Abstract:
A method and apparatus for reducing fuel consumption in conveyor ovens by creating a barrier to the infiltration of heated air from a heating section of the oven into a cooling section of the oven thereby reducing the loss of heated air and reducing consumption of fuel otherwise required to maintain the selected oven temperature. The barrier is formed by directing a stream of pressurized air into a transition section between the heating and cooling sections of the oven.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/574,918, filed May 27, 2004. 

   BACKGROUND OF THE INVENTION 
   The invention relates to product dryers or ovens and in particular to air convection conveying dryers or ovens having a cooling section connected to a heating section. 
   Various types of products are dried, baked or toasted in convection type conveying dryers or ovens such as charcoal, pet foods, fish foods, foods for human consumption such as breakfast cereals and snack foods and other particulate type materials which may also include material in granular and flake forms. As used herein, the terms dryer and oven may be used interchangeably. A typical convection dryer includes one or more conveyer-driven product passes, convection or circulation fans, burners for elevating the temperature within a heating or drying section, and a cooling section. An upper conveyor may receive product to be dried, carry it the length of the drying section for a first product pass and then deposit it upon a lower conveyor that carries the product back through the length of the drying section for a second product pass. The cooling section is often constructed as an extension of the lower portion of the drying section that houses the second, lower conveyor in order to reduce fabrication costs and provide a more compact assembly, as compared to a separate dryer and a separate cooler. In such configurations, the lower conveyor is typically longer than the upper conveyer so that it may project into the cooling section. The conveyors are generally porous or perforated, with pores or openings sized large enough to permit heating and cooling air to pass through the conveyor and the bed of particulate material supported thereon, but small enough to prevent the particulate material from falling therethrough. 
   To initiate the drying process, product is introduced into the drying zone of the dryer and deposited on the conveyor. A process air stream in the drying section consists of a moving stream of heated air that removes moisture from the product as it is carried through the process air stream on the conveyor bed. 
   It is important that the temperature of the process air be controllable to avoid over or under heating of the process air which would lead, respectively, to detrimental effects on the product being dried or reduction in process efficiency and greater energy cost. In addition, it is important that the heated process air be contained in the dryer until exhausted through a dryer exhaust, and not lost to the cooling zone where it would hamper cooling of the product. During a steady state operating condition any lost or exhausted process air from the dryer must be compensated for through the introduction of freshly heated makeup air. 
   Dryers and other types of ovens are available with various airflow configurations, including some that cause air to flow upward through the product (air-up), some that cause air to flow downward through the product (air-down) and various combinations in which the air may flow both up and down through the product bed in different sections of the dryer depending upon the requirements of the end user and the product to be dried and cooled. The purpose of the convection or circulation fan or fans associated with the drying section is to force heated air through the product bed. Therefore, a positive pressure is exerted on one side of the product bed by the air flow from the fans and a negative pressure is created on the other side of the product bed. 
   Typically, a dryer inlet and transition section separates the drying section from the cooling section of the dryer. Cooling sections move fresh air through the dried product to cool it and are generally built in an air-down configuration, such that the air pressure beneath the product bed is less than the air pressure above the product bed during the cooling process. Baffles are usually installed in the transition section in order to minimize the amount of heated air that migrates from the drying section to the cooling section, but such baffles are only partially effective because the product bed is moving along a conveyor and the openings for the conveyor prevent forming of an effective seal between the drying section and the transition section. 
   Because the area beneath the product bed in the cooling section is typically at a relative negative pressure compared to the pressure beneath the bed in an air-up configured drying section, a large amount of heated air tends to migrate from the drying section to the cooling section. This unnecessarily increases fuel consumption in the heating or drying section. 
   SUMMARY OF THE INVENTION 
   The present invention includes the equipment and processes necessary to neutralize the pressure differential between the heating section and the transition section, typically through partial pressurization of the transition section. Partial pressurization of the transition section effectively neutralizes the pressure differential between the heating section and the cooling section. Pressurized air may be delivered to the transition section by a diffuser inserted below an upper or conveying run of a conveyor which extends from the heating section, through the transition section and into the cooling section. The pressurized air for the diffuser may be generated by an auxiliary fan or may be supplied by diverting a portion of the exhaust from the cooling section fan. In one embodiment, the diffuser may include upper and lower projections or baffles to span a substantial portion of the gap between the upper and lower runs of the conveyor to function as a physical barrier to the movement of heated air from the heating section to the cooling section of the dryer. 
   Other advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example embodiments of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal diagrammatic view of a first embodiment of a prior art conveyor dryer. 
       FIG. 2  is a transverse cross-sectional diagrammatic view of a drying section of the conveyor dryer taken along line  2 - 2  in  FIG. 1 . 
       FIG. 3  is a transverse cross-sectional diagrammatic view of a cooling section of the conveyor dryer taken along line  3 - 3  of  FIG. 1 . 
       FIG. 4  is a transverse cross sectional diagrammatic view of a drying section of a second embodiment of a conveyor dryer. 
       FIG. 5  is a transverse cross sectional diagrammatic view of a drying section of a third embodiment of a conveyor dryer. 
       FIG. 6  is a diagrammatic cross-sectional view of a conveyor dryer of the type shown in  FIG. 1 , taken generally along line  6 - 6  of  FIG. 1 , with a transition section pressurization assembly added thereto for providing pressurized air to the transition section. 
       FIG. 7  is an enlarged and fragmentary cross-sectional view taken generally along line  7 - 7  of  FIG. 6 , showing a diffuser of the transition section pressurization assembly extending into the transition section of the conveyor dryer. 
       FIG. 8  is an enlarged and fragmentary cross-sectional view taken generally along line  8 - 8  of  FIG. 6  showing the diffuser in the transition section between upper and lower runs of a product conveyor. 
       FIG. 9  is a longitudinal diagrammatic view of an alternative embodiment of a pressurization assembly for a conveyor dryer of the type shown in  FIG. 1  whereby a portion of the cooling fan exhaust air stream is used to supply relatively pressurized air to the transition section. 
       FIG. 10  is a reduced scale cross-sectional view taken generally along line  10 - 10  of  FIG. 9 . 
       FIG. 11  is a longitudinal diagrammatic view of another alternative embodiment of a pressurization assembly for a conveyor dryer of the type shown in  FIG. 1 . 
       FIG. 12  is a cross-sectional view similar to  FIG. 6  showing an alternative embodiment of a pressurization assembly for a conveyor dryer of the type shown in  FIG. 1 , whereby a portion of the cooling fan exhaust air stream is used to supply relatively pressurized air to the transition section through a diffuser. 
   

   DETAILED DESCRIPTION 
   As required, a detailed embodiment of the present invention is disclosed herein; however, it is to be understood that the disclosed embodiment is merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
   Referring now to the drawings, and with particular reference to  FIG. 1 , there is shown, for the purposes of later comparison to the developments of the present invention, a longitudinal, diagrammatic view of a two-pass, conveyor dryer  1  of the prior art. The conveyor dryer  1  includes a heating or drying section  2  separated from a cooling section  3  by a transition section  4 . Product is conveyed through the dryer on two sets of conveyors, an upper conveyor  6  and a lower conveyor  7 . Each of the conveyors  6  and  7  generally comprises a continuous loop formed from a series of perforated pans linked together. The upper conveyor includes an upper, conveying run  6 A and lower, return run  6 B and the lower conveyor  7  includes an upper, conveying run  7 A and a lower, return run  7 B. 
   Each of the conveyor dryer sections is enclosed by a housing, typically formed of sheet metal, including a drying section housing  10 , a cooling section housing  11  and a transition section housing  12 . The upper conveyor  6  extends from the transition section  4  and through the drying section  2 , from the front to the rear of the drying section house  10 . The lower conveyor  7  generally runs the entire length of the dryer  1  from the rear of the drying section  2 , through the transition section  4  and into the cooling section  3  to a front end thereof. 
   Referring to  FIG. 1  and following the course of product to be dried as it passes through the dryer  1 , the product is first introduced into the dryer  1  through a product spreader  15  into the transition section  4  where it falls upon the conveying run  6 A of upper conveyer  6 . As illustrated in  FIG. 1 , the upper conveyor  6  is moving product from left to right (from front to back). The product falls into the perforated or porous pans of the upper conveyor  6  as the pans pass beneath the spreader  15 . As shown in  FIG. 2 , the product forms a product bed  18  on the pans of conveyor  6 . The product bed  18  on the upper conveyor  6  may be referred to as an upper product bed  18 A and the product bed on the lower conveyor  7  (discussed hereafter) may be referred to as a lower product bed  18 B. 
   The upper conveyor  6  carries the product into the drying section  2 . The rear end of the upper and lower conveyors  6  and  7  extend into a dryer return section  20  at the rear of the drying section  2 . As the product in the upper product bed  18 A is conveyed to the rear end of the upper conveyor  6 , the product falls from the upper conveyor  6  onto the lower conveyor  7 . A guide plate  21 , extending across the end of the drying section  2 , may be utilized to direct the product onto the lower conveyor  7  where it forms the lower product bed  18 B. 
   In the drying section  2 , air is circulated by dryer circulation fans or blowers  25  past burners  26  and through the product bed  18  and the perforated pans forming the upper and lower conveyors  6  and  7  to dry the product. The dryer circulation fans  25  are powered by electric motors  27  mounted below the fans  25 . The drying section housing  10  forms an enclosure of the drying section  2  to retain heated air as it flows past lower and then upper product beds  18 B and  18 A respectively. 
     FIG. 2  is a transverse cross sectional view of the drying section  2  taken along line  2 - 2  of  FIG. 1 , including arrows which illustrate the air flow pattern in the drying section  2 . Due to the action of the dryer circulation fans  25 , an area of relatively low air pressure  30  is created in the portion of the drying section  2  leading from the burners  26  to the fans  25 , and an area of relatively high pressure  31  is created in the area or zone leading from the fans  25  to the lower product bed  18 B. As illustrated, the dryer circulation fans  25  draw air past the burners  26  which heat the air. The heated air is then drawn through the fans  25  and forced outward by the fans  25  between the upper and lower runs  7 A and  7 B of the lower conveyor  7  and then upward through the upper run  7 A of lower conveyor  7 , through the lower product bed  18 B and then through both runs  6 A and  6 B of the upper conveyor  6  and the upper product bed  18 A. Air is then returned across the top portion of the drying section  2 . A portion of the return air is exhausted through a duct or vent (not shown) to remove moisture evaporated from the product bed  18 . The remaining air is circulated past the burners  25  to reheat the air prior to recirculation through the fan and then the product beds  18 . 
   Referring to  FIG. 8 , which is a modified version of the dryer  1 , as shown in  FIG. 1 , incorporating a diffuser as discussed in more detail below, the lower conveyor  7  extends from the drying section  2  into the transition section  4  through a drying section outlet or first passageway  35  extending between the drying section housing  10  and the transition section housing  12 . A drying section baffle  37  typically extends across the drying section outlet  35  between the upper and lower runs  7 A and  7 B of the lower conveyor  7 , to generally separate the drying section outlet into upper and lower dryer section outlet openings  35 A and  35 B. The lower conveyor  7  further extends into the cooling section  3  through a second passageway or cooling section inlet  40  extending between the transition section  4  and the cooling section  3 . The cooling section inlet  40  may, or may not contain its own baffling system similar to what is illustrated in drying section outlet  35 . Referring again to  FIG. 1  with reference to the transition section  4 , as enclosed by the transition section housing  12 , no heating elements or burners are located in the transition section  4 . In addition, no fan or blower or other air circulating means is located in the transition section  4  for circulating air in the transition section. 
   Referring to  FIG. 3 , which is a transverse cross-sectional view of the cooling section  3  of the dryer  1  taken along line  3 - 3  of  FIG. 1 , a cooling fan or blower  44  driven by motor  46  pulls air through the cooling section housing  11  and the product bed  18 B to cool the product on the lower conveyor  7  in the cooling section  3 . The blower  44  creates an area of low pressure  47  on its inlet side causing ambient air to be pulled through vents  52  in the top of the cooling section housing  11 , down through the product bed  18 B and the upper, conveying run  7 A of lower conveyor  7  as indicated by arrows  53 . The cooling air is then drawn into the blower  44  through a blower inlet  54  from between the upper, conveying and lower return runs  7 A and  7 B. The blower  44  then exhausts the air to atmosphere through an exhaust duct  56 . A portion of the air exhausted through duct  56  may be redirected back to the drying section  2 . After cooling, the lower product bed  18 B is conveyed to a discharge section  60  (see  FIG. 1 ) where the product may be offloaded from the dryer  1 . 
   The dryer outlet openings  35 A and  35 B (as shown in  FIG. 8 ) and the cooling section inlet  40  allow heated air from the drying section  2  to migrate through the transition section  4  from the area of relatively high pressure  31  in the drying section  2  to the area of relatively low pressure  47  in the cooling section  3 . The migration of hot air from the drying section  2  to the cooling section  4  decreases the overall efficiency of the dryer  1   
     FIG. 4  is a transverse cross sectional diagrammatic view of an alternative embodiment  60  of the conveyor dryer, having a modified drying section  61 . The drying section  61  includes an inner housing  62  and an outer housing  63 . The inner housing  62  encloses product beds  64 A and  64 B on upper and lower conveyors  65  and  66  respectively. The inner housing  62  is enclosed by the outer housing  63 . Air flow is created by a top-mounted blower  68  powered by motor  69  and which blows air downward in a gap  71  formed between the inner and outer housings  62  and  63 . An opening  73  is formed in the inner housing such that pressurized air generated by blower  68  passes between upper and lower runs  66 A and  66 B of lower conveyor  66  and then upward through the lower product bed  64 B and then through the upper conveyor  65  and the upper product bed  64 A. The air then passes through air balancing vents  75  formed in the floor  76  of heating chamber heating chamber  80  which is contained in the inner housing  62 . In the heating chamber  80  air flows past burners  78  to heat the air. The heated air is then drawn into the blower  68  through an opening in a roof of the heating chamber  80  and distributed back through the drying section in the manner discussed above. An area of relatively high pressure is created in the lower portion of the drying section  61  beneath the upper, conveying run  66 A of the lower conveyor  66 . An area of relatively low pressure is created in the area immediately below the blower  68 . 
     FIG. 5  illustrates a further embodiment  85  of the conveyor dryer as shown in  FIG. 2  in which the burner  86  and blower  87  are mounted on transversely opposing sides of the drying section housing  88 . Each of the embodiments described creates an area of relatively high pressure below the upper, conveying run  66 A of the lower conveyor  7  and typically between the upper conveying run  66 A and the lower, return run  66 B. The pressurized heated air migrates out of the drying section  2 , through the transition section  4 , and toward the area of relatively low pressure in the cooling section  3  created by the cooling fan  44 . 
   To prevent or resist such migration, and with reference to the embodiment shown in  FIGS. 7-9  and  11 , relatively cool pressurized air, typically ambient air, is introduced into the transition section  4  below the upper, conveying run  7 A of the lower conveyor  7  and typically between the upper conveying run  7 A and the lower return run  7 B of the lower conveyor  7 . In a preferred embodiment, as shown in  FIGS. 6-8 , the pressurized air is introduced into the transition section through a transition section pressurization assembly  101  having a diffuser or nozzle  102  which is inserted in the transition section  4  between the upper conveying run  7 A and the lower return run  7 B of the lower conveyor  7 . The diffuser  102  is connected to a pressurization fan or auxiliary blower  103  on the discharge side thereof and the blower  103  is driven by a motor  105 . The blower  103  draws in ambient air and distributes it through the diffuser  102  and into the transition section  4 . As shown in  FIGS. 6 and 7 , the diffuser  102  preferably extends across most or a substantial portion of the width of the transition section  4  and generally distributes an even flow of air along the width of the transition section  4 . 
   The diffuser  102  is formed from a cylindrical duct or conduit  110  with a pair of V-shaped baffles  112  and  113  connected to and extending across a substantial portion of that section of the cylindrical duct  110  positioned within the transition section  4  of the dryer  1 . One of the baffles  112  is mounted on and extends above the duct  110  and the other baffle  113  is mounted on and extends below the duct  110 . To maintain conveyor clearances, the distance between the top of baffle  112  and the bottom of baffle  113  is just shorter than the narrowest distance between the upper and lower runs  7 A and  7 B of lower conveyor  7 . For example, the gap between the baffles  112  and  113  and the upper and lower runs  7 A and  7 B respectively may be approximately three-eighths of an inch. 
   The duct  110  with the baffles  112  and  113  mounted thereon, presents a physical barrier to the flow of air from the transition section  4  into the cooling section  3 . The upper gap may also be described as being formed between the upper edge of the baffle  112  and the lower edge of the path of travel of the upper, conveying run  7 A of conveyor  7  and the lower gap as being formed between the lower edge of the baffle  113  and the upper edge of the path of travel of the lower, return run  7 B of conveyor  7 . It is also foreseen that the duct  110  by itself could be sized and shaped to span a substantial portion of the gap between the upper and lower runs  7 A and  7 B of conveyor  7  to serve as the duct and baffle. It is also foreseen that the structure used to form the baffles  112  and  113  could be formed in various shapes and configurations. As shown in  FIG. 7 , brackets  114  may be used to connect the diffuser duct  102  to the transition section housing  12 . 
   An air discharge opening or slot  115  is formed along a front surface of the duct  110  on the side facing the drying section  2 . Reinforcing straps  118  may be mounted to the duct  110  on opposite sides of and bridging the gap across the air discharge slot  115  to reinforce the duct  110  along the slot  115 . Blowing ambient air into the transition section  4  through duct  110  creates an area of relatively neutral pressure in the transition section  4  in relation to the pressure in the drying section  2  interrupting or resisting the flow of heated air from the drying section  2  through the transition section  4  and into the cooling section  3 , and thereby reducing the amount of heat transferred from the drying section  2  to the cooling section  4  and increasing the efficiency of the system. As shown in  FIG. 8  by the three arrows A extending through the air discharge opening  115  in duct  110 , air is directed or diffuses out of the duct  110  toward the heating section  2  and in a direction opposite to the direction or path of travel of the upper, conveying run  7 A of conveyor  7 . 
   The pressurization fan or blower  103  is preferably mounted on a stand  125 , in which the legs or feet may be height adjustable so that the height of the pressurization assembly may be adjusted vertically to fit the diffuser  102  into an opening typically created in the wall of the transition section  4 . Other blower mounting positions are possible, if required by physical barriers encountered adjacent to the cooling section  3 . 
   Referring to  FIGS. 9 and 10 , an alternative embodiment is shown for distributing a stream of air into, or pressurizing, the transition section  4  to interrupt or resist migration of pressurized hot air from the drying section  2  through the transition section  4  into the cooling section  3 . In the embodiment shown in  FIGS. 9 and 10 , a portion of the air blown out exhaust duct  56  by the cooling fan  44  is redirected into the transition section  4  by a transition duct  130 . The transition duct  130  may be connected to the transition section  4  through an opening  132  cut in a side of the transition section housing  12 . The opening  132  is preferably positioned to deliver the pressurized air between the upper and lower runs  7 A and  7 B of the lower conveyor  7 . It is also to be understood that the transition duct  130  extending off of exhaust duct  56  may be connected to a diffuser extending into the transition section  4 , similar to diffuser  102 . 
   A damper  133  may be incorporated into the transition duct  130  of the embodiment shown in  FIGS. 9 and 10 ), transition duct  138  of the embodiment of  FIG. 11 , or in the diffuser duct  110  of the previously discussed embodiment, to permit control of the air flow therethrough. Control of the transition section air pressure may be manual or may include an automated system wherein sensors and actuators are used to adjust damper position or fan speed to maintain a pressure set point or to provide electronically pre-programmed pressure set-points. 
   Referring to  FIG. 11 , a further alternative embodiment is shown, wherein a separate pressurization blower  135  and blower motor (not shown) are provided for supplying pressurized air through a separate transition section duct  138  into the transition section  4 , through an opening in a sidewall of the transition section housing  12 . The pressurized air from transition duct  130  or duct  138  functions to increase the air pressure in the transition section  4  to resist or interrupt the flow of pressurized hot air from the drying section  2  into the cooling section  3 . The pressurized air delivered through ducts  130  or  138  create a pressurization zone in the lower portion of the transition section  4  to neutralize the pressure differential between the drying section  2  and the transition section  4 . 
     FIG. 12  is a cross-sectional view similar to  FIG. 6  showing an alternative embodiment of a pressurization assembly  140  for a conveyor dryer of the type shown in  FIG. 1 , whereby a portion of the exhaust air from a cooling fan  144  is directed into the transition section  4 , through a transition duct  145  and a diffuser  146 , similar to the diffuser  102  discussed previously. 
   In the embodiments discussed above, the air blown into the transition section  4  is either ambient air or the exhaust from the cooling fan  44 , which is generally warmer than ambient air due to the heat picked up by the air stream in cooling the product on the conveyor  7 . Both the ambient air and the exhaust from the cooling fan  44  are cooler than the heated air from the drying section  2 . 
   In a further embodiment, not shown, heated air leaving the drying section  2  through the dryer outlet  35 , as shown in  FIG. 8 , is allowed to escape the drying section  2  prior to entering the cooling section  3  through provision of an open vent in the side wall of the transition section  4 . In another embodiment, of the type shown in  FIGS. 9-11 , a physical barrier or cooling section baffle (not shown), could be installed to extend across the space between the upper, conveying run  7 A and the lower return run  7 B of lower conveyor  7  generally across the cooler section inlet opening  40 . The cooling section baffle (similar to baffle  37 ) helps to hold the pressurized air delivered through transition duct  130  in the transition section  4 . 
   It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown. For example, although in the embodiments shown, a stream of pressurized air is directed into the transition section  4  between the upper and lower runs  7 A and  7 B of the lower conveyor  7 , it is foreseen that the pressurized area could enter into different locations in the transition section  4 , or in an area that might otherwise be referred to as the front of the cooling section  3 . In addition, although the dryers shown all comprise two pass dryers, it is to be understood that the improvements of the present invention could also be used with a single pass conveyor system or multiple pass systems. In particular, the improvements could be utilized with a single conveyor that extends from the drying section to the cooling section. It is also to be understood that the improvements could be utilized with a system in which the lower, return run of the lower conveyor (or only conveyor) extends out of and runs generally beneath the floor of the dryer housing. In such an application, pressurized air is preferably introduced into the transition section between the upper, conveying run and the floor of the transition section. It is also to be understood that the improvements could be used with ovens which utilize other heat sources, including steam heat, electric heaters and hot water or oil heaters.