Patent Publication Number: US-8535054-B2

Title: Convection combustion oven

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. patent application Ser. No. 11/701,254, filed Feb. 1, 2007, which claims priority to Provisional patent Application Nos. 60/839,082, filed Aug. 21, 2006; 60/807,875, filed Jul. 20, 2006; and 60/814,632, filed Jun. 16, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates toward an inventive oven for curing coatings applied to an object. More specifically, the present invention relates to a convection combustion oven having a simplified design for curing coatings applied to an object. 
     Various types of ovens are used to cure coatings, such as, for example, paint and sealers, that are applied to articles in a production setting. One example is decorative and protective paint that is applied to automotive vehicle bodies in a high volume paint shop known to process vehicle bodies at rates exceeding one per minute. 
     A typical oven uses combustion fuel to provide the necessary amount of heat to cure paint applied to a vehicle body. Generally two types of ovens are presently used, a convection oven and a radiant heat oven. Occasionally, a combination of convection and radiant heat is used in a single oven to meet paint curing specifications. A convection heat oven makes use of a heat source such as natural gas flame that heats pressurized air prior to delivering the heated air to an oven housing. A first type of convection heating applies combustion heat directly to pressurized air prior to delivery to the oven housing mixing combustion gases with the pressurized air. A second type of convection heating uses an indirect heating process where combustion heat is directed into a heat exchanger that heats the pressurized air without mixing the combustion gases with the pressurized air. 
     An alternative source of heat is provided inside the oven housing by a radiant heater that transfers heat to the vehicle body by way of proximity to the vehicle body. As known to those of skill in the art, a radiant heater is generally a metal panel that is heated by circulating hot air into a space located behind a radiator. 
     The conventional convection and radiant ovens have proven to be exceedingly expensive to construct and do not provide energy efficiencies desirable in today&#39;s high-cost energy market. A conventional oven design is generally shown at  10  in  FIG. 1 . The conventional oven assembly  10  generally includes two main components, a heater box  12  and an oven housing  14 . The heater box  12  is generally spaced from the oven housing  14  and includes components (not shown) to provide heat and pressurized air to the oven housing  14  through hot air duct  16 . The heater box  12  includes a return duct that draws a significant portion of air from the interior of the oven housing  14  for recirculation through the oven housing  14 . Up to ninety percent of the air passing through the heater box  12  is derived from the interior of the oven housing  14  through return duct  16 . Generally, only ten percent of the air delivered to the oven housing  14  through hot air duct  16  is fresh air drawn from outside the oven housing  14 . Hot air is directed through hot air headers  20  toward the vehicle body through nozzles  22  to optimize a uniform heat transfer to cure the coating applied to the vehicle body. Generally, the vehicle body is heated to about 275-340° F. at a predetermined time to adequately cure the applied coating. Some coatings, such as electrodeposition primers, require temperatures at the higher end of this range. As is known to those of skill in the art, more heat must be directed toward heavy metal areas of the vehicle body to derive the desired baking temperature. 
     A typical oven zone of about eighty feet in length of a conventional oven requires an actual air volume of about 30,000 cfm when using a heater box. This high air volume is required to transfer the necessary heat to the vehicle body to cure the applied coating. The air temperature at the nozzle  22  in a conventional oven is generally 444° F. requiring an air velocity at the nozzle  22  of 4,930 fpm to transfer the desired amount of heat energy. The operating parameter set forth above generally provides 1,595,000 BTU/hr at a momentum of 4.9×10 6  ft-lb/sec 2 . Because hot air is recirculated by the fan located in the heater box  12 , and because the recirculated air is often reheated prior to being pressurized by the fan, the fan requires an overlying robust design adding to operation and installation costs. 
     The volumes and flow rates presently used in conventional ovens require heavy duty fans and heater systems that are not believed necessary to obtain the required heat transfer. This is in part due to the recirculation of hot air through the fan and back into the oven housing  12 . Furthermore, due to the recirculation, a substantial amount of insulation  24  is required around the heater box  12  and the hot air duct  16  to reduce heat loss and protect workers from physical contact. Therefore, it would be desirable to design a simplified oven assembly that does not require extensive insulation and complex apparatus associated with conventional heater boxes. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an oven assembly for curing a coating applied to an article being conveyed through the oven assembly. A transporter extends through an oven housing for conveying the article through the oven assembly. A fan provides pressurized air into the oven housing drawn substantially from outside the oven housing. A duct includes a first element extending into the oven housing and a second element interconnected with the fan for transporting pressurized air from the fan into the oven housing. A burner is disposed generally between the first element and the second element for heating the pressurized air being transported into the oven housing. The first element defines a plurality of air outlets spaced throughout the oven housing for directing heated air toward the article. The first element is substantially insulated inside the oven housing reducing the escape of heat generated by the burner from the duct except through the air outlet. The burner heats the pressurized air being directed into the oven housing to a temperature of about three times the curing temperature of the coating that is applied to the article. 
     The inventive oven assembly solves the problems associated with the prior art, or conventional oven assembly. Particularly, the size of the ventilator or fan used to provide pressurized air to the oven housing for transferring heat to the article being baked is significantly reduced for two reasons. First, the fan primarily draws ambient temperature air as the present design does not circulate heated air back into the oven housing and, therefore, does not need to be heat resistant. Furthermore, the heater or burner used to heat the ambient temperature air prior to the introduction to the first element of the duct is configured to heat the air to about two to four times the curing temperature of the coating applied to the vehicle body adjacent the oven housing. This temperature air, when introduced to the oven interior at a high nozzle velocity, reduces the air volume of a conventional 80 foot long oven zone from about 30,000 acfm to about 2,000 scfm. At this combination of air volume, air temperature, and air velocity, a substantially similar amount of BTUs per hour is delivered to the oven as a conventional oven while using less energy to drive the ventilator and having a significantly simplified ventilation and heating apparatus. Specifically, the complex heater box presently used in conventional ovens is no longer necessary and is, therefore, completely eliminated substantially simplifying the construction and design of a production oven. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  shows a prior art oven assembly; 
         FIG. 2  shows an inventive oven assembly having two heaters positioned on opposing sides of an oven housing of the oven assembly, wherein each heater provides heat to opposing first elements; 
         FIG. 3  represents the spaced locations of nozzles on an upper header and a lower header of the oven assembly; 
         FIG. 4  represents a cross-sectional view of one of the upper header and lower header; 
         FIG. 5A  represents a perspective view of the shape of the nozzles; 
         FIG. 5B  shows an alternative nozzle having a swivel; 
         FIG. 6  shows an alternative nozzle in the form of an eductor or venturi nozzle; and 
         FIG. 7  shows still another alternative embodiment of the nozzle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an inventive oven assembly is generally shown at  30 . The oven assembly includes an oven housing  32  through which an article such as, for example, a vehicle body  34  is conveyed on a transporter  36 . The transporter  36 , as is known to those of skill in the art, is generally designed as a conveyor that conveys a carrier  38  upon which the vehicle body  34  is secured. 
     In a production paint shop, a coating is applied to the vehicle body  34  providing decorative and protective paint finish to the vehicle body  34 . Different coatings have different baking or curing requirements that, along with vehicle body type and production volume, dictate the length and thermal requirements of the inventive oven assembly  30 . For example, electrodeposition primers typically cure at about 340° F. for about twenty minutes and decorative top coat and clear coats cure at about 285° F. also for about twenty minutes. For simplicity, the explanation of the inventive concepts of the present oven assembly  30  will assume a typical eighty foot long oven zone requiring a delivery of heat of about 1,595,000 British thermal minutes per hour (BTU/hr). 
     Pressurized air is delivered into the oven housing  32  through a duct  40  by a ventilator  42 . Preferably, the ventilator  42  is a conventional fan capable of providing the transfer of ambient air at a volume of about 2,000 scfm. The duct  40  includes a first element  44  generally extending inside the oven housing  32  and a second element  46  generally extending from the ventilator  42  to the first element  44 . A heater  48  is disposed between the first element  44  and the second element  46  to provide heat to the pressurized air passing through the duet  40  as delivered by the ventilator  42 . Preferably, the heater  48  is a gas fired burner sized to provide the desired amount of heat to the pressurized air passing through the duct  40  to adequately cure the coating applied to the vehicle body  34 . However, it should be understood by those of skill in the art, that alternative heaters may also be used to provide heat to the pressurized air as set forth above. 
     As will be explained further below, the heater increases the temperature of the pressurized air to about 1,100° F. or hotter. One range contemplated is between about 700° and 1,100° F. The desired temperature is selected to be between about two and four times the curing temperature of the coating as will be explained further below. The heater is located, preferably, adjacent to or nearly adjacent to the oven housing  32  so that the heated, pressurized air travels only through the interior of the oven housing  32 . This reduces the need to insulate the duct  40 , or more specifically, the second element  46  of the duct  40  further reducing assembly costs. However, insulation  50  covers the first element  44  of the duct  40  inside the oven housing  32  to prevent the escape of heat through the first element  44  into the oven housing  32  except where desired. 
     The oven assembly  30  represented in  FIG. 2  shows two heaters  48  positioned on opposing sides of the oven housing  32 , each providing heat to opposing first elements  44 . Therefore, the first element  44  of the duct  40  is disposed on opposing sides of the vehicle body  34  being transported through the oven housing  32 . However, it should be understood that a single heater  48  is contemplated to provide heat to each of the opposing first elements  44  of the duct  40  by locating the heater  48  generally midway between each of the opposing first elements  44 . 
     Each first element  44  defines an upper header  52  and a lower header  54  that extend in a generally horizontal direction. Nozzles  56  are spaced along each of the upper header  52  and lower header  54  through which pressurized, heated air is projected toward predetermined locations on the vehicle body  34 .  FIG. 3  best represents the spaced locations of the nozzles  56  on the upper header  52  and lower header  54 , the configuration of which will be explained further below. As best represented in  FIG. 3 , a feed header  58  extends between the heater  48  and the lower header  54  of the first element  44 . The feed header  58  serves as a mixer providing distance between the first of the nozzles  56  and the heater  48  so that the combustion gases produced by the heater  48  have ample time to mix with the pressurized air provided by the ventilator  42 . In this example, about eight feet in length of the feed header  58  has proven to provide ample mixing time for the combustion gases generated by the heater  48  in the pressurized air provided by the ventilator  42  for an eighty foot oven zone. Different size oven assemblies with different heat requirements may require different lengths of feed headers  58 . The first element  44  shown in  FIG. 3  shows in connection serially, the feed header  58  with the lower header  54 , which is connected to the upper header  52  by a connection header  60 . In this configuration, the pressurized air travels a single path through the feed header  58  to the lower header  54 , through the connection header  60 , terminating at a distal end  62  of the upper header  52 . It should be understood by those of skill in the art that a heater  48  placed in a lower portion of the oven assembly  30  connects first to the upper header  52  via feed header  58  reversing the direction of the pressurized air through the first element  44 . 
     Referring again to  FIGS. 2 and 3 , vertical temperature probes  68  extend downwardly from the roof of the oven housing  32  to measure the interior temperature of the oven housing  32 . The vertical temperature probes  68  communicate with a controller (not shown) that signals the heaters  48  to adjust, when necessary, the interior temperature of the oven housing  32 . Horizontal temperature probes  70  are spaced below the vertical temperature probes  68  and measure temperature in a similar manner as the vertical temperature probes  68  the temperature of the oven in the lower regions of the housing  32 . Header temperature probes  72  extend into the feed header  58  to measure the temperature of the pressurized air inside the feed header  58  in a manner similar to that explained for the vertical temperature probe  68  above. Each of the probes interact with the controller to control the temperature of the interior of the oven housing  32 . Additional header temperature probes  72  may be spaced along the second element  46  if necessary. For faster response, vertical or horizontal probes  68 , 70  may be located directly in front of a nozzle  56 , spaced from the nozzle  56  between one to three feet. 
     Referring to  FIG. 4 , a cross-sectional view of one of the upper header  52  and lower header  54  is shown. As set forth above, insulation  50  surrounds a header wall  74  reducing the heat loss through the header wall  74  into the oven housing  32 . The nozzles  56  are located inside the header wall  74  and define a decreasing diameter from a distal end  76  toward a terminal end  78  located generally adjacent the header wall  74 . Therefore, the nozzle  56  defines a generally concave, frustoconical shape so that the pressurized air passing through the nozzle  56  accelerates due to decreasing area upon exit from the first element  44 . The shape of the nozzles  56  is best represented in the perspective view shown in  FIG. 5A .  FIG. 5B  shows an alternative nozzle  57  having a swivel  80  that allows the alternative nozzle  57  to be articulated inside the first element  44  enabling the pressurized air to be directed to the predetermined location in a more accurate manner. 
     An alternative nozzle in the form of an eductor or venturi nozzle is shown at  82  in  FIG. 6 . The eductor  82  is shown in  FIG. 6  having a mating surface  86  that is affixed to header wall  74  outside of the header  52 ,  54 . The mating surface  86  defines a pressurized air inlet  88  that receives pressurized air from one of the upper and lower header  52 ,  54 . The pressurized air passes through venturi chamber  90  and exits the eductor  82  through eductor nozzle  92  directing the pressurized air toward the predetermined location of the vehicle body  34  as set forth above. Hot air is drawn from the interior of the oven housing  32  through venturi inlet  94  and is forced into the eductor nozzle  92  by the pressurized air passing through the venturi chamber  90  via venturi effect as is known. This increases the volumetric flow of air toward the predetermined location of the vehicle body  34  further reducing the energy requirements of the ventilator  42 . 
     A further embodiment nozzle is shown as an air amplifier  96  at  FIG. 7  where like numerals will be used with  FIG. 6  for simplicity. The air amplifier  96  includes an air inlet  88  where pressurized air is forced from one of the upper and lower headers  52 ,  54 . The pressurized air passes through the venturi chamber  90  and into the amplifier nozzle  92  and directs the pressurized air toward a predetermined location of the vehicle body  34 . Heated air is drawn from the interior of the oven housing  32  through venturi inlet  94  via the venturi effect causing an increase in the volumetric flow of heated air directed toward the vehicle body  34  again reducing the energy requirements of the ventilator  42 . 
     The embodiments set forth above are desirable to heat heavy metal areas of the vehicle body  34 , which have higher heat requirements than thin or sheet metal areas of the vehicle body  34 . In these embodiments, the eductor  84  and the air amplifier  96  are each directed at a predetermined location of the vehicle body drawing heated air from inside the oven housing  32  maximizing the amount of heat energy directed toward the heavy metal area of the vehicle body  34 . As explained above, pressurized air passes through the header  52 ,  54  through air inlet  88  and into the venturi chamber  90  prior to exiting through the nozzle  92 . Hot air is drawn into venturi inlet  94  via the venturi effect increasing the volumetric flow rate of hot air being directed toward the vehicle body  34 . 
     Table 1 shows the operational parameters of the inventive oven assembly  30  that provides the benefits set forth above. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Conventional Oven 
                 New Oven 
                 New Oven 
                 New Oven 
               
               
                   
                 Nominal 
                 Nominal 
                 Low Velocity 
                 High Velocity 
               
               
                   
                 Design 
                 Design 
                 Case 
                 Case 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Heat Delivered 
                 BTU/hr 
                 1,595,217 
                 1,595,217 
                 1,595,217 
                 1,595,217 
               
               
                 Momentum Delivered 
                 ft 
                 1,365 
                 1,365 
                 836 
                 1,643 
               
               
                   
                 lbm/sec2 
               
               
                 Delivery Volume - Actual 
                 acfm 
                 30,000 
                 6,000 
                 6,000 
                 6,000 
               
               
                 Delivery Volume - Standard 
                 scfm 
                 17,584 
                 2,000 
                 2,000 
                 2,000 
               
               
                 Air Delivery Temperature 
                 F. 
                 444 
                 1,100 
                 1,100 
                 1,100 
               
               
                 Number of Nozzles 
                   
                 72 
                 72 
                 44 
                 97 
               
               
                 Nozzle Diameter 
                 in 
                 4.528 
                 0.676 
                 1.100 
                 0.531 
               
               
                 Air Velocity at Nozzle 
                 fpm 
                 3,727 
                 32,000 
                 20,000 
                 40,000 
               
               
                 Nozzle Velocity/Volume 
                 l/ft2 
                 9 
                 401 
                 150 
                 650 
               
               
                 Nozzle Velocity/Area 
                 l/ft-sec 
                 556 
                 219,000 
                 50,000 
                 427,000 
               
               
                 Air Volume/Oven Length 
                 scfm/ft 
                 220 
                 25 
                 25 
                 25 
               
               
                   
               
            
           
         
       
     
     The data shown in Table 1 is based upon a typical 80 foot long oven section (i.e., heat up zone) at a typical vehicle body  34  production rate. In each example, the required heat delivery is about 1,595,000 BTU/hr. The first column shows the various operating requirements to produce the heat required in a conventional oven design and the following columns indicate the inventive oven nominal design, with a lower limit velocity and an upper limit velocity establishing the general operating range. 
     Most notably, a significant reduction in the standard delivery volume is realized in standard cubic feet per minute (ambient temperature). Those of skill in the art will understand that delivery volume in a conventional oven is generally 30,000 acfm because hot air is recirculated through the oven by the heater box  12  shown in  FIG. 1 . Therefore the reduction in delivery volume enabling a significant reduction in fan capacity is actually from 30,000 acfm to 2,000 scfm. To maintain the required heat delivery at the reduced delivery volume, the air delivery temperature at the nozzles  56  is increased to about 1,100° F. in the new oven design exceeding the conventional air delivery temperature at a conventional nozzle  22  of about 444° F. Additionally, the nozzle diameter is reduced from a conventional diameter of about 0.38 feet to about 0.06 feet resulting in an increase in air velocity at the nozzle from 3,727 fpm to about 32,000 fpm in the nominal oven assembly  30 . This provides a nominal nozzle velocity per area of nozzle of about 219,000 ft-sec, much higher than the conventional nozzle velocity per area of about 556 ft-sec. Therefore, the inventors have determined that a momentum requirement for delivering heat energy remains constant when pressurized air is delivered at up to three times higher than the curing temperature of the coating applied to the vehicle body at higher air velocities and significantly lower delivery volume. Based upon studies, it is believed that temperatures of between two and four times the curing temperature in Fahrenheit degrees with a coating applied to the vehicle body is a preferred operating range while still providing enough heat energy to cure or bake the coating applied to the vehicle body. Furthermore, the ratio set forth above makes use of an air velocity to air volume ratio at the nozzles  56  of between about 150 and 650 to 1, with a nominal ratio of about 400 to 1. Furthermore, the ratio of air velocity in feet per second to a nozzle area is determined to be between about 50,000 and 400,000 to 1, with a nominal velocity of about 220,000 to 1. 
     Further operating parameters proven to achieve desired heat and momentum requirements include providing the volume of air to the oven housing at less than about 25 scfm per foot of oven housing. An alternate embodiment provides a volume of air to the oven housing of less than about 50 scfm per foot of oven housing. A still further alternate embodiment provides a volume of air to the oven housing at a rate of about 75 scfm per foot of oven housing. This is significantly less than a conventional oven design which requires about 220 scfm per foot oven length, requiring higher energy usage than the inventive oven assembly  30 . 
     An additional benefit of heating the pressurized air to about 1,100° F. is the ability to clean the oven  30  by combustion of coating byproducts known to coat oven walls. This eliminates the need to manually wash oven walls, which is labor intensive. 
     The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather, than of limitation. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, wherein reference numerals are merely for convenience and are not to be in any way limiting, the invention may be practiced otherwise than as specifically described.