Note: Descriptions are shown in the official language in which they were submitted.
1057048
The present invention relates to ovens for drying or
curing coatings such as paints, adhesives containing oxidizable
solvents and the like applied to a workpiece moving through such
oven, such as for example strip material.
BACKGROUND OF THE INVENTION
- The oven under consideration is generally known in
the trade as a "recirculating convection oven".
Such ovens are usually divided into oven zones. In each
zone recirculating fans and ducts are provided to continuously
recirculate the zone atmosphere and produce gas flow around the
workpiece. Such gases are heated during recirculation to main-
tain over~ tem~rature. A prc>p~rtic~n c>f such qases is continuously
withdrawn as "ef~uent", ana re~acea with make u~ air or qa~
~e c~ c~at~ ~s cc~ca~ S~ t ~e~'cs ~ s~c~ a~
tho8e applied to strip sheet metal, presents a number of prob-
lems. In the first place, the solvent fumes must be rendered
harmless before venting to atmosphere so as to avoid envinron-
mental pollution. This can be done in some cases by solvent
extraction, condensing the solvent vapour~ from the effluent
from the oven for reuse or by use of a catalyst such as pla-
tinum. However, in the majority of cases the solvent vapours
are simply oxidized by passing the effluent through an incin-
erator chamber which in most cases must be capable of operation
at a solvent~oxidation level of 98% to 99~ in order to meet
prevailing standards for emissions from this type of equipment.
Usually such incinerators are gas fired incinerators, and the
fuel consumption requried in order to operate at these levels
of efficiency with the very large volumes of oven effluent is
a major consideration in the design of such an oven.
Recovery of the heat generated by such incinerators
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1057048
for use in -the oven, or for use elsewhere will somewhat reduce
the operatlng cost of the incinerator, but ln many cases it is
not possible to use the heat recovered from the incinerator in
an economical manner, or to its full extent.
It is therefore desirable where possible to reduce the
volume of emissions to atmosphere, and thereby reduce the
size of the incinerator for treating such emissions. Proposals
have been made vor continuously recycling oxidized gases from
the exhaust incinerator, and returning them within the system,
so as to reduce the volume of exhaust going to atmosphere, but
this is of only limited value, and leaves large volumes of ox-
idized gases which cannot be used in the oven due to tempera-
ture control limitations required for individual zones in the
oven. Since any exhaust volume must be replaced by fresh air,
which must eventually be heated to incineration temperature,
an unnecessary heat load is incurred.
A second major factor in the construction of such
ovens i~ the manner in which the various zones in the oven are
heated. Various different heating systems have been used, a
common ~ystem being the use of gas ~urners heating the recir-
culating gas0s in the various zones. Clearly, the fuel cost for
heating such gases is a further major factor in the cost of
operating the oven. Some systems have been proposed for reduc-
ing the fuel requirement for heating the gases in the various
zones by recycling the oxidized gases exiting from the incin-
erator back through the zones, and such systems have met with
some degree of success. However, they introduce further problems.
In particular, the gases exiting from a typical incinerator will
be at about 1,400F. At these temperatures, conventional steel
duct work, fans, dampers and the like are no longer usable, and
~ os704~
special alloys must be employed to withstand such temperatures.
This of course greatly increases the construction costs of the
oven and requires more frequent maintenance, and reduces reli-
ability.
A further major factor in the design of such ovens is
the ability to control the temperature of the gases in the var-
ious zones, and to regulate the gas temperatures in the different
zones progressively so that the coating on the strip is cured
in the most advantageous manner. Such coatings may employ
several different solvents having different boiling points so
that the coating dries progressively from the inside out to pro-
duce the desired finish. Similarly, some types of paints have
solvents with relatively high boiling points therefore requiring
relatively high temperatures in the oven, and other forms of
coatings such as some adhe~ives use solvents with relatively
low boiling points requiring lower temperatures.
Accordingly, in order to build an oven which is capable
of handling a wide range of different paints, coatings, adhesives
and the like over a relatively wide range of temperatures, it is
essential that the gas temperature~ in the various zones may be
closely regulated and controlled. The controlling of gas temp-
erature in the different zones of an oven, where the gases are
heated even partially by mean~ of recycled incinerator gases at
high temperatures, becomes particularly difficult, since the
regulation of the temperature will depend upon the proportioning
of a mixture of fresh air, and incinerator gases introduced
into each 20ne, so as to produce the correct gas temperature
within the zone. As mentioned above, the handling of inciner-
ator gases at the high temperatures experienced, it is both dif-
ficult and relatively expensive in terms of the equipment required
1057048
In ad~ition to convection heating of the strip byrecirculating hot gases, i~ i 5 also desirable at some point in
the curing line to provide for radiant l,eating of the strip so
as to actually heat up the strip metal itself and thereby cure
the paint or other coating material from the inside to the out-
side of the coating layer. In the past, such radiant heating
was usually effected, if at all, by means of gas radiants or
by means of electrical radiants located within the oven. Such
radiant heating systems involved the use of still furtller fuel
input adding still further to the cost of the operation of the
system.
BRIEF SUMMARY OF THE INVENTION
It is therefore a general objective of the present inven-
tion to provide an oven and method of treating a workpiece of the
type described in which the various disadvantages and inefficiencies
of earlier oven systems are eliminated or at least reduced. A
major proportion of the oven zone effluent containing solvent vapours
i5 recycled within the system and returned to the zones. Prior to
being fed into the zones, the effluent is passed through individual
zone incinerators located at the entrance to each of the oven zones,
thereby heating such incoming effluent gases to a controlled
elevated temperature for oxidation of the solvent vapour. Sur-
ficient ozidized gas volume is admitted to each zone to maintain
a predetermined desired zone gas temperature. A further incin-
erator is provided in the exhaust system for the oven, through
which a certain proportion of the oven effluent is passed, to
permit venting of some of the oven effluent to atmosphere, the
exhaust incinerator oxidizing the solvent vapour content to
avoid environmental damage. A further portion of the oven ef-
fluent containing solvent vapours, passes through a bypass
1057048
incinerator and the oxidized gases then enter a radiant heatingsystem and provide heat input for the radiant heating system which
is in turn transmitted as radiant heat directly to the strip.
The oxidized gases are then discharged from the radiant heater
into the oven, providing additional ventilation.
; In an alternate form of the invention the exhaust in-
cinerator discharges into separate radiant header ducts which
displace the first convective zone of the oven, and the oxidized
fumes are then discharged up the stack.
More particularly, it is an objective of the invention
to provide a curing oven having the foregoing advantages in which
the effluent from one or more low solvent release zones is re-
introduced directly through a high solvent release zone, whereby
to increase the ventilation of such zone.
I~ore particularly, it is an objective of the invention
to provide a curing oven having the foregoing advantages in
which effluent from low solvent zones is reintroduced in cas-
cade form progressively through other zones of higher solvent
release whereby to still further increase the ventilation of
these further zones.
More particularly , it is an objective of the invention
to provide an oven having the foregoing advantages in which the
bypass incinerator discharges to a radiant duct system extend-
ing above and below the part of the workpiece, and having in-
wardly directed radiant surfaces for radiating heat from said
duct work onto both sides of said wor~piece. The duct work pro-
vides an extended oxidation space thereby prolonging the dwell
time and ensuring adequate oxidation of recycled oven effluent.
The solvent vapours thus provide a portion of the fuel for pro-
viding the heat input for said radiant heating unit. The bypass
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~057048
incinerator incorporates means for varying the fuel input to the
incinerator, and further control means for varying the input of
recycled oven effluent to the incinerator, whereby to provide
for oxidation of greater or lesser amounts of solvent vapours.
This provides a fully flexible system in which a fixed proportion
of oven effluent i~ exhau~ted to atmosphere, and the balance
of effluent is taken either by the zone incinerators, or by
the bypass incinerator, depending on the volume of effluent re-
quired by the zone incinerators to satisfy the heat input demand
of the zones. In this way, the system avoids any variation in
the exhaust to atmosphere and fresh air intake.
More particularly, it is an objective of the invention
to provide a curing oven having the foregoing advantages in which
the individual oven zones are provided both with individual zone
incinerators for oxidizing the recycled oven effluent prior to
reintroduction into the zones, and in addition, are provided with
supplementary heaters, for providing a rapid warm up of the
various zones, and providing supplementary heat if requlred.
The various features of novelty which characterize
the invention are pointed out with particularity in the claims
annexed to and forming a part of this disclosure. For a better
understanding of the invention, its operating advantages and
specific ob~ects attained by its use, reference should be had
to the aocompanying drawings and de~criptive matter in which
there are illustrated and described preferred embodiments of
the invention.
IN THE ~RAWINGS
Figure 1 is a schematic illustration showing the
layout of a curing oven system according to the invention in
which provision is made for simultaneous curing of a strip
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1057048
havinq a prime coat, and a further strip having a finish coat;
Figure 2 is an enlarged illustration showing one-half
of the oven system -~hown in ~igure 1, in greater detail;
Figure 3 is an enlarged illustration showing the other
half of the oven system shown in Figure 1 in greater detail;
Figure 4 i~ an enlarged schematic elevational view
showing the radiant heating means of the oven of Figure 2: and
Figure 5 is a schematic plan view showing an alternate
form of the radiant heating means.
D~SCRIPTION OF A PREFERRED EMBODIMENT
Referring now to Figure 1, this illustration shows in
schematic form the layout of an oven installation suitable for
a strip coating plant, where strip sheet metal is painted, or
coated with a prime coat and a finish coat. Obviously, however,
oven installations of the same general type will be suitable for
other purposes such as the curing of other forms of coatings, and
the curing of adhesives, with only minor modifications a~ will
suggest themselves to persons skilled in the art.
As shown in Figure 1, the oven installation comprises
a prime coat oven 10 and a finish coat oven 12, which are sepa-
rate from one another, but are interconnected with the same ven-
tilation and heating system, whereby they may be operated simul-
taneously, f~or curing two different strips at the same time.
The prime coating oven 10 will be seen to be divided
nominally into four zones, namely lOA, lOB, lOC and 10D. The
finish coating oven will be seen to be nominally partitioned into
five zones, namely 12A, 12~, 12C, 12D and 12E.
The organization and arrangement of the ovens 10 and 12
is essentially the same, and accordingly detailed description
will be given initially of oven 10, it being understood that the
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l~S7048
i
finish coating oven 12 i8 provided with essentially the same
equipment, which will be described somewhat more briefly herein- ;
after.
The four zones 10A, 10B, 10C and lOD of oven 10 are all
constructed a~ a single continuous unit essentially in the form
of a tunnel or elongated chamber of suitable dimensions to ac~
cept passage of a strip of sheet metal or other strip workpiece
moving therethrough. Each of the oven zones lOA, lOB, lOC and
lOD are provided with oven zone gas recirculation chambers 14,
16, 18 and 20, through which the zone gas mixtures is continu-
ously recirculated into the zone in a manner known per se, by
any suitable circulating fans and duct work, not shown.
Effluent gas is extracted from each of the oven zones
through respective exhaust conduits 22, 24, 2~ and 28. In order
to increase the ventilation through zone lOB, the exhaust con-
, duits 26 and 28 are united at conduit 30, which reintroduces the
effluent from oven zone~ lOC and lOD back into oven zone lOB.
Oven zone lOA will receive fresll air input throughfresh air conduit 32 wh~ch in turn will receive fresh air through
20 the fresh air supply system 34. This fresh air supply system 34
may be connected so as to receive fresh air from the atmosphere.
Alternatively, however, and in the preferred case, it will in
fact be conn~cted to the air ventllation and exhaust system for
the coater room3, where the strip is coated. In this way, the
atmosphere of the coater rooms is kept fresh and breathable, and
: any solvent fumes-which may evaporate within the coater room
will be contained and used within the oven system, so that the
heat available from the solvent vapours may be used in the ovens,
and will not be simply vented to atmosphere from the coater rooms
them~elves. Thls system also avoids the requirement for attach-
_ g _
1057048
ing an incinerator to the coater room ventilation system which
might otherwise be necessary in certain jurisdictions to avoid
environmental pollution, A branch fresh air duct 32a may be
connected to the entrance to the oven zone lOA so as to pass
-fresh air directly into the opening at the front of the oven
zone lOA.
The exhaust conduit 22 is located at the main oven ex-
haust section 22a in zone lOA and is connected to a central mix-
ing chamher or plenum 36. From the plenum 36 a main supply duct
38 connects with branch supply ducts 40, 42 and 44 respectively.
The branch ducts 40, 42 and ~4 in turn are connected with the
zone recirculation chambers 16, 18 and 20 of the oven zones lOB,
lOC and lOD.
The p~enum 36 also connects with the exhaust duct 46,
feeding the exhaust incinerator 48 which is then directed to the
exhaust stack 50. A heat recovery system 52 may be provided on
the outpu~ to the incinerator 48, and the heat recovered may be
used for any heating purpose around the plant, and may also be
used for preheating the incoming air on the air supply system 34
by means for example of the air preheater 54.
It will be understood that the plenum 36 will be re-
ceiving oven effluents which are essentially the combined efflu-
ent output of zones lOD, lOC, lOB and lOA.
In accordance with the invention, some of the vapours
; in the effluents going to the plenum 36 are utilized for heating
the zones lOB, lOC, and lOD. In order to achieve this purpose,
each of the zones lOB, lOC and lOD respectively is provided with
its own incinerator 56, 58 and 60 respectively. Each of the in-
cinerators 56, 58 and 60 is supplied with oven effluent carrying
solvent fumes, by means of the branch ducts 40, 42 and 44. The
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11057048
incinerators function to oxidize the solvent vapours, and the
high temperature oxidized gases are then mixed with the recircu-
lating zone gases entering the oven zones through the respective
zone recirculation chambers 16, 18 and 20 and at the same time
reducing the percentage of solvent vapours in ~uch recirculating
. zone gases. A branch input duct 62 communicate~ with air cur-
tain at 20a o oven zone lOD to supply hot gases to the exit end
of the æone lOD.
. In order to control the temperature within each of the
. 10 zones lOB, lOC and lOD, control dampers 64 are located in the
. ducts 40, 42 and 44, and are controlled by suitable tempexature
controls 66, connected to suitable temperature sensors ~not
shown) located within the respective zones lOB, lOC and lOD.
. Vaxiation in the zone temperature due to varying heat load will
. produce variation in the volumes of oven effluent gas ~upplied
to the incinerators, by operation of the dampers 64, thereby
varying the flow of hot oxidized gases from the respective inci-
nerators 56, 58 and 60. In this way, the temperature of each
. zone can be regulated to a desired preset level.
In accordance with well known practice in the art, the
incinerators 56, 58 and 60 will be fired normally by natural gas
or other ~uitable fuel, more or less being required dependent
upon the per~centage of solvent vapour content in the gas 6uppli-
ed through the ducts 40, 42 and 44. In order to control the tem-
perature of the oxidized gas from the incinerator~ 56, 58 and 60,
suitable temperature controls 68 are provided and connected to
sensors (not shown) for sensing the temperature of the gases exit-
ing from the respective incinerators, thereby controlling the fuel
input.
In order to provide for a rapid warmup of the oven
,
. - 11 -
1057048
zones lOA, lOB, lOC and lOD, supplementary heaters 70 are pro-
vided for the respective recirculation chambers. It will of
course be appreciated that such supplementary heaters will be
used mainly during the initial start-up phase of operation, and
that in the great majority of cases, once the gases in the ducts
40, 42 and 44 are carrying their regular volumes of solvent va-
pours, the operation of the individual incinerators 56, 58 and
60 will be sufficient to provide all the heat required for the
zones lOB, lOC and lOD and the heaters 70 will remain on low
fire or will be shut down. Such supplementary heaters will usual-
ly be fired by natural gas for example although any other suit-
able fuel, capable of providing adequate heat at the location
may be substituted. Any suitable control may be provided, the
details of which are omitted for sake of clarity.
A heater 72 is provided on the air conduit 32 for
heating the combined incoming fresh air and effluent gases en-
tering the zone lOA of the oven. Heater 72 i8 not an incinera-
tor and does not oxidize the solvent vapours in the effluent
gases at this point.
In order to regulate the flow of exhaust gases out of
the zones lOC and lOD, dampers 74 are provided in the exhaust
conduits 26 and 28, and pressure sensitive control means 76 are
provided fo~ sensing the pressure in the zones lOC and lOD, and
varying the position of the dampers 74 accordingly. Similarly,
the exhaust conduit 24 is also provided wlth a control damper 78,
and pressure sensing means 80 for sensing the pressure in the
zone lOB and varying the opening of the damper 78 accordingly.
A damper 74 is also provided in the conduit 30.
In order to provide for radiant heating of the strip,
a radiant heating unit indicated generally as 82, and ~hown in
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1057048
greater detail in Figure 4, is located between zone lOA and
zone lOB. As shown in Figure 4, the radiant heating unit 82 will
be seen to comprise a generally U-shaped loop of duct work, hav-
ing a lower portion 84 and an upper portion 86 and a return U-
bend 88. The lower and upper portions are adapted to extend on
the lower and upper sides of a strip or workpiece passing through
the oven 10, and are spaced apart a suitable distance to accom-
modate any variation in the position of the strip during opera-
tion of the oven. The portions 84 and 86 of the duct work are
provided with inwardly directed radiant surfaces 90, which are
preferably formed, at least on the interior of the duct work,
with any suitable heat exchange surface formation such as fins,
ridges, or any other suitable formation. Insulation is provided . .
elsewhere around the duct work, to retain heat therein.
Bypass supply duct 92 extends from the plenum 36 to by-
pass incinerator 94 which in turn discharges into one end of the
radiant heat~ng unit 82, which end may be either that of the low-
er portion 84 or the end of the upper portion 86. The other end
¦ of the unit 82 discharges into zone lOA at exhauet section 22a
and the ga~es then enter exhaust conduit 22 which communicates
with the plenum 36 as described above. The bypass inclnerator 94
is located in the bypass supply duct 92 for oxidizing the solvent
vapours entr.ained in gases coming from the plenum 36. Such by-
pass incinerator will normally be fired by natural gas, or any
other suitable fuel. The temperature of the oxidized gas from
the bypass incinerator 94 is controlled by means of a temperature
sensitive controller 96 sen8ing the temperature oP the gases
exiting from the incinerator.
The duct work portions B4, 86 and 88 provide.an elon-
gated oxidation chamber eneuring a long dwell time for oxidation
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~057048
of solvent vapours. The bypass incinerator 94 can thus be opera-
ted at a somewhat lower temperature while still achieving effi-
cient oxidation.
Supply of gases to the bypass incinerator 94, from the
plenum 36, is controlled by means of the damper 98 and pressure
sensitive controller 9g, ~ensing the pressure in the interior of
zone lOA of the oven.
In order to provide for rapid cooling, and rapid vent-
ing in an emergency situation, a series of quick cooling fresh
air vents are provided throughout the oven system which wlll ad-
mit fresh air to the oven at a number of different locations.
Such quick cool vents are indicated as 100 and consist essenti-
ally of dampers which may be opened or closed either manually on
command, or automatically by any suitable emergency control.
Typically, such emergency controls will comprise gas analyzers
located at various points within the oven, and operable to give
an alarm signal if the solvent vapour content should exceed the
desired percentage of the L.E.L. for that particular solvent.
An additional sealing damper 102 is provided to back-
up the quick cool damper lO0 so as to prevent any risk of a leakof the oven effluent gases from the conduit 26 to atmosphere at
this point ~n the system.
A ~ressure relief duct 104 extends from a point be-
tween the damper lO0 and the sealing damper 102, back to the up-
stream side of a fan F feeding gases from the conduit 26 to the
conduit 30 so that any leakage through the damper lO0 can be re-
, cycled bac~ into the conduit 30.
Throughout the sy~tem numerous fans are provided whichare shown schematically, the function of which will be apparent
to those skilled in the art, and e~eentially maintain flow of
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gases through the system.
The finish coat oven 12 is provided with essentially
the same system of zone incinerators and recycliny of zone ex-
haust, the respective zone recirculating chambers being shown as
114, 116, 118, 120 and 122, for zones 12A to 12E respectively.
Similarly, branch supply ducts 124, 128, 130 and 132 communicate
from the main supply duct 38 with the respective zone recircula-
ting chambers 12B to 12E. Individual zone incinerators 134, 136,
138 and 140 provide for oxidation of the solvent vapours suppli-
ed to them through the branch ducts, and provide heat input forgas recirculating in their respective zones.
Controls similar to those shown in the case of oven 10
will of course be incorporated, the details being omitted for
the sake of clarity.
Similar supplementary heaters 142 are provided for
zones 12A to 12E. A heater 150 is provided on the inlet duct
152 for heating the combined incoming fresh air and effluent
gase~ entering the zone 12A of the oven 12. ~eater 150 is not
an incinerator and does not oxidize the solvent vapours ln the
effluent at thi~ point.
A radiant heating unit 154 i9 provided in the main
exhaust section of zone 12A, similar to the radiant heating unit
82 and provided with a bypass incinerator 156 controlled in the
same manner as bypass incinerator 94 of the radiant heating unit
82.
Similar quick cool ventilation is provided by means of
ventilators 158. Effluent from the zones 12C, 12D and 12E ex-
hausts through respective exhau t ducts 160, 162 and 164 all of
which are controlled by dampers, as described ln connection with
zone~ lOC and lOD. A common return duct 166 feeds the effluent
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lOS7048
of zones 12C, 12D and 12E bac~ into zone 12B.
The effluent exhaust from zone 12B is removed through
duct 168, uniting with fresh air duct 152, for supplying a mix-
ture of fresh air and oven effluent directly into zone 12A. :
Effluent from the main oven section in zone 12A is removed
through duct 170 and flows back into the common plenum 36.
A similar sealing damper 172 is provided to back-up the
quick cool vent 158 on duct 166.
STATEMENT OF OPERATION
During normal operation, i.e. when a steady state has
been achieved, the plenum 36 and duct 38 will normally contain
oven effluent containing solvent vapours at or close to the
desired lower explosive limit. These gases flow down the duct 38
and up the bxanch ducts 40, 42 and 44. It will of course be
understood that similar gases will also flow up the branch supply
ducts 124, 128, 130 and 132. However, the operation of the oven
12 will not be described in detail for the sake of simplicity
since it is essentially the same as the operation of oven 10, and
takes place simultaneously.
As the effluent gases flow down the branch ducts, they
pass through the incinerators 56, 58 and 60 where the solvent
vapours are oxidized in known manner. The combustion of the sol-
vent vapours~ together with the heat input from the incinerator
burners, raises the temperature of the exiting oxidized gases
sufficiently so as to maintain the desired temperature level in
the gases recirculating within the zones 10B, 10C and 10D. It
will of course be borne in mind that the temperatures in the zones
will in the preferred case be maintained at different levels and
consequently different heat inputq will normally be required.
3Q The temperatures of the zones, as described above, are controlled
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~OS7048
through the operation of the dampers 64 in the branch ducts 40,
42 and 44, which are in turn controlled by the temperature sen
sitive controls 66. In order to reduce the temperature in a
particular zone, its respective damper is closed down thereby
shutting off some of the supply of effluent gases containing
solvent fumes to the incinerator for that zone.
In this way, the temperature of each zone can be con-
trolled accurately.
A proportion of the recirculating gases i5 removed as
oven effluent from the zones lOC and lOD, continuously through
their respective exhaust conduits 26 and 28, and is returned
through the conduit 30 and reintroduced back into the zone lOB.
At the same time, zone lOB i5 also receiving oxidized incinera-
tor gases from its incinerator 56, as described above, and zone
lOB will therefore normally be subjected to approximately three
times the ventilation passing through zone lOC or lOD. This is
desirable since there will be a greater volume of solvent vapours
, evaporated and removed in zone lOB, than in zones lOC and lOD.
.I Th~ exhaust from zone lOB is removed through the ex-
¦ 20 haust conduit 24, and reintroduced into the gases recirculating
in zone lOA. A certain proportion of fresh incoming air re-
quired by the system i8 introduced here ~o that the temperature
t input to th~ gases in zone lOA can be controlled and at the same. time the overall percentage of solvent vapours in the ga~ mix-
. ture circulating in zone lOA i8 somewhat diluted, by the mixture
of fre~h air, without incinerating the gases passing into zone
. lOA. It will be understood that the heater 72 does not function
as an incinerator but simply operates to maintain a desired
stable temperature within the gases circulating in zone lOA.
: 30 This is desirable since in the majority of cases zone lOA should
~0570~8
be operated at a somewhat lower temperature than zones lOB, lOC
and lOD. The ventilation passing through zone lOA will be the
sum of the entire exhaust fr. l zone lOB together with the fresh
air input.
The entire exhaust from zone lOA is r4moved at the main
oven exhaust 22a through conduit 22 and passes into the plenum 36
where it i8 again available for recycling down the supply duct 38.
Throughout this operation, a varying proportion of the
gases in the plenum 36 is contlnuously withdrawn through the by-
pass duct 92 and passed through the bypass incinerator 94. The
oxidized gases are fed into the radiant heating unit 82. The
gases will circulate through the duct portions 84, 88 and 86, and
give up some of their heat to the heat exchange surfaces ~0,
I which will then radiate heat directly onto the strip passing be-
I tween them, from both sides.
Ga~es exiting from the radiant heating unit 82 will
discharge into the main oven exhaust section 22a and mix with
cooler gases entering the exhaust duct 22 and be returned to the
! plenum 36.
Throuqhout this operation a continuous fixed portion
¦ of gases i~ removed from the plenum 36 through the exhaust duct
46 and fed through the incinerator 48, and the heat recovery sys-
tem 52 and o~ut through the stack 50.
Preferably, the percentage of gases passed through the
incinerator 48 and up the stack 50 will be kept as low as possi-
ble consistent with the volume of fresh air entering the system,
and the volumes of solvent vapours and products of combustion
evolved during operation.
It will be noted that the very high temperature incine-
rator gases, i.e. gases at about 1,400F, will occur only after
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1057048
`~ passage through the incinerators 56, 58 and 60, and incinerators
94 and 4~. In the zones 10B, lOC and lOD, as soon as the gase~
pass through the incinerators, they will enter their respective
zonas, mix with the recirculating gases and lose their excess
heat. Consequently, when they are again drawn into the duct
work, they will be at a considerably lower temperature. In the
case of the gases discharged from the radiant heating unit 82,
the gases will immediately be mixing with gases at a lower tem-
perature in the oven exhaust section. Consequently, the problem
of handling gases flowing through the system and duct work at
very high temperatures is eliminated. In fact, the maximum nor-
mal operating temperature experienced throughout the main portions
of the duct work, fans and the like, will not exceed 900F. At
these temperatures, conventional duct work materials and fan ma-
terials can be used without suffering damage.
During the steady state operation of the oven unit,
the volume~ of gases entering the individual zones lOB, lOC and
, lOD will be more or less stable, or subject only to quite minor
.~ variations in respon6e to operation of the various temperature
controls.
Similarly, the volume of gases exiting to atmosphere,
through the incinerator 48 and stack 50 will also be maintained
at a stable~percentage of the total oven effluent. Thi~ percen-
tage may vary somewhat depending on the nature of the solvents
and the oxygen re~uirements of the system. If the system can
operate with a lower percentage of oxygen while ~till maintaining
effective oxidation of the solvent vapours, then less fresh air
is required and less exhaust gases will be vented to atmosphere.
As mentioned above, the oven is highly flexible in its
operation, and can accept different types of materials and dif-
: - 19 -
~~~
1057048
ferent types of coatings and solvents. In the case of some ma-
terials and solvents, the operation of the individual zone in-
cinerators 56, 58 and 60, will consume a larger proportion of
effluent and thus reduce the volume required to be handled by
the bypas~ incinerator to a minimum.
In other cases the demand of the zone incinerators 56,
58 and 60 will be lower, and in these cases the bypass incinera-
tor 94 will be regulated to consume a proportionally increased
volume of the effluent so as to reduce the solvent vapour percen-
tage and thus maintain the desired level throughout the system.
~ hus, once a steady state operation is achieved in thezones lOB, lOC and lOD, the volume of gases passing through the
bypass incinerator 94 will be regulated to whatever level is
necessary to consume the balance of the solvent vapours in the
system. In this way, the bypass incinerator provides a wide
range of flexibility in the o~erall operation of the entire oven
system without the necessity for increasing the volumes of gases
which are exhausted to atmosphere in a wasteful manner.
In some cases, it may be desirable to provide an alter-
nate form of radiant heating unit, as is ~hown in Figure 5.
In thi~ embodiment, the zone lOA of the oven i5 modi-
fied to eliminate the convection heating of the workpiece, and
the recircu~ating fans and duct work, and this iR replaced by
more exten8ive radiant heating throughout the zone lOA. Thus
zone lOA is shown having a fresh air branch air duct 32a which
receives fresh air from any source such as a coater room, fresh
air or the like. The plenum 36 is connected with the zone lOA
through a gas input duct 200. The duct 200 supplies solvent
laden gases from the plenum 36 to an incinerator 202 and gases
from such incinerator 202 are supplied through radiant duct work
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- . .. ~
1057048
and these factors still further mitigate against the use of
recycled incinerated gases for maintaining the ga~ temperature
in each of the oven zones.
A further factor in the design of such curing ovens
is that for safety reasons it is essential that the solvent
vapour content of any effluent in the oven duct work shall be ;
at or below a desired percenta~e of the lower explosive limit
(the so-called L.E.L.) for any particular solvent. Normally,
this is achieve~ by ensuring that the make up gases entering
the zones contain negligible amounts of solvent vapours, and
maintain a sufficient level of ventilation in the zone. If an
unusual situation should arise and excess solvent vapours
should become entrained with the gases and the solvent limit
is exceeded, then emergency measures must be taken to vent the
oven and reduce the solvent vapour content of the gases present
in the system to avoid the danger of an explosion. Obviou91y,
if such emergency measure~ have to be taken at all frequently,
then the operation fo the system is not commercially sound
since each time the sy~tem i~ shut down, there will be consi-
derable wastage of product and machine down-time.
It iB however desirable that in any such an oven
system provision should be made for rapid venting and cooling
of the sy~tem, with a minimum of di~ruption to the operation
of the coating line, so a8 to permit rapid change-overs from
one colour to another for example, and at the same time provid-
ing for emergency venting of the system if the solvent limit
is inadvertently exceeded. Earlier oven systems did not gener-
ally speaking have this flexibility combined with the safety
features mentioned, and relatively lengthy procedures were
necessary to effect a change-over of colour for example.
1057041~
204 consisting essentially of upper and lower passage~ extending
lengthwise along the zone lOA above and below the path of a
strip or workpiece passing through the zone lOA. A return duct
206 communicates with the other end of the radiant duct work -
204, and connects with the fresh air pr~heater 54 and then to the
stack 50.
~ n effluent gas duct 210 receives effluent gases from
zone 10B (not shown) and connects with the branch air duct 32a
and provides a joint gas input 212 entering the zone 10A, to
provide a gas mixture consisting of combined hot effluent gases,
and cooler fresh air for ventilation of the oven lOA. This pro-
vides ventilating gas flow parallel to the workpiece having only
a minimal heating effect.
Exhaust from the oven lOA is withdrawn through the
exhaust conduit 22 and returned to the plenum 36.
Additional fresh air if required for the purposes of
cooling the radiant duct work 204 may be introduced through a
secondary fresh air intake 216.
As will be seen, the gases exiting from the incinera-
tor 202 are first of all employed for radiant heating of thestrip itself, through the radiant duct work 204, and are there-
after passed to the exhaust duct 206 and stack S0. Thus the in-
cinerator 2Q2 replaces the exhaust incinerator 48 of the embodi-
ment of Figure 1 producing further economies ~n heat recovery.
The bypass ~ncinerator 94 and radiant heating unit 82
remain unaffected and function as before.
During the start up condition of the oven, ~ince there
will be only a small percentage of solvent vapours in the incom- -
ing fresh air, or possibly none at all, the supplementary burners
70 will be turned up to a point where sufficient heat input is
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. ..
.
.
1057048
provided for each of the zones to provide a satisfactory cure or
treatment of the workpiece and coating thereon. As the effluent
content of the gases circulating in the zones builds up, then
the heat input from the zone incinerators will become greater,
and it will then be possible to turn the supplementary heaters
down to their low fire condition.
In the event that the line is shut down for some rea-
son either for a changeover from one colour or coating to anoth-
er or for some other reason, then the quick cool venting system
100 will be put into operation whereby to admit fresh air direct-
ly into the system at the various points where the quick cool
dampers are provided, thereby rapidly reducing the temperature
of the gases in the various zones.
In this way, it is possible to effect a relatively
rapid colour change for example by shutting down the line, opera-
ting the quick cool damper system, and then when the line is
started up again the supplementary burners are put into action
to rapidly increase the heat in the zones to their wor~ing tem-
perature.
The foregoing description of the invention is given
here by way of example only with reference to the drawings here-
in. It is not intended that the invention shall be limited to
any of the ~pecific features as described or shown, but compre-
hends all such variations as come within the scope of the append-
ed claims.
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~057048
SUPPLEMENTARY DISCLOSURE
In the original disclosure of this patent application,
we have described and claimed certain heat treatment oven appara-
tus as well as a method for the heat treatment of a workpiece
carrying a coating containing a vapourizable and oxidizable sol-
vent. The apparatus and method of the original disclosure pre-
sent the important advantage that they permit a very significant
reduction in the volume of waste gases discharged into the at-
mosphere from such an oven apparatus compared to those previous-
ly known. As explained in the original disclosure, such reduc-
tion in exhaust gas volume leads to a considerable saving in the
amount of fuel required for operating such an oven apparatus.
The heat treatment oven apparatus described in the
original disclosure can broadly be defined as comprising a plu-
rality of oven zones, a workpiece being movable through such
zones in sequence; gas circulation means in those zones for cir-
culating gases therein continuou~ly around such a workpiece
within the zone~; a gas input at each such zone for continuously
introducing gases into such zone; a gas exhaust at each such
zone for contlnuously removing a portion of the gaseou~ atmos-
phere from such zone~ a gas-transferring means connected ~etween
the gaa exhaust of one of the zones and the gas input of a dif-
ferent one of the zones for transferring gases exhausted from
such one of the zones and introducing them into the different
one of the zones; and an incinerator di~posed essentially at the
different one of the zones for oxidizing solvent vapour contain-
ed within the gases exhausted from the one of the zones on intro-
duction into the different one of the zones.
The method described in the original disclosure can
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1057048
broadly be defined as comprising the following steps of moving a
workpiece carrying a coating containing a vapourizable solvent,
oxidizable to provide at least part of the heat required for
such heat treatment, sequentially through a plurality of zones
of an oven; circulating gases in the oven zonesi removing gases
from one of the zones, transferring the gases as removed from
such one of the zones to an incinerator disposed essentially at
a different one of the zones; incinerating such gases in that in-
cinerator; and discharging such gases after incineration into
such different one of the zones.
In tne preferred embodiment specifically described in
the original disclosure, solvent-rich vapours are transferred
from an upstream zone of an oven apparatus into one or more down-
stream zones thereof where they are incinerated to provide both
additional heat input to those zones and ventilation of those
zones. The transfer of gases from a downstream zone to an up-
stream zone to ventilate such upstream zone was also described
in the original aisclosure.
It has now been found that additional benefits can be
obtained by incinerating at such an upstream zone of such an oven
the gases transferred to such an upstream zone from one or more
downstream zones. Such additional incineration will frequently
further red~ce fuel requirements and will often lessen the risk
of the solvent vapour concentration within such an upstream zone
exceeding the lower explo~ive limit.
Further advantages in the way of more uniform zone tem-
peratures are presented by providing the incinerators already
considered so that they discharge generally into the intakes of
respective ones of zone gas recirculating fans.
Heat losses are further reduced by disposing the means
- 24 -
~C~57048
such as ducts for transferring gases from oven zone to another
within the oven apparatus itself.
Other features of these further developments and the
advant~ges presented thereby will become apparent as the descrip-
tion proceeds with reference to Figures 6 to 9 of the accompany-
ing drawings in which:
Figure 6 is a top plan view partly in section of a
strip treatment oven in accordance here-
with;
Figure 7 is a side elevation partly in section of the
oven of Figure 6;
Figure 8 is a partially sectioned view along the line
8-8 of Figure 7; and
Figure 9 is a schematic perspective illustration par-
tially cut away of an alternate embodiment.
Referring to Figures 6, 7 and 8, the invention is there
shown in the form of an oven for curing coatings applied to strip
sheet metal. The oven comprises three oven zones indicated as
300, 302 and 304, respectively.
An inlet end 306 and an outlet end 308 form the two
ends of the oven.
The oven zones are not separated from one another ~ut,
in fact, ar~ all contained within a single integral housing which
is continuous from one end of the oven to the other.
The zone 300 comprises the lower temperature high sol-
vent release zone, and the zones 302 and 304 will be respective-
ly at somewhat higher temperatures, with the volume of solvent
release progressively decreasing. ~Jithin each of the zones 300,
302 and 304 a separate fan and duct recirculating system is pro-
vided for continuously recirculating the atmosphere within that
1057048
zone and redirecting it onto the strip shown as S in Figure 7.
Each of the zones is thus provided with a fan 310 having an in-
take 312 and an outlet duct 314. The outlet duct 314 in turn
supplies upper and lower discharge ducts 316 and 318. These
ducts 316 and 318 run lengthwise along the zones 300, 302 and 304
respectively, thereby to provide continuous discharge of zone
atmosphere onto both the upper and lower surfaces of the strip S
simultaneously along substantially its entire length as it passes
through each of the zones. Fans 310 are driven by separate power
sources such as electric motors or the like. The upper and lower
discharge ducts 316 and 318 will usually be provided with suit-
able dampers or the like. Preferably, the fan 310 and the outlet
ducts 314 are all located in a small chamber indicated as 320
formed at one side of each of the zones 300, 302 and 304 respec-
tively. An air flow baffle 322 is disposed between the intake of
each fan 310 and the interior of a respective one of the zones
300, 302 and 304 t~ avoid undesirable air flow patterns within
the zones. Each of the zones 300, 302 and 304 respectively are
provided with incinerator flame tubes 324, 326 and 328 respect-
ively. The flame tubes are in turn supplied internally with in-
cinerator burners 330 which are typically fired by natural gas,
or other suitable fuel.
Fans 332a, 332b and 332c are provided for forcing por- t
tions of zone atmosphere through the flame tubes 324, 326 and 328
respectively.
In order to supply zone atmosphere to the incinerator
tube 324, a zone atmosphere transfer duct 334 is provided extend-
ing down the length of the interior of the oven from the upstream
end of the tube 324, in zone 3G0, and having its free open end at
the downstream ~nd of the zone 304. In this way, the fan 332a
- 26 -
:
~L057048
supplying the tube 324 will draw zone atmosphere from the down-
stream end of the zone 304, and will pass it all of the way up
the oven through zones 302 and 300 and supply it to the flame
tube 324.
Supply for the flame tube 328 is provided by the oven
atmosphere transfer duct 336, communicating between the fan 332c
and a point midway between zones 300 and 302. In this way, zone
atmosphere will be drawn from the transition between zone 300
and 302 and supplied to the flame tube 328.
The flame tube 326 of the zone 302 is supplied through
port 338, communicating with the fan 332b. The port 338 draws
zone atmosphere from about the same point as the transfer duct
336. In this way, zone atmosphere is withdrawn simultaneously
by both the fans 332b and 332c from about the same point, i.e.
the transition between zone 300 and 302.
The outlets of the respective incinerator flame tubes
324, 326 and 328 are located more or less adjacent the intakes
312 of the fans 310 in the zones 100, 302 and 304 respectively.
In this way, incinerator gases exiting from the tubes 324, 326
and 328 respectively will mix with oven atmosphere gases before
being drawn into the intakes 312 of the fans 310 thereby modify-
ing the temperature of both gases, and achieving the two desir-
able objectives, namely, raising the temperature of the oven at-
mosphere as a whole, while reducing the temperature of the in-
cinerator gases themselves, thereby overcoming problems caused
by the handling of high-temperature incinerator gases.
This significant advantage is achieved at least in part
by the location of the flame tubes 324, 326 and 328 essentially
within the zones 300, 302 and 304 respectively where they are
surxounded by the zone atmosphere which is already at an elevated
- 27 -
.- . . . . . .
1057048
temperature and in a s-tate of considerable turbulence caused by
the rapid high volume circulation of the oven atmosphere induced
by the fans 310. In this way, as soon as the high temperature
incinerator gases are discharged from the incinerator flame tubes
they are immediately mixed with the surrounding oven atmosphere
which is at a considerably lower temperature without the need for
providing costly high temperature ductwork and control dampers.
In order to admit oxygen for combustion, and control
the L.E.L. of the oven atmosphere, a certain volume of the oven
atmosphere must be withdrawn and a certain volume of fresh air
introduced into the oven continuously. An exhaust stack 340 is
therefore provided, which will communicate with a further inci-
nerator 342, where the exhaus~ gases are incinerated to oxidize
the solvent vapour content prior to venting such incinerated
exhaust gases to the surrounding ambient atmosphere. If desired,
some form of heat recovery can be incorporated in the exhaust
stack, downstream of the incinerator 342, to recover some of the
heat. Possibly, such heat recovery system may be used to preheat
incoming fresh air, although in the majority of cases this will
not be necessary and the heat recovery system will merely pro-
vide, for example, steam for heating the building.
Admisqion of fresh air in volumes essentially equal to
the volume o~ oven atmosphere exhausted through the stack 340
may be provided in various ways. Where only limited volumes of
fresh air are required, then it can enter simply through either
end of the oven, i.e. through the strip entry 306 and the strip
exit 308. In this way, the air flow pattern within the oven will
always be inward with respect to either end thereby substantially
completely preventing the escape of oven atmosphere through the
open end of the oven with consequent pollution of the atmosphere
- 28 -
.
105704~
within the buildings surroundins~ the oven.
However, where larger volumes of fresh air are requir-
ed, due to larger voiumes of exhaust gases being exhausted up
the ~tack, then fresh air inlets (not shown) may be provided.
Adjacent the strip exit 308, there may be provided an
additional circulating fan 310a, and ductwork 312a, 316a and
318a, for maintaining continuous circulation of the zone atmos- , -
phere at the exit end. In the majority of cases, the zone atmos-
¦ phere at this point will not require the provision of a further
¦ 10 zone incinerator, and consequently none is illustrated in the
embodiment of Figures 6 and 7.
In some circumstances, it may be desirable to provide
for additional direct heat input to the strip. This may be
achieved by radiant heating by means of the additional radiant
heater as shown in Figure 9. Figure 9 illustrates a radiant
heating unit in the form of a generally square box-like duct 344
having a rectangular opening 346 therethrough for passage of the
strip S therethrough.
The box-like duct 344 is connected to, for example,
the incinerator flame tube 328 for receiving the high tempera~
ture incinerator gases directly therefrom throùgh suitable high
temperature ductwork (not shown).
Th~ high temperature incinerator gases will flow
through the duct 344.
Within the duct 344, four rectangular wall members
348 are provided, providing an open-ended passageway extending
through the duct 344, communicating with the openings 346 through
which the strip S passes.
In the upper and lower walls 348, gas outlet holes 350
may be provided, through which jets of high temperature incinera-
- 29 -
..
1057~48
tor gases may pass downwardly and upwardly and impinge directly
on the upper and lower surfacPs of the strip S.
In addition, the four wall surfaces 348 will be heat-
ed to an elevated temperature by the high temperature incinera-
tor gases, and will thus subject the strip S to radiant heat as
well as heating by the action of the impingement of the gases
themselves.
The radiant heater duct may be provided with or with-
out the holes 350, depending on its location in the oven. Where
no such holes 350 are provided, then the only heating effect
will be a radiant heating effect. In this case, it will usual-
ly be located in the first, high-solvent release zone 300, and
will heat the strip sheet metal itself without heating the
coating to the same extent. The coating will then tend to cure
from the inside outwardly.
Where additional heat input is required in, for exam-
ple, zone 302, having a higher temperature and lower solvent re-
lease, then the provision of holes 350 is acceptable since the
coating is already partially cured and can withstand the impinge-
ment of the high temperature gases.
In accordance with the invention, a novel form of fuel
control is provided for the incinerators 330. Each of them is
provided wit~ a separate fuel control 352 which will typically
be an electrical type control, possibly in the form of a synchro-
nous motor operating a fuel supply valve 354.
The motor 352 is in turn operated by a relay 356. The
relay 356 is in turn connected with two temperature-responsive
signal generators 358 and 360.
Signal generatox 358 is connected with a temperature
sensor 362, and signal generator 360 is connected with a tempera-
- 30 -
.
,: . . : . .
1057048
ture sensor 364. The signal generators 358 and 360 are respon-
sive to predetermined high and low temperatures to operate the
motor 352 so as to supply either less or more fuel t~ the burn-
èr 330.
The burners 330 are thus responsive to ~a) variations
in the -~emperature of their own output and to (b) variations in
the temperature of the oven atmosphere.
In this way, each of the burners 330 is solely respon-
sible f or maintaining a predetermined temperature level in its
respective zone of the oven.
It will be appreciated that, while only one such sys-
tem of temperature control is illustrated for one of the burners
330, the same system of controls is provided in fact for each of
the burners.
Alr doors 366 and 368 are provided at the entry and
exit respectively to the oven chamber and prevent the escape of
oven atmosphere at these points.
Numerous variations and modifications in the structure
hereinbefore specifically described are of course possible.
-
- 31 -
