Abstract:
A regenerative firing system is disclosed which functions in a heat treating furnace or other high temperature technology. The system comprises a plurality of regenerator heat transfer boxes which absorb the heat contained in high temperature exhaust from the furnace. Each regenerator box transfers this absorbed heat to a flow of ambient air. The now heated air flows from the regenerator box into a common air stream which is then fed to a plurality of burners. The preheated air stream is supplied to a common air stream that is then simultaneously provided to each of a plurality of burners. In addition, the current invention comprises a method of heat recovery for a furnace utilizing the inventive system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a regenerative furnace firing system. More particularly, the invention relates to a regenerative heat transfer system for use in heat treating furnaces and reheating furnaces. 
     2. Description of the Prior Art 
     Reheating, forging and Heat treating furnaces are utilized to alter the physical, and sometimes chemical properties of a material. The most common application of such furnaces are metallurgical, although heat treatment is often used with other materials such as glass. These furnaces are employed to heat and chill materials, notably steel, often to extreme temperatures, to achieve a desired result such as hardening or softening of the material. Heating methods include softening for forging or rolling, annealing, case hardening, precipitation strengthening, tempering and quenching. 
     Because reheating and heat treating furnaces operate at a variety of high temperatures, often in excess of 2000° F., they require an increased amount of energy for their operation. Large quantities of hot flue gasses are produced by heating furnaces. The recovery of some of this heat and its reuse in the heating process results in the reduction of the amount of primary fuel needed to run the system, and therefore increases efficiency. An example of such waste heat recovery is the preheating of the combustion air used to fire the burners. 
     Typically, waste heat recovery from large furnaces utilizes some type of heat exchanger. A heat exchanger is a device built for the efficient transfer of heat from one medium to a second medium. The media may be separated or may mix with other components of the devoce during the heat exchange process. Heat exchangers are commonly used in heating and refrigeration systems, power plants or chemical plants. A heat exchanger may be utilized to retain the waste heat produced by a heating furnace so that it may be reused to reduce fuel costs. 
     Gas fired fuel furnaces traditionally employ two types of heat recovery systems. Recuperators generally utilize a metallic heat exchanger and have the ability to preheat combustion air to about 800° F.-1000° F. The preheated air and fuel mixture is continuously adjusted as the furnace heats and cools to allow for the proper air/gas combustion ratio. This ratio is constantly monitored and changed as a result of volume expansion and contraction. Adjustment of the preheated air temperature is mainly controlled by the injection of dilution air into the combustion mix. As a result, recuperator systems work well with furnaces that run at a steady state temperature, for example reheating furnaces, forging temperature furnaces and other types, which operate at higher temperatures for extended periods of time. Recuperators are generally not economically practical with heat treating furnaces that require numerous temperature changes, i.e., temperature ramping and cooling cycles. 
     Regenerator heat recovery systems are more fuel efficient than recuperators and have the ability to operate with higher temperature furnaces, for example 2000° F. or higher. The airflow through a regenerative heat exchanger is cyclical and periodically changes. Hot exhaust air is directed from the furnace through the regenerator where it heats up a stationary medium. This medium may comprise a metallic or ceramic material. The incoming flow of hot waste air stops and cooler combustion air is then passed over the heated medium, which heats the air before it mixes with combustion gas and is directed to the burners. Current heat treatment technology requires that each furnace burner be connected to a single paired regenerator or regenerates within the burner itself during operation. In the case of single paired regenerators, each burner ceases firing at the time when the flow of preheated combustion air stops and the regenerator receives hot waste air from the furnace. Each regenerator therefore does not supply a continuous supply of preheated combustion air to its dedicated single burner. Other types of regenerative (burner) firing systems simultaneously fire and exhaust through the burner itself. However, these systems are many times not economically practical due the expense of each individual burner and the size of the furnace. Using this cyclic firing of the burners for heat treating often causes non-uniform heating of the furnace and too large of a firing footprint to meet uniformity requirements, an undesirable condition for the heat treatment of metals and other materials. The currently developed regenerators are expensive to install because of the need for a regenerator for each pair of burners and the typical space limitations due to the physical size of such regenerators. The temperature uniformity requirements of treating systems are easier to achieve with a greater number of small burners. The use of multiple burners/regenerator pairs also raises the capital investment costs due to the increased hardware cost per unit. As a result, it is generally cost prohibitive to utilize regenerators with heat treating furnaces because of the need for a large number of smaller burners. 
     There remains a need, therefore, in the art of heat treating, forging and reheating furnaces for a heat recovery system that utilizes burners and regenerators that do not require firing in pairs and therefore have the ability to utilize a number of small burners that achieve heating uniformity and fuel savings. Specifically, there is a need for a system that uses regenerator heat transfer boxes that are not dedicated to a single burner. Such a system allows for the continuous firing of all burners or the flexibility to pulse burners in various configurations that are not dedicated to a specific hardware arrangement, thus providing greater fuel efficiency and more precise temperature control. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a system that utilizes regenerative firing technology employing various numbers of burners that operate independently of two or more regenerators. The system is useful in the operation of heating furnaces or other high temperature technologies. The inventive system utilizes various numbers of burners that receive a continuous supply of preheated combustion air and therefore can constantly fire or perform variable pulse heating independent of the regenerators, allowing for more uniform heating of the furnace. The steady supply of combustion air provides precise temperature control and is thus adaptable for use in applications that require multiple or rapid temperature changes. The use of a small number of regenerators results in lowered fuel requirements, thus reducing fuel costs. 
     The system comprises a plurality of regenerator heat transfer boxes that absorb the heat contained in high temperature exhaust from the furnace. Each regenerator box transfers this absorbed heat to a flow of ambient air. A media, for example aluminum spheres, accomplishes the absorption and transfer of heat from the furnace air to the ambient air. The now heated air flows from the regenerator box into a common air stream, which then feeds to a plurality of burners. Therefore, the preheated air stream produced by each of the regenerators of the present system is not dedicated to a single furnace burner. Instead, the preheated air stream is supplied to a common air stream that is then simultaneously fed to each of a plurality of burners, for instance two or more burners. As a result, it is unnecessary for the individual burners to stop firing when a regenerator box is receiving high temperature exhaust from the furnace. The use of multiple numbers of small burners provides greater control over furnace heat uniformity. In addition, the use of a limited number of regenerator boxes decreases the amount of fuel required, thus reducing operational costs. 
     Because each burner operates independently from a single paired regenerator, it is unnecessary for the regenerators to be placed in direct proximity to a single burner. This allows for the placement of each regenerator in any convenient location around the furnace and the ability to place additional regenerators to a preexisting system. The regenerative system also does not require the addition of burners. Rather, the decoupling of the regenerators to their respective paired burners is accomplished through the addition of pipes or lines. The system is therefore easily adapted to current heating furnaces. 
     In an additional aspect, the current invention comprises a method of heat recovery for a furnace. The method comprises a cycle of transferring furnace exhaust heat to an ambient air supply, providing the now heated air supply to common combustion air supply to the burners, and then again transferring additional waste furnace exhaust heat to the ambient air supply. In the first step of the cycle, a flow of heated furnace air is directed from a furnace through a valve to a regenerator for a specified period of time. The heated furnace exhaust is directed through a media, for example, tabular alumina spheres, located within the regenerator. This media has the capability to absorb and transfer the waste heat. The temperature within the regenerator is controlled with a thermocouple which provides a feedback control mechanism. When the temperature of the regenerator reaches a predetermined set point, the valve closes and the flow of heated exhaust air into the regenerator stops. Next, a second valve opens, allowing a stream of cold ambient air to enter the regenerator box and contact the heated media. The absorbed heat contained in the media is transferred from the media to the stream of ambient air. The flow of ambient air is halted at the end of a complete cycle by closing an additional valve. The now heated air is directed, through a series of valves, from the regenerator to a common combustion air supply. The common combustion air supply then flows to a plurality of burners. The cycle then begins again with the flow of waste furnace heat into the regenerator. This cycle of transferring waste furnace heat to a common combustion air supply continues until the plurality of burners reach a maximum allowable temperature. Like the regenerators, the temperature of the burner combustion air is controlled through the use of a thermocouple which provides a feedback control mechanism. 
     The heated air produced from each regenerator flows to each of the burners through the common combustion air supply. The preheated air stream produced by the regenerators of the present method is not dedicated to a single furnace burner. Instead, the preheated air stream is supplied to a common combustion air stream that is then provided simultaneously to each burner. Each regenerator performs a different step of the method at any given time, thus providing a constant supply of combustion air which is directed to each burner. As a result, it is unnecessary for the individual burners to stop firing when a regenerator box is receiving high temperature exhaust from the furnace. Thus, each of the burners may continuously fire or pulse in random cycles not dictated by the regenerator boxes. The use of a number of small firing burners provides greater control over furnace heat uniformity. In addition, the use of a limited number of regenerator boxes decreases the amount of fuel required, thus reducing operational costs. The duration of the flow of heated furnace exhaust into the media within the regenerator box may be shortened or eliminated in order to control the temperature of the common air supply reaching the burners. 
     The inventive method allows for a number of burners to receive a continuous supply of preheated combustion air and therefore constantly fire or pulse in random cycles, allowing for more uniform heating of the furnace. The steady supply of combustion air provides more precise temperature control and is thus adaptable for use in applications that require multiple or rapid temperature changes. By using a limited number of regenerator boxes results in lowered fuel requirements, thus reducing fuel costs. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of the regenerative firing system in accordance with the present invention; 
         FIG. 2  is an alternative schematic view of the regenerative firing system in accordance with the present invention; 
         FIG. 3  is a section view of  FIG. 2  taken along line AA-AA; 
         FIG. 4  is a section view of  FIG. 2  taken along line BA-BA; 
         FIG. 5A  is a top view of the regenerator box of the invention; 
         FIG. 5B  is a section view of  FIG. 5A  taken along line A-A; 
         FIG. 5C  is a section view of  FIG. 5A  taken along line B-B; and 
         FIG. 5D  is a section view of  FIG. 5B  taken along line C-C; 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates the regenerative firing system  1  of the invention for a high temperature furnace having gas fueled burners. A heat treating furnace  2  is illustrated having a number of component parts. Furnace  2  is provided with four walls  4  which are constructed from a material, such as metal or concrete, capable of withstanding the elevated temperatures necessary for the heat treating process. For example, walls  4  may be constructed from ceramic fiber or a hard refractory material. Walls  4  may be insulated with an appropriate insulating material so that furnace  2  maintains the proper temperature. Such insulation may comprise refractory materials which are chemically and physically stable at high temperatures, resistant to thermal shock and are chemically inert. For example, the insulating material may be ceramic fiber. Furnace  2  is provided with door  6  which opens and securely closes to allow for the entry and exit of the material to be treated. Door  6  may be provided with appropriate sealing and closing mechanisms (not shown) to allow for proper heating and cooling of the furnace  2 . Furnace  2  may be constructed at any size and dimension, depending on the intended use. As an example, furnace  2  may be of the dimension of 12′ long×8′ wide×5′ high to allow for industrial applications. 
     Furnace  2  is provided with one or more burners  10 . In one embodiment, each burner  10  may be mounted within furnace wall  4 ; i.e., burner  10  is integrated within wall  4 . In an additional embodiment, burner  10  may be supported on wall  4  or the ceiling portion of furnace (not shown) through an appropriate mounting apparatus. Each burner  10  is in flow communication with a fuel source  28  and a source of preheated combustion air  24 , as discussed below. Burners  10  may be placed at locations that are convenient as dictated by the size and use of furnace  2 . Burners  10  are known in the art, and may comprise for example, a Tempest® burner with refractory burner block and spark initiator. Furnace  2  may be further provided with a safety control system  11  comprising a UV scanner. The safety control system  11  utilizes a UV scanner to detect the presence or absence of a flame in burner  10 . Burner  10  may also comprise an ignition source  13 , for example an ignition transformer, for ignition. 
     Referring again to  FIG. 1 , furnace  2  comprises a plurality of furnace ports  8  which are placed integrally within furnace wall  4 . Heated furnace exhaust gases, for example at temperature of 500° F. to 2000° F., exits from these ports and is recycled as detailed below. Furnace  2  is also provided with one or more exhaust flues  12  which are in flow communication with the interior of the furnace and may be utilized to vent hot furnace air into the atmosphere in order to adjust the furnace pressure. Exhaust flue  12  may be placed in a location that allows for the best furnace temperature uniformity. 
     Regenerative firing system  1  further comprises a plurality of regenerator boxes  14 . Each regenerator box  14  is in flow communication with furnace  2 . The regenerator boxes  14  may be placed at varying locations around furnace  2  depending on the space requirements of furnace  2 . As discussed in detail below, individual regenerator boxes  14  are not dedicated to a single burner  10  and are in flow communication with all burners  10 . Therefore it is unnecessary to locate the regenerator box  14  in direct proximity to burner  10 . 
     The function and operation of the regenerative firing system  1  of the invention will now be described in detail. Referring now to  FIGS. 1 and 2 , heated furnace exhaust  16 , generally at a temperature of between 500° F. and 2000° F., is expelled from the furnace  2  through furnace port  8 . When valve  80  is turned to the open position, heated furnace exhaust  16  flows to regenerator box  14 . The regenerator box  14  of the invention is illustrated in greater detail in  FIGS. 5A-5D . Referring to  FIG. 5A , regenerator box  14  is shown with a generally square configuration. The regenerator box  14  is constructed of a material which is chemically and physically stable at high temperatures. For example, regenerator box  14  may be constructed from an insulating castable material. Metallic or ceramic stiffening bars  42  are provided to add support and handles  40  allow for ease of movement and installation. Regenerator box  14  may be any size appropriate for its intended application. For instance, the regenerator box  14  may be 2′-3′ wide and 2′-3′ tall. 
     Referring now to  FIGS. 5B and 5C , regenerator box  14  is further provided with a heat absorbing air permeable media  46 . Media  46  absorbs heat rapidly from the heated furnace exhaust. In the reverse flow, media  46  rapidly gives up the absorbed heat to the ambient regenerator supply air, described in greater detail below. In one embodiment, media  46  comprises tabular alumina spheres approximately ¾ inch in diameter. It is to be understood that media  46  may comprise any material that is capable of absorbing and transferring heat. Media  46  may be any shape or configuration that is allows air to flow around and through the media. In one embodiment, media  46  is a round or spherical configuration to allow for maximum air flow through regenerator  14 . Media  46  is contained within regenerator box  14  through the use of grate  44  ( FIG. 5D ). Grate  44  may be constructed from the same material as regenerator box  14 , or alternatively may comprise a metal. Grate  44  contains a plurality of openings  48  which are smaller than the diameter of media  46 . Openings  48  are small enough to prevent media  46  from exiting regenerator box  14  but large enough to allow for adequate air flow around media  46  to promote heat transfer. 
     Referring again to  FIGS. 1 and 2 , heated furnace exhaust travels along line  16  from furnace  2  and enters regenerator box  14  through incoming air pipe  52  ( FIG. 5B ). The flow of heated furnace exhaust along line  16  continues for a period of time until media  46  is heated to an appropriate temperature. The appropriate temperature is specific to the type of media  46  in use and the intended use of the furnace. The flow of heated furnace exhaust along line  16  into regenerator box  14  is then stopped by closing valve  80 . It is to be understood that the operation, i.e., opening and closing, of all valves described herein may be accomplished by any appropriate means known in the art. For instance, the operation of valves may be performed manually, or alternatively may be controlled by a computer system. It is also to be understood that the various component parts of system  1  are in flow communication with each other, as is illustrated in detail in the accompanying figures. For example, furnace  1  is in flow communication with regenerator box  14 , while all burners  10  are in flow communication with each regenerator box  14 . The movement of air throughout the present system is accomplished with the use of pipes or other instruments as will be known to one of skill in the art. 
     The cold ambient regenerator air supply enters the regenerative firing system  1  from outside the furnace  2  through combustion air inlet  86 . The cold regenerator air supply travels along line  20  and is in flow communication with regenerator box  14 . The cold regenerator air supply flows along line  20  through the system  1  by the use of combustion air fan  88 , which is also in flow communication with the regenerator boxes  14 . After the regenerator  14  reaches a preset temperature, the flow of heated furnace exhaust into regenerator box  14  is stopped by closing valve  80 . Valves  80   a  are then opened and cold regenerator air supply then enters regenerator box  14  through air supply/discharge pipe  56  ( FIG. 5B ). The heat from media  46  within regenerator box  14  is transferred to the cold regenerator air, resulting in the production of hot regenerator air. Valve  80   b  is then closed, ceasing the stream of cold regenerator supply air into regenerator box  14 . The hot regenerator air is streamed along line  24  from regenerator box  14  through hot discharge pipe  54  ( FIG. 5B ). 
     It is contemplated that the regenerative firing system  1  of the present invention comprises a plurality of regenerator boxes  14  in its operation. In a preferred embodiment, the present regenerative firing system  1  employs two or more regenerator boxes  14  each of which is in flow communication with and supplies preheated air to each of multiple burners  10 . Referring now to  FIG. 3 , two regenerator boxes  14   a  and  14   b  are illustrated. Each box is in flow communication with a furnace port  8 . Valves  80  are opened and each regenerator receives heated furnace air along lines  16   a  or  16   b  (respectively) through air supply/discharge pipes  56   a  or  56   b . The media  46  (not shown) contained in regenerator boxes  14   a  and  14   b  absorbs the heat from the heated furnace air. The flow of heated furnace exhaust along lines  16   a  or  16   b  into regenerator boxes  14   a  and  14   b  continues for a specified period of time as is appropriate for the particular application and the types of regenerators and burners in use. The flow of heated furnace exhaust is then stopped by closing valves  80 . Valves  80   a  are opened and cold regenerator air supply travels along lines  20   a  and  20   b  and enters the regenerator boxes  14   a  and  14   b  through air supply/discharge pipes  56   a  and  56   b . The cold regenerator air supply receives heat from media  46  (not shown), resulting in the production of hot regenerator air discharge. Valves  80   a  are then closed at an appropriate time, ceasing the stream of cold regenerator supply air along lines  20   a  and  20   b  into regenerators  14   a  and  14   b . Valves  80   b  are then opened. The hot regenerator air discharge travels along lines  24   a  and  24   b  and flows into a common hot regenerator air discharge stream, referred to here as a combustion air stream. The combustion air stream flows along line  22 . Oxygen sensor  58  may be utilized to monitor the concentration of oxygen in the combustion air stream  22 . The oxygen concentration of the combustion air may be adjusted by the addition of recirculated exhaust gases from the exhaust fan  30  along line  72  directly back into line  22  into the heated combustion air stream. The addition of recirculated exhaust gases into the heated combustion air stream assists in the reduction of certain emissions in the flue gases, for example Sox and NOx, and may be controlled with the use of a valve in line  32  ( FIG. 2 ). Valve  80   c  ( FIGS. 1 and 2 ) is opened and the heated combustion air is then directed via line  22  to each of the plurality of burners  10 . Burners  10  receive a flow of fuel supply along line  28  which combines with the heated combustion air to allow for the ignition of the burners. It will be appreciated from the above description that each regenerator box  14  provides heated regenerator air along line  24  to all of the burners  10  in regenerative firing system  1 . Each regenerator box  10  supplies heated regenerator air along line  24  to a common combustion air supply, which is subsequently directed to each burner  10 . Although the above discussion contemplates the use of three regenerator boxes  14 , it will be appreciated by one of skill in the art that any number of such boxes may be utilized in system  1 . For instance, two regenerator boxes  14  may provide heated regenerator air  24  to common combustion air supply  22 . The number of regenerator boxes  14  will vary depending on the intended use of the furnace and the number and type of burners  10  in use. 
     Each regenerator box  14  further comprises a temperature thermocouple  82  which provides an individual temperature control loop for regenerator box  14 . The temperature loop manages, commands, directs and regulates the flow of air into and out of regenerator box  14 . The temperature of the heated air produced by the regenerator box  14  is constantly monitored and adjusted to maintain a temperature according to user-defined settings. Thermocouple  82  operates as a feedback control. When the temperature of the heated regenerator air discharge falls below a set point, the stream of heated furnace exhaust is directed to media  16  within regenerator  14  for a longer period of time. Likewise, when the temperature of the heated regenerator air discharge exceeds a certain level, the stream of heated furnace exhaust is directed to media within regenerator  14  for a shorter period of time, or may be discontinued. The control loop varies the amount or temperature of heated furnace exhaust entering the regenerator box  14  so that various components of the system are not damaged by excessive heat. 
     In a preferred embodiment, the regenerative firing system  1  of the invention utilizes at least two regenerator boxes  14 . The regenerator boxes  14  work in sequential order. As described in detail above, each regenerator box  14  cycles through a process of: (1) opening a set of valves  80  and providing heated furnace exhaust to a media  46 ; (2) transferring heat from the furnace exhaust to the media  46 ; (3) discontinuing the supply of heated furnace exhaust to the matrix by closing valves  80 ; (4) simultaneously opening valves  80   a  and providing a cold regenerator air supply to the media  46 ; (5) discontinuing the supply of cold regenerator air by closing valves  80   a ; (6) transferring heat from the media  46  to the cold regenerator air supply; and (7) opening valves  80   b  and expelling the now heated regenerator air discharge to a common combustion air stream. The present system  1  is configured so that each regenerator box  14  is simultaneously performing a different step in this cycle. For example, while a first regenerator box  14  is supplying heated regenerator air discharge to the common combustion stream, a second regenerator  14  is simultaneously receiving heated furnace exhaust. Additional regenerator boxes  14  may be added to the system as dictated by the temperature requirements of the particular application. However, each regenerator box  14  is configured to operate at a different part of the heating and exhaust cycle. This type of overlapping cycling ensures that there is a continuous supply of heated regenerator air discharge to the common combustion air stream and thus a continuous supply of heated combustion air to all burners  10 . None of the regenerator boxes  14  are dedicated to a single individual burner  10 . Instead, regenerator boxes  10  collectively supply a stream of heated regenerator air discharge to a common combustion air supply, which simultaneously feeds each burner  10 . Unlike prior art burners, it is therefore unnecessary for the present burners  10  to cycle on and off due to a cessation of the combustion air supply from a single dedicated regenerator  14 . Thus, the burners  10  of the present system may constantly fire with a proportional modulation control system. There is no need to provide a large number of redundant burners in an attempt to compensate for the lack of constantly firing burners. The present system therefore allows for enhanced furnace temperature uniformity and a savings in fuel cost due to the efficiency of the regenerator boxes  14 . 
     The plurality of regenerators  10  will cycle through the steps of opening valves  80  and accepting heated furnace exhaust, transferring heat to a matrix, closing valves  80 , opening valves  80   a  and heating cold regenerator air supply, closing valves  80   a , and opening valve  80 , supplying heated regenerator air discharge to the common combustion air stream, and providing heated combustion air to each individual burner  10  until the burners  10  have heated to a maximum operating temperature. This maximum temperature is specific to and will vary with the type of burner employed. For example, exceeding temperatures of 1000° F. may damage the components of burner  10 . Each burner  10  further comprises temperature thermostat  82  (not shown) which provides an individual temperature control loop for the burner  10 . Similar to the temperature control loop  82  for regenerator box  14 , this control loop regulates the temperature of the regenerator air discharged entering burner  10 . The temperature of the heated air entering burner  10  is constantly monitored and adjusted to maintain a temperature that will not damage the burner  10 . As described previously, thermocouple  82  operates as a feedback control. When the temperature of the heated regenerator air discharge exceeds a certain set point, the stream of heated furnace exhaust is directed to media  16  within regenerator  14  for a shorter period of time, or may be discontinued. The control loop varies the amount or temperature of heated furnace exhaust entering the regenerator box  14  so that various components of the system are not damaged by excessive heat. 
       FIG. 4  illustrates the temperature loop control system described above. Here, the direction of the flow of heated regenerator air discharge is used to regulate the temperature of burners  10 . As illustrated in  FIG. 4 , the amount of cold regenerator air supply moving along line  20  into regenerator  14  is halted by closing valves  80   a . In addition, valves  80   a  are closed while valve  80   d  is opened. As a result, the flow of heated regenerator air discharge along line  24  is no longer directed to a common combustion air stream. Instead, the opening of butterfly valve  80   d  allows the heated regenerator air discharge to flow out of regenerator box  14  via air supply/discharge pipe  56 . The heated regenerator air discharge, now referred to as regenerator exhaust, is directed along line  26  and away from burners  10 . The regenerator exhaust is moved along line  26  through and out exhaust flue  12  (not shown) and into the atmosphere through the use of regenerator box exhaust fan  70 . Regenerator box exhaust fan  70  is in flow communication with regenerator  14 . The regenerator exhaust may undergo additional treatments before discharge into the atmosphere to lessen adverse environmental impacts, for example exhaust gas recirculation for NOx reduction (not shown). 
     As is clear from the above discussion and the accompanying figures, regenerator boxes  14  are not directly connected to a single burner  10 . As a result, it is not necessary to place regenerator boxes  14  in direct proximity to burners  10 . It is instead possible to place regenerator boxes  14  at any position around furnace  2  that is convenient. Movement of the heated regenerator air discharge from each regenerator box  10  may be easily directed to the common combustion air stream through the use of pipes or other mechanisms known in the art. This creates flexibility in space utilization and allows for advantageous furnace exhaust flue  12  location. In addition, firing system  1  may be adapted to a previously installed conventional furnace system because use of the present regenerator boxes  14  does not require significant structural changes or modifications to the existing furnace. 
     Finally, one preferred embodiment of the invention has been described hereinabove and those of ordinary skill in the art will recognize that this embodiment may be modified and altered without departing from the central spirit and scope of the invention. Thus, the embodiment described hereinabove is to be considered in all respects as illustrative and not restrictive. The scope of the invention being indicated by the appended claims rather than the foregoing descriptions and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced herein.