US3437321A - Regenerative paint drying system for continuous strip - Google Patents

Description

April 8, 1969 3,437,321

REGENERATIVE PAINT DRYING SYSTEM FOR con'rmuous STRIP D. M. WILKINSON Sheet Filed May 26, 1967 INVENTOR DW/GHZ' M WILKINSON ATTQRNEXW April 8, 1969 o. M. WILKINSON ,3

REGENERATIVE PAINT DRYING SYSTEM FOR CONTINUOUS STRIP Filed may 26, 1967 Sheet 8 of 4 FIG. 2

INVENTOR DWIGHT M. WILKINSON ATTORNEY April 8, 1969 o. M. WILKINSON 3,437,321

REGENERATIVE PAINT DRYING SYSTEM FOR CONTINUOUS STRIP Filed lay 26, 1967 Sheet 3 of 4 SIGNAL GENERATOR s| v 5e POWER SUPPLY FIG. 4

v INVENTOR DWIGHT M WILKINSON ATTORNEY April 8, 1969 n. M. WILKINSON REGENERATIVE PAINT DRYING SYSTEM FOR CONTINUOUS STRIP Sheet Filed lay 26, 1967 INVENTOR DWIGHT M. WILKINSON ATiTORMEZ.

United States Patent US. Cl. 263-3 8 Claims ABSTRACT OF THE DISCLOSURE This specification discloses a paint drying system in which the vapours of the volatile solvents are burned to eliminate pollution of the atmosphere and the heat generated by such combustion is employed to preheat the incoming fresh air thereby reducing the fuel required to dry the paint.

This invention relates to apparatus for drying and baking paint on a continuous painting line.

The continuous painting or coating of strip material such as sheet metal requires the use of high efliciency continuous oven dryers which apply heat in various stages to evaporate the volatile solvents and to bake the paint. In the normal case the paint used contains considerable quantities of volatile solvents which are evaporated in large quantities and constitute a serious disposal problem.

In the past, it has been common practice to vent these volatile solvents directly into the atmosphere. As a result, municipal regulations have been adopted in certain areas prohibiting the dumping of these solvent efi'luents which otherwise constituted a serious source of air pollution. Operators of such coating plants have therefore been forced to adopt some system for the removal of the volatile eflluents down to a predetermined maximum level. Unlike sooty carbon deposits, volatile efiluents are difiicult to filter, and, although they may be recovered by a distillation or condensation process, the equipment required for such processes is costly to manufacture and difficult to install in existing plants and may be subject to variations in the efiiciency of eifiuent extraction due for example to different concentrations of effluents encountered in the waste air stream.

An alternative method of efiluent removal is simply to oxidize the effluents by burning them in the waste air stream. Such oxidization is again subject to variations due to variations in the eflluent content of the waste air system and although the equipment required is very much cheaper than the distillation or condensation equipment referred to above, there is no efiluent recovery to offset such cost and the process becomes increasingly wasteful.

Accordingly, it is an objective of the present invention to provide a paint drying and baking system incorporating high efiiciency ovens for effective and rapid drying and baking of the paint on a continuously moving strip and further incorporating effluent oxidizing means which is regenerative in nature and assists in maintaining the efiiciency of the ovens.

More particularly, it is an objective of the present invention to provide a system having the foregoing advantages in which the ovens are automatically self-regulating to maintain a constant temperature level at the surface of the metal strip with a minimum of temperature fluctuations.

3,437,321 Patented Apr. 8, 1969 adjustable over a wide range to insure substantially complete oxidizing of the volatile eflluent constituent of the waste air stream at all levels of concentration encountered.

More particularly, it is an objective of the present invention to provide a system having the foregoing advantages in which the heat generated by oxidizing of the volatile efiluents is employed for preheating the incoming air stream prior to entry into the oven system thereby conserving the overall energy required for drying and baking the paint and minimizing the losses due to waste of volatile solvents.

The foregoing and other advantages will become apparent from the following description of a preferred embodiment of the invention which is given here by way of example only, with reference to the following drawings in which like reference devices referred to like parts thereof throughout the various views and diagrams and in which;

FIGURE 1 is a schematic block diagram showing the overall system of ovens, oxidizing means, and heat exchange system;

FIGURE 2 is a sectional side elevational view of a typical oven as used in the system of FIGURE 1 showing alternative positions of members therein in phantom form;

FIGURE 3 is a plan view partially cut away and sectioned of a portion of one of the burners of the oven of FIGURE 2;

FIGURE 4 is a schematic block diagram showing the operation of control system of the oven of FIGURE 2;

FIGURE 5 is a sectional side elevation of an efiiuent burner used for oxidizing the volatile efiluents in the system of FIGURE 1;

FIGURE 6 is greatly enlarged sectional side elevation of a portion of a burner element as shown in FIGURE 5;

FIGURE 7 is a sectional side elevational view of an alternative form of effluent burner; and

FIGURE 8 is a schematic view of a further embodiment of efiluent burner.

Referring to FIGURE 1 the apparatus shown comprises a Number 1 Heater Oven 11, Number 2 Heater Oven 12, and Number 3 Heater Oven 13. Material, which in this case is strip metal 14 is fed through the ovens in order to exit as at 15 with the paint dried. It should be understood that the apparatus shown in FIGURE 1 is an intermediate portion of a complete painting line and that in operation the various types of cleaning and spraying operations would take place prior to the entrance of the strip as at 14 and cooling and other finishing operations as maybe required would take place after the strip leaves as at 15, the drying apparatus shown in the figure.

Each of the drying ovens is quite similar in construction and is provided with a heating unit 16, 17 and 18 respectively, the ovens in this case being gas fired. Details of construction for such gas fired ovens are described below with reference to FIGURES 2, 3, and 4.

The apparatus shown in FIGURE 1 is utilized for drying paint containing volatiles and does so by a pre-heating and partial drying by means of the oven 11, a second heating by means of the oven 12 in which the remainder of any of the heavier volatiles are driven from the paint, and a final drying or baking process by means of the oven 13.

The air intake to oven 11 is pre-heated by means described below for the primary evaporation and it is then preferable to transfer the heated air from oven 11 to oven 12 by means of the pump 19 operating through the pipes 20 and 21. By utilizing the heated air from oven 11 introduced into oven 12 where additional heat is supplied by means of heater 17 economy of operation with regard to fuel input is improved. The efiluent from the drying of paint entrained with waste air exist through the piping at 22 and driven by the fan unit 23 is forced through a burner unit 24.

The burner unit 24 utilizes a variable fuel input through the intake 25 and by raising the heat of the effluent from the fan 23 oxidizes the solvents contained therein.

The considerable amount of heat generated, both from the burning of the fuel input from 25 and from the solvents from the effluent from the drying process, is utilized by means of heat exchanger 26 heating the incoming air as at 27 utilized in the entire drying process. Waste gases and oxidized effiuent exit at 28, the burner effectively reducing the percentage content of solvents to acceptable limits.

The burner unit 24 employed in the present invention is described below with reference to FIGURES 5, 6, 7 and 8.

A fan unit 29 is used to force the incoming pre-heated air from exchanger 26 into the first oven 11. In some cases it may be necessary to provide an additional heat input by means of the heater 30 supplied by the line 31 with fuel should the heat available from the heat exchanger 26 not prove suflicient for particular installations. The heater 30 can be gas fired similar to the ovens 11, 12 or 13 or if desired other means of heating the air introduced into oven 11 can be resorted to.

With reference to FIGURE 2 an oven such as 11 is utilized for drying a painted strip shown and indicated generally as 14, adapted for moving through the oven, having paint on both sides which is dried during the passage therethrough. A plurality of upper burner assemblies at 32 and a plurality of lower burner assemblies at 33 are pivotally supported within the oven and operably connected to respective operating pistons 34 and 35 of any suitable type.

The radiating burners such as burners 36 or 37 can be of any suitable type and can for instance be gas fired infrared burners such as the type employing surface combustion phenomena to produce the heat radiation. Each individual burner is pivotally supported within the oven and is interconnected by means of linkages, such as linkages 38 and 39 in order to be operable in unison with the respective operating cylinder. The operable position is as shown in solid line and the inoperable or non-heating position is as shown in the chain dotted line as at 40 and 41. It should be noted that if desired other arrangements of infrared heating units can be resorted to, the arrangements shown in the drawing being done so by way of example only.

The air intake and outlet for oven have not been shown in this illustration for the sake of clarity, since the construction of the ovens 16, 17 and 18 will be essentially similar. However, while in FIGURE 1 there has been shown an arrangement of ovens 16 and 17 connected for seires flow of air from one to the other, it will readily be understood that other connections might be arranged such as a connection in parallel whereby each oven received fresh preheated air from fan 29 and delivered solventsaturated effluent separately to fan 23.

Referring to FIGURE 3 a preferred method of mounting an individual infrared burner which in this case is a gas rfired infrared burner, is shown. The oven wall 42 supports in bearing 43 a hollow shaft 44 connected through fitting 45 to the infrared burner 46. A simple trunnion mount is utilized at the extreme other end of the burner 46 this not being shown in the drawing.

A slip joint assembly 47 is sealably secured to end 48 of the shaft 44 and is sealed gas tight by means of the seal 49 and to be retained by pin 50 engaged within the annular slot 51. A gas valve 52 is supplied in the gas supply line 53 supplying fuel to the burner 46.

Referring to FIGURE 4 a schematic of the device is shown wherein cylinders 34 and 35 are hydraulic and are supplied selectively from the three ways valve 54 which receives hydraulic power through the lines 55. The selective position of valve 54 is controlled by means of the switch unit as at 56 having an opened position thus to give the two positions for operation of valve 54 and the respective extended or retracted position of the cylinders 34 and 35. Control of the valve 54 from switch 56 is eifected from the main control source of the painting line whereby upon the line being stopped the burners 36 and 37 are moved to the inoperative position and upon the line being started the burners are moved to the operative position as described above.

The gas burners 36 and 37 are controlled by means of the turn down valve 57 which in turn is responsive to the sensor 58. This sensor is positioned adjacent to the work as it leaves the furnace as shown in FIGURE 2 and senses the heat radiated from the work and thus is an indication of the temperature of the work leaving the oven. Suitable signal conditioning means in the form of amplifier 59 and variable signal output 60 supplied by power supply 61 provide a signal for the valve 57 in order to regulate the fuel supply to the burners from supply line 62.

In use the work 14 can be fed through the oven and during normal passage of the work therethrough the burners are in a position to radiate heat onto the work and thus to dry the paint. Upon stoppage of the line, or even a major slow down in the line, the switch means 56 will through valve 54 cause the cylinders 34 and 35 to actuate and move the radiant burners away from the work.

In order to control the temperature at which the work leaves the oven, the sensor 58, responsive to radiation from the work 14 provides a variable signal to the amplifier and output system thus to vary the supply of fuel to burners 36 and 37.

An additional sensor (not shown) is preferably located within each oven which senses the ambient (air) temperature, which is a function of the heat of incoming air and additional heat of the gas burners, and is connected in the same way as sensor 58 to control the supply of fuel to the burners. Thus, even where the line is temporarily halted, the fuel supply to the gas burners will be controlled notwithstanding that sensor 58 is rendered temporarily useless.

Thus by utilizing the gas valve 57 controlled by means of the sensor 58, the heat supplied to the work piece as it moves through the drying oven can be controlled.

Additionally should it become necessary to control the heat within the oven during which times the radiant burners are not directed at the work, vent means such as for instance the controllable vent 63 as shown in FIGURE 2 can be utilized. In most cases the heat resulting from convection off the burners will be considerably below that resulting from radiation and normal heat loss through the walls of the oven will be sufficient to maintain a low temperature therewithin. However, in some cases where the heat loss is not sutficient or where the input due to the burners is excessive, venting means such as described may be resorted to.

The volatile effluent burner system shown in FIG- URES 5 and 6 will be seen to be located in the effiuent flow duct at 24 and according to this preferred embodiment of the invention it is provided with a gas flow barrier comprising, in this preferred embodiment at least two flat panels 64 formed of porous ceramic material. Panels 64 are supported in a generally V-shaped orientation within the duct at 24 by resilient support means 65 fastened to the side walls of the duct and resilient support means 66 fastened across the side walls of the duct and supported at intervals as by supporting bar 67. Burner means are provided in the form of gas burners 68 supplied with fuel through the piping system 69. The burners 68 are positioned at the upstream end of the panels 64 the flame thereof being directed along the inside surface as at 70.

With particular reference to FIGURE 6 the action of the flame 70 panels 64 is shown diagrammatically. The hot gases and flames from the burners 68 are indicated at 70 and the waste air laden with volatile hydrocarbons is indicated at 71. The porous ceramic material of panels 64 allows passage of the hot products of combustion from the flame and the waste gas 71 therethrough and causes instant mixing of the waste gas with the hot flame so as to ensure oxidization of the volatile solvents. The eflluent gas indicated at 72 on the downstream side of the panels 64 is thus a mixture of both the flame gas and the waste gas in which the solvents have been burnt.

With reference to FIGURE 7 a further configuration for a barrier member such as for instance the panel 73 is shown placed within a gas duct 74 and has a pair of burners 75 and 76 directing flames therefrom along the upstream side of the barrier 73. Thus for a simple application a single barrier member together with one or more gas flame burners can be utilized to effectively oxidize any solvents existing in the waste gas from a drying process. While a single barrier 73 is shown in FIGURE 7 and a pair of barriers 64 are shown in FIGURE 5 it should be understood that such arrangements can be used in plurality within a particular duct in order to increase the effective capacity of the installation.

With reference to FIGURE 8 such a system employing a plurality of barriers is shown wherein the barrier members as indicated at 77 are mounted within a large duct 78 and are provided with a plurality of burners as at 79. The waste gas containing volatile solvents is introduced at 80 and passing through the barriers 77 where the solvents oxidize and exit at 81. Thus for particularly large installations a certain size of overall area of barrier and burner unit can be used in plurality to effect complete handling of the full capacity of the duct system. It should also be noted that the plurality arrangement as shown in FIGURE 8 can be varied according to individual requirements for space and design conditions. Other configurations will occur to those skilled in the art to suit particular installations and the configurations shown are done so by way of example only.

As an example of a typical installation for oxidizing air laden with volatile solvents from a paint drying process the following conditions and dimensions for a burner are given. For a duct height of approximately twelve inches and of a width suitable to handle the required air flow the configuration as shown in FIGURE 5 can be used to advantage. Each burner should have a rating of approximately 350,000 B.t.u. per hour per lineal foot of burner to provide a sufiiciently hot gas blanket over the length of the barrier members. It is preferred to form the barriers out of a one-inch thick plate of silicon carbide porous media having a grade of 210 as specified by the Carborundum Company, Bond Abrasive Division, PO. Box 37, Niagara Falls, NY. This would handle an air flow of approximately 210 cubic feet per minute over each square foot of barrier media at a two inch W.C. pressure drop through the barrier. The fuel would be natural gas.

The temperature of the combustion gases from the burners required for efficient oxidization of the solvents will vary between 900 F. and 1350 F. To some extent this may depend also upon the residence time during which the solvent vapours remain in contact with the hot combustion gases, the object being to achieve the highest rate of oxidisation with the shortest possible residence time. The intimate mixing of the solvent vapours and the combustion gases greatly increases the efliciency and this mixing is significantly improved by the use of porous barriers such as those described above. In some instances aluminum oxide porous barriers may be acceptable although at reduced temperatures.

In some instances it may be desirable to regulate the pressure drop through the system in order to effectively control complete oxidization of the solvents contained in waste gas. In such a case an adjustable orifice as indicated generally at 82 in FIGURE 5 can be resorted to. The orifice as shown in FIGURE 5 comprises a pair of plates 83 and 84 slidably mounted within the mounting 85 and moveably responsive to a suitable transducer to effectively close or open the opening indicated at 86 to throttle the air stream moving therethrough. Thus while control of the burner flame, by using available equipment, will control the temperature at which the Waste gas comes in contact with the flame, the adjustable orifice, provided with suitable automatic control interconnected with the transducer, will control the pressure drop through the porous media and thus control the time of contact of the solvent volatiles with the hot gas flame thus to control the degree of oxidization of these volatiles.

Other means for the control of pressure drop within the duct and thus the time of contact of the hot gas flame and the solvents can be resorted to, the means shown in the drawings being done by way of example. Thus many well-known types of dampers can be employed within the duct system either upstream or downstream of the efiluent burner unit to control the pressure drop within the complete unit. Essentially any means tending to constrict the area of air passage through the device will affect the pressure drop through the system and control of this means will control the pressure drop and the time of contact.

From the foregoing it will be seen that the present invention provides means for effectively oxidizing volatile hydrocarbons in a gas stream. The invention provides for variation in temperatures at which the hydrocarbons are oxidized and also provides for variation in time of contact of the hydrocarbons with the high temperature regions thus to effectively oxidize the hydrocarbons to the desired degree.

What I claim is:

1. Regenerative paint drying system for use in association with a continuous strip painting line and comprising: heating oven means including at least two heating ovens adapted to enclose at portion of said continuous strip spaced apart along said strip and evaporate solvents from paint thereon; variable strip heating means within each said oven; intake and exhaust conduit means for conducting air to part of said oven means, and for conducting solvent-laden efliuent air therefrom; solvent burning means in said exhaust conduit means for oxidizing solvent vapours in said eflluent; heat exchange means in thermal communication with the eifluent emerging from said solvent burning means for pre-heating intake air entering said intake conduit means; and heat regulating means sensing the temperature in the first said oven resulting from the combined effect of said strip heating means and said heated intake air and varying the heat input of the second said oven strip heating means.

2. Regenerative paint drying system as claimed in claim 1 including further heat exchange means associated with said intake conduit means operable to further preheat said air flowing therein.

3. Regenerative paint drying system as claimed in claim 1 wherein said variable oven heating means includes radiant burner means fired by gas fuel, said burner means being pivotally mounted within each said oven for movement between an operating position in which the radiant heat is directed towards said continuous strip and a non operating position in which the radiant heat is directed away from the continuous strip, and means for moving said burner members between said two positions.

4. Regenerative paint drying system as claimed in claim 3 including ambient temperature sensing means within said oven means, and means for sensing the temperature of said continuous strip, said sensing means providing a control signal, and gas flow valve means responsive to said control signal to regulate the flow of gas to said burner means.

5. Regenerative paint drying system as claimed in claim 1 wherein said solvent burning means includes a burner chamber; a porous barrier within said burner chamber formed of high temperature resistant material and providing a predetermined cross sectional area for flow of effluent therethrough; and fuel burner means on the upstream side of said barrier member providing combustion gases at a predetermined temperature, said combustion gases and said eflluent being intimately mixed one with the other in passage through said porous barrier means.

6. Regenerative paint drying system as claimed in claim 1 including gas fiow regulating means in said exraust conduit means operable to regulate the flow of effluent therethrough.

7. Regenerative paint drying system as claimed in claim 1 wherein said solvent burning means includes at least one porous barrier member of high temperature resistant material of fiat planar construction arranged diagonally relative to the flow axis of effluent within said conduit means whereby to define a continuously decreasing flow area on the upstream side of said conduit means and a continuously increasing flow area on the downstream side of said conduit means, and including fuel burner means on the upstream side of said barrier means located closely adjacent thereto at about the point of maximum flow area and oriented to direct combustion gases along the upstream surface of said barrier member towards the point of minimum flow area.

8. Regenerative paint drying system as claimed in claim 1 wherein said solvent burning means includes at least two porous barrier members of flat planar construction located within said exhaust conduit and arranged in a V-shaped configuration the apex of said V being oriented towards the downsream portion of said conduit, and fuel burner means arranged adjacent the barrier members at the widest portion of said V-shape on the upstream side thereof.

References Cited UNITED STATES PATENTS 2,658,742 11/1953 Suter et al 2633 2,804,694 9/1957 Clipsham 2633 3,106,386 10/1963 Harris 2633 3,183,605 5/1965 Argue et a1. 2633 X JOHN J. CAMBY, Primary Examiner.

U.S. Cl. X.R.

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