U.S. patent number 3,909,953 [Application Number 05/446,737] was granted by the patent office on 1975-10-07 for paint drying method and apparatus.
This patent grant is currently assigned to Midland-Ross Corporation. Invention is credited to Klaus H. Hemsath, Arvind C. Thekdi, Frank J. Vereecke.
United States Patent |
3,909,953 |
Hemsath , et al. |
October 7, 1975 |
Paint drying method and apparatus
Abstract
A high-efficiency system for drying solvent-based paint coatings
on articles, with minimum fuel requirements and pollution-free
emissions, comprises a paint drying oven in which the painted
article is dried in a heated atmosphere which is maintained
substantially oxygen-free or inert so as to be above the upper
explosion limit at all times during the drying process. The
combustible solvent vapors which evaporate from the paint coating
during the drying process and are contained in the flue gases
exhausted from the oven are conveyed into and incinerated under
stoichiometric conditions in a rich fume incinerator, containing
auxiliary gas-air pilot burners also operating under stoichiometric
conditions, to produce substantially oxygen-free nonpolluting inert
gaseous combustion products at least a part of which, after cooling
in a heat exchanger for heat recovery purposes, is recycled back
into the drying oven to supply thereto all or part of the heat
required for the paint drying process as well as the essentially
inert gas atmosphere required therein. The unused portion of the
incinerator combustion products may be vented directly to the
atmosphere as nonpolluting emissions.
Inventors: |
Hemsath; Klaus H. (Sylvania,
OH), Thekdi; Arvind C. (Sylvania, OH), Vereecke; Frank
J. (Palmyra, MI) |
Assignee: |
Midland-Ross Corporation
(Cleveland, OH)
|
Family
ID: |
23773659 |
Appl.
No.: |
05/446,737 |
Filed: |
February 28, 1974 |
Current U.S.
Class: |
34/450; 34/72;
422/198; 34/479; 422/182 |
Current CPC
Class: |
F23G
7/065 (20130101); F26B 23/022 (20130101); F26B
13/10 (20130101); Y02P 70/10 (20151101) |
Current International
Class: |
F26B
23/00 (20060101); F26B 13/10 (20060101); F26B
23/02 (20060101); F23G 7/06 (20060101); F26B
005/00 () |
Field of
Search: |
;34/26,28,32,72,155,223,36,37,242 ;23/277C |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dority, Jr.; Carroll B.
Assistant Examiner: Schwartz; Larry I.
Attorney, Agent or Firm: Nawalanic; Frank J.
Claims
What is claimed is:
1. A method for drying solvent-based coatings on articles moving
through a drying chamber by means of exhaust gases coming from a
combustion apparatus having an intake in fluid communication with a
gaseous exhaust duct in said drying chamber whereby a furnace
atmosphere generated within said drying chamber is drawn into said
combustion apparatus, said method comprising the steps of:
injecting combustion air into said combustion apparatus at a fixed
rate sufficient to mix stoichiometrically with said furnace
atmosphere drawn from said exhaust duct when said drying chamber is
operating under constant load;
initially mixing, during startup, a gaseous fuel from an auxiliary
burner in stoichiometric proportions with said combustion air;
igniting said gaseous fuel and said combustion air in said
combustion apparatus by means of at least one pilot burner to
produce an inert type gas mixture possessing not more than five
percent oxygen by content;
circulating at least a portion of said inert gas mixture leaving
said combustion apparatus through a heat exchange mechanism to cool
same to a predetermined value;
piping at least a portion of said inert gas mixture leaving said
cooling mechanism into said drying chamber to purge same;
introducing said articles through an article inlet end into said
drying chamber to produce a furnace atmosphere containing solvent
emissions and removing said articles through an article outlet end
of said chamber;
continuously removing a first portion of said furnace atmosphere
through said exhaust duct and burning said removed solvents in said
combustion apparatus to continuously produce an inert type
off-gas;
sensing the temperature of said off-gas to regulate the flow of
fuel to said auxiliary burner to insure that said off-gas possesses
no more than five percent oxygen by content;
circulating a portion of said off-gas through said heat exchange
mechanism to cool same to a predetermined temperature;
introducing a portion of said cooled off-gas to said drying chamber
to insure that said furnace atmosphere is outside the explosive
range;
exhausting a second portion of said furnace atmosphere from said
drying chamber through a vent;
recirculating said second portion of said furnace atmosphere into
said drying chamber after mixing same with said portion of inert
type off-gas being circulated into said drying chamber;
controlling the flow of said off-gas to produce a slight
overpressure in said drying chamber;
circulating a portion of said inert type off-gas downstream from
said heat exchange mechanism into a plenum arrangement at said
article inlet end and said article outlet end; and
ejecting said inert type gas mixture from said plenum arrangement
in a plurality of gaseous streams directed generally normal to the
flow of said articles at a pressure sufficient to establish gas
flow patterns through said article inlet and outlet ends whereby
said overpressure is maintained and said furnace atmosphere is
substantially sealed.
2. Apparatus for drying solvent-based coatings on articles, said
apparatus comprising:
a drying oven having a drying chamber, an article inlet opening, an
article outlet opening, a gaseous inlet opening for injecting a
furnace type gaseous atmosphere into said oven, an exhaust outlet
opening for removing a first portion of the furnace atmosphere, and
a vent for removing a second portion of the furnace atmosphere;
combustion means in fluid communication with said gaseous exhaust
outlet for burning furnace fumes exiting therefrom to produce an
inert type off-gas mixture which contains not more than five
percent oxygen by content;
said combustion means including combustion air supply means preset
to deliver a fixed quantity of combustion air to said combustion
means to achieve approximately stoichiometric combustion when said
apparatus is operated under constant load, pilot burner means to
ignite said combustion air with said exhaust furnace fumes to
produce said inert type off-gas mixture, auxiliary fuel means
effective during startup of said apparatus to mix with said
combustion air in said combustion means to produce an inert type
off-gas suitable for purging said drying chamber and effective
during normal operation of said apparatus to control said oxygen
content within said five percent limit in said inert off-gas
mixture, and temperature control means associated with said
combustion means for controlling the fuel quantity supplied said
combustion means by said auxiliary fuel means;
recycle conduit means for recycling at least a portion of said
inert gas mixture from said combustion means to said gaseous inlet
opening in said drying chamber;
said recycling means including a heat transfer mechanism for
reducing the temperature of said inert off-gas mixture to a
predetermined value, first conduit means upstream of said heat
transfer mechanism for venting a portion of said inert off-gas
mixture directly to the atmosphere as nonpolluting emissions,
second conduit means in fluid communication with said gaseous inlet
of said drying chamber for conveying and mixing a second portion of
said inert type off-gas mixture with the solvents emitted from said
coatings to produce a furnace gas atmosphere which is above the
upper explosive limits of said atmosphere at all times during the
drying the said coatings;
recirculating means including a third conduit in fluid
communication with said vent for circulating a second portion of
said furnace atmosphere with said inert type gaseous mixture
whereby the heat sensibility of the furnace mixture is conserved
and temperature uniformity of the furnace atmosphere is
promoted;
pressure means for maintaining said drying chamber at a slight
overpressure;
aerodynamic sealing means at said article inlet opening and said
article outlet opening directing a plurality of streams of said
inert type off-gas mixture in a direction generally normal to all
surfaces of said articles at a flow rate sufficient to establish
gas flow into said drying chamber to maintain said drying chamber
substantially sealed from the outside atmosphere; and
said recycling means including a fourth conduit supplying said
inert gas mixture to said aerodynamic seal means.
Description
BACKGROUND OF THE INVENTION
The present invention relates, in general, to method and apparatus
for carrying out heating operations which release flammable solvent
vapors, and more particularly to method and apparatus for drying
solvent-based paint coatings on articles of various form.
Solvent-based paints normally contain from 40 to 60 percent organic
solvents usually in the form of aromatic hydrocarbon compounds,
such as benzene, toluene, xylene and other high-flash napthas which
are derived from coal-tar distillates and which are evaporated from
the paint coating during the drying process. Because these solvent
vapors are highly flammable and can form explosive mixtures when
mixed with air, they represent potential fire and explosion
hazards. For this reason, insurance and fire prevention regulations
require that for safe operation of drying ovens open to the
atmosphere the solvent vapor concentration in the oven exhaust
gases must be lower than 25 percent of the lower explosion limit
(L.E.L.) thereof. To comply with these regulations therefore, and
independent of the drying process heat demand, the solvent vapors
evolved in such ovens must, before their exhaustion therefrom, be
diluted to 25 percent or less of their lower explosion limit. In
conventional paint drying ovens, this dilution is obtained by
introducing into the oven at least 10,000 cu. ft. of air per gallon
of evaporated solvent. Prior to the enactment of recent pollution
control regulations, these solvent-air mixtures containing
undesirable pollutants were discharged directly into the
atmosphere.
The heat required to operate such a conventional paint drying
system is used not only for heating the paint coated article to the
required paint drying temperature, usually from 200.degree. to
600.degree.F., but also is used up in heating the dilution air to
the oven exhaust temperature. The added heat energy thus expended
in heating the required large volume of dilution air represents a
considerable proportion of the total fuel requirement for the
system and therefore a considerable added operating expense.
Because of the elevated levels of the pollutants contained in the
solvent-air mixtures exhausted from conventional paint drying
ovens, recently enacted governmental pollution control regulations
no longer permit the emission of these mixtures directly into the
atmosphere. To comply with these air pollution control regulations,
therefore, it has been necessary to equip paint drying ovens with
some type of pollution abatement equipment, the most common form of
which has been a thermal fume incinerator for incinerating the
exhaust gases from the paint drying oven. Such thermal incineration
of these oven exhaust gases normally requires temperatures between
1,250.degree.F. and 1,450.degree.F. In a typical fume incinerator,
the fumes are mixed with auxiliary fuel, e.g., natural gas, and the
mixture temperature is raised to 1,250.degree.-1,450.degree.F. to
cause the incineration of the fumes. With proper design of the
incinerator, the gaseous products of combustion are essentially
clean, i.e., free of air pollutants. The auxiliary fuel
requirements for the incineration process depend on the exhaust
fume temperature and on the initial fume loading. In most paint
drying applications employing such exhaust gas incineration,
however, the fuel consumption for the paint drying operation
increases more than twice compared to that used in the conventional
paint drying system not equipped with a fume incinerator.
To reduce the fuel consumption of such incinerator-equipped paint
drying systems, it has been proposed to include a fume preheater in
which the exhaust gases from the incinerator are used to heat the
fumes and raise their temperature before entering the incinerator.
However, the addition of such a fume preheater increases the
capital cost for the system by an amount comparable to or higher
than the cost of the incinerator itself. In many cases, economic
considerations make it impractical to raise the preheater
efficiency beyond 50 to 60 percent, with the result that the
additional fuel consumption for the operation of the incinerator is
still substantial. A further proposal for reducing the fuel
consumption still further has been to employ a liquid-to-gas heat
exchanger to recover additional heat from the exhaust gases leaving
the fume preheater. But such a system is considerably more
expensive and its use is limited to very large installations. Even
with such an elaborate heat recovery system, however, the fuel
consumption is higher than the simple conventional system without
fume incineration equipment.
Many small and medium scale paint drying installations cannot
justify from an economic standpoint the use of a complete heat
recovery system, with the result that they have to use considerably
more fuel than in the past in order to operate the required
pollution abatement equipment or incinerator. Moreover, the present
fuel supply shortages and the continuously rising fuel costs are
threatening the paint drying industry with higher production costs,
insufficient fuel allocations and production cutbacks or
interruptions. As alternatives to the use of solvent-based paints,
the use of powder coatings and water-based paints is being
considered as a way of reducing or eliminating air polluting
emissions and hence the additional fuel consumption in the
incinerator, but the use of any one of these two alternatives still
requires substantial fuel usage in the drying or heating oven.
Moreover, so far as known, none of these alternatives has been
tried as yet on a large scale so that their ultimate use in large
scale coating operations may not occur for a considerable time.
SUMMARY OF THE INVENTION
As mentioned hereinabove, conventional solvent-based paints contain
from 40 to 60 percent or higher of organic solvents which are
highly inflammable and have heating value of 120,000 to 140,000
B.t.u. per gallon of solvent. These paints are customarily applied
as a thin coating or film on the article or object to be painted,
the film thickness in most applications being of the order of 0.5
to 2 mils or thereabouts. During the drying process most of these
solvents are evaporated, and the pigments of the paint form a
protective coating on the article. The heating value of the solvent
vapors evaporated during the paint drying process is usually many
times more than the fuel requirements in a typical paint drying
oven. Accordingly, a paint drying system designed to take advantage
of this heat energy of the solvent vapors, while maintaining the
basic requirements of safety, pollution free exhaust emissions and
unchanged coating or product quality, will provide a drastic
reduction in the fuel demand of the paint drying industry.
In conventional paint drying systems which operate with oven
atmospheres below the lower explosion limit, the dilution air which
must be added to the solvent vapors before exhausting them from the
oven in order to comply with safety regulations uses up a major
portion of the total heat requirement for the system. Moreover,
bringing the exhaust gases to an incineration temperature of
1,250.degree. to 1,450.degree.F. to comply with air pollution
regulations can more than double the heat requirements of systems
not provided with oven exhaust incineration means. By eliminating
or reducing the dilution air without endangering the safety
requirements of the paint drying system, a large part of the
intrinsic heat energy in the solvent vapors released from the paint
on drying can be utilized for heating the drying oven and the
painted article therein to be dried. In the design of such a
system, however, considerations relating to safety and perfect
combustion require an understanding of the fundamentals of the
flammability characteristics of gas mixtures, such as are formed in
paint drying ovens, by reference to the established flammability
curves therefor. These curves show that mixtures of common
flammable gases including hydrogen, carbon monoxide, hydrocarbons
and vapors of common paint solvents, with inert gases such as
nitrogen, carbon dioxide and water vapor, are nonflammable if the
oxygen concentration in the mixture is less than about 5 percent.
Accordingly, and as an alternative to the use of large quantities
of dilution air to control potential explosion hazards, the
gas-solvent vapor mixture formed in paint drying ovens during the
drying operation will remain nonexplosive if an atmosphere with a
low oxygen content, or an inert atmosphere with essentially no
oxygen content, is maintained in the oven during the solvent
evaporation from the paint coating. In this way, it is not
necessary to use any dilution air at all because the gas-solvent
mixture is outside and considerably away from the flammability
region, i.e., it is above the upper explosion limit (U.E.L.)
thereof. While, from a safety standpoint, the volume ratio of inert
gases to the solvent vapors may in such gas be any value, in most
instances the required volume of inert gas per gallon of evaporated
solvent is much less than the dilution air required in conventional
paint drying systems.
The nonexplosive gas-solvent mixture or rich fumes exhausted from
the drying oven can be utilized to supply all the heat required for
the paint drying operation by incinerating the oven exhaust gases,
under stoichiometric conditions, in an incinerator to convert them
into off-gases of substantially inert character and high
temperature, e.g., from 1,400.degree. to 1,650.degree.F. or
thereabouts. Because of their inert character, these hot exhaust
gases from the incinerator then can be recycled, after cooling,
back into the paint drying oven to not only supply thereto all, or
if desired only a part of the heat required for the paint drying
operation but to also supply thereto the required amount of inert
gases to assure the maintenance in the oven of a gas-solvent vapor
mixture above the upper explosion limit thereof at all times during
the paint drying operation. Since the volume and heat energy of the
exhaust gases generated in the incinerator are substantially
greater than that required for the paint drying operation in the
oven, only a part of the total volume of these exhaust gases need
be recycled back into the oven. In view of their inert and
therefore nonpolluting character, the other part of these exhaust
gases can be vented directly to the atmosphere without any
polluting effect thereon. The portion of the hot incinerator
off-gases recycled to the oven can be cooled to the lowered
temperature required for the paint drying operation in the oven in
a suitable heat recovery system or heat exchanger such as a water
or an air heater, or a low pressure steam boiler, or a metal
preparation section of the article coating line. Thus, the
incinerator is used as a means of energy recovery and serves the
multi-purpose function of an inert gas generator, a heat generator
or burner, and as pollution abatement equipment. Moreover, the
incinerator does not require any auxiliary fuel for its normal
operation except for the very small amount needed to operate pilot
burners to maintain the required ignition conditions in the
incinerator for assuring the complete combustion of the solvent
vapors contained in the oven exhaust gas mixture introduced into
the incinerator.
It is an object of the invention, therefore, to provide a system
for drying solvent-based paint and other solvent-containing
coatings which is of increased thermal efficiency.
Another object of the invention is to provide a system for drying
such solvent-based coatings which not only is of improved thermal
efficiency but also is characterized by pollution-free
emissions.
Still another object of the invention is to provide a system for
drying solvent-based coatings which is substantially
self-supporting in respect to its fuel energy requirements.
A further object of the invention is to provide a system for drying
such solvent-based coatings in a substantially oxygen-free or low
oxygen content inert atmosphere maintained well above the upper
explosion limit at all times during the drying operation and
complying with standard fire safety regulations therefor.
A still further object of the invention is to provide a system for
drying such solvent-based coatings in which the latent heat energy
in the solvent vapors evaporated from the coating during the drying
operation is used to supply all or part of the heat required for
such operation.
Another object of the invention is to provide a system for drying
such solvent-based coatings in which the solvent vapors evaporated
from the coating during the drying operation are incinerated in a
manner to produce substantially oxygen-free or low oxygen content
inert gaseous combustion products for recycling back into the
drying oven to supply an inert atmosphere therein.
Still another object of the invention is to provide a system for
drying such solvent-based coatings which does not require the
addition of any dilution air to the gases exhausted from the drying
oven during the drying operation in order to maintain them outside
their explosion limits so as to comply with standard fire safety
regulations.
A further object of the invention is to provide apparatus for
effectively carrying out the drying of solvent-based paint and
other such coatings in accordance with the above referred to
systems.
Briefly stated, in accordance with one aspect of the invention, the
drying of solvent-based paint and other such type coatings on
articles are carried out in a drying oven in which a heated
atmosphere is maintained of substantially oxygen-free or low oxygen
content inert character so as to be above the upper explosion limit
at all times during the drying operation. The solvent vapors
evaporated from the coating and mixed with the inert gases
exhausted from the drying oven are incinerated under stoichiometric
conditions in an incinerator to form substantially oxygen-free or
low oxygen content nonpolluting inert off-gases at elevated
temperatures of 1,400.degree. to 1,650.degree.F. which may be
vented directly to the atmosphere as nonpolluting emissions from
the system. At least a part of the off-gases, after cooling down in
a suitable heat recovery system to the required temperatures of
from 200.degree. to 600.degree.F. for the coating drying operation,
is recycled back into the drying oven to not only supply thereto
all or part of the heat required by the oven for the drying
operation but also to supply the inert atmosphere for the oven.
In accordance with a further aspect of the invention, the only
auxiliary fuel requirement of the coating drying system according
to the invention during the normal operation thereof is the small
amount required for the operation of pilot burners within the
incinerator combustion chamber to maintain the required ignition
conditions therein for assuring the complete combustion of the
solvent vapors introduced thereinto from the oven.
Further objects and advantages of the invention will appear from
the following detailed description of species thereof and from the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical cross-sectional view, partly schematic, of a
paint drying system according to the invention;
FIG. 2 is a flow chart of the basic system shown in FIG. 1;
FIG. 3 is a flow chart of a modified paint drying system according
to the invention; and
FIG. 4 is a vertical cross-sectional view, on an enlarged scale, of
one of the aerodynamic seals which are located at the article inlet
and outlet openings in the drying oven shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawings, the invention is therein illustrated as embodied
in a system for the continuous drying of solvent-based paint
coatings on metallic strip stock such as metallic coiled strip. It
should be understood, however, that the invention is applicable as
well to the drying of solvent-based paint coatings on articles of
various other types such as, for example, automobile bodies, metal
cabinets for household appliances, etc., and also to other heating
processes in which high B.t.u. vapors are released as in chemical
coating, pyrolysis of carbonaceous material, carbon baking
operations, and resin coating processes.
Referring to FIG. 1, there is shown at 10 an oven in which heating
operations are to be carried out which result in the release of
flammable vapors of high B.t.u. content, in the particular case
illustrated the heating of metallic strip stock material 11 such as
steel or aluminum strip coated with a layer or film 12 (FIG. 4) of
a solvent-based paint on one or both sides. In the exemplary
continuous metallic strip paint coating process as shown in FIG. 1,
the metallic strip material 11 is uncoiled from a supply roll or
coil 13 thereof as by means of a pair of feed rolls 14 between
which the strip 11 passes and from which it is then dipped or
immersed in a bath 15 of solvent-based paint contained in a coating
tank 16 to form the coating 12 on the strip. It should be
understood, however, that the coating 12 of paint on the strip
material 11 could be applied thereto by methods other than the
illustrated dip-coating method, e.g., by spraying, rolling, etc.
From the coating tank 16 the coated strip 11 passes between and is
supported by a pair of squeegee rolls 17 which squeeze the excess
coating material from the strip so as to leave the desired
thickness film or coating 12 thereon which, in the case of
conventional paint coatings, is usually from 0.5 to 2.0 mil
thickness. The coated strip 11 passes from the squeegee rolls 17
into the interior or drying chamber 18 of the oven 10 through an
inlet or entrance opening 19 in an end wall 20 of the oven and
then, after the drying of the coating 12 on the strip 11, out of
the oven chamber 18 through an outlet or exit opening 21 in the
other end wall 22 of the oven. The emerging strip 11 from the oven
10 and on which the coating 12 is now dried passes between and is
supported by a pair of idler rolls 23 and is then coiled on a
take-up roll 24. The oven chamber 18 is supplied with a heated
atmosphere at the required temperature for the particular drying
operation or other heating operation to be carried out therein,
e.g., a temperature in the range of from about 200.degree. to
600.degree.F. in the case of conventional solvent-based paint
coatings 12 to be dried.
As mentioned hereinabove, where any heating operation carried out
in an oxygen-containing atmosphere in an oven results in the
evolution of vapors in the oven which are highly flammable and of
explosive character, such as the solvent vapors which are
evaporated from solvent-based paint coatings 12 during the drying
thereof, the applicable insurance and fire regulations require that
for safe operation of the oven the solvent vapors evolved therein
must, before the exhausting thereof from the oven, be diluted to 25
percent of, i.e., well below, the lower explosion limit (L.E.L.) of
the solvent vapor-containing gas mixture formed in the oven during
the heating operation. In conventional paint drying ovens, this
dilution is obtained by introducing into the oven 10,000 cu. ft. of
air per gallon of evaporated solvent. Because this dilution air
introduced into the oven must, of necessity, be also heated to the
temperature required for the heating operation to be performed in
the oven, the added heat energy thus expended in heating the
dilution air represents a considerable proportion of the total
heating fuel requirement of the oven and therefore a considerable
added operating expense.
In accordance with the invention, the need for and added expense of
heating such dilution air is entirely eliminated by carrying out
the paint drying or other heating operation to be performed in the
oven in a substantially oxygen-free or low oxygen content inert
atmosphere which is above the upper explosion limit (U.E.L.) of the
particular gas mixture formed in the oven during the heating
operation instead of below the lower explosion limit (L.E.L.) of
such gas mixture as has been customary practice heretofore. The
flammability characteristics of any gas mixture can be determined
by reference to the established flammability curves for various gas
mixtures. These curves show that a mixture of common flammable
gases including hydrogen, carbon monoxide, hydrocarbons and vapors
of common solvents such as employed in conventional solvent-based
paints, with inert gases such as nitrogen, carbon dioxide and water
vapor, is nonflammable if the oxygen concentration in the mixture
is less than 5 percent. For the purposes of the invention,
therefore, the oven 10 is supplied with a substantially inert gas
atmosphere having an oxygen concentration of less than 5 percent
and preferably less than 2 percent. The inert gas atmosphere can be
supplied to the oven 10 from a separate source of supply thereof.
Preferably, however, and to realize the fullest benefits of the
invention from an economic standpoint, the inert gas atmosphere
supplied to the oven 10, as well as the heat required therein for
the paint drying or other solvent vaporizing heating operation
conducted in the oven, is supplied in the manner shown by the flow
chart in FIG. 2, by the exhaust or off-gases from an incinerator 25
in which the mixture of inert gas and solvent vapor exhausted from
the oven 10 is incinerated in a manner as described hereinafter. At
least a part of these off-gases, after cooling to the temperature
required for the paint drying or other heating operation to be
performed in the oven 10, is recycled back into the oven to supply
the inert gas atmosphere and all or part of the heat required
therein. Thus, the paint drying or other solvent vaporizing system
according to the invention may be substantially self-supporting in
respect to its fuel energy requirements.
To maintain the desired low oxygen concentration in the oven
atmosphere at all times during the continuance of the paint drying
or other heating operation carried out in the oven 10, it is
necessary to isolate the drying chamber 18 of the oven and thus the
atmosphere therein from the ambient atmosphere to prevent leakage
of air into the oven. To this end, the walls of the oven 10 are
made of proper design to be substantially air tight in themselves.
Thus, as shown in FIG. 1, the end walls 20, 22, side walls 26 and
top and bottom walls 27 and 28, respectively, may be formed as a
sheet metal shell or casing 29, the inner side of which may be
provided with a lining 30 of a suitable heat refractory material
such as, for example, ceramic fiber type block insulation. Further
assurance against leakage of ambient air into the oven chamber 18
is afforded by maintaining the inert gas atmosphere therein at a
slight overpressure of, for example, 0.05 inches w.c. (water
column) or so. Also, where, as in the particular paint drying
operation illustrated, the work to be processed in the oven 10 is
continuously advanced into and out of the oven through inlet and
outlet openings 19 and 21 therein, the isolation of the oven
chamber 18 from the outside atmosphere requires that these openings
19, 21 also be sealed off from the outside atmosphere. For this
purpose, the oven 10 may be formed at its inlet and outlet openings
19, 21 with aerodynamic seals 31 of suitable form.
As shown more particularly in FIG. 4, where the work to be
processed in the oven 10 is in the form of a continuous strip such
as the paint coated metallic strip material 11, or in the form of
articles conveyed into and out of the oven on a conveyor belt or
similar such means, the aerodynamic seals 31 may in such case be
formed by curtains or screens 32 of inert gas directed normal to
and against the opposite flat sides of the strip material 11 or
conveyor means immediately outside each of the openings 19 and 21
in the oven. For the gas screens 32 to be effective to seal off the
openings 19, 21 from the outside atmosphere, the inlet and outlet
openings 19, 21 in the oven walls 20, 22 may be in the form of
narrow rectangular shaped slots about 6 inches high, for example,
and wide enough to accept, with a slight clearance at each side,
the maximum width strip material 11 or conveyor to be advanced
through the openings. The inert gas screens 32 may be directed
against the advancing strip 11 or work carrying conveyor from
respective pairs of plenums 33 and 34 mounted on the outside of the
oven end walls 20, 22 at locations immediately above and below the
inlet and outlet openings 19 and 21 so as to be positioned on
opposite sides of the advancing strip 11. The plenums 33 and 34 are
of an extent conforming to the width of the inlet and outlet
openings 19, 21 in the oven walls 20, 22 and they are provided with
opposed flat plenum outlet plates 35 of perforated form, and
disposed parallel to and facing the strip 11, to provide outlet
openings for the inert gas forming the gas stream curtains 32. The
plenum outlet plates 35 may have a dimension longitudinally of the
advancing strip 11 suitably around 6 inches or so, and they may be
spaced apart around 9 inches or thereabouts so as to be each spaced
approximately the same distance of around 41/2 inches or so from
their respective side of the advancing strip 11. The perforated
outlet plates 35 may have small diameter gas discharge or outlet
holes (not shown), the centers of which are spaced apart a distance
of two outlet hole diameters and are arranged in staggered pattern
to provide approximately 33 percent open surface area in the plates
35. To prevent leakage of the sealing-off streams 32 at the side
ends of the space between the plenums 33, 34, end plates 36
bridging the plenums at their respective ends and closing off the
space therebetween are provided on the plenums.
The inert gas for the plenums 33, 34 is supplied thereto by
respective gas inlets or ducts 37 which open into the plenum
chambers and are connected to a suitable source of inert gas which,
in accordance with the invention, may be the exhaust gases from the
incinerator 25. Sufficient inlets 37 to each of the plenums 33, 34
should be provided to prevent uneven gas distribution therein.
Satisfactory results are obtained in this respect with one side
inlet 37 per foot of extent of the plenum along the width of the
respective slot-shaped inlet or outlet opening 19, 21. With the
particular dimensional openings 19, 21 and plenums 33, 34 as
described above, the quantity of inert gas required to be supplied
to the plenums 33, 34 to produce an effective aerodynamic seal 31
at the oven openings 19, 21 is much less than the off-gas
by-products from the incinerator 25. A sufficient excess of such
inert off-gases from the incinerator 25 is therefore available at
all times to supply the needed atmosphere for the plenums 33, 34 to
maintain the aerodynamic seals 31 at the inlet and outlet openings
19, 21 of the oven 10.
In the operation of the aerodynamic seals 31, the inert gas streams
32 discharged from the plenums 33, 34 in directions normal to the
advancing strip 11, after impinging on the strip, then turn and
proceed along and more or less parallel to the strip 11 as
indicated by the arrows in FIG. 4, part of the gas stream flowing
inwardly into the oven 10 and the other part flowing outwardly
thereof. This flow pattern operates to effectively prevent outside
air from entering the oven 10 through the oven inlet and outlet
openings 19 and 21 as well as to prevent the inert oven atmosphere
from escaping to the outside atmosphere through these openings.
For proper operation of the illustrated paint drying operation, the
paint coated strip 11 is advanced through the heated oven chamber
18 at a rate, dependent upon the specific operating temperature of
the oven atmosphere and the thickness and particular composition of
the paint coatings 12, such as will assure the substantially
complete drying of the paint coatings 12 on the strip before the
exiting thereof from the oven chamber 18 through the oven outlet
opening 21. During this paint drying operation, the solvents in the
paint coatings 12 are evaporated therefrom and the resulting
solvent vapors mix with the inert gas atmosphere maintained in the
oven chamber 18. Because this gas-solvent vapor mixture is
maintained below approximately 5 percent oxygen content at all
times during the continuance of the paint drying operation, it
therefore is above the upper explosion limit and outside the
flammability region of the flammable components thereof and thus
can be directly exhausted safely from the oven 10 without the need
of diluting it with any air whatsoever to comply with insurance and
fire regulations such as has been customary practice heretofore
with prior conventional paint drying systems. As shown, the inert
gas-solvent vapor mixture is exhausted from the oven chamber 18,
preferably at a region of the roof thereof near or at the back or
outlet end of the oven, through an exhaust duct 38 (FIG. 1) and as
shown by the flow line 38a in the flow charts of FIGS. 2 and 3. The
gas-solvent vapor mixture is suitably exhausted from the oven
chamber 18, as by means of an exhaust pump or fan 39 in duct 38, at
the required rate to maintain the solvent vapor concentration in
the oven atmosphere at the desired level. In this regard, while
from the viewpoint of safety requirements the volume ratio of inert
gases to the solvent vapors in the oven atmosphere can assume any
value, the vapor condensation and other considerations may require
that this ratio be maintained below a definite numerical value. In
most cases, however, the required volume of inert gas per gallon of
the evaporated solvent is in any event many times less than the
dilution air required in conventional solvent-based paint drying
systems.
The oven atmosphere may be maintained at the aforementioned slight
positive or overpressure of around 0.05 inch w.c. by a conventional
pressure controller 40 (FIG. 1) which is actuated by a pressure
measuring unit 41 in the oven chamber 18. The pressure controller
40 is arranged to control the position of either a damper 42 in the
gas exhaust flue 38 from the oven chamber 18 or a damper 43 in the
inert gas supply intake 44 therefor.
The inert gas-solvent vapor mixture or so-called rich fume
exhausted from the oven 10 is, in accordance with a further aspect
of the invention, introduced into a special design incinerator 25,
known as a rich fume incinerator, where the rich fume is
incinerated at a temperature of at least 1,400.degree.F. and under
approximately stoichiometric conditions to convert the rich fume
into clean, i.e., nonpolluting, inert gaseous products of
combustion or off-gases such as nitrogen, carbon dioxide and water
vapor and having an oxygen concentration of less than about 5
percent, preferably about 2 percent in the particular paint drying
process illustrated, such as are permissible to vent to the
atmosphere under existing pollution control regulations. For the
purposes of the invention, the rich fume incinerator 25 may be of
the type described and claimed in U.S. application Ser. No.
274,406, K. H. Hemsath and A. C. Thekdi, filed July 24, 1972 and
assigned to the assignee of the present application. As therein
disclosed, the incinerator 25 comprises in general a stack 45 (FIG.
1) which is lined on the inside with refractory bricks in order to
preserve heat and into which the rich fume exhausted from the oven
10 is discharged. Disposed about and above the generally
cylindrical stack 45 is a housing 46 having a lower chamber 47 and
an upper or combustion chamber 48, the housing also being lined
with refractory material. The lower chamber 47 has a restricted
cylindrical passageway 49 extending upwardly and opening into the
upper chamber 48 and terminating a short distance above the upper
end of the stack 45 with which it is concentric and which it
closely surrounds to form therebetween a narrow annular air supply
channelway 50. The combustion chamber 48 is abruptly widened at the
upper end of the restricted passageway 49, as by means of the step
shoulder 51 in the wall of the chamber 48, to provide flame
stabilization means for promoting the complete combustion of the
rich fume and pump air mixture within the combustion chamber. The
lower chamber 47 also has an opening 53 for an air supply pipe 54
provided with a manual valve 55. The air supply pipe 54 provides
the required quantity of pump air for the approximate
stoichiometric combustion of the rich fume in the upper or
combustion chamber 48 of the housing 46. The pump air directed into
the lower end of the combustion chamber 48 from the restricted
annular channelway 50 is discharged into the combustion chamber at
a velocity greater than that of the rich fume exhaust discharged
thereinto from the stack and thus acts as an air pump to aspirate
into, and enhance the discharge or emission of the rich fume from
the stack 45 into the combustion chamber. Additionally, the pump
air from the annular channelway 50 promotes uniform mixing thereof
with the rich fume discharged from the stack 45 so as to better
assure the complete combustion in the combustion chamber 48 of all
the combustibles in the rich fume introduced thereinto from the
oven chamber 18.
The upper or combustion chamber 48 of the incinerator 25 is
provided at its lower end, at the region of the intermixing therein
of the pump air from channelway 50 with the rich fume from the
stack 45, with a number of pilot or auxiliary gas burners 56
directed tangentially to the combustion chamber and operated with
air and natural gas mixtures proportioned for approximate
stoichiometric combustion thereof. The pilot burners 56 provide a
pilot flame or ignition source in the combustion chamber 48 and
assist in maintaining the combustion chamber at the required
temperature of at least about 1,400.degree.F., and preferably about
1,650.degree.F., for assuring the complete combustion or
incineration therein of the combustibles in the rich fume
discharged thereinto from the stack 45. The supply of combustion
air and gas to the pilot burners 56 is preset by manual adjustment
of valves 57 and 58 in the air line 59 and gas line 60,
respectively, to provide the ignition source for the rich fume-pump
air mixture introduced into the incinerator. Since the solvent
concentration in the exhaust gases or rich fumes from the oven 10
is, in the process according to the invention, considerably higher
than that in conventional paint drying systems because of the
nonrequirement for the addition of any dilution air to the oven
exhaust gases, the heat released by the solvent vapor combustion in
the incinerator 25 is ordinarily high enough to itself bring the
fume-air mixture in the combustion chamber 48 to the required
incineration temperature of from 1,400.degree. to 1,650.degree.F.
Thus, for the normal operation of the paint drying system according
to the invention, the incinerator 25 does not require any auxiliary
fuel except for the very small amount for a pilot flame from the
burners 56. The fuel requirement of the oven 10 is additionally
lowered, compared to that required by conventional systems, because
the inert gas heat load is much less than what is required for the
conventional dilution air paint drying systems.
To permit the use of the off-gases from the incinerator 25 as the
source of supply for the low oxygen content atmosphere, i.e., no
more than about 5 percent oxygen content, required to be maintained
in the oven chamber 18 for the practice of the invention, it is
therefore necessary that these incinerator off-gases contain not
more than about 5 percent oxygen. Since the pilot burners 56 in the
incinerator 25 are operated at approximately stoichiometric
air-fuel ratio, they release little if any oxygen into the
off-gases from the incinerator. However, the solvent vapors in the
rich fumes introduced into the incinerator 25 from the drying oven
10 need oxygen for their combustion in the incinerator, this oxygen
being supplied by the pump air introduced into the lower chamber 47
of the incinerator. The solvent concentration in the gas mixture
introduced into the incinerator combustion chamber 48 is maintained
in such a way that the heat generated by the perfect, i.e.,
stoichiometric combustion of the solvent vapors in this mixture is
enough to raise it to at least the 1,400.degree. to 1,650.degree.F.
temperature required to be maintained in the combustion chamber for
the complete combustion of the solvent vapors. To assure that the
off-gases from the incinerator 25 normally contain no more than
about 5 percent and preferably around 2 percent oxygen content, the
pump air supply to the incinerator is preset by manual valve 55 in
accordance with the design solvent vapor emissions from the oven
10.
For maintaining the oxygen content in the flue or off-gases from
the incinerator 25 at the desired low set value, e.g., 2 percent,
the electrical signal from a temperature controller means can be
used for this purpose. The temperature controller means may
comprise a thermocouple 61 in the combustion chamber 48 connected
by a lead 62 to a temperature measuring and recording instrument
63. The signal from the temperature measuring and recording
instrument 63 is fed through a lead 64 to a fuel controller unit 65
which then actuates the valve 66 in an auxiliary gas inlet 67 to
the rich fume stack 45. As previously stated, the pump air supply
54 to the incinerator 25 is maintained constant for a given type of
load in the oven 10. This constant flow of pump air is
pre-established by design conditions of solvent vapor emissions
from the oven 10. When the solvent concentration in the rich fume
from the oven 10 decreases, the temperature of the oven 10 also
drops with correspondingly less solvent evaporated therein, causing
the oxygen concentration in the incinerator flue products to rise.
When this occurs, the signal from the temperature controller means
61- 63 will cause the fuel controller unit 65 to open the auxiliary
gas supply valve 66 so as to add more gas fuel and thus more heat
to the incinerator 25 by its combustion with the available excess
oxygen therein and so reduce the oxygen content in the incinerator
flue products. Since the heat release from the auxiliary fuel
replacing the drop-off in the amount of the solvent vapors in the
rich fumes, and the oxygen requirement for the oxidation or
combustion of the solvent vapors and auxiliary fuel are all
linearly proportional, the rate of temperature rise in the
incinerator combustion chamber 48 and corresponding flue products
therefor will be substantially the same. A conventional type oxygen
indicator 68 with a high-low limit, e.g., 0 to 5.0 percent oxygen
content actuating range, may be used for the purpose of providing
an electrical signal for actuating an alarm when the oxygen
concentration in the incinerator flue gases drops below the set
oxygen concentration limit, in which case an operator will then
investigate the possible problems in the system, such as
insufficient pump air. When the oxygen indicator 68 senses an
oxygen concentration above its high limit oxygen concentration
setting, the entire system is then shut down by the automatic
closure of all system emergency shut-off gas valves (not shown)
which are actuated by the oxygen indicator 68. In this case, the
entire oven, incinerator and gas recirculation system is
automatically purged with an adequate supply of an inert atmosphere
such as nitrogen for safety against explosion hazard. Under normal
operating conditions, however, including changeover of the paint
coated strip 11 or breakage thereof, the fuel supply to the
auxiliary gas inlet 67 will be enough to reduce the oxygen percent
in the incinerator flue gases below the maximum set limit.
The incineration of the rich fumes from the oven 10 in the
incinerator 25 in the manner according to the invention as
described above results in the formation of so-called clean
incinerator flue or off-gases such as nitrogen, carbon dioxide and
water vapor which are of inert and nonpolluting character and which
therefore are permissible to vent directly to the atmosphere under
present pollution control regulations. In accordance with a further
aspect of the invention, however, these off-gases or by-products
from the incinerator 25, because of their inert and substantially
oxygen-free composition, i.e., containing no more than about 5.0,
and preferably around 2.0 percent oxygen in this particular case,
are recycled at least in part back into the oven chamber 18 by
recycling means comprising exhaust flue 69 to supply to the oven
chamber the inert gas atmosphere as well as the heat required for
the conduct of the particular solvent-evaporating operation to be
carried out therein, such as the illustrated solvent-based paint
drying operation. Although the recycled portion of the off-gases
may be enough to supply only a part of the total heat and total
inert gas requirement of the oven 10, it is preferable from
economic considerations to employ a sufficient amount of the
off-gases from the incinerator to supply all of the total heat and
inert gas requirement of the oven 10.
Inasmuch as the flue or off-gases exiting from the incinerator 25
through flue 69 are normally at a temperature in excess of
1,400.degree.F. which is considerably above the 450.degree. to
600.degree.F. temperature required in the oven chamber 18 for the
paint drying or other solvent-evaporating operation carried out
therein, the off-gases to be recycled back into the oven 10
therefore must first be cooled down to such lowered oven operating
temperature as by passage of the gases through one or more suitable
heat recovery systems or heat exchangers 70 of the recycling means,
such as a water or air heater, or a low pressure steam boiler, or a
heater for a metal preparation solution employed to prepare the
metal strip 11 for the application of the paint coating 12 thereto.
The heat thus recovered in the heat exchanger 70 can be
advantageously employed, as a further economic benefit of the
process comprising the invention, for various supplementary heating
operations.
The flue gases cooled in the heat exchanger 70 are exhausted
therefrom and discharged, as by a recirculating fan 71, into the
inert gas intake 44 of the oven 10 to thereby supply the inert gas
atmosphere for the oven chamber 18. As shown in FIG. 1, the gas
intake 44 may be formed with a manifold 72 from which the inert
gases from the incinerator 25 are distributed to, and introduced
into the oven chamber 18 through a series of gas inlet or supply
openings 73 in the side wall 26 of the oven 10. Before their
introduction into the oven 10, however, the cooled flue gases from
the heat exchanger 70 are preferably first mixed with some of the
existing inert gas atmosphere in the oven chamber 18 for the
purpose of assuring better temperature homogeneity of the inert gas
atmosphere introduced into the oven chamber 18 with that present
therein during the operation of the system. This intermixing may be
achieved by exhausting a portion of the inert gas atmosphere from
the oven chamber 18 through an exhaust opening 74 in the side wall
26 of the oven 10 and conveying the so-exhausted oven atmosphere
through a duct 75 into a suitable mixing means 76 where it is mixed
with the cooled inert flue gases from the heat exchanger 70. The
resulting gas mixture is then circulated by the fan 71 into the
oven chamber 18 through the manifold 72 and gas supply openings 73
in the oven wall 26. The portion of the cooled inert off-gases
leaving the heat exchanger 70 that is not recycled back into the
oven 10 may be supplied to the plenums 33, 34 for the production of
the aerodynamic seals 31 at the article inlet and outlet openings
19 and 21 of the oven 10. Manually set dampers 77 and 78 in the
respective ducts 79 and 80 which supply the recirculated cooled
inert gases from the heat exchanger 70 to the oven 10 and
aerodynamic seals 31, respectively, are employed to properly
balance the flow of the off-gases from the incinerator 25 to the
seals 31, to the atmosphere, and recycled to the oven 10.
FIG. 3 illustrates a modification of the invention wherein all the
off-gases from the incinerator 25 are first passed through one or
more heat exchangers 70 before the unused portion thereof not
employed for the heat requirements of the oven 18, or for the seals
31, is vented to the atmosphere. In this way, heat recovery from
all the incinerator off-gases may be obtained instead of just from
the unvented portion thereof. In the particular case illustrated,
the heat recovered in the heat exchanger 70 is shown as being
conducted through a duct 81 to a metallic strip preparation section
82 of the illustrated strip paint coating line, such as for heating
a metal preparation solution used to prepare the metallic strip 11
for the application of the paint coatings 12 thereto. Thus, a
further reduction in the total heat required for the paint coating
and drying system is obtained.
In the start-up of the paint drying or other solvent vapor
releasing process according to the invention, the oven 10 must be
purged of atmospheric air. For this purpose, any inert gaseous
medium such as, for example, nitrogen, steam, or combustion
products from burners 56 and auxiliary fuel 67, can be used to
purge the oven chamber 18. An oxygen indicating device should be
used to indicate when the oxygen content of the oven atmosphere is
less than the maximum permissible concentration thereof of no more
than 5.0 percent, e.g., about 2.0 percent, before starting the
introduction into the oven 10 of the work to be processed therein.
The oven seals 31 are activated before the metallic strip 11 or
other work conveyor is started into the oven. The burners 56 and
the auxiliary fuel 67 in the incinerator 25 will supply the heat
for bringing the system to the required oven and incinerator
temperatures and the inert atmosphere to the oven seals 31.
Auxiliary fuel 67 will be required during start-up due to the
addition to the incinerator 25 of the constant and pre-set amount
of pump air through the supply pipe 54 provided with the manual
valve 55. The incinerator temperature controller means 61-63 will
adjust the fuel input rate to the auxiliary fuel inlet 67. Once the
operational conditions are settled, the metallic strip 11 or the
work conveyor is started into the oven 10.
The economic advantages of the paint drying system comprising the
invention will be readily apparent from a comparison of its
requirements of fuel, air and other utilities with those of a
conventional solvent-based paint drying system in which the drying
oven is operated with an atmosphere below the lower explosion limit
of the gas mixture formed in the oven during the drying operation
instead of above the upper explosion limit thereof as in the
process according to the invention. For example, in a conventional
system used for comparison purposes and including a fume
incinerator, a preheat recuperator, and a heat transfer liquid
medium for total heat recovery, the gas consumption thereof is
approximately 41,000,000 B.t.u. per hour, and the heat transfer
rate to the work is approximately 7,000,000 B.t.u. per hour. For
the same heat demand in the paint drying oven 10, a properly
designed paint drying system according to the invention reduces the
fuel consumption to less than 5,000,000 B.t.u. per hour or only 12
percent as much fuel requirement. The fresh air requirements of the
system according to the invention are only 200,000 s.c.f.h.
(standard cubic feet per hour) or about 13 percent of the 1,500,000
s.c.f.g. required by the conventional system. In addition, the
lower fume volume of approximately 525,000 s.c.f.h. for the process
comprising the invention compared to the 1,300,000 s.c.f.h. for the
conventional system makes it possible to employ smaller size
exhaust fans, piping, and the incinerator itself, thus further
effecting economies in the form of reduced cost for the required
equipment.
Insofar as the quality of the finished paint coating is concerned,
comparative tests of thermosetting type paint samples indicate that
the composition of the atmosphere in the drying oven 10 does not
have any effect on the finished paint properties or color
characteristics. As a matter of fact, the inert atmosphere employed
in the paint drying oven 10 in accordance with the invention
retains the paint qualities when the paint is exposed to such inert
atmosphere for a long time, whereas with high oxygen-containing
atmospheres such as are employed in the drying ovens of
conventional paint drying systems the paint pigments are oxidized
and become darkened. Thus, if anything, the drying of solvent-based
paint coatings in inert gas atmospheres in accordance with the
invention actually results in better paint finishes and
quality.
* * * * *