Abstract:
A continuous controlled atmosphere brazing system includes a dry-off oven for driving off moisture from materials to be brazed, a pre-heat section for pre-heating the material and a brazing furnace for brazing the materials. Heated gas produced in the pre-heat section and the brazing furnace is conveyed along a flowpath to the chamber of the dry-off oven to provide substantially the sole source of heat for the oven. The flowpath for the heated gas is defined by a series of tubes in communication with each component of the brazing system and intermediate manifolds disposed between components. The discharge mouths of the tubes open into the oven chamber at the suction side of recirculation fans operating within the oven. The overall temperature of the oven can be regulated by controllably mixing ambient air with the heated gas in relation to the oven temperature.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to now abandoned provisional application, Serial No. 60/384,049, filed on May 29, 2002, having the title “Waste Energy Recuperation System”, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to waste energy or heat recovery system for use with controlled atmosphere environments, such as a controlled atmosphere brazing system. The invention particularly pertains to a system for recovering heat from the heated sections of the brazing system for use in a dry-off oven. 
     Brazing is a commonly used technique for joining metal parts with close fitting joints. Typically, a flux material is disposed at the location of the joint and then melted within a furnace or oven to flow into the gap between adjacent parts. Most commercial brazing operations are carried out on a continuous conveyor belt that passes through heated sections, or furnaces, of the brazing system. The furnaces are usually fitted with a muffle disposed within a refractory structure. The muffle is heated by the use of natural gas and/or electric heating elements outside the muffle. The brazing environment within the muffle is maintained as a controlled atmosphere, meaning that the atmosphere is maintained to facilitate the brazing process and to prevent oxidation or coloration. Typically, the controlled atmosphere is maintained by continuously pumping nitrogen into the muffle. 
     In order to ensure an optimum braze, it is first necessary to eliminate any moisture from the metal parts of the flux. Thus, most brazing systems include a dry-off or dehydration oven between the fluxer and the controlled atmosphere furnaces. The purpose of the dry-off oven is to raise the core part temperature sufficiently high to evaporation off all moisture from the part. Typically, a dry-off oven will raise the part temperature to 150° C. (350° F.) in an air atmosphere. 
     Thus, it can be appreciated that the standard continuous belt brazing system will include a number of ovens or furnaces that include their own heating element(s). The braze furnace will often be heated by a combination of gas fired zones and electric zones. Where a pre-heater is employed to increase the flux and core temperature, the pre-heater furnace will usually be gas fired, but may also include additional electrically heated zones. Finally, the conventional dry-off oven can be gas fired and/or electrically heated. Each one of these units produces hot gas and products of combustion that must be exhausted to the outside of the building housing the brazing system. These hot gases require special handling, such as alloy ducts, insulated ducts and high temperature exhaust fans. Moreover, energy contained within these hot gases is lost to the atmosphere. 
     Much effort has been expended to make the brazing process more efficient and to reduce the overall energy requirements for the brazing system. More efficient gas-fired or electrical heating elements have reduced the fuel requirements and provided more efficient heating of the various sections of the brazing system. Improved venting systems are better able to discharge the waste gases from the various brazing system sections. However, there remains a need for even greater improvements to the heating of the brazing system and to the handling of the waste gases associated with the system. 
     SUMMARY OF THE INVENTION 
     In order to address these needs, the present invention contemplates a waste energy, or heat, recovery system that is integrated into a serial closed-atmosphere process. A system of pipes extract hot gas from the heated downstream components of the serial process and feed them to heated components at the upstream end of the process. This recycled hot gas provides a portion or even all of the heating requirements for the upstream heated component. In one embodiment, the recovery system extracts hot gas from each downstream heated component, and returns that hot gas to the upstream component. It should be understood that the recovery system can redirect hot gas from any component of the system to any other component, not just simply from downstream component to upstream component. 
     In another aspect, the recovery system extracts all of the hot gas from the downstream components and circulates all of the hot gas through the upstream component. In order to modulate or control the temperature of this upstream component, ambient air can be fed into the upstream component to mix with the recycled hot gas. A temperature sensor can be provided at the upstream component to monitor the temperature and regulate the inflow of ambient air to mix with the recycled hot gas. 
     In one feature of the invention, the waste energy recovery system of the present invention is integrated into a continuous controlled atmosphere brazing system. The brazing system can include one or more downstream heated components, such as a brazing furnace or a pre-heat furnace. The brazing system also includes an upstream dry-off oven. Rather than provide the dry-off oven with its own heat source, the recovery system of the present invention recirculates the hot gas from each of the downstream components back to the interior of the dry-off oven. In one aspect, this recirculation can be accomplished by a series of tube extending along the process path and projecting into the heated portion of each downstream component. An insulated manifold can be disposed between adjacent heated components with the recirculation tubes from each component opening into the interior of the manifold. 
     The waste energy recovery system includes a flow device that draws the hot gas from the heated portion of each heated component of the brazing system and directs that hot gas through the upstream component. In certain embodiments, the flow device can include fans disposed within the upstream dry-off oven with the end of the recirculating tubes at the suction side of the fans. Thus, the fans continuously draw the hot gas from the downstream components. In addition, the fans can be sized and positioned to draw ambient air into the dry-off oven. In some embodiments, the ambient air is provided through the inlet to the oven and/or through an additional air inlet. 
     The air inlet into the upstream dry-off oven can be modulated by a control valve. The control valve can control entry of air into the oven at the suction side of the fans. In certain aspects, the control valve can be regulated by a temperature sensor disposed within the dry-off oven. The temperature signal from the sensor can be used by the control valve to close or open a valve within the airflow path into the oven, or to modulate the position of the valve and therefore the flow rate of ambient air. The valve can be a variable position butterfly valve within an air intake plenum. 
     In certain embodiments, the recirculation tubes can extend into the heated portions of the downstream heated components. Multiple tubes can be provided, with each tube extending into a different heating zone within the component. Similarly, at the upstream end, multiple tubes can extend from an upstream manifold into the dry-off oven. A greater number of tubes can exhaust at the intake end of the oven to more quickly raise the part and flux temperature. 
     It is one object of the present invention to provide a system for recovery and using waste energy or heat energy from a process that would otherwise be exhausted form the process. In the context of a continuous brazing system, it is an object of the invention to utilize the hot gases generated in heating a brazing furnace and/or a pre-heater. 
     One benefit of the present invention is that is can significantly reduce the energy requirements for a continuous heated process. More specific to a continuous brazing system, the present invention beneficially allows the use of a “burnerless” dry-off oven. 
     Another benefit achieved by the recovery system of the present invention is that it reduces the requirements for exhausting hot gas outside the process facility. These reduced requirements can translate into lower cost for building the process facility, as well as reduced environmental effects. 
     Other objects and benefits of the present invention will become apparent upon consideration of the following written description, taken together with the accompanying figures. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a general perspective view of one type of continuous brazing system incorporating the waste energy recovery system of one embodiment of the present invention. 
     FIG. 2 is side elevational view of the continuous brazing system shown in FIG.  1 . 
     FIG. 3 is a top elevational cut-away view of the continuous brazing system shown in FIG.  2 . 
     FIG. 4 is an end perspective view of the interior of the braze furnace component of the continuous brazing system shown in the prior figures, illustrating the placement of a recirculation tube within the furnace. 
     FIG. 5 is an end perspective view of the interior of a transition manifold of the waste energy recovery system shown in FIGS. 1-3. 
     FIG. 6 is a top perspective view of an ambient airflow apparatus of the waste energy recovery system shown in FIGS. 1-3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     A continuous brazing system  10 , shown in FIG. 1, includes a braze section or brazing furnace  12  at the discharge end of a continuous serial process. The brazing furnace can be of conventional construction with a muffle  14  disposed within a refractory furnace containment  13 . Stock material and flux are fed into the muffle  14  through an inlet  16  and exit the brazing system  10  through outlet  18 . In addition, the system  10  can include a pre-heat section  20  that also includes a muffle  22  within the furnace containment  23 . Stock material and flux enter through the inlet  24  and exit through the outlet  26  to be conveyed to the brazing furnace  12 . 
     As is typical with most continuous brazing systems, the muffles  14 ,  22  provide a controlled atmosphere and include means for maintaining that controlled atmosphere within the interior of the muffle. In an aluminum brazing system, the atmosphere is primarily composed of nitrogen. In order to maintain this controlled atmosphere, the system  10  is provided with a vestibule  42  between the outlet  26  of the pre-heat section  20  and the inlet  16  of the brazing furnace. Likewise, a vestibule  44  can be provided at the inlet  24  of the pre-heat section  20 . The vestibules  42 ,  44  can be of conventional construction. 
     With the downstream components of the system  10  described, attention can turn to the dry-off oven  30  at the upstream end of the process. The dry-off oven receives stock material and flux after it has left the fluxer. The oven  30  includes an inlet  32  and an outlet  34  that provide a path for the material through the oven. It is understood that the dry-off oven  30 , as well as the downstream pre-heat section  20  and brazing furnace  12  can be integrated with a continuous conveyor system extending through the respective inlets and outlets. 
     The dry-off oven  30  can include exhaust unit  35  that are operable to exhaust spent gas from the chamber  36  of the oven. (Note: for clarity, the exhaust unit  35  is not depicted in FIGS. 2,  3 ). The exhaust unit  35  can be of a variety of configurations to exhaust the gases from the oven to the atmosphere. Typically, the exhaust unit  35  comprises one or more rotary fans connected to discharge outlets or shrouds at the ends of the oven  30 , or more particularly shrouds situated around the perimeter of the inlet  32  and outlet  34 . The fans can feed the exhaust gas to one or more exhaust stacks outside the building housing the system  10 . 
     The brazing furnace  12  and pre-heat section  20  can be of conventional design. More specifically, these downstream components of the system  10  can be heated in a conventional manner, such as by gas fired or electrical heating elements, or both. As shown in FIG. 2, the brazing furnace can include rails  38  to support the muffle  14  within the furnace containment  13 . The furnace walls  37  (FIG. 4) define a heating region  39  that surrounds the muffle. The heating elements are typically disposed at the bottom wall of the containment  13 . Most brazing furnaces include multiple zones of heating, sometimes followed by a cooling zone. In the illustrated example, the brazing furnace includes five temperature zones. The pre-heat section  20  also defines a heating region similar to the brazing furnace, although the pre-heat section will usually include fewer temperature zones. 
     Regardless of the manner in which the heating region  39  is heated, hot gas is produced within the brazing furnace  12  and pre-heat section  20 . In the conventional continuous brazing system of the prior art, these hot gases are discharged to the atmosphere. In these prior systems, high temperature exhaust components are required to handle the extremely high temperature (exceeding 1000° C.) gases produced within the furnaces. In addition, a separate exhaust stack is usually required for each component. This is where the present invention departs from these prior systems. 
     In particular, the present invention contemplates a waste energy, or heat, recovery system  50 . The system  50  draws the hot gas from the heated stages of the system  10  and circulates the gas back to the dry-off oven  30 . More specifically, the hot gases from the brazing furnace  12  and pre-heat section  20  are recycled back to the oven  30 , thereby supplying the oven with all, or at least some portion, of its heating requirements. Ideally, the oven  30  can be a “burnerless” oven, meaning that it does not require any separate heating source, such as a gas-fired or electric heating element. Instead, the high temperature gas drawn from the downstream components of the system  10  are sufficient to raise the temperature within the chamber  32  of the oven  30  to well-above the desired temperature. 
     In accordance with an embodiment of the invention, the recovery system  50  includes a number of exhaust recovery tubes  52  disposed within the heating region  39  of the brazing furnace  12 , as shown in FIGS. 2 and 3. The tubes  52  communicate with a transition manifold  54  disposed between the furnace  12  and the pre-heat section  20 . Additional exhaust recovery tubes  56  are situated within the heating region of the pre-heat section  20 , each opening into an upstream transition manifold  58 . The upstream manifold is disposed between the pre-heat section  20  and the dry-off oven  30 . 
     The exhaust recovery tubes  52  can include more than one tube, such as tubes  52   a  and  52   b . As seen best in FIG. 3, tube  52   a  extends farther into the furnace  12  than the other tube  52   b . In one feature of the invention, the number of recovery tubes can have their respective mouths disposed in different heating zones within the furnace. In the illustrated embodiment, the two tubes  52   a ,  52   b  draw hot gas from two different heating zones. As shown in FIG. 4, the longer of the two tubes, tube  52   a , is disposed on the bottom wall  37  of the furnace containment  13 , and below the rails  38  that support the muffle  14 . The mouth of the tube  52   a  can be provided with suitable filtering. 
     The tubes  56  associated with the pre-heat section  20  can include a number of tubes, such as tubes  56   a ,  56   b , that extend entirely through the section. More particularly, these tubes  56   a ,  56   b  communicate with the transition manifold  54  which receives hot gas from the brazing furnace  12 . The transition manifold  54  and tubes  56   a ,  56   b  provide a generally leak-proof path for the brazing furnace gas to pass through the pre-heat section. In this way, the hot gas recovered from the downstream brazing furnace will not affect the temperature within the pre-heat section. The tubes  56   a ,  56   b  can be insulated to further reduce the possibility of convection heating of the pre-heat section by hot gas flowing through the tubes. The waste heat from the pre-heat section can be scavenged by additional tubes  56   c ,  56   d . Like the brazing furnace, the pre-heat section may include multiple temperature zones, in which case one of the tubes  56   c ,  56   d  may extend farther into the interior of the pre-heat section. In the illustrated embodiment, a single heating zone is employed, so both tubes  56   c ,  56   d  are directly adjacent the inlet to the section  20 . 
     In the illustrated embodiment, the gas flows from each downstream component  12 ,  20 , to the upstream component  30  in the flow direction designated by arrow G in FIGS. 2 and 4. This flow direction is counter to the product conveyance direction through the muffles, as designated by the arrow P. This gas flow direction is dictated by the placement of the components of the system  10  that are heated to the higher temperatures, relative to the components that require lesser temperatures. In some continuous brazing systems, a cool-down section may be provided downstream of the brazing furnace  12  which is heated to above ambient temperature in order to gradually cool the brazed materials. In this instance, the waste energy recovery system  50  can include tubes and transition manifolds directing hot gas to the cool-down section. The cool-down section can be constructed like the dry-off oven  30 , as described above and in more detail below. 
     Referring not to FIG. 5, details of the transition manifolds can be discerned. The transition manifold  54  is disposed between the brazing furnace  12  and pre-heat section  20 . Thus, tubes  52   a ,  52   b  enter the manifold  54  at one end, while tubes  56   a ,  56   b  exit the manifold from its opposite end. As seen in the figure, the manifold  54  provides an open chamber  66  in communication with the mouths of the various tubes  52   a ,  52   b ,  56   a ,  56   b . In the illustrated embodiment, the manifold  54  is formed from two manifold halves, such as half  62 . The two halves  62  can be joined and sealed around a bolt flange  64 . The manifold  54  can be mounted to the adjacent components by way of end mounting flanges  60 . The manifold halves  62  can be joined at the bolt flange  64  and the mounting flanges  60  engaged to the system components by a conventional nut and bolt construct. Alternatively, the components can be welded for a more permanent connection. 
     The interior chamber  66  of the manifold  54  is lined with insulation  68 . Since the transition manifolds are exposed to ambient conditions, it is desirable that the manifolds include at least some insulation to prevent heat loss through the manifolds. Preferably, the manifolds  54 ,  58  are constructed of stainless steel, as are the various tubes. The insulation is preferably a high-grade material capable of sustaining the high gas temperatures (exceeding 1000° C.) exiting the brazing furnace. For instance, the insulation material can be a loose-fill insulation such as alumina-silica fiber, or a castable cement with refractory fibers. 
     As seen in the top view of the system  10  in FIG. 3, the recovery system  50 , and specifically the manifolds and tubes, are offset to one side of the system components. This offset is principally for maintenance and construction convenience. Offsetting the recovery system  50  toward one side provides easier access to the system for installation of the system and for replacement of system components. In addition, offsetting the recirculation tubes toward one side may better accommodate the heating elements within the furnace  12  or pre-heat section  20 . 
     Referring again to FIGS. 1-3, additional details of the recovery system and its integration into the dry-off oven will be described. The upstream manifold  58  is connected between the inlet end of the pre-heat section  20  and the outlet end of the oven. Hot gas flows through the manifold  58  and into recirculation tubes  70  within the dry-off oven. It should be understood that these recirculation tubes  70  can replace the conventional heating elements that have been used to heat prior dry-off ovens. Thus, the placement of the tubes  70  within the oven is not constrained by other heating elements. Instead, the primary goal in placing the recirculation tubes  70  is to ensure uniform heating of the chamber  36  through which the feed materials pass along the continuous conveyor. 
     In the illustrated embodiment, the recirculation tubes  70  include four tubes  70   a - 70   d  that are dispersed along the length of the oven  30 . In one feature, two tubes  70   a ,  70   b  are situated adjacent the inlet  32  to pump the majority of the hot gas into the oven at the inlet side. The remaining two tubes  70   c ,  70   d  can be evenly spaced from the front two tubes, and can be positioned to leave a zone at the outlet of the oven without any recirculation tubes. With this arrangement, the heat within the oven is concentrated toward the inlet side so that the conveyed materials are fully heated as they pass through the oven  30 , thereby ensuring that all moisture will be evaporated by the time the materials exit the oven. 
     The waste energy recovery system  50  further contemplates means for drawing the hot gas through the system in the flow direction G, or more specifically from the heated components of the system to the “burnerless” dry-off oven  30 . To that end, one or more gas flow devices or gas pumps can be disposed at locations throughout the system  50 . A preferred gas flow means has been found to be recirculation fans, such as fans  72 , disposed within the oven. In this instance, the outlet mouth of the recirculation tubes  70   a-d  open at the negative pressure side of a corresponding one of the fans  72 , as depicted in FIG.  2 . With this configuration, operation of the fans draws a suction pressure along the tubes  70   a-d , and through the upstream elements of the recovery system  50 . Thus, the gas moving elements of the system  50  are contained in one location and are more readily accessible than flow devices dispersed throughout the system. In the illustrated embodiment, the fans  72 ,  73  can be 16″ diameter, 4500 CFM units. 
     The recirculation fans  72  thus draw heated gas from the upstream portions of the recovery system, namely from the heating region of the furnace  12  and pre-heat section  20 . An additional recirculation fan  73  can be provided near the outlet of the oven  30  to provide gas circulation at the exit of the oven. Ambient air naturally flows through the inlet  32  and outlet  34  openings. The ambient air/hot gas mixture naturally has a lower temperature than the gases drawn from the brazing system components  12 ,  20 . However, the supply of ambient air through the opening  32  may not be sufficient to reduce the temperature of the resulting gas mixture to acceptable levels. 
     In order to address this difficulty, an ambient airflow apparatus  80  can be provided. In general terms, this apparatus  80  provides for controlled flow of ambient air into the oven  30  to achieve a predetermined temperature within the oven. In one embodiment, the apparatus  80  can include an ambient air inlet  75  associated with at least one of the recirculation fans  72 ,  73 . Ambient air is fed to the inlet through an intake plenum  82  that is preferably mounted to the side of the oven  30 . The mouth of the plenum  82  can be covered by a screen  83  to prevent entry of unwanted materials into the oven, as shown in FIG.  6 . 
     A valve element  85  (FIG. 6) is disposed at the mouth of the plenum  82  to control the flow of ambient air into the plenum. In one embodiment, the valve  85  can be a butterfly valve that rotates about its long axis from a position substantially closing the plenum mouth to a position in which the mouth is substantially unobstructed. In other words, the valve  85  can be pivoted from a no flow to a full flow position. The ambient airflow apparatus  80  further contemplates means for controlling the movement of the valve  85  between its two extreme positions. In one aspect, this means can include an actuator  89  that controls the movement of the valve  82  through a linkage  87 . The actuator  89  can be an on-off type actuator, such as a solenoid, that is only capable of moving the valve  82  between its full open and full closed positions. In other embodiments, the actuator  89  can capable of incremental movements to accomplish fully controlled opening and closing of the valve. Thus, the actuator  89  can include a stepper motor or a lead screw mechanism. 
     Operation of the actuator  89  can be controlled by a temperature sensor (not shown) disposed within the chamber  36  of the oven  30 . The temperature sensor can ascertain the temperature within the oven, or can provide a reading relative to a pre-determined temperature set point. For instance, the temperature sensor can include a thermocouple or a thermistor that generates an electrical signal in proportion to the sensed temperature. The actuator  89  can include circuitry to operate on the temperature sensor generated electrical signal to ultimately control the movement of the actuator. Temperature controlled actuators are widely known and a variety of such actuator and temperature sensor combination can be implemented within the ambient airflow apparatus  80  of the present invention. Ordinarily the oven  30  can withstand a wide temperature range, so high precision control of the valve  82  is not essential. 
     Although one ambient airflow apparatus  80  is shown, additional units can be provided. Specifically, another apparatus  80  can be situated adjacent the fan  73 , or any of the other fans  72 . The actuator associated with the additional airflow apparatus can be controlled by the same temperature sensor as the apparatus  80 , or can include its own temperature sensor. 
     As may be appreciated, the brazing system  10  incorporating the waste energy recovery system  50  of the present invention requires only a single exhaust for the entire system. In particular, the exhaust units  35  mounted on the oven  30  handle the discharge of all gases within the system. Therein lies one benefit of the present invention, namely that there is no need to exhaust hot gas from each section of the brazing system. Instead, all of the hot gases generated in the brazing furnace  12  and pre-heat section  20  can be fed through and exhausted from the dry-off oven  30 . 
     In addition to providing a centralized exhaust for the brazing system, the recovery system  50  also provides means for tempering the gas discharged into the atmosphere. In particular, since the temperature of the dry-off oven is significantly lower than the downstream components  12 ,  20 , the temperature of the discharged gas is naturally lower. This lower temperature means that high temperature exhaust components are not required, and that the environmental impact of the exhaust is minimized. 
     The various elements of the waste energy recovery system  50  can be sized for the anticipated rates of hot gas production within the brazing system components, the length of travel of the hot gas to the dry-off oven, the size of the oven and the temperatures in the brazing furnace  12 , pre-heat section  20  and dry-off oven  30 . In the illustrated embodiment, the recirculation tubes are 6″ diameter 304SS pipes. The tube  52   a  has a length of 113″ and is preferably supported near its free end. 
     In addition, in the-illustrated embodiment, the two transition manifolds  54 ,  58  have respective lengths of 34″ and 172″. The chamber  66  within each manifold is 6⅝″ high, with the width determined by the number of tubes in communication with the chamber. For the manifold  54 , the chamber width is 17¾′, while the larger manifold  58  has a chamber width of 24⅞″. The insulation in both manifolds is nominally 7″ thick around the entire chamber  66 . The intake plenum  82  can provide an intake flow area of about 85 sq.in. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.