTION 
Referring now to the Drawings, wherein like reference numerals designate 
corresponding elements throughout the views, there is shown an oven 10 
incorporating the invention. Oven 10 is particularly adapted for curing 
wire coated with enamel on a continuous basis to form magnet wire, 
however, it will be appreciated that the invention can easily be adapted 
for drying or curing other types of items continuously conveyed along an 
input path. For purposes of illustration and discussion, the invention 
will thus be discussed only in reference to a wire enameling oven. 
Oven 10 comprises a generally rectangular casing 12 having spaced apart 
front and rear walls 14 and 16, side walls 18 and 20, and top and bottom 
walls 22 and 24. The front wall 14 includes a plurality of vertically 
spaced apart horizontal cross members 26 extending between side walls 18 
and 20, and removable access doors 28. The walls and doors comprising 
casing 12 are preferably of double wall construction including inner and 
outer layers of metal with insulation therebetween. 
The interior of casing 12 is divided into several chambers and passageways 
by means of partitions. Three vertical insulated partitions 30, 32 and 34 
extend between side walls 18 and 20 in vertical alignment and cooperate 
with portions of the top and bottom walls 22 and 24 to form an elongated 
work chamber 36 with the front wall 14. Work chamber 36 is preferably 
separated by a vertical divider 38 into lateral pairs of upper curing 
zones 40 and lower evaporating zones 42. In spaced apart side-by-side 
arrangement, lines of wire 44 coated with liquid insulator, such as 
enamel, pass upwardly through the work chamber 36 of oven 10 first through 
the evaporating zone 42 and then through the curing zone 40. Separations 
are provided at the vertical ends of the insulated partitions 30, 32 and 
34 to direct air flow through the work chamber 36 as will be explained 
more fully hereinbelow. 
A first horizontal partition 46 extends between the inside surfaces of 
partition 30, side walls 18 and 20, and back wall 16 to define an upper 
combustor chamber 48 and an intermediate chamber 50 within the casing 12. 
A second horizontal insulated partition 52 extends between a vertical duct 
102, which is spaced inwardly from the vertical partitions 32 and 34, and 
side walls 18 and 20 and back wall 16 to separate the outlet combustor 
chamber 56 from the intermediate chamber 50. An insulated partition 58 
extends between partition 52 and the inside surfaces of bottom wall 24 and 
side walls 18 and 20 in spaced relationship behind partition 34 to form a 
space therebetween for the duct 102, which is described more fully below. 
An elongated combustor tube 60 is mounted inside casing 12. The combustor 
tube 60 extends through partitions 46 and 52 and chamber 50 therebetween, 
interconnecting the combustor chambers 48 and 56 in fluid communication. 
Radial vanes 62 are provided around the upper end of the combustor tube 
60, which is spaced downwardly from the inside surface of the upper casing 
wall 22 for expansion purposes. 
A burner 64 extends downwardly through an insulated, removable plug 65 in 
wall 22 and into the upper end of the combustor tube 60 in the inlet 
combustor chamber 48. Burner 64 can be constructed similar to the burner 
shown in U.S. Pat. No. 3,810,736, the disclosure of which is incorporated 
herein by reference; however, any suitable burner can be utilized. Primary 
combustion for the burner 64 is provided by natural gas, or other suitable 
fuel. As burner 64 burns downwardly, air within the chamber 48 is pulled 
through vanes 62 and around the upper end of tube 60. The resultant 
superheated combustion produces are drawn downwardly through tube 60 and 
onto heat shield 66 in outlet combustor chamber 56. A sight glass 68 can 
be mounted in the bottom wall 24 of casing 12 for looking through an 
opening (not shown) in shield 66 to monitor the flame of burner 64. It 
will thus be apparent that burner 64 combusts gas from chamber 48 and 
directs the resultant superheated combustion products downwardly through 
tube 60 into the outlet combustor chamber 56. 
A heat exchanger 70 is located inside casing 12 between the combustor tube 
60 and the back wall 16. The heat exchanger 70 comprises a substantially 
conventional parallel flat plate unit defining two separate fluid flow 
paths. The heat exchanger 70 is mounted in suitable openings in partitions 
46 and 52 in fluid communication with chambers 48 and 50. Superheated 
combustion products from the outlet combustor chamber 56 enter the lower 
end of the exchanger 70 and passes upwardly along one flow path to a 
plenum 72 mounted on the other end of the heat exchanger. Simultaneously, 
air from work chamber 36, together with volatile vapors from the enamel on 
wires 44, enter the intermediate chamber 50 through the passageway 74 
defined between adjacent ends of partitions 30 and 32 and pass along the 
other fluid path through the heat exchanger 70 and on into the upper 
combustor chamber 48 for combustion by burner 64. It will thus be 
appreciated that "process" air from the work chamber 36 together with 
volatile vapors given off by the coating on wires 44 as it evaporates and 
cures are first preheated in the heat exchanger 70 before being directed 
to the burner 64 to effect oxidation of the fumes. This preheating step of 
the process is an important feature of the present invention. 
Air flow through the oven 10 is effected by a single exhaust fan 76 and 
multiple recirculation fans 78, both of which are connected directly or 
indirectly to the plenum 72. The exhaust fan 76, which is preferably of 
the constant speed type, is connected to plenum 72 through an exhaust pipe 
80 and expansion joint 82, and serves to exhaust excess hot combustion 
gases which have passed through the heat exchanger 70 from chamber 56. The 
recirculation fans 78, which also are preferably of the constant speed 
type, are indirectly connected in plenum 72 via mixing boxes 84. Each 
mixing box 84 in turn is connected directly to the plenum 72 by duct 86, 
as well as to an open outside air intake 88 extending through the back 
wall 16 by duct 90. The purpose of recirculation fans 78 is thus to draw 
into their associated mixing boxes 84 cold outside air from intakes 88 as 
well as hot combustion gases from plenum 72. Motorized dampers 92 and 94 
are provided in each associated pair of ducts 86 and 90, respectively, to 
control the relative volumes of cold outside air and hot combustion gases 
drawn into their corresponding mixing boxes 84. Another motorized damper 
96 is located in the exhaust pipe 80 for controlling the volume of excess 
hot combustion gases exhausted from the heat exchanger 70. A separate 
recirculation fan 78, mixing box 84, and motorized damper assembly 
including dampers 92 and 94, are provided for each evaporating zone 42 and 
curing zone 40 in the work chamber 36 of oven 10. This comprises another 
significant feature of the present invention, as will become more apparent 
hereinafter. 
The inlet sides of recirculating fans 78 are connected to their associated 
mixing boxes 84, while the outlet sides thereof are connected to 
appropriate ductwork leading to the work chamber 36. In the case of the 
curing zones 40, the outlet sides of their recirculation fans 78 are 
connected to ducts 98 extending an opposite sides of the combustor tube 60 
in chamber 48 to the lateral passageways 100 defined between the upper 
ends of partitions 30 and the upper casing wall 22. In the case of the 
evaporating zones 42, the outlet sides of their corresponding 
recirculation fans 78 are connected by ducts 102 to passageways 106 and 
108 opening onto the work chamber 36 near the inlet end thereof. 
Passageway 106 is defined by the separation between the adjacent ends of 
partitions 32 and 34, while passageway 108 is defined by the separation 
between the bottom edge of partition 34 and bottom wall 24. 
Dampers 110 and 112, which can be either fixed in preset positions or 
adjustable by means of shafts and knobs 111 and 113, respectively, located 
outside casing 12, are provided in ducts 98 and 102, respectively, for 
volumetric control purposes. If desired, dampers 110 and 112 can be 
motorized and automatically controlled similarly to dampers 92 and 94, but 
responsive to preprogrammed or other conditions. Dampers 110 and 112 can 
thus be fixed or manually or automatically adjusted to the desired 
settings in accordance with the operating conditions of oven 10. 
Although the preferred embodiment as illustrated incorporates constant 
speed recirculation fans 78 together with volumetric control dampers 110 
and 112, it will be apparent that variable speed fans could be used. 
Variable speed fans are considered fully equivalent to constant speed fans 
and volumetric control dampers. Similarly, a variable speed fan could be 
substituted for exhaust fan 76 and volumetric control damper 96. 
A damper 114, which can be manually adjustable with knob 116 located on the 
outside of the oven casing 12, is preferably provided in the throat of 
each vertical duct 102 to adjust relative flow through passageways 106 and 
108 as necessary for evaporative temperature gradient control in the 
associated zone 42. As with dampers 110 and 112, dampers 114 also can be 
motorized and automatically adjusted to the desired settings in accordance 
with the particular operating conditions of oven 10. 
Oven 10 operates as follows. Assume that exhaust fan 76 and recirculation 
fans 78 are energized, that burner 64 is ignited and burning downwardly 
through the combustor tube 60, and that wire 44 coated with liquid enamel 
is being fed through the work chamber 36. The process air, which comprises 
a relatively small amount of room air inspirated through the ends of the 
work chamber 36 together with the heated mixtures of gas flowing through 
passageways 100, 106 and 108 and volatile fumes carried thereby from the 
enamel on wire 44, is drawn inwardly along the work chamber and through 
passageway 74 into chamber 50. The small amount of room air inspirated 
through the open ends of work chamber 36 would typically be at about 
70.degree. F., whereas the "dirty" process air drawn through passageway 74 
into chamber 50 would typically be at 700.degree. to 850.degree. F. From 
chamber 50, the process air passes through the heat exchanger 70 for 
preheating before entering chamber 48 for combustion by burner 64 so that 
the volatile vapors from the solvent in the enamel on wire 44 are 
substantially completely oxidized to the point of being neither 
objectionable nor harmful. The superheated combustion gases, typically at 
about 1,350.degree. F., are directed downwardly through tube 60 to chamber 
56 and heat exchanger 70 for preheating the process air from work chamber 
36. After passage through the heat exchanger 70, the "clean" combustion 
gases at about 1,200.degree. F. enter the hot air plenum 72. Hot gases 
from the hot air plenum 72, together with room air at about 70.degree. F. 
from the intakes 88, are drawn by recirculation fans 78 into the mixing 
boxes 84 to form a gas mixture of 700.degree. to 1,100.degree. F. which is 
then directed through ducts 98 and 102 and respectively discharged into 
the curing and evaporating zones 40 and 42 of the work chamber 36 for 
temperature control thereof. Typically, the temperature within work 
chamber 36 would be about 400.degree. F. along partition 34 and about 
700.degree. F. along partition 32, in the evaporating zone 42, while the 
temperature would be relatively higher at about 950.degree. F. along 
partition 30 in the curing zone 40. Excess hot combustion gases from the 
heat exchanger 70 are exhausted from the hot air plenum 72 past the 
volumetric control damper 96 by the exhaust fan 76. The proportionate 
mixture of outside air and combustion gases drawn into mixing boxes 84 is 
individually controlled by the corresponding pair of motorized dampers 92 
and 94. Temperature gradient control along the evaporating zone 42 is 
controlled by the manually adjustable damper 114. 
If desired, gravity biased doors 117 can be provided in partition 52 
together with gravity biased access doors 118 in side walls 18 and 20 
adjacent to chamber 50 for pressure relief and thus safety purposes in the 
event of an explosion. 
Referring now to FIG. 9, there is shown the control system 120 for the wire 
enameling 10 herein. Although dampers 92 and 94 in each corresponding pair 
could be separately actuated, each pair of dampers is preferably mounted 
on a common shaft operated by a single motor 122 or other suitable 
actuator, as is best seen in FIG. 6. Motors 122 in turn are controlled by 
microprocessor controllers 124 responsive to temperature sensors or 
thermocouples positioned in the work chamber 36. As illustrated, a 
separate microprocessor controller 124 is provided for each pair of curing 
zones 40 and each pair of evaporating zones 42; however, a separate 
microprocessor controller could be provided for each zone or a single 
central microprocessor with adequate capacity could be employed to control 
all of the zones, and it will be understood that the use of multiple 
controllers is not critical to practice of the invention. A suitable 
microprocessor would be the Honeywell UDC 500 digital controller, for 
example. 
Motors 122 for the dampers 92 and 94 corresponding to the curing zone 40 
are automatically controlled by the microprocessor controller 124 
responsive to temperature sensors or thermocouples 126 located in the work 
chamber 36 near passageways 100, while the motors for the mixing dampers 
corresponding to the evaporating zone 42 are controlled by the 
microprocessor responsive to thermocouples 128 located in the work chamber 
near passageways 106. As shown in phantom lines in FIG. 9, the volumetric 
control dampers 110 associated with the rcirculation fans 78 are operated 
by motors 129 automatically controlled by the microprocessor 124, instead 
of manual knobs 111. Dampers 114 can be adjusted as necessary by reference 
to temperature sensors or thermocouples 130 located in the work chamber 36 
near passageways 108. 
Dampers 114 may also be motorized and automatically controlled by the 
microprocessor controller 124 responsive to thermocouples 130. If desired, 
a display 132 can be connected to the microprocessors 124 to present a 
visual readout of the sensed conditions within oven 10. 
The box labeled D.T.O. in FIG. 9 represents the direct thermal oxidizer 
comprised of burner 64 and combustor tube 60. 
With respect to the exhaust fan 76, which operates at substantially 
constant speed, the volumetric damper 96 is actuated by a motor 134 
controlled by a controller 136 responsive to a pressure sensor 138 located 
in chamber 50 of the oven 10. Controller 136 preferably comprises a 
digital device like that shown in U.S. Pat. No. 3,677,335, also assigned 
to the assignee hereof, the disclosure of which is incorporated herein by 
reference. In order to contain the pollutants therein the oven 10 must 
operate at a slightly negative pressure relative to ambient air pressure. 
Since more outside air through inlets 88 is required if lower operating 
conditions are to be maintained in the work chamber 36, then more hot 
gases must be exhausted by the exhaust fan 76 to maintain the oven at a 
predetermined negative pressure. Fan 76 thus operates at substantially 
constant speed and damper 96 provides volumetric control of the exhaust. 
From the foregoing, it will thus be apparent that the present invention 
comprises a curing oven and control system therefor having numerous 
advantages over the prior art. Pollution control is accomplished by direct 
thermal oxidation of volatile fumes released by the coating of enamel. The 
system herein can be classified as constant volume/variable temperature as 
opposed to the constant temperature/variable volume systems of the prior 
art. Separate motorized dampers, mixing boxes and recirculation fans are 
provided for each zone in the work chamber, and proper temperature control 
is automatically maintained by a microprocessor responsive to temperature 
sensors in the oven. Maintenance of the oven at a predetermined pressure 
less than ambient for environmental control and burner operating economy 
is achieved by an exhaust fan and motorized damper responsive to a 
pressure sensor inside the oven. Other advantages will be evident to those 
skilled in the art. 
Although particular embodiments of the invention have been illustrated in 
the accompanying Drawings and described in the foregoing Detailed 
Description, it will be understood that the invention is not limited to 
the embodiments disclosed, but is intended to embrace any alternatives, 
equivalents, modifications and/or rearrangements of elements falling 
within the scope of the invention as defined by the following claims.