Abstract:
A radiant convection oven for baking a coating on an article which includes an oven enclosure receiving coating articles, a fan receiving fresh air and directing the fresh air to a burner or burners, a plurality of heat radiators each having an internal chamber receiving heated fresh air from the burner. The radiators are located within the oven opposite a coating article, radiating heat energy to the coated article, and a plurality of nozzles connected to the internal chamber of the radiators directing fresh heated onto the article, transferring convection heat energy to the coated article. In the disclosed embodiment, the radiators include radiating side and bottom walls having a relatively shallow depth, increasing the flow rate through the radiators, reducing the size and cost of the radiators and the fan.

Description:
RELATED APPLICATIONS 
     This non-provisional patent application claims priority to a provisional patent application Ser. No. 60/995,542, filed on Sep. 27, 2007 and is a continuation-in-part application of Ser. No. 11/701,254 filed on Feb. 1, 2007, which application claims priority to provisional application Ser. Nos. 60/814,632, filed Jun. 16, 2006 and 60/807,875, filed Jul. 20, 2006 and 60/839,082, filed Aug. 21, 2006. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a radiant convection oven for baking or curing a coating on an article. As used herein, the term “baking” is defined as heating an object or article for the purpose of drying or curing the coating, including paint. Paint is defined as a decorative, protective or performance enhancing coating or sealant. 
     BACKGROUND OF THE INVENTION 
     A paint bake oven typically includes an oven enclosure into which coated articles are received, typically on a conveyor, a heating system to provide heat for drying or curing the coating and an exhaust system to ventilate fumes and smoke from the oven enclosure. The heating system provides thermal energy to the oven and transfers that thermal energy to the coated object or article. There are two types of heating systems presently in wide use for paint bake ovens of this type, namely convection and radiation. Occasionally, a combination of convection and radiant ovens are used. 
     A convection heating system transfers heat to the coated object or article by blowing heated convection air onto the coated article, transferring convection heat energy to the coated article. The volumetric flow rate, temperature and velocity of the convection air are controlled to provide the desired rate of heat gain in the coated object. A convection heating system includes a fan or blower for moving the convection air and a heat source for heating the convection air. Filtration is often provided in the convection heating system to remove dirt particles from the convection air before it is blown onto the coated object. 
     A radiant heating system transfers heat to the coated article by positioning a hot radiator or radiating wall or duct adjacent the coated object. Electromagnetic radiation, primarily in the form of infrared radiation, is exchanged between the radiator and the object. The radiator size, distance from the object and temperature of the radiator are controlled to provide desired rate of heat gain to the coated object. The radiator is typically a metal wall or panel that is heated by circulating hot air into a space behind the radiator from typically a passage or chamber within the radiator. A radiant heating system, similar to a direct fired convection heating system, heats and circulates the hot air inside a passage or chamber within the radiator. Typically, a radiant heating system will include some convection heating directed at specific heavy metal areas. For automotive bodies, for example, this convection heat is directed at the door sill area. The door sill area is typically made from multiple layers of thicker material, such as steel, and the radiant heating alone is not sufficient to properly heat this area. The convection air may also be directed from a fresh supply for the radiant portion of the oven. 
     In either type of heating system, a heater box is typically used to house the filters and a heat exchanger (if required) to provide a place to connect the recirculation fan and burner. The heater box also provides a closed space to allow mixing of the burner heat with the recirculating air. The heater box is connected to the oven enclosure by ductwork for conveying air between the oven and the heater box. The heater box or boxes must be insulated to reduce heat loss and reduce the burn hazard to personnel in the area. Further, regardless of the type of heating system used, fresh air make-up is required for the oven. The purpose of fresh air make-up is to replace the air removed from the oven enclosure by the oven exhaust system used to remove combustible gases. With a conventional heating system, the fresh hair make-up is provided by drawing some pressure into the convection heater box. With a radiant heating system, fresh air is provided by a separate fresh air heating system. The fresh air heating system is essentially a convection system with capacity sufficient for the fresh air needs of the oven. It is also possible to allow fresh air to leak into the oven without heating; but this is generally not done because (1) it can lead to condensation problems when the cold air mixes with the hot air inside the oven, and (2) may carry dirt into the oven which would contaminate the coated article. 
     SUMMARY OF THE INVENTION 
     As set forth above, this invention relates to a radiant convection oven for baking a coating on an article, such as the paint on a vehicle body. The new heating system used in the convection radiant oven of this invention may use the fresh air make-up only to transfer heat to the coated object or article. The fresh air make-up is used to both heat the radiators and is then delivered to the oven as convection air. In addition, this air is heated by burners mounted to the oven enclosure instead of a separate heater box. 
     The radiant convection oven for baking a coating on an article of this invention includes an oven enclosure receiving the coated articles, a plurality of radiators, including heat radiating walls, having an internal chamber receiving heated air from the burner, wherein the radiators are located within the oven opposite the coated article, radiating heat energy to the article, and a plurality of nozzles in communication with the internal chamber of the radiators, directing fresh heated air under pressure onto the coated article, transferring convection heat energy to the article. As would be understood by those skilled in this art, for the radiant convection oven of this invention to a heat a coated object at the same rate as a conventional radiant heating system, it must produce an equivalent amount of radiant and convection energy delivered to the coated object surface as does a conventional radiant oven. This is accomplished when the convection air volume, temperature and velocity as well as the size, position, surface temperature and emissivity of the radiators are generally equal. 
     In the radiant convection oven of this invention, the volumetric flow rate of the fresh air make-up is small compared to the recirculating air flow rate in a convectional oven radiator. Therefore, the temperature of the air circulating through the inventive oven radiator must be greater than the air circulated through a conventional radiant oven. The temperature and velocity of the radiator air for the radiant convection oven of this invention has been designed to provide radiation heat delivery equal to a convection radiant heat oven as described in more detail below. 
     In the disclosed embodiment of the radiant convection oven of this invention, the internal chamber of the radiating walls has a depth of less than five inches and an air velocity of greater than two thousand feet per minute. More preferably, the velocity of the air through the internal chamber of the radiating walls has a velocity of greater than two thousand five hundred feet per minute and may exceed three thousand feet per minute and the depth of the radiating walls or radiator may be three to five inches or less, significantly reducing the requirement for fresh make-up air while maintaining radiation heat delivered to the coated article equal to conventional radiant bake ovens. Further, in the disclosed embodiment, the internal chamber of the radiating walls have an air volume flow rate of less than five thousand cubic feet per minute. In the disclosed embodiment of the radiant convection oven of this invention, the burner is located outside of the oven enclosure, preferably adjacent or attached to the oven enclosure, and the oven includes a combustion chamber located within the oven receiving hot combustion gas from the burner. In the enclosed embodiment, the radiant convection oven includes two burners, including a burner located adjacent each side wall of the enclosure and the radiating walls include a bottom wall and radiating sidewalls on opposed sides of the oven. Further, the disclosed embodiment of the convection radiant oven of this invention includes radiant ducts on opposed sides of the oven, adjacent the sill area of the vehicle body, receiving hot fresh air from the radiating side walls, directing hot convection air onto the coated article, such as the sill area of a coated vehicle body. 
     As would be understood by those skilled in this art, various modifications may made to the radiant convection oven of this invention within the purview of the appended claims and the following description of the preferred embodiments and the appended drawings are for illustrative purposes only and do not limit the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side, partially cross-sectioned, partially-schematic view of one embodiment of the radiant convection oven of this invention; 
         FIG. 2  is a partial end elevation of the radiant convection shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As set forth above, the radiant convection oven of this invention utilizes fresh air make-up to transfer heat to the coated object. The fresh air make-up is used to the heat radiators and is then delivered to the oven by the radiators as convection air. In the disclosed embodiment, the fresh air is heated by burners mounted to the oven enclosure instead of a separate heater box. The radiant convection oven of this invention may be used for baking or curing any coating on an object, including but not limited to decorative and protective coatings and adhesives as used, for example, by the automotive industry. Although the radiant convection bake oven of this invention may be utilized for any application, it is particularly useful for mass production applications, such as utilized by the automotive industry to cure paint and adhesives on an automotive body. In order for the radiant convection oven of this invention to heat a coated object at the same rate as a conventional radiant heat oven, it must produce an equivalent amount of radiant and convection energy delivered to an object as does a conventional radiant oven. This is accomplished when the convection air volume, temperature and velocity as well as the size, position, surface temperature and emissivity of the radiators are generally equal. 
       FIGS. 1 and 2  illustrate one embodiment of the radiant convection oven of this invention. As set forth above, however, the disclosed embodiments are for illustrative purposes only and do not limit this invention except as set forth in the appended claims. 
     The embodiment of the radiant convection oven  20  shown in  FIGS. 1 and 2  includes an oven enclosure  21  which may be conventional. A typical paint bake oven used by the automotive industry has a length of about eighty to one hundred feet or greater. A fan or blower  22  shown in  FIG. 1  draws fresh air as shown by arrow  24  preferably through a filter (not shown) and delivers fresh air under pressure through duct  26  to combustion burners  28 . However, as discussed further below, the radiant convection oven of this invention may include only one burner or more than two burners depending upon requirements of the application. A portion of the fresh air goes through the burner or burners  28  as combustion air and a portion may bypass the burner and mix downstream in the combustion chambers  30 . The heated air mixture enters insulated combustion chamber  30  located inside the oven enclosure  21  for distribution to a plurality of radiators. Within a preferred embodiment, the combustion chambers  30  include the insulated walls  32  to avoid overheating light metal parts of the vehicle body that pass close to the combustion chambers  30 . The design of the combustion chambers  30  should be such that there is adequate mixing of the burner heat before air enters the radiators. In the disclosed embodiment, the hot fresh air from the combustion chambers  30  pass through the radiators in one or several passes. A three-radiator configuration is illustrated in  FIG. 1 . 
     The hot fresh air and combustion gases are delivered from the combustion chambers  30  to three types of radiators. The hot fresh air is delivered through the first radiators  34 , which, in the disclosed embodiment, are radiating bottom walls. The hot fresh air is then delivered from the first radiators  34  to the second radiators  36  which, in the disclosed embodiment, are radiating sidewalls. The hot fresh air is then delivered through duct  38  to the third radiators  40  which, in the disclosed embodiment, are horizontal ducts  40  which extend the length of the paint oven as best shown in  FIG. 2 . As will be understood from  FIG. 1 , the primary function of the horizontal ducts  40  is to delivery heated fresh air from the radiators  36  to the coated article through nozzles  44 . However, the heated ducts  40  also radiate heat to the coated article. Thus, the radiant convection oven  20  in the disclosed embodiment includes three radiators, including the radiating bottom walls  34 , the radiating sidewalls  36  and the radiating horizontal ducts  40 . As will be understood, the preferred location and orientation of the radiators will depend upon the application. In this embodiment, wherein a vehicle body “V” is conveyed through the oven on a conveyor “C,” the horizontal radiating ducts  44  are located adjacent the sill area of the vehicle body and hot fresh air is directed to the sill area by the nozzles  44  because of the greater requirement for heating the multi-layered metal sill area as would be understood by those skilled in this art. The nozzles  42  are communicating with the interior of the radiating sidewalls  36  direct heated fresh air through the lighter metal areas of the vehicle body. Thus, as will be understood by those skilled in this art. The location and orientation of the radiators and the nozzles will depend upon the particular application. 
     As set forth above, the volumetric flow rate of the fresh air is relatively small in the disclosed embodiment of the radiant convection oven  20  of this invention compared to the recycling air flow rate in a conventional radiant oven. Thus, the temperature of the air circulating through the oven radiators must be greater than the air circulated through the conventional radiators. Table 1 below compares the radiator flow rate, temperature and velocity for a single, eighty-foot long zone of conventional automotive radiation oven to that of the radiant convection oven of this invention. The temperature and velocity of the radiated air for the radiant convection oven of this invention has been selected to provide radiation heat delivery equal to a conventional design. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Conventional 
                 New 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Heat Delivered by Radiators 
                 BTU/hr 
                 578,700 
                 578,700 
               
               
                 Radiator Air Volume - Actual 
                 acfm 
                 15,000 
                 4,673 
               
               
                 Radiator Air Volume - Standard 
                 scfm 
                 8,325 
                 2,435 
               
               
                 Radiator Air Inlet 
                 F 
                 495 
                 557 
               
               
                 Radiator Surface Area 
                 ft2/ft-oven 
                 10 
                 10 
               
               
                 Radiator Depth 
                 In 
                 16 
                 3 
               
               
                 Radiator Air Velocity 
                 fpm 
                 800-1,500 
                 2,470-3,115 
               
               
                 Sill Convection Air Temperature 
                 F 
                 325 
                 325 
               
               
                 Sill Convection Air Volume - 
                 scfm 
                 2,435 
                 2,435 
               
               
                 Standard 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, the radiator air volume of the radiant convection oven of this invention is significantly reduced compared to a conventional radiator air volume. The radiators for the radiant convection oven of this invention are also designed so that the temperature of the air exiting from the radiators is at the same temperature as the sill convection air for a conventional radiation oven. In this way, the sill convection air nozzles  44  may be of the same design as a conventional radiant oven resulting in equal performance. 
     The radiant convection oven of this invention has several important advantages over conventional radiant paint ovens. First, the reduced volumetric flow rate of the radiators heating air reduces the size of the radiator panels or walls  34 ,  36  within the oven enclosure. This not only reduces the cost of the radiators, but also allows the oven enclosure to be smaller in size, further reducing the cost. The reduced radiator heating air volume further reduces the size of the fan necessary to move fresh air as part of the heating system. Because the heating system is a “once-through system” rather than a recirculating heating system, the fan  22  moves ambient temperature fresh air (from ˜70° F.) instead of radiated outlet temperature air (˜350° F.). This further reduces the size of the fan. Further, because the fan is operating at a lower temperature, it is simpler and less costly construction. The combination of lower air volume and temperature results in a reduced electrical power consumption for the fan, even though the fan for the radiant convection oven of this invention delivers it at a higher pressure. Another advantage of the radiant convection oven of this invention is that the heated box and associated ductwork is eliminated, further reducing costs. The elimination of the heater box and ductwork also eliminates the heat losses associated with its components, further reducing the fuel consumption of the oven and the cost. In addition, the elimination of the heater box eliminates any burn hazard associated with the hot surfaces. 
     As set forth above, various modifications may be made to the disclosed embodiment of the radiant convection oven of this invention within the purview of the appended claims. For example, after exiting the radiators, the fresh air may be directed to a heated ceiling. A heated ceiling is often used at the entrance and exit of a paint bake oven to prevent condensation on the ceiling. Condensation on the ceiling in this area is known to drip onto the object being baked and ruin the finish. If this is done, air exiting the radiators ducted to a hard cavity over the ceiling of the oven entrance or exit. In another alternative design, the air exiting the radiators may be directed to an air seal. An air seal is a system that blows a curtain of air across the open ends of the oven to prevent smoke and fumes from the oven from drifting out of the oven enclosure into the paint shop general area. Another option for controlling the radiation intensity is to manufacture the radiators with an internal shield between the hot air and the radiator surface. This may be accomplished by making a second internal radiator wall in the area where it is desired to reduce the radiation intensity. The hot fresh air may also be channeled through the radiating walls by channels located within the walls to provide a multiple pass radiator. Temperature sensors may also be provided in the combustion chamber to monitor the temperature of the hot air entering the radiator panels. The signal from the sensor is then used to control the heat output from the burners. Finally, as set forth above, a single burner channeling combustion air into the combustion chambers  30  may be preferred in certain applications. However, in a preferred embodiment of the radiant convection oven of this invention, the burners  28  are preferably located on the upper wall of the oven enclosure  21  as shown in  FIG. 1  or on one or both of the sidewalls of the oven enclosure. Having described the preferred embodiment of the radiant convection oven of this invention, the invention is now claimed as found.