Abstract:
A conveyor oven having a first mode of operation and a second mode of operation is provided. The conveyor oven generally includes an oven chamber in which food is cooked, a conveyor movable to convey the food through the oven chamber, a burner to generate heat for the oven chamber, at least one blower to circulate air within the oven chamber, and a controller. The burner has a combustion airflow rate, and operates at a first output during the first mode of operation and at a second output that is greater than the first output during the second mode of operation. The blower operates at a first speed during the first mode of operation and at a second speed that is faster than the first speed during the second mode of operation. The controller is responsive to at least one of a burner output and an internal temperature of the oven chamber, and increases the speed of the blower from the first speed during the first mode of operation to the second speed during the second mode of operation responsive to at least one of an increase of the burner output and an increase of the internal temperature of the oven chamber.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/612,522, filed on Sep. 12, 2012, which is a continuation of U.S. patent application Ser. No. 12/463,051, filed on May 8, 2009, which is a continuation of U.S. patent application Ser. No. 10/546,104, filed on Jul. 19, 2006, which is a U.S. National Phase Application of International Application No. PCT/US04/05153, filed Feb. 20, 2004, which claims priority of U.S. Provisional Patent Application No. 60/449,545, filed on Feb. 21, 2003. 
         [0002]    This application is also a continuation of U.S. patent application Ser. No. 12/463,051, filed on May 8, 2009, which is a continuation of U.S. patent application Ser. No. 10/546,104, filed on Jul. 19, 2006, which is a U.S. National Phase Application of International Application No. PCT/US04/05153, filed Feb. 20, 2004, which claims priority of U.S. Provisional Patent Application No. 60/449,545, filed on Feb. 21, 2003. 
         [0003]    U.S. patent application Ser. No. 13/612,522 published as U.S. Publication No. 2013/0000628 on Jan. 3, 2013; U.S. patent application Ser. No. 12/463,051 published as U.S. Publication No. 2009/0223503 on Sep. 10, 2009; U.S. patent application Ser. No. 10/546,104 published as U.S. Publication No. 2007/0006865 on Jan. 11, 2007; and International Application No. PCT/US04/05153 published as International Publication No. WO 2004/076928 on Sep. 10, 2004. The entire contents of each of the foregoing applications and publications are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0004]    The present invention relates to a gas-fired tunnel oven, and particularly a gas-fired conveyor oven. 
       BACKGROUND 
       [0005]    Commercial, gas-fired tunnel ovens equipped with conveyors produce, among other things, pizzas, cookies, bread, cakes and donuts. Each oven routinely processes a large volume of food products and, as a result, becomes rather dirty. Bits of the food products themselves, burned food products, and soot from the burners are typical sources of contamination that accumulate during use. 
         [0006]    Gas-fired tunnel ovens traditionally have been cleaned manually with detergent and acid solutions. The oven must be taken apart for cleaning by these methods. In addition to the oven walls, roof, and floor, the conveyor used with the tunnel oven must be cleaned, as well as any jet-impingement convection fingers, convection blowers, and fired burners. Cleaning by the traditional methods is tedious and expensive. 
       SUMMARY 
       [0007]    In theory, a gas-fired, commercially-sized tunnel oven might be cleaned by installing electrical heaters at critical points to raise the internal temperature to a range that reduces virtually all contamination to ash. In practice, cleaning a gas-fired tunnel oven by raising the temperature with electrical heaters requires an estimated 50-100 amperes of electricity for each oven. Commercial bakers do not normally have access to this much electrical current, and the cost of installing high current electrical service is a significant financial barrier for most bakers. 
         [0008]    Accordingly, there is a need for a self-cleaning, gas-fired tunnel oven suitable for use with a conveyor that can be cleaned without need of disassembly, manual cleaning, or detergents. Commercial bakers would welcome a self-cleaning, gas-fired tunnel oven. 
         [0009]    In some embodiments, a conveyor oven having a first mode of operation and a second mode of operation is provided. The conveyor oven generally includes an oven chamber in which food is cooked, a conveyor movable to convey the food through the oven chamber, a burner to generate heat for the oven chamber, at least one blower to circulate air within the oven chamber, and a controller. The burner has a combustion airflow rate, and operates at a first output during the first mode of operation and at a second output that is greater than the first output during the second mode of operation. The blower operates at a first speed during the first mode of operation and at a second speed that is faster than the first speed during the second mode of operation. The controller is responsive to at least one of a burner output and an internal temperature of the oven chamber, and increases the speed of the blower from the first speed during the first mode of operation to the second speed during the second mode of operation responsive to at least one of an increase of the burner output and an increase of the internal temperature of the oven chamber. 
         [0010]    Also, in some embodiments, a conveyor oven generally includes an oven chamber in which food is cooked, a conveyor movable to convey the food through the oven chamber, a gas burner configured to generate heat for the oven chamber, at least one blower to circulate air within the oven chamber, and a controller. The gas burner has an adjustable combustion airflow rate and an adjustable gas flow rate. The controller is responsive to at least one of a burner output and an internal temperature of the oven chamber, and increases the combustion airflow rate response to at least one of an increase in the burner output and an increase in the internal temperature of the oven chamber. 
         [0011]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a self-cleaning oven with both first and second conveyor extension sections extended, each of the conveyor sections being depicted without its mesh belt for clarity; 
           [0013]      FIG. 2  is a partial perspective view of the oven of  FIG. 1  showing a drive shaft, a main conveyor section and the first and second conveyor extension sections, each of the conveyor sections being depicted without its mesh belt for clarity; 
           [0014]      FIG. 3  is a side view of the oven of  FIG. 1  with one of the side walls removed, showing one of the blower and motor assemblies and two air-impingement fingers; 
           [0015]      FIG. 4  is a perspective view of the oven of  FIG. 1  with the first conveyor extension section extended and the second conveyor extension section retracted, the first conveyor extension section being shown without its mesh belt for clarity; 
           [0016]      FIG. 5  is a view of the oven of  FIG. 1  with the front access door removed, showing both conveyor extension sections in their retracted positions; 
           [0017]      FIG. 6  is a front elevation view of the oven of  FIG. 1  with both conveyor extension sections retracted; 
           [0018]      FIG. 7  is an overhead perspective view of the oven of  FIG. 1  with the ceiling and overhead insulation removed, showing a roof-mounted burner assembly, two blower and motor assemblies, and a vent arrangement; 
           [0019]      FIG. 8  is a close-up perspective view of the burner assembly of  FIG. 3  showing a gas shut-off valve, a gas valve, an air valve, a valve link that coordinates the action of the gas valve with the action of the air valve, and a burner; 
           [0020]      FIG. 9  is a rear perspective view of the oven of  FIG. 1  showing the two blowers and the vent arrangement; and 
           [0021]      FIG. 10  is a perspective view of walls and a tubular frame support surrounding a cooking chamber of the oven of  FIG. 1 . 
           [0022]      FIG. 11  is a partial perspective view of the oven of  FIG. 1  showing a drive shaft, the main conveyor section and the first conveyor extension section, each of the conveyors depicted without its mesh belt for clarity. 
           [0023]      FIG. 12  is an overhead view of the oven of  FIG. 1  with the ceiling and overhead insulation removed, showing a roof-mounted flame tube assembly and two blower and motor assemblies. 
           [0024]      FIG. 13  is a view of the main conveyor section through an oven opening with one of the conveyor extension sections removed. 
           [0025]      FIG. 14  is a view of the main conveyor section and its connection to the front wall of the oven depicted in  FIG. 1 . 
           [0026]      FIG. 15  is a view of one of the conveyor extension sections separated from the oven of  FIG. 1 . 
           [0027]      FIG. 16  is a side view of the mesh belt of one of the conveyor extension sections. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    According to one embodiment, a pyrolytically self-cleaning, gas-fired, conveyor oven  10 , as shown in  FIG. 1 , includes an oven housing  12  supported on four legs  14 . The oven housing  12  surrounds a cooking chamber  16  through which food products are passed on a conveyor assembly  18 . The oven  10  also includes a front access door  40  that can be opened using a front access door handle  41 . 
         [0029]    As best seen in  FIG. 2 , the conveyor assembly  18  includes powered rollers  20  that drive a wire mesh conveyor belt (not shown in the Figures) that conveys food through the cooking chamber  16 . The powered rollers  20  can be driven in either direction so that, as viewed in  FIG. 2 , the conveyor belt can convey food through the cooking chamber  16  from left-to-right or right-to-left. Food products can be transported by the conveyor assembly  18  into a first oven opening  37  and out of a second oven opening  38  or, alternatively, into the second oven opening  38  and out of the first oven opening  37 . In either case, the motion of the conveyor drive motor (not shown) and, consequently, the motions of the conveyor assembly  18  are precisely and continuously controlled in order to provide the optimum cooking time for the food products. The speed and direction of the conveyor assembly  18  are input by an operator through a control station (not shown). 
         [0030]    As the oven  10  is shown in  FIGS. 1 and 2 , it is configured for cooking food products. That is, the conveyor assembly  18  extends out of the cooking chamber  16  at both ends. Food is placed on the conveyor assembly  18  at either end of the oven  10  and is carried through the cooking chamber  16  to the other end of the oven  10 . As best seen in  FIG. 2 , the conveyor assembly  18  includes a main conveyor section  30  and first and second conveyor extension sections  32 ,  34  extending out of the cooking chamber  16 . Over time, as food products travel back and forth over the conveyor assembly  18 , the various sections  30 ,  32 ,  34  of the conveyor assembly  18  clog with food debris and otherwise become dirty. Additionally, food particles that drop onto various surfaces and components within the cooking chamber  16  become dirty. To clean the oven  10 , the first and second conveyor extension sections  32 ,  34  can be disconnected from the main conveyor section  30  and inserted into the cooking chamber  16 . 
         [0031]    The main conveyor  30  is driven by a direct current electric motor operating through a gear reducer. A pulse-controlled conveyor drive motor (not shown) turns a drive shaft  86  which is rigidly attached to a drive gear  88 , which are depicted in  FIG. 11 . The drive motor sends well-defined pulses of electrical energy to move the drive shaft  86  in either a clockwise or counterclockwise direction. Each electrical pulse of the motor moves the drive shaft  86  a reproducible increment. For example, a single pulse may be adjusted to advance the drive shaft  86  by a predetermined number of angular degrees. The frequency of electrical pules determines the speed of the drive shaft  86 , and consequently the speed of the conveyor assembly  18 , in either direction. The drive gear  88  turns the main conveyor section  30  and the first and second conveyor extension sections  32 ,  34  by means of follower gears  90 ,  92  (only one is shown for the first conveyor extension section  32 ). The follower gears  90 ,  92  cause conveyor axles  110  to turn, which creates the conveyor motion. The speed of all the conveyor sections, and ultimately, the cooking time of food products traveling through the oven  10 , is regulated by the drive motor. The drive motor for oven  10  is controlled by a digital control unit (not shown). 
         [0032]      FIGS. 1 and 2  depict both the first conveyor extension section  32  and the second conveyor extension section  34  in an extended and locked position, the conveyor extension sections  32 ,  34  being both collapsible and extendable. The first conveyor extension section  32  is accompanied by a first insulated door  35  and the second conveyor extension section  34  is accompanied by a second insulated door  36 . Both the first and second conveyor extension sections  32 ,  34  can be separated from the oven housing  12  and inserted into a first oven opening  37  and a second oven opening  38 , respectively. After the conveyor extension sections  32 ,  34  have been inserted into the oven housing  12 , the first insulated door  35  can be shut to close the first oven opening  37  and the second insulated door  36  can be shut to close the second oven opening  38 . 
         [0033]      FIG. 11  is a partial perspective view of oven  10  in which only selected components are shown in order to better communicate the invention.  FIG. 11  shows the relationship of the main conveyor section  30  to the first conveyor extension section  32  when the first conveyor extension section  32  is in the extended position. The first and second conveyor extension sections  32 ,  34  (only one is shown in  FIG. 11 ) each include an upper notch  78 , sized and shaped to receive an upper peg  80 , which is attached to an inside wall of the oven (not shown in FIG.  11 ). The first and second conveyor extension sections  32 ,  34  also each include a lower notch  82  for receiving a lower peg  84 , which is also attached to the inside wall (not shown). Lifting the first and second conveyor extension sections  32 ,  34  causes them to rotate about the upper pegs  80  until the lower pegs  84  disengage from the lower notches  82 . 
         [0034]    With the lower notches  82  disengaged, the first and second conveyor extension sections  32 ,  34  can be separated from oven  10  and inserted into the first and second oven openings  37 ,  38  so that the first and second insulated doors  35 ,  36  close the first and second oven openings  36 ,  38 , as shown in  FIG. 6 . 
         [0035]    In order to assemble the conveyor assembly  18  for baking, the first conveyor section  32  is partially inserted into the first oven opening  37  and locked in an extended position with respect to the main conveyor section  30 . The first conveyor section  32  is locked by inserting the pair of upper notches  78  formed by the sides of the first conveyor section  32  under a pair of upper pegs  80  mounted in the oven  10 . A pair of lower notches  82  also formed by the sides of the first conveyor extension  32  are then rotated onto a pair of lower pegs  84  mounted in the oven  10 . The second conveyor extension section  34  is similarly inserted into the second oven opening  38  and locked in an extended position with respect to the main conveyor section  30 . 
         [0036]    The first conveyor extension section  32  is separated from the oven  10  in  FIG. 13 , providing a close-up view of the first oven opening  37  and the main conveyor section  30 . The drive shaft  86  of the main conveyor section  30  extends between two side plates  96 , although only one of the side plates  96  is visible in  FIG. 13 .  FIG. 13  also depicts five of the six drive sprocket wheels  100  attached to the conveyor axle  110  of the main conveyor section  30 . A mesh belt  102  is shown as an endless chain engaged with the drive sprocket wheels  100 . One of the upper pegs  80  and one of the lower pegs  84 , which cooperate for locking the first conveyor section  32  (not shown in  FIG. 13 ) in an extended position, are also visible in  FIG. 13 . 
         [0037]    The sixth of the six drive sprocket wheels  100  of the main conveyor section  30  is shown in  FIG. 14  along with one of the two side plates  96 . A bracket  106  extends from one of the side plates  96  and is fastened to the front wall  66  for supporting the main conveyor section  30 . The front wall  66  also supports one of the upper pegs  80  and one of the lower pegs  84 . 
         [0038]    A close-up, partial perspective view of the first conveyor extension section  32  is presented in  FIG. 15 . The mesh belt  102  of the first conveyor extension section  32  tends to sag if not supported, as illustrated in  FIG. 16 .  FIG. 15  depicts four guides  108 , which are provided to support the mesh belt  102 . The guides  108  are in turn supported by guide supports  98 , which extend the width of the first conveyor extension section  32 .  FIG. 15  also shows the conveyor axle  110  and the six drive sprocket wheels  100  for the first conveyor section, which are used to facilitate the progress of the mesh belt  102 . 
         [0039]      FIG. 4  shows the second conveyor extension section  34  inserted into the cooking chamber  16  and a second insulated door  36  closed to seal off the second oven opening  38  through which the second conveyor extension section  34  previously extended. 
         [0040]      FIG. 5  further illustrates that the main conveyor section  30  supports the first conveyor extension section  32  when the first conveyor extension section  32  is inserted into the first oven opening  37 . Inserting the first conveyor extension section  32  into first oven opening  37  allows the first insulated door  35  to close the first oven opening  37 . Similarly, the main conveyor section  30  supports the second conveyor extension section  34 , when the second conveyor extension section  34  is inserted into the second oven opening  38  so that the second insulated door  36  can close the second oven opening  38 . With the insulated doors  35 ,  36  closed, the cooking chamber  16  of the oven  10  is completely sealed, as shown in  FIG. 6 . The cooking chamber  16  can then be superheated to approximately 900°, turning all food debris in the oven  10  to ash. When the food debris has been burned and turned to ash, the front access door  40  can be opened using the front access door handle  41  and the ash can be cleaned from the oven  10 . 
         [0041]    As seen in  FIGS. 13-16  each of conveyors includes endless stainless steel mesh belts  102  capable of traveling in either direction and at variable speeds. Crumb trays (not shown) are removably installed underneath the first and second conveyor extension sections  32 ,  34 . 
         [0042]    As food travels through the cooking chamber  16 , it is cooked by the impingement of hot air that is directed at the main conveyor section  30  through nozzles  22  located on fingers  24 . As shown in  FIGS. 2 and 3 , the depicted conveyor oven  10  includes two fingers  24 , a lower finger having nozzles  22  directing air upward at the bottom of the conveyor assembly  18  and an upper finger having nozzles  22  (not seen in  FIG. 2 ) directing air downward at the top of the main conveyor section  30 . The fingers  24  contain an inner distributor plate (not shown) and a perforated outer plate containing the nozzles  22  that collimate the heated air and evenly distribute the heated air across the main conveyor section  30  on which the food products ride. The oven  10  depicted in  FIG. 3  includes two fingers  24  (one above the conveyor and one below), however, the oven  10  can accommodate a number of bottom fingers  24  and top fingers  24 . Any combination or deletion of fingers may be employed. 
         [0043]    The hot air directed through the fingers  24  is heated by a burner assembly  42  (best seen in  FIGS. 7 and 8 ) located under an instrument panel  39  ( FIG. 1 ) on the front of the oven  10 . The burner assembly  42  creates the heat used by the oven  10  during both cooking (baking) and self-cleaning The burner assembly  42  heats the hot air that flows through the fingers  24  to cook food products passing along the conveyor assembly  18 . The burner assembly  42  burns a gas and air mixture at a burner  44 , which shoots a flame down a flame tube  46 . The flame heats the air contained in the flame tube  46 , and the heated air exits the flame tube through an outlet  47  and into a plenum  94 , as seen in  FIG. 12 . The open space of the plenum  94 , located in front of the back wall  70  of the oven  10 , provides the hot air with a directed passageway toward a blower housing  74  where it will be circulated throughout the cooking chamber  16 . 
         [0044]    Because the burner  44  is called upon to satisfy a wide range of heat output requirements, it is necessary to control the flow of gas and air supplied to the burner  44 . While the burner  44  is operating, the flow of both air and heating gas to the burner  44  is modulated by a combined control system. With this combined modulating control system for combustion air and heating gas, optimum combustion conditions within the burner  44  can be maintained approximately constant over a range of heat outputs. With this combined modulating control system, the ratio of combustion air flow to heating gas flow can be optimized and maintained so that combustion is thermally efficient and environmentally sound, producing a minimum of objectionable byproducts. 
         [0045]    The ratio of combustion air to heating gas can be optimized to produce, for example, environmentally clean burning and the ratio will remain close to the optimum value whether the programmable controller (not shown) calls for high heat or low heat. Alternatively, the ratio may be optimized for optimal fuel consumption, optimal heat-up time or any other results that the operator desires and the ratio will not vary substantially with heat output. This modulating control system for combustion air and heating gas over a range of heat output is especially advantageous for a self-cleaning oven, such as the oven  10 , where a range of heat outputs is required. 
         [0046]    The burner assembly  42  includes an actuator  48  that operates an air valve  50  that regulates the amount of air entering the burner  44  from a combustion air blower  52 . The actuator  48  controls the position of the air valve  50  based on signals received from other control instruments and sensors (not shown) included in oven  10 . A valve link  54  coordinates the movement of the gas valve  56  with that of the air valve  50 . The gas valve  56  receives gas from an automatic gas shut-off valve  57  and modulates the flow of this gas so that the ratio of heating gas to combustion air is relatively constant for a wide range of heating loads. The valve link  54  connects the air valve  50  to the gas valve  56  so that as the actuator  48  opens and closes the air valve  50 , the gas valve  56  is correspondingly opened and closed, proportionally mixing the air and gas as they enter the burner  44 . The air and gas mixture is then ignited inside the burner  44  and a flame shoots down the flame tube  46 . 
         [0047]    One of the advantages of modulating air and gas control, as provided by the valve link  54 , is that the amount of excess air in the flame tube  46  remains substantially the same during high and low heating load periods. This advantage is particularly important in a self-cleaning, pyrolitic oven, which exhibits a significantly higher heating load during self-cleaning than conventional cooking heating loads. Although the valve link  54  depicted in  FIGS. 6 ,  7  and  8  is mechanical, it is also contemplated that the positions of an air valve and a gas valve in modulating air and gas control systems may alternatively be coordinated by, for example, utilizing electronically-controlled actuators for each of the valves and coordinating their positions by means of one or more electronic controllers. 
         [0048]    The burner  44  may be mounted anywhere in the oven. Preferably, the burner  44  is roof-mounted as shown and sends its flame along the inside of the flame tube  46  mounted adjacent the ceiling of the oven  10 . During operation of the oven  10 , contaminants tend to accumulate most heavily on and near the oven floors. Thus, the roof-mounted burner system is more likely to progressively incinerate—and less likely to ignite—the floor accumulation as compared to conventional floor-mounted and wall-mounted burner configurations. 
         [0049]    During baking and self-cleaning operations, the flame tube  46  becomes very hot and radiates heat energy throughout the inventive oven  10 . It is contemplated that a diffusing tube (not shown) may be employed around the burner for processing food products that tend to discolor or otherwise deteriorate when subjected to intense radiant heat. 
         [0050]    The oven  10  has two relatively large blowers  26 ,  27  (see  FIG. 7 ) to move the heated air created by the burner  44  through the fingers  24  and onto the product so that the most efficient bake is achieved for each food product processed in the oven  10 . More specifically, the oven  10  employs collimated, vertical air streams to give uniform and intensive heating. The collimated, vertical streams of air that emerge from the fingers  24  provide an exceptional heat transfer rate and generally bake foods faster and at lower temperatures than in conventional convection hot air or infrared heating ovens. 
         [0051]    The hot air is circulated through the oven  10  by the two blowers  26 ,  27  located at the back of the oven  10  (see  FIG. 9 ). The two convection blowers  26 ,  27  are located in the blower housing  74  (see  FIG. 10 ). The blowers  26 ,  27  are each powered by a blower motor  75  (only one is shown in  FIG. 9 ), which is mounted on the back wall  70 , and connected to the blowers  26 ,  27  by a shaft (not shown). In order to protect the shafts from the pyrolitic temperature of the self-cleaning operation, the shafts may be fitted with heat-slingers (not shown) or other cooling apparatuses. A heat slinger is a type of fan arrangement mounted on the shaft. Each blower motor  75  may be equipped with a dedicated speed controller (not shown), preferably including an electrical power inverter. With the benefit of individual speed control, the blowers  26 ,  27  can be individually accelerated and decelerated to optimize electrical current inrush, the burner  44  firing or convective heat loss. The speed of the blowers  26 ,  27  may also be individually controlled in order to create distinguishable heating zones within the oven  10  to optimize the baking of particular food products. 
         [0052]    In another embodiment, the blowers  26 ,  27  may be variable speed blowers that are controlled together so that their speeds, while variable, are always the same as each other. 
         [0053]    There are also two cooling fans  13 ,  15  located on the front of oven  10  as depicted in  FIGS. 1 ,  4 ,  6 , and  7 . These fans blow cool air in through the machinery compartment and out the side walls. The cooling fans  13 ,  15  draw air from the surroundings through the instrument panel  39  for cooling the instruments located behind the instrument panel  39 . A portion of the discharge air from the cooling fans  13 ,  15  may enter the combustion air blower  52  and be delivered to the burner  44  as combustion air. The remainder of the discharge air from cooling fans  13 ,  15  enters passages that extend between the external sheeting of the oven  10  and an inside wall, which supports insulation. The flow of air in these passages serves to cool the external sheeting of the oven  10  below preferably about 125 degrees F. 
         [0054]    Referring to  FIG. 9 , a vent arrangement  58  is located at the back of the oven  10 . The vent arrangement  58  includes a vent valve  60  that is positioned between a vent tube  62  and a T-shaped tube  64  that communicates with the high-pressure sides of the blowers  26 . During a normal cooking cycle, the vent valve  60  is closed so that no air passes through the vent valve  60  into the vent tube  62 . In this way, during cooking, air that is heated is directed solely into the cooking chamber  16  for efficient cooking of food in the cooking chamber  16 . However, when it is desired to clean the oven  10 , the vent valve  60  is opened and the oven openings  37 ,  38  are closed, as discussed above. By opening the vent valve  60 , enough heated air is exhausted through the vent tube  62  to maintain a slight negative pressure within the cooking chamber  16 . In this way, the smoke and soot that develops during a self-cleaning cycle is exhausted through the vent tube  62  and the passage of smoke and soot through small openings and cracks in the oven housing  12  is prevented. 
         [0055]    As shown in  FIG. 10 , the cooking chamber  16  is bounded by a front wall  66  and two side walls  68  that are connected to a back wall  70 . The front wall  66 , two side walls  68 , and back wall  70  are all screwed together to form a box surrounding the cooking chamber  16 . The back wall  70  of this box is fixed to a tubular frame  71 , which is connected to a platform  72 . However, while the back wall  70  of the box is fixed to the tubular frame  71 , the front wall  66 , and two side walls  68  are free-floating. That is, the front wall  66  and two side walls  68  are connected to the back wall  70 , but are not connected to the tubular frame  71 . The perimeters of the front wall  66  and the two side walls  68  include lips  73  that sit on the various members that make up the tubular frame  71 , but are not fixedly connected to those members. In this way, the front wall  66  and two side walls  68  are free to move relative to the tubular frame  71  so that during cooking, and particularly during self-cleaning when the temperatures in the cooking chamber  16  are relatively high, the front wall  66  and side walls  68  of the cooking chamber  16  are free to expand and slide on the members of the tubular frame  71 , thereby preventing buckling and warping of the walls of the cooking chamber  16 . 
         [0056]    A unified display control station (not shown) for the oven  10  can include a blower selector, a heat selector, a conveyor selector, two or more conveyor speed controllers and a digital temperature controller. Additionally, a machinery compartment access panel safety switch disconnects electrical power to the controls and the blowers when the machinery compartment access panel is opened. 
         [0057]    In order to start up the oven  10 , an operator confirms that the front access door  40  is closed. The operator then turns the blower and conveyor selectors to the “on” position. If necessary, the operator adjusts the conveyor speed setting by pushing appropriate selectors on the conveyor speed controller. The operator adjusts the temperature controller to a desired temperature and selects normal operation. A heat switch on a control station (not shown) of the oven  10  activates the combustion air blower  52 . The burner  44  is a direct ignition burner. The main gas valve  57  is opened while starting a spark in the burner  44 . A sensor then monitors whether a flame is present within the burner  44 . If a flame is not detected within 6 seconds, the main gas valve  57  is shut down, the burner  44  is purged, and the ignition cycle is repeated. Referring to  FIGS. 7 and 8 , a gas bypass tube  76  provides enough gas to the burner  44  to maintain a minimum flame even when the gas modulation valve  56  is closed. 
         [0058]    The oven  10  will typically heat to a desired heating set-point temperature within a matter of minutes. While the oven  10  is heating, the control station (not shown) displays the actual temperature. One or more thermocouples (not shown) in the interior of the oven  10  send signals to a programmable controller (not shown) that controls the actuator  48 . If the programmable controller (not shown) calls for more heat, the actuator  48  rotates to open the air valve  50  and more combustion air is permitted to pass from the combustion air blower  52  to the burner  44 . Simultaneously, the valve link  54  moves under the influence of the actuator  48  to further open the gas valve  56 , permitting more heating gas to pass from gas line  55  to the burner  44 . If the programmable controller (not shown) calls for less heat, the valve link  54  causes the air valve  50  and the gas valve  56  to close simultaneously and proportionally. Consequently, the ratio of combustion air flow to heating gas flow entering the burner  44  remains approximately constant over a range of heat output. 
         [0059]    As mentioned, the speed of the blowers  26 ,  27  can be varied. For example, the speed of the blowers may be two-thirds full speed during start-up and self-cleaning cycles and full speed during a cooking cycle to promote heating efficiency during each of the cycles. For heating the oven  10  to baking or self-cleaning temperatures, one or both of the blower motors  75  (only one is shown in  FIG. 9 ) start and routinely ramp up to a desired operating speed in a programmable period of time. Programming the start-up time of convection blower motors  75  makes firing of the burner  44  more reliable and promotes better combustion, among other things. When the blowers  26 ,  27  are turning, the burner  44  is initially fired with a minimum heat output and ramped up to the baking or self-cleaning heat output over a period of time by, for example, a programmable controller (not shown). When the desired heat output has been achieved, the blower motors  75  are accelerated to operating speed in a programmable period of time. 
         [0060]    The start-up procedure (i.e., ramping up the speed of one or both of the blowers  26 ,  27 ) prevents an objectionable current inrush situation that is observed in conventional ovens, which commonly start two or more blower motors at full speed simultaneously. This startup procedure is also quieter, and requires less electricity and heating gas, than the startup of conventional ovens. Because the blowers  26 ,  27  draw more electrical current when the oven is cold and the air in the oven is relatively dense, operating both blowers at low speed during heat-up (start-up) saves electricity. Also, because increased convection on the inside surfaces of the oven walls promotes heat loss to the kitchen, operating only one of the convection blowers during heat-up saves heating gas. 
         [0061]    Preferably, each of the blowers  26 ,  27  is equipped with an electrical power inverter (not shown), which alters the frequency and/or voltage of the electrical current to control the speed of the blower  26  or  27 . In that case, the blower motor  75  can be either ramped up to operating speed over a programmable period such as, for example, about thirty minutes, or held at an optimal intermediate speed until the oven  10  reaches baking or cleaning temperature and then accelerated. These variations conserve still more energy by providing appropriate programmable blower speeds depending on the current operation of the oven  10 . When the oven  10  is, for example, baking (cooking), self-cleaning, warming up, or cooling down, the blowers  26 ,  27  can operate at specific speeds best suited for each individual activity. 
         [0062]    Furthermore, for baking, the speed of the blower motors  75  (only one is shown in  FIG. 9 ) may be separately adjusted to create two or more different heating zones (not shown) within the oven  10 . These heating zones (not shown) can be created at will and utilized to optimize the baking process and, consequently, the finished quality of a particular food product. The oven  10  may be equipped with two or more thermocouples (not shown) or other temperature sensors to individually monitor and adjust these heating zones (not shown). The manner in which the signals from these thermocouples (not shown) are averaged or otherwise interpreted by the programmable controller can be varied to suit the food product. 
         [0063]    In order to shut down the blowers  26 ,  27 , the operator selects standby on the control station. The blowers  26 ,  27  will remain in operation until the oven  10  has cooled to below 200 degrees F. and then cease turning. 
         [0064]    When it is determined that the oven  10  should be cleaned, it is cooled to a temperature below about 140 degrees F. The operator then disengages the first conveyor extension section  32  and withdraws the first conveyor extension section  32  from the first oven opening  37 . The first conveyor extension section  32  is then inserted into the first oven opening  37  so that the first conveyor extension section  32  is supported by the main conveyor section  30  and the first insulated door  35  closes the first oven opening  37 . The second conveyor extension section  34  is similarly separated from the oven  10  and inserted into the second oven opening  38  and the second insulated door  36  is closed. Because the first and second conveyor extension sections  32 ,  34  are inserted into the interior of the oven  10 , they are cleaned by pyrolitic heat during the self-cleaning cycle. The vent valve  60  (best seen in  FIG. 9 ) is opened and the blowers  26 ,  27  are then brought up to operating speed and the burner  44  is fired to raise the oven  10  to self-cleaning temperature. During the self-cleaning cycle, oven  10  operates under the control of temperature sensors and controllers (not shown) that are specifically designed to operate in the range of about 650-1000 degrees F. These may be the same sensors and controllers used for baking (not shown) or a separate set. 
         [0065]    In either case, the programmable control system actuates a set of safety interlocks adapted for cleaning temperature operation. For example, the oven overrides the baking cycle high temperature shutdown limits, which are typically set at values less than 600 degrees F. As another example, the programmable control system actuates door locks that deter people from opening the oven doors during the pyrolitic self-cleaning cycle. 
         [0066]    The programmable controller also initiates corrective action if unsafe or undesirable conditions are detected. For example, upon detecting excessively high temperatures, high smoke levels or low oxygen levels within the oven, the programmable controller shuts down the burner  44  and the blowers  26 ,  27 . 
         [0067]    As mentioned, during cleaning, the interior of the oven  10  is kept under a negative pressure compared to the surrounding atmospheric pressure. In the illustrated embodiment the opening of the vent valve  60  and the operation of the blowers  26 ,  27  create the negative pressure in the interior of the oven  10 . As mentioned earlier, when the vent valve  60  is opened and the blowers  26 ,  27  are operating, enough circulating hot air escapes through the vent valve  60  to create the negative pressure inside the cooking chamber  16  necessary to force the smoke and soot created during the cleaning cycle through the vent tube  62 . In another embodiment, an inducer blower (not shown) maintains the interior of the oven  10  under a negative pressure during cleaning as compared to the surrounding atmospheric pressure. The inducer blower creates this negative pressure by drawing air from the blower housing  74 . The blowers  26 ,  27  actually assist the inducer blower in creating this negative pressure because the discharge flow from the blowers  26 ,  27  is impelled directly into the inducer blower. The combined effect is similar to that of a two-stage blower. The discharge flow from the inducer blower is sent to the vent arrangement  58 . 
         [0068]    The inducer blower could also take suction from the interior of the oven  10  during normal baking The entry of the inducer blower opens directly into the blower housing  74 . The inducer blower may be positioned directly in the path of the discharge air flow from each of the blowers  26 ,  27  so that the two sets of blowers work in tandem to reduce the pressure in the interior of the oven  10 . Alternatively, the inducer blower may be mounted anywhere in the interior of the oven  10 . The discharge flow of air from the inducer blower is sent to the vent arrangement  58  for disposal. 
         [0069]    Maintaining negative pressure in the interior of the oven  10  during both cooking and self-cleaning enhances energy efficiency and safety. Maintaining negative pressure in the interior of the oven  10  during the cooking and self-cleaning operations insures that little or no heated air escapes to the kitchen. Minimizing heated air loss makes the oven  10  more energy efficient. Any loss or discharge of heated air from the interior of the oven  10  necessitates the combustion of additional heating gas. By directing all exhaust flows from the oven  10  to the vent arrangement  58  and ultimately the vent tube  62 , the loss or discharge of heated air can be better controlled and minimized. Also, the negative pressure system promotes safety because negative pressure retains burning gases in the interior of the oven  10  rather than permitting them to escape into the kitchen. Additionally, maintaining negative pressure in the oven  10  tends to prevent any smoky residue from building up on the exterior of the oven  10  during normal cooking and self-cleaning operations. The exterior surfaces of the oven  10  remain clean longer because they are not subjected to smoke, which commonly escapes from the atmospheric cooking chambers of conventional ovens. 
         [0070]    The blowers  26 ,  27  turn at a relatively low speed during a first incineration period of the cleaning cycle. This low speed uniformly distributes heat throughout the interior of the oven  10  while minimizing convective heat loss through the walls of the oven  10 . The first incineration period generally continues for about one hour, although it may be longer or shorter based on factors such as the cleaning temperature and the amount and type of contamination in the oven  10 . 
         [0071]    During a second incineration period, which is generally about one to three hours in duration, the blowers  26 ,  27  operate at a relatively higher speed to promote complete incineration of the contamination or debris accumulation. The temperature of the oven  10  is increased to a peak temperature at least once during the second incineration period. 
         [0072]    After the incineration periods, the programmable controller cools the oven, disengages the safety interlocks and arranges the control system for cooking operation. Due to the combination of high temperature and convective air flow in the inventive oven during the self-cleaning cycle, any contamination accumulation that is in the oven is reduced to harmless and sterile ash. This ash may be collected on drip pans provided for that purpose, which can be accessed through the front access door  40  and carried away to disposal. Alternatively, the ash may be collected in a vacuum cleaner system that is built into or independent of the inventive oven. 
         [0073]    It is contemplated that collection of the ash from the lower fingers may be facilitated by constructing the mesh belt  102  of the main conveyor section  30  so that it is close to or touching the perforated plates of the lower fingers  24 . The mesh belt  102  thus pushes or scrapes the ash from the lower fingers  24  for collection by a drip pan or vacuum system. Preferably, the perforations are formed so that the lower fingers  24  present a nonabrasive surface to the mesh belt  102 . 
         [0074]    Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.