US 3737280 A
A paint line oven and heat source comprising a stack having fuel-fired burners in the lower portion thereof, and a solvent-burner section at the bottom of the stack. A blower intake duct is connected to the stack above the burners and draws off part of the hot exhaust from the stack. This is mixed with fresh air and circulated through the oven, where it evaporates paint solvent. A portion of the solvent-laden air is bled off to the solvent-burner section, where the solvent vapor is ignited and burns, adding its heat to the heat of the burners. A first temperature sensor in solvent-burner section actuates a valve to increase the amount of oven atmosphere bled off to the solvent-burner section as the temperature rises. A second sensor responds to the temperature in the upper part of the stack to reduce the heat output of the burners as the heat of solvent combustion raises the stack temperature. A third sensor in the blower outlet duct actuates valves to vary the relative proportions of hot flue gas and fresh air, according to the temperature in the oven.
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Description (OCR text may contain errors)
United States Patent [1 1 Cromp nu 3,737,28ii
1 1 June 5., 1973 [541 EMISSION-CONTROLLED PAINT LINE HEAT SOURCE  Inventor: Theodore Q. Cromp, Placentia,
 Assignee: Hunter Engineering Co., Inc., Riverside, Calif.
 Filed: Apr. 14, 1972  Appl. No.: 244,149
 U.S. Cl ..432/41  Int. Cl ..F27b 9/40  Field of Search ..432/41, 21, 8
 References Cited UNITED STATES PATENTS 2,288,129 6/1942 Feldhausen et al ..432/4l 2,648,531 8/1953 Shorek et al. ..432/4l 3,472,498 10/1969 Price et al. ..431/21 Primary ExaminerJ0hn J. Camby AttorneyHerbert E. Kidder l 57] I ABSTRACT A paint line oven and heat source comprising a stack having fuel-fired burners in the lower portion thereof, and a solvent-burner section at the bottom of the stack. A blower intake duct is connected to the stack above the burners and draws off part of the hot exhaust from the stack. This is mixed with fresh air and circulated through the oven, where it evaporates paint solvent. A portion of the solvent-laden air is bled off to the solvent-burner section, where the solvent vapor is ignited and burns, adding its heat to the heat of the burners. A first temperature sensor in solvent-burner section actuates a valve to increase the amount of oven atmosphere bled off to the solvent-burner section as the temperature rises. A second sensor responds to the temperature in the upper part of the stack to reduce the heat output of the burners as the heat of solvent combustion raises the stack temperature. A third sensor in the blower outlet duct actuates valves to vary the relative proportions of hot flue gas and fresh air, according to the temperature in the oven.
4 Claims, 1 Drawing Figure EMISSION-CONTROLLED PAINT LINE HEAT SOURCE BACKGROUND OF THE INVENTION The present invention pertains generally to paint line ovens for evaporating the volatile solvent from painted metal strip, and then polymerizing the resin in the paint. Such ovens are usually maintained at'temperatures ranging from 500F to about 750F, and heat is supplied by blowers, which take in hot flue gas from an adjacent stack having burners discharging flame into the lower portion thereof, and mix it with a large volume of fresh air before discharging the mixture into the oven. The oven atmosphere is continuously recirculated by the blowers, and a volume of the oven atmosphere equal to the incoming volume of fresh air and hot flue gas is bled off into the stack and thence to the atmosphere.
Heretofore, it has been the practice to ventilate the ovens with an excessive amount of fresh air in order to dilute the concentration of combustible solvent fumes down to a level well below the explosive level. This eliminates the explosion hazard, but one major objection is that it is extremely costly because of the excessive amount of fuel that must be used to heat up the large amount of air that is required. Another objection is that because the solvent fumes are so dilute in the oven atmosphere discharged into the stack, there is relatively little combustion of the solvent, and most of the fumes are discharged into the atmosphere as pollution.
SUMMARY OF THE INVENTION The primary object of the present invention is to provide an emission-controlled heat source for paint line ovens, in which the amount of fuel required to heat the ovens is reduced by more than 50 percent.
Another important object of the invention is to provide a heat source for paint line ovens, in which raw solvent fumes are substantially eliminated from the stack emissions, thereby greatly reducing this source of atmospheric pollution.
These objects are achieved in the present invention by providing a solvent-burner section and pilot light in the bottom of the stack at the point where the oven atmosphere is discharged into the stack, and a temperature sensor in the solvent-burner section, which actuates the by-pass valve that regulates the amount of oven-atmosphere bled off into the stack, so that as the temperature in the solvent-burner section rises above a predetermined level owing to the heat of combustion of the solvent, a greater amount of oven atmosphere is bled off. At the same time, a second temperature sensor in the upper portion of the stack responds to any rise in the stack temperature (owing to combustion of the solvent) by reducing the heat out-put of the fuelfired burners. The system is completed by a third temperature sensor located in the blower outlet duct, which responds to temperature fluctuation by actuating a control device that increases the amount of flue gas taken in, while simultaneously decreasing the amount of fresh air taken in, or vice versa, until the desired temperature has been reached. The total amount of heat obtained from combustion of the solvent fumes is substantial, and once the system has been brought up to operating temperature, most of the heat required for the ovens can be supplied by the solvent fumes, with only a small amount of fuel being consumed by the burners and pilot light. While the amount of fresh air taken in by this system is still relatively large in order to dilute the concentration of solvent fumes in the oven down to a level below the explosive level, it is closely regulated by the two ternperature-sensor-controlled valves, which accurately vary the proportion of fresh air to hot flue gas admitted to the oven, according to oven temperature requirements, and at the same time vary the amount of oven atmosphere bled off to the stack according to the solvent fume concentration, as reflected by the temperature in the solvent-burner section. Thus, the total amount of fresh air added to the system is closely proportioned to the amount of solvent being evaporated at any given time, and the amount of fresh air to be heated up to oven temperature is greatly reduced. At the same time, the heat of combustion of the solvent is efflciently utilized in the system, with a net saving of more than 50 percent of the total fuel required, as compared with the conventional paint line oven.
These and other oejects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment thereof.
BRIEF DESCRIPTION OF THE DRAWINGS The one FIGURE of the drawing is a schematic representation of a paint line oven with associated heat source, in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawing, the oven is designated in its entirety by the reference numeral 10, and the heat source is indicated by the numeral 12. The oven 10 comprises an elongated housing 14, through which freshly painted metal strip 16 passes in the direction perpendicular to the plane of the drawing. Extending parallel to the strip 16 above and below the latter are hot air ducts l8 and 20, having outlets on the sides thereof facing the strip, so that streams of hot air blow from the duct onto the strip, as shown by arrows. Ducts 18 and 20 are connected by other ducts 22 and 24 to the outlet 26 of a blower 28. The inlet 30 of the blower is connected by a duct 32 to a horizontal duct 34 above the oven, one end of which is connected at 36 to the interior of the oven, and the other end of which is connected to a mixing chamber 38.
Mixing chamber 38 has a fresh air intake 40 controlled by a butterfly valve 42, and a flue gas intake duct 44 controlled by a butterfly valve 46; duct 44 being connected to the interior of stack 12 near the upper end of the later, as shown. valves 42 and 46 are linked together by an operating connection 48, and are simultaneously actuated in the opposite directions by a valve actuator 50, so that valve 42 is opened when valve 46 is closed, and vice versa. Actuator 50 is controlled by a temperature sensor 52 mounted inside duct 24 close to the outlet 26 of the blower. Sensor 52 is preferably a thermocouple, and is connected by wire 53 to actuator 50.
Heat source 12 is preferably a vertical stack 54 having a discharge outlet 56 at the top end, and burners 58 near the lower end thereof. Burners 58 may be fueled by gas or oil, and are supplied with fuel by lines 60, and with combustion air by lines 62. The burners 58 discharge their flames into the interior of the stack54, to
heat up the interior of the stack to around 1,400F. A temperature sensor 64, preferably in the form ofa thermocouple, is located in the upper portion of the stack 54 and is connected by electrical wire 66 to the control mechanism 68 of the burners. Sensor 64 responds to temperature increase in the stack 54 above a predetermined level by reducing the heat output of the burners 58, so as to maintain the temperature in the stack at a more or less constant level.
At the bottom end of the stack 54 is a solvent-burner section 70, which is connected to the blower duct 24 by a by-pass duct 72. At the point where the by-pass duct 72 discharges into the solvent-burner section 70' is a pilot light burner 74, which is also connected to the fuel and air lines 60 and 62. A temperature sensor 76 (preferably a thermocouple) is located in the solventburner section 70, and is connected by an electrical wire 78 to an actuator 80, which operates a butterfly valve 82 in duct 72. Another butterfly valve 84 in duct 24 serves as a restrictor, to maintain a slightly elevated back pressure in duct 24 so that a portion of the output of the blower will be diverted to the duct 72 and thence to the solvent-burner section 70. Sensor 76 is shielded from the radiant heat of the burners 58 by an infra-red screen 85, which may be in the form of a perforated steel plate extending across the width of the stack between sensor 76 and the lower burner 58.
Located in duct 34 between the oven outlet 36 and the junction with blower intake duct 32 is another butterfly valve 86, which also serves as a restrictor at that point so that the relative proportions of ovenatmosphere recirculated to the blower 28 and mixed flue gas and fresh air coming from the mixing chamber 38 can be controlled and regulated. Thus, the blower 28 constantly recirculates a portion of the hot oven atmosphere with its burden of solvent vapor, and mixes this with a certain proportion of hot flue gas and fresh air, the proportions of cool fresh air and hot flue gas from the stack being determined by the temperature sensor 52, which is set to maintain the oven temperature at 350F to 800F, depending upon how fast the painted strip 16 is moving, and the length of time it is exposed to the oven temperature. The average oven temperature is usually around 500F to 550F.
For each volume of fresh air and hot flue gas added to the system from the mixing chamber 38, an equal volume of oven atmosphere must be discharged through duct 72 to the stack 54. As the solvent-laden air enters the solvent-burner section 70, it is ignited by the flame of the pilot light burner 74 and begins to burn. The solvent vapors continue to burn as they rise through stack 54 and mix with excess air supplied by the burners 58. Combustion is further accelerated as the burning solvent vapors pass upwardly through the flames of the burners 58, which raises the temperature of the burning gases well above the ignition temperature. At the same time, other volatile constitutents of the oven atmosphere, such as aldehydes, ketones, olefins, various hydrocarbons, and inorganic matter, are oxidized and degraded as they rise in the stack, and the exhaust issuing from the stack is almost entirely free of objectionable atmospheric pollutants.
The temperature in the solvent-burner section'70 is a function of the concentration of solvent vapor in the air discharged through duct 72, and as the temperature rises (indicating an increase in solvent concentration) sensor 76 reacts by opening the valve 83 further, so
that more of the blower output is bled off through bypass duct 72, and less of it goes to the oven. This results in an increased amount of air and flue gas mixture being drawn from the mixing chamber 38 to dilute the solvent vapors in the oven atmosphere down to a level below the explosion level. If the additional incoming air causes the temperature of the blower outlet to drop belowa predetermined level, sensor 52 operates the valve actuator 50, causing the fresh air intake valve 42 to close somewhat, and the flue gas intake valve 46 to open a corresponding amount, so that a larger proportion of hot flue gas is mixed with a smaller proportion of cool fresh air, to raise the temperature at sensor 52 up to the desired level.
Combustion of the solvent vapor in the burner section and in the stack 54 adds a considerable amount of heat to the output of burners 58, with the result that the stack temperature tends to rise. However, as the temperature starts to rise, sensor 64 reacts by shutting down the burners 58 a corresponding amount, so that the total heat generated within the stack remains more or less constant, but with different proportions coming from the burners 58 and from the combustion of the solvent vapors in burner section 70 and in the stack 54.
The operation of the system is believed to be clearly evident from the foregoing description. The chief advantage of the system is the great saving in fuel cost, which is a considerable item. The total heat generated by combustion of the solvent in a typical paint line oven handling painted metal strip, in which 50 gallons per hour of solvent is evaporated from the paint, amounts to approximately-7,000,000 BTU per hour, which is a substantial proportion of the total heat required. Moreover, the emissions coming out of the stack are almost entirely free of raw solvent, and consist of approximately percent or more of N 1 percent or less of O and the balance of CO CO, and other minor constituents.
While I have shown and described in considerable detail what I believe to be the preferred form of my invention, it will be understood by those skilled in the art that the invention is not limited to the exact details shown, but might take various other forms within the scope of the claims.
1. An emission-controlled paint line oven for evaporating paint solvent from freshly painted metal strip, comprising:
an enclosed oven through which said painted strip passes;
a heat source comprising an enclosure having means therein for generating heat;
a mixing chamber having a hot air duct connected to said heat source enclosure and a fresh air ductfor taking in atmospheric air;
a blower having an intake duct and an outlet duct, said outlet duct discharging into said oven and said intake duct including a first branch connected to said oven and a second branch connected to said 'mixing chamber;
proportioning valve means for regulating the relative proportions of hot air and cool fresh air admitted to said mixing chamber, said valve means including an actuator;
a first temperature sensor located in a position to respond to the temperature of the air being circulated to said oven by said blower, said sensor being operatively connected to said proportioning valve means actuator to increase or decrease the respective proportions of hot air and cold air admitted to said mixing chamber responsive to temperature changes in the air being circulated by the blower; said heat source enclosure having a solvent-burner section adjacent said heat generating means;
a by-pass duct connected at one end to said blower outlet duct and at the other end to said solventburner section;
igniter means within said solvent-burner section for igniting the solvent vapors delivered thereto by said by-pass 'duct;
by-pass valve means and an actuator therefor to regulate the amount of blower air sent to said solventburner section;
a second temperature sensor located in said solventburner section and operatively connected to said by-pass valve means actuator to open said bypass valve means wider as the temperature in the solvent-burner section increases, and to close it as the temperature decreases; and
a third temperature sensor located in said heat source enclosure in a position to measure the total heatoutput due to said heatgenerating means and to the combustion of solvent, said third temperature sensor being operatively connected to said heatgenerating means to reduce their heat output as the heat of combustion of said solvent vapor causes the temperature in said enclosure to rise above a predetermined level.
2. An emission-controlled paint line oven as in claim 1, wherein said first temperature sensor is located in the outlet duct of said blower.
3. An emission-controlled paint line oven as in claim 1, wherein said heat source enclosure comprises an elongated stack having fuel-fired burners discharging flame into the same near one end thereof, and combustion gases exhausting from the other end thereof, said solvent-burner section being located in said stack adjacent said one end whereby burning solvent vapors ignited in said solvent-burner section pass through the flame of said fuel-fired burners and continue to burn as they travel lengthwise in said stack, said third temperature sensor being located in said stack downstream of said burners.
4. An emission-controlled paint line oven as in claim 3, wherein said mixing chamber hot air duct is con nected to said stack on the side of said burners toward said other end, said third temperature sensor being located in the area adjacent the point where said mixing chamber hot air duct connects to the stack.