Abstract:
One aspect provides a heating device comprising a firebox having a hearth therein and first and second heat exchange chambers, and a heat exchanging plate having a first surface and a second opposing surface. The heat exchanging plate is suspended above the hearth, such that the first surface is located between the hearth and the second surface. The heat exchanging plate has lower protrusions extending from the first surface and into the first heat exchange chamber, and upper protrusions extending from the second surface and into the second heat exchange chamber. A method of manufacturing a heating device is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/446,396, filed by Joseph A. Benedetti on Feb. 24, 2011, entitled “INTEGRATED HEAT EXCHANGING WOOD STOVE FIRE BOX TOP,” commonly assigned with this application and incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This application is directed, in general, to a heating device and, more specifically, to a heat exchanging, wood stove fire box top. 
       BACKGROUND 
       [0003]    Wood burning stoves have become commonplace in today&#39;s building trades for both residential and commercial applications, whether for providing heat or for value enhancement. Where a more massive fireplace is not desired or feasible, wood stoves are a highly desirable option. Stoves are often preferred over open fireplaces because many wood stoves have the capability to heat large spaces efficiently from a center-room location. Most of these stoves are able to burn for extended periods of time, such as over night, without refueling or reloading, further enhancing the preference over conventional masonry fireplaces. The fact that the stove fully contains the fire while providing heat in a full circle around the stove makes the wood stove highly desirable. In general, wood stoves are much less expensive than a comparable masonry fireplace. However, these stoves have seen little effort directed toward improving the efficiency of heat transfer into the room. 
       SUMMARY 
       [0004]    One aspect provides a heating device comprising a firebox having a hearth therein and first and second heat exchange chambers, and a heat exchanging plate having a first surface and a second opposing surface. The heat exchanging plate is suspended above the hearth, such that the first surface is located between the hearth and the second surface. The heat exchanging plate has lower protrusions extending from the first surface and into the first heat exchange chamber, and upper protrusions extending from the second surface and into the second heat exchange chamber. 
         [0005]    In a further aspect, a method of manufacturing a heating device is provided comprising forming a firebox having a hearth therein and first and second heat exchange chambers, and suspending a heat exchanging plate above the hearth. The heat exchanging plate has a first surface and a second opposing surface, such that the first surface is located between the hearth and the second surface. The heat exchanging plate has lower protrusions extending from the first surface and into the first heat exchange chamber and upper protrusions extending from the second surface and into the second heat exchange chamber. 
     
    
     
       BRIEF DESCRIPTION 
         [0006]    Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0007]      FIG. 1A  is a plan view of a first surface of one embodiment of a wood burning stove heat exchanging plate; 
           [0008]      FIG. 1B  is a plan view of a second opposing surface of one embodiment of a wood burning stove heat exchanging plate; 
           [0009]      FIG. 2A  is a sectional view of a round airfoil in a free-stream, laminar airflow; 
           [0010]      FIG. 2B  is a sectional view of a symmetric low-speed airfoil in the same free-stream, laminar airflow as in  FIG. 2A ; 
           [0011]      FIG. 3  is a right side, vertical sectional view of one embodiment of a stove employing the heat exchanging plate of  FIG. 1 ; 
           [0012]      FIG. 4  is a plan view of the first surface of one embodiment of the wood burning stove heat exchanging plate with combustion products flow depicted; 
           [0013]      FIG. 5  is a plan view of the second opposing surface of the heat exchanging plate  100  with heating air flow depicted; 
           [0014]      FIG. 6A  is a top view of the stove of  FIG. 3 ; 
           [0015]      FIG. 6B  is a front elevation view of the stove of  FIG. 3 ; 
           [0016]      FIG. 6C  is a right side elevation view of the stove of  FIG. 3 ; and 
           [0017]      FIG. 7  is a table of efficiency results for the heat exchanging plate versus a conventional flat plate. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The principles described in this discussion directed to a heating device, while described with reference to a wood burning stove, are equally applicable to other heating devices, e.g., fireplace inserts, etc. 
         [0019]    Referring initially to  FIGS. 1A and 1B , illustrated are plan views of a first surface and a second opposing surface, respectively, of one embodiment of a wood stove heat exchanging plate  100 . The heat exchanging plate  100  comprises a plate body  105  having a first surface  110 , a second opposing surface  120 , a flue aperture  130 , a flow diverter  140 , coupling apertures  150 , and first and second regions  161 ,  162 , respectively. The first surface  110  may have a plurality of lower protrusions  111  extending therefrom while the second surface  120  may have a similar plurality of upper protrusions  121  extending therefrom. In one embodiment, each of the upper protrusions  121  may overlie a corresponding, polar opposite, lower protrusion  111 ; however, in other embodiments, the upper and lower protrusions  121 ,  111  may be off-set from one another. 
         [0020]    In one embodiment, the plurality of upper protrusions  121  and corresponding polar opposite lower protrusions  111  may be arrayed in upper arcs  122   a - 122   i  and lower arcs  112   a - 112   h,  respectively, around the flue aperture  130 . However, it should be noted that other embodiments provide that the protrusions may be arranged in straight line or off-set formations. The upper and lower arcs  122   a - 122   i  and  112   a - 112   h,  respectively, are not necessarily concentric to the flue aperture  130 . In one embodiment, the upper and lower arcs  122   a - 122   i  and  112   a - 112   h  are concentric to a point  170 . Positioning of the flow diverter  140  may require that certain of the lower protrusions  111  be foregone, i.e., construction or forming of the flow diverter  140  prevents forming of certain of the lower protrusions  111 . The flow diverter  140 , in one aspect, may comprise a first wishbone-shaped forward diverter  141  and a second arcuate rear diverter  142 . The first wishbone-shaped forward diverter  141  and second arcuate rear diverter  142  may be separated by first and second gaps  145 ,  146 , respectively. 
         [0021]    In one embodiment, the heat exchanging plate  100  including the plurality of lower and upper protrusions  111 ,  121 , respectively, the flue aperture  130 , and the flow diverter  140 , may be simultaneously formed of cast iron by traditional methods. The height and geometric configurations of the protrusions  111 ,  121 , may vary. For example, in one embodiment, the heights of the protrusions may gradually increase from one region of the heat exchanging plate  100  to another region of the heat exchanging plate  100 . In another example, the upper protrusions  121  within the first region  161  may be substantially equal in height above the second surface  120  as the lower protrusions  111  are in height below the first surface  110 . In one aspect of this embodiment, the lower protrusions may be 1.3 inches in height while the upper protrusions  121  within the first region  161  may be 1.5 inches in height. Conversely, the upper protrusions  121  within the second region  162  may be substantially shorter in height above the second surface  120  than the lower protrusions  111  are in height below the first surface  110 . For example, in one embodiment, the upper protrusions within the second region  162  may be 0.375 inches in height. 
         [0022]    Cross sections of airfoils referenced in this description are taken parallel to the surface  110  or  120  of the heat exchanging plate  100 .  FIG. 2A  illustrates a cross section of one geometric configuration that the protrusion might take. In this embodiment, the geometric configuration is a round airfoil  210  in a free-stream, laminar airflow  230 . A free-stream, laminar airflow  230  is generally representative of the flow of combustion products and room air over the surfaces  110 ,  120  of the heat exchanging plate  100  in heat exchanging chambers to be described below. Note that the airflow around the round airfoil  210 , as might be achieved by affixing round rods sticking up from the surfaces of a heat exchanging plate, separates from free-stream laminar flow and becomes turbulent just prior to points  211 ,  212  on the surface of the rod/round airfoil  210 . Points  211 ,  212  are found by constructing a diameter d that is normal to the airflow through the center of the rod/round airfoil  210 . Of course, the actual points  211 ,  212  will vary as no flow is perfectly laminar. One who is of skill in the art will recognize that low speed airflow  230  around the cylinder  210  will be laminar flow around the leading edge of the cylinder  210  and turbulent flow from points  211 ,  212  on the surface of the cylinder  210  and beyond. 
         [0023]    Referring now to  FIG. 2B  illustrated is a sectional view of another geometric configuration that the protrusions  111 ,  121  might take. In this particular embodiment, the configuration is a symmetric low-speed airfoil  220  in the same free-stream, laminar airflow as in  FIG. 2A . In this case, the symmetric low-speed airfoil  220  has a maximum thickness d equal to the diameter d of the rod  210  of  FIG. 2A . The symmetric low-speed airfoil  220  is representative of one of the lower and upper protrusions  111 ,  121 , respectively. In one embodiment, the lower and upper protrusions  111 ,  121  may comprise an airfoil cross section tapering in thickness d toward the tip much as a low-speed wing cross section has a decreasing thickness toward the wing tip. In a preferred embodiment, the lower and upper protrusions  111 ,  121  may comprise an airfoil cross section that is symmetric about the chord line of the airfoil. The chord line being defined as a straight line drawn from the leading edge of the airfoil to the trailing edge. In contrast to the rod/round airfoil  210  of  FIG. 2A , airflow around the symmetric low-speed airfoil  220  remains laminar along the first and second surfaces  223 ,  224  of the low-speed airfoil  220  until at points  221 ,  222  almost at the trailing edge  225  of the low-speed airfoil  220 . Because of the laminar flow around most of the low-speed airfoil  220 , air flow remains in contact with the surfaces  223 ,  224  of the low-speed airfoil  220  for a greater time than with the rod/round airfoil  210 ; thus ensuring significant heat transfer between the airflow  230  and the low-speed airfoil  220 . The same principle will be used in the transfer of heat from the second side of the heat exchanging plate with upper protrusions to the room air as will be described below. 
         [0024]    Referring now to  FIG. 3 , with continuing reference to  FIGS. 1A and 1B , illustrated is a right side, vertical sectional view of one embodiment of a wood burning stove  300  employing the heat exchanging plate  100  of  FIG. 1 . The stove  300  comprises a stove cabinet  310 , a firebox  320 , a hearth  330 , a flue baffle plate assembly  340 , a firebox door  350 , a fan  360 , a flue  390  and first and second heat exchange chambers  391 ,  392 , respectively. 
         [0025]    The heat exchanging plate  100  may be coupled to the stove cabinet  310  and the firebox  320  with mechanical fasteners  370  through coupling apertures  150 . In one embodiment, the flue baffle plate assembly  340  may be a ceramic plate; however, other heat retaining materials, such as metal and alloys thereof may be used. In a preferred embodiment, the flue baffle plate assembly  340  may comprise first and second ceramic plates  341 ,  342 , respectively. The first heat exchange chamber  391  is bounded from below by the flue baffle plate assembly  340  and from above by the first surface  110  of the heat exchanging plate  100 . The second heat exchange chamber  392  is bounded from below by the second surface  120  of the heat exchanging plate  100  and from above by a stove cabinet top  311 . The first heat exchange chamber  391  is bounded also by the side walls (not shown) of the firebox  320 . The second heat exchange chamber  392  is, in a like manner, bounded by the side walls (not shown) of the cabinet  310 . In a preferred embodiment, the stove cabinet top  311  has a first section  312  and a second section  313  at different heights above the heat exchanging plate  100  to accommodate the different heights of upper protrusions  121  in the first and second heat exchanging plate regions  161 ,  162 , respectively. 
         [0026]    In general operation, the stove  300  houses a fire  380  on the hearth  330 . The fire  380  generates heated combustion products  385  that circulate via pathway  387  through the first heat exchange chamber  391  and out the flue  390 . Ambient air is drawn in through the fan  360 , forced through a duct  365  into the second heat exchange chamber  392 , across protrusions  121  and out the front of the stove cabinet  310  as two conditioned airflows  367   a,    367   b,  collectively  367 . 
         [0027]    Referring now to  FIG. 4  with continuing reference to  FIG. 3 , illustrated is a plan view of the first surface  110  of one embodiment of the wood burning stove heat exchanging plate  100  with combustion products  385  flow depicted. Shown thereon is the path of the combustion products  385  across the first surface  110  and around the plurality of lower protrusions  111 . Note that the leading edges (blunt end) of the lower protrusions  111  are positioned into the prevailing combustion products flow  385 . The combustion products  385  are deflected by and around the first wishbone-shaped forward diverter  141 . The forward diverter  141  combined with the second arcuate rear diverter  142  causes the combustion products  385  to flow toward a back of the first heat exchange chamber  391  and then through the first and second gaps  145 ,  146  and up the flue  390 . As the combustion products  385  flow through the first heat exchange chamber  391 , heat is transferred from the combustion products  385  to the first surface  110 , the plate body  105  and the plurality of lower protrusions  111 . The forward diverter  141  generally assures that the combustion products  385  do not immediately exit the first heat exchange chamber  391  through the flue  390  without at least transferring some heat to the back part of the heat exchanging plate  100 . Heat is then further transferred by conduction to the second opposing surface  120  and to the plurality of upper protrusions  121 . 
         [0028]    Referring now to  FIG. 5  with continuing reference to  FIG. 3 , illustrated is a plan view of the second opposing surface  120  of the heat exchanging plate  100  with heating air flow depicted. Shown thereon is the path of the ambient room air  363  drawn in through fan  360  and directed through duct  365  to the second heat exchange chamber  392 , across the second opposing surface  120 , around the flue  390  and the plurality of upper protrusions  121 . Air flowing across the second opposing surface  120  and ejected into the room is designated conditioned air  367  and shown in  FIG. 3  as conditioned air  367   a,    367   b.    
         [0029]    Referring now to  FIGS. 6A-6C , illustrated are a top, front and right side elevation views, respectively, of the stove  300  of  FIG. 3 . The stove  300  illustrates three points in the vicinity of the stove where temperature data was collected to compare a conventional steel firebox top to the heat exchanging plate  100  of the present discussion. The first temperature collection point  611  is that of ambient air being drawn into the fan  360  of the stove  300 . The second temperature collection point  612  is within the flue  390 . The third temperature collection point  613  corresponds to the heated air  367  being expelled from the top front of the stove  300 . 
         [0030]    For comparative testing, a conventional steel firebox top was provided of 0.25″ thick, hot rolled steel. The steel firebox top was intended as the baseline of conventional design to be compared to the heat exchanging design of the present disclosure. A cast iron prototype of the heat exchanging plate  100  was formed to provide comparative data on the new design. 
         [0031]    Three test runs of the conventional steel firebox top without protrusions were accomplished and the temperature results are shown as follows: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Ambient 
                 Flue Temp 
                 Heated Air 
                 ΔT = Heated − 
               
               
                 Sample Sets 
                 Air ° F. 
                 ° F. 
                 ° F. 
                 Ambient 
               
               
                   
               
             
             
               
                 Steel 1 
                 80 
                 317 
                 111 
                 31 
               
               
                 Steel 2 
                 82 
                 326 
                 115 
                 33 
               
               
                 Steel 3 
                 79 
                 327 
                 109 
                 30 
               
               
                   
               
             
          
         
       
     
         [0032]    Four test runs of the cast iron heat exchanging plate  100  were made with the temperature results as shown: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Ambient 
                 Flue Temp. 
                 Heated Air 
                 ΔT = Heated − 
               
               
                 Sample Sets 
                 Air ° F. 
                 ° F. 
                 ° F. 
                 Ambient 
               
               
                   
               
             
             
               
                 Heat 
                 88 
                 321 
                 135 
                 47 
               
               
                 Exchange 1 
               
               
                 Heat 
                 79 
                 308 
                 130 
                 51 
               
               
                 Exchange 2 
               
               
                 Heat 
                 73 
                 307 
                 120 
                 47 
               
               
                 Exchange 3 
               
               
                 Heat 
                 78 
                 315 
                 123 
                 45 
               
               
                 Exchange 4 
               
               
                   
               
             
          
         
       
     
         [0033]    These temperatures can be converted to approximate 
         [0034]    BTUs into the conditioned space with the formula: BTU/hr=CFM*ΔT*1.08. For the cast iron heat exchanging plate of the present discussion, the average temperature increase in the heated air over the ambient air is: ΔT=47.5° F. For the conventional steel firebox top, the average temperature increase in the heated air over the ambient air is: ΔT=31° F. The heat output results are: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 CFM 
                 ΔT 
                 BTU/hr 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Heat Exchange 
                 50 
                 47.5 
                 2565 
               
               
                   
                 Conv. Steel 
                 50 
                 31.3 
                 1690 
               
               
                   
                   
               
             
          
         
       
     
         [0035]    Heat output may be compared to that of the conventional stove top by dividing the heat (BTU/hr) increase of 875 BTU/hr by the conventional steel firebox top output of 1690 BTU/hr. The result is a heat output increase of 52.3%. Thus, the cast iron heat exchanger significantly improved heated air output by more than a 50% increase over a conventional steel firebox top design. 
         [0036]    Stove efficiency can be expressed as: 
         [0000]      Efficiency=(100−T.A.R.)−[(0.343/CO2 m +0.009)*Δ T] 
 
         [0000]    where T.A.R. is Theoretical Air Requirement which for propane gas, the fuel used, equals 23.86. CO2m is measured CO2, ΔT is the flue loss temperature, i.e., flue temperature minus room temperature in ° C. and the ° F. to ° C. conversion is: 
         [0000]      ° C.=5/9*(° F.−32).
 
         [0000]    Thus efficiency results for the cast iron heat exchanging plate vs. steel firebox top are shown in  FIG. 7 . 
         [0037]    The average efficiency of the heat exchanging plate is 47.1% vs. the average efficiency of the steel firebox top being 43.3%. Thus, the efficiency improvement is (47.1%−43.3%)/43.3%=8.8% improvement. 
         [0038]    Thus, a wood stove, as an example of a heating device, comprising a heat exchanging plate defining the boundary between the combustion products and conditioned/circulating room air has been described. The heat exchanging plate comprises aerodynamic protrusions on lower and upper surfaces thereof to better transfer heat from the combustion products to the heat exchanging plate in the first heat exchange chamber, thence through the heat exchanging plate and to the circulating room air in the second heat exchange chamber. 
         [0039]    For the purposes of this discussion, use of the terms “providing” and “forming,” etc., includes: manufacture, subcontracting, purchase, etc. Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.