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.

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.

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.