Abstract:
A wood burning stove in the shape of a barrel having both primary and secondary air inlets for supplying clean combustion. Primary air is fed into the stove from above the door. Secondary air is fed into a heating duct which extends along the bottom of the stove to be preheated. The heating duct empties the preheated air into a plenum formed in the end of the stove remote from the door. Air admitted to the plenum is additionally heated by exposure to a series of heat transfer fins within the plenum. The shape of the plenum is such that the velocity of air in the central portion of the plenum is the slowest, allowing for efficient heat transfer from the plenum walls and fins to the air passing through. A baffle mounted in the stove above the fire shields the fire from the exhaust outlet. Secondary air is admitted into the interior of the stove from the plenum from a first set of ports near the top of the plenum. Additional ports located in the baffle and spaced some distance from the plenum feed tertiary air into the stove at some distance from the plenum. The entire interior surface of the stove is coated with a suitable glass material.

Description:
FIELD OF INVENTION 
     This invention is directed to a heating stove which is not limited to but preferably to stoves adapted to the burning of wood. 
     BACKGROUND OF THE INVENTION 
     Mankind has made use of heating appliances generally categorized as stoves for hundreds of years. For the most part, the primary function of stoves until recently was for cooking and heating. While this is still true today, other factors have driven the design of stoves to produce heat more efficiently while discharging less of the undesirable byproducts of combustion into the atmosphere. Prior art stoves have traditionally had the capability of producing large amounts of heat, whilst simultaneously producing large undesirable amounts of noxious substances which were expelled into the atmosphere by the burning fuel. 
     In order to provide an acceptable life for most of the prior art woodburning stoves, manufacturers usually provided a metallic shell in which firebricks and/or heavy iron castings were formed and fitted into a firebox in order to protect and shield the inner surface of the outer metallic shell of the stove from the burning fuel. The resultant stove tended to be quite massive, slow to heat and difficult to move. Because of the massiveness of these stoves, considerable heat energy is required just to raise the temperature of the stove to the desirable operating temperature. 
     Recently stove manufacturers resorted to producing an “airtight” stove which limited the amount of combustion air allowed to the firebox so that a firebox filled with wood could be made to burn at a controlled rate for many hours. 
     Because of the lack of oxygen supplied to the burning wood, these “airtight” stoves tended to produce copious amounts of creosote and other gaseous products resulting from incomplete combustion of the burning fuel because of oxygen starvation. The low temperature of the emitted flue gas also allowed creosote and other noxious substances to be deposited in the cold chimney flue. 
     Continued use of these “airtight” stoves usually resulted in a chimney fire from time to time. Because of the problems associated with this type of heating appliance, environmental authorities had little choice but to introduce stringent restrictions on the types of stoves which could be sold in each jurisdiction. 
     In 1988 the U.S. Environmental Protection Agency introduced a set of standards for New Residential Wood Heaters under Title 40—Code of Federal Regulations Part 60, which has had a great influence on the design of stoves which have been and are to be introduced into the U.S. market. The presence of these Regulations has provided stove manufacturers all over the world with a set of guidelines to measure the efficiency of any wood burning stove and the resulting production of any undesirable emitted materials produced by the stove under test during a monitored burning operation so as to enable a comparison of the test stove results against a (set of) given standard(s). 
     It is with a view to the production of a stove which is able to easily meet the 40 C.F.R. (60) regulations that this invention is directed. 
     DISCUSSION OF PRIOR ART 
     U.S. Pat. No. 4,941,451 Jul. 17, 1990 
     A stove having a firebox which is surrounded by multiple air chambers is described. Primary air enters the front of the stove just below the door and is ducted to the top of the firebox where it is directed downwardly from a point well above the burning fire to induce combustion of the fuel in the firebox. 
     Cooling air for the stove also enters the stove in an opening in the bottom of the stove below the firebox floor. A fan is shown propelling air entering the opening into three separate streams. 
     A first stream is ducted up the back of the stove behind the firebox and across the top of the stove and out to the room via louvres. 
     A second stream is ducted upwardly in a pair of riser tubes to empty from a manifold above the fire but below the hollow baffle. Air leaves a secondary manifold to ignite and burn unburned gases. 
     A third stream enters the hollow baffle from a side space. This air cools the baffle and exits through a series of holes above the second secondary stream. 
     A slider type draft control adjusts the amount of primary air fed to the firebox. The secondary air is pressurized by a fan in the plenum beneath the firebox floor. 
     U.S. Pat. No. 4,832,000 
     This patent uses separate primary and secondary airflows to improve the combustion of the fuel in the firebox. Both primary and secondary airflows are preheated. 
     U.S. Pat. No. 4,665,889 
     A stove having a baffle and separate primary and secondary airflow paths is illustrated. The primary air is not really heated, but the secondary air is heated during its passage through the secondary duct work. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a stove which is extremely lightweight (in comparison to the heavy stoves of recent vintage) and typically uses sheet steel as the basic material for forming an enclosure for a typical stove fire box. The interior of the sheet material forming the firebox is preferably coated with a layer of a preselected material which is resistant to break down due to exposure to high temperature and the products of combustion present in a firebox. The sheet steel which forms the firebox of the stove of this invention is typically coated with a protective layer of a suitable glass material on the inside surface to protect the steel sheet from the effects of exposure to the high temperatures existing in a firebox and the combustion byproducts produced therein. The sheet steel is typically a mild steel with low carbon content which lends itself to the glass coating process which must be carried out in an oven at temperatures approaching 1500° F. The glass coating is selected to be a high temperature glass which contains a small amount of titanium (up to about 8%) which tends to have the effect of making the interior glass surface of the firebox self cleaning. The glass film and the metallic sheet steel base material must have similar coefficients of expansion in order that the glass coating steadfastly adheres to the base material during the many temperature excursions to which the glass coated sheet steel will be subjected over the life of the stove. 
     The stove is provided with primary and secondary inlet air passages which are designed specifically to control the quantities of primary and secondary invitiated air allowed to enter the combustion chamber of the stove during a normal combustion process. The secondary inlet air is ducted through passages in the stove which are placed so as to be in excellent heat transfer relationship with the burning fuel in the combustion chamber of the stove so as to efficiently heat the air in the duct work to a temperature approaching or matching that existing in the combustion chamber of the stove. 
     The primary air (unheated) enters the stove above the access door and is ducted downwardly so as to sweep downwardly against the inside surface of the glass on the access door. This tends to prevent any buildup of smoke particles on the glass in the door. Because of the difference in density of the cold inlet air and the hot air near the burning fuel, the inlet air tends to make its way to the bottom of the firebox to promote primary combustion. 
     The stove of this invention is provided with a forwardly extending baffle which extends from the rear of the combustion chamber and which is fastened into the combustion chamber at each side of the baffle to the interior of the stove at some distance beneath the exhaust vent. This baffle prevents the hot air produced during the burning process from exiting directly from the fire into the exhaust vent and up the flue. Because the hot gasses produced by pyrolysis must linger longer in the hot combustion chamber, the chances for oxidation of these gasses to occur is much greater in the presence of the baffle. 
     The secondary air enters the stove through a draft control (at the front of the stove) and passes through a heat exchanger duct or preheat heat exchanger to the rear of the stove which allows the secondary air to undergo a preheating operating during its passage to the rear of the stove. This preheated air next enters a heat exchanger which defines a plenum or chamber (at the back of the stove) where the air inside the heat exchanger is heated to a temperature approaching the maximum temperature in the combustion chamber. This heated air is allowed to exit from the heat exchanger from preferably two sets of exit ports. 
     Some of the heated secondary air exits the heat exchanger of the stove from exit ports formed in the heat exchanger just below the point of insertion of the baffle. The balance of the secondary air may de ducted forwardly in the stove toward the front of the combustion chamber in a duct associated with the baffle and which is provided with suitable exit parts in the baffle so that heated air is expelled from these exit ports near the front of the stove. 
     It is the combination of the admittance of these predetermined volumes of primary and secondary air in the presence of the baffle which determines the efficiency and the U.S.E.P.A. rating of the stove during a burning operation. 
     Prior art stoves have found it all but impossible to yield acceptable heating efficiencies and E.P.A. rating produced by this stove while simultaneously meeting the emission criteria of 40 C.F.R. Part 60 during a monitored burning operation. 
     In a first embodiment of this invention, there is provided a wood-burning stove having a combustion chamber which is the general shape of a barrel resting on its side. The interior surfaces of the stove which are exposed to the hot exhaust gases are coated with a suitable glass material. The two “ends” of the “barrel” are specially designed closure members designed to improve the efficiency of the stove. The front closure member has an opening formed therein for providing access to the combustion chamber. The rear closure member is formed into a heat exchanger. The above structure is supported on a base which is incorporated into the structure, and which is provided with a set of legs. The combustion chamber is provided with a baffle to control the flow of the hot gases before exit through the exhaust vent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the stove of this invention. 
     FIG. 2 is an exploded view of the front closure assembly of the stove of FIG.  1 . 
     FIG. 3 is an exploded view of the rear closure member of the stove of FIG.  1 . 
     FIG. 4 is a partial sectional perspective view of the stove of FIG.  1 . 
     FIG. 5 is a side elevational sectional view of the stove of FIG. 1 showing the airflow pattern. 
     FIG. 6 is a top partial sectional view of the stove of FIG.  1 . 
     FIG. 7 is a representation of the airflow in the rear chamber formed in the stove of FIG.  1 . 
     FIG. 8 shows a perspective view of a heat sink fin. 
     FIG. 9 shows a perspective view of an alternate heat sink fin. 
     FIG. 10 shows a sectional view of the stove-baffle interface. 
     FIG. 11 shows a perspective view of an alternative stove construction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The stove  10  of this invention is shown in perspective in FIG.  1 . The stove  10  comprises a combustion chamber  12  (firebox) which is sealingly attached to a front closure member  14  and a rear closure member  16  (not shown in FIG.  1 ). The front closure member comprises a front outer panel  18  which is attached to inner panel member  20  (see FIG.  2 ). Front closure member is a composite structure which provides closed passages for admission of primary and secondary air into the combustion chamber formed within shell  12 . A draft control  22  controls the flow of primary air and draft control  24  controls the flow of secondary air into the combustion chamber  12 . 
     Front outer panel  18  provides a framework to which legs  26  are attached to support the front portion of the stove. 
     The front outer and inner panels  18  and  20  are provided with flanges  28  and  30  respectively which may be welded or otherwise joined together to form a closed composite assembly. Sliding dampers  22  (primary) and  24  (secondary) are assembled into the composite before final welding takes place. Damper  22  is enclosed in a closed box formed by member  32  within front closure member  14 . 
     A pair of side shields (may or may not be required)  34  are attached at one end to front closure member  14 . An ashtray  35  may not be necessary) is also attached to front closure member  14 . 
     A door  36  is hingedly mounted on front closure memeber  14  on hinges  38 . Door  36  has a glass  40  (in this instance) held in place by inner frame  42 . 
     FIG. 3 shows an exploded view of the rear closure member  16  and combustion chamber shell  12 . Rear closure member  16  is also a composite, comprising an inner plate  50  and an outer plate  52  which are joined together at their peripheries In a sealed fashion by press fit, welding etc. to from a sealed A series of heat exchanger ( 88 ) which defines a heat exchanger chamber  17 . Fins  56 ,  58  are mounted in chamber  17  as will be described later. 
     An insulation disc  60  is mounted in intimate contact with the outer surface of rear outer plate member  52 . A rear frame member  62  having a heat shield  64  (may not be necessary) serves to support the combustion shell  12  as well as the rear closure member  16  on legs. 
     Lastly, combustion chamber shell  12  supports a “flat top”  70  on the top thereof, and chamber  12  has an aperture  72  formed therein in which collar  74  is fitted so as to form an exhaust gas vent in combustion chamber shell  12 . 
     A baffle  76  (see FIGS. 4 and 6) is provided for illustration in combustion chamber shell  12  by any convenient method, is this instance a plurality of projecting abutments  77  are formed in the Inside of combustion chamber shell  12  which hold the baffle in its installed position. Baffle  76  is provided with a plurality of recesses  75  to allow easy Installation of the baffle in the combustion chamber  12 . 
     A floor  78  is provided to be sealingly mounted in the lower region of combustion chamber shell  12 . 
     Referring specifically to FIG. 1 door  36  is shown mounted on hinges  38  to cover the aperture formed in front closure member  14  for fueling the stove  10  and removing the ashes produced in combustion chamber of stove  10  therefrom. The front closure member  14  and the rear closure member  16  when assembled with shell  12  provides a sealed combustion chamber. The side shields  34  are mounted on the two closure members  14  and  16  so as to provide some clearance between the combustion chamber shell  12  and shields  34  to permit natural air to flow there between. 
     Referring now specifically to FIG. 4, the various parts of the stove are shown in a partly sectioned perspective view of the stove  10 . Here the opening  80  into combustion chamber  82  is clearly shown. The composite construction of front closure member  14  is also clearly shown, slider draft members  22  and  24  having been omitted for clarity. Ports  21  and  25  are shown in closure member  14 . 
     Floor  78  is sealed into combustion chamber shell  12  as well as to front and rear closure members  14  and  16 . 
     The inner plate member  50  of rear closure member  16  is clearly shown as is a portion of outer plate  52 . Baffle  76  is shown mounted in combustion chamber shell  12 . 
     A series of apertures  90  are shown in inner plate  50  to permit the escape of heated air into combustion chamber  82  from the rear closure member  16 . A duct  84  is shown mounted on baffle  76 . 
     FIG. 4 shows the enlarged view of a section of shell  12  having a glass coating  13  formed thereon. (The thickness of the glass coating  13  is typically 6 to 12 thousandths of an inch.) 
     FIG. 5 shows the primary, secondary and tertiary airflow in stove  10 . 
     Primary air enters front closure member  14  through ports  23 , past slider damper  22  and down through ports in upper flange  28 ,  30  of door opening  80  to enter the combustion chamber  82  just above door glass  40 . The cold air sweeps downwardly past glass  40  and curves toward the burning fuel in combustion chamber  82  to provide oxygen for the burning of the fuel. 
     At the same time secondary (and tertiary) air is admitted into ports shown as  25  in front closure member  14  and past slider damper  24  through ports  21  in inner panel  20  of front closure member  14  to enter a preheat heat exchanger  86  formed beneath floor  78  and above shell  12 . As this secondary air travels through preheat heat exchanger  86  it is heated by the burning fuel. Preheat heat exchanger  86  ends at port  87  in inner rear panel member of rear closure  16 . The heated air leaves preheated heat exchanger  86  and enters into chamber  17  of heat exchanger formed between plates  50  and  52  of rear closure member  16 . Here plate  50  forms part of the combustion chamber  82  and this is deliberately exposed to intense heat from the burning fuel in the chamber. 
     The preheated air passes from preheat heat exchanger  86  and into port  87  of heat exchanger  88  and gathers more heat during passage therethrough. Secondary heated air exits at ports  90  formed in plate  50  near the top of the combustion chamber  82  and passes into the upper part of combustion chamber  82 . 
     The construction of the heat exchanger of rear closure member  16  as shown in the exploded view FIG. 3 will not be described in detail. Plates  50  and  52  (form the heat exchanger) are joined at the outer periphery to form an enclosed plenum or chamber  17  therein. The plates  50  and  52  are preferably formed as surfaces of revolution (similar to bottom closure members in commercial hot water tanks). 
     Fins  56  and  58  may be curved to match the surface contour of heat exchanger  88  and are provided with teeth  100  which are separated by recesses  102 . In the scheme shown the heat transfer fins  58  and  58  are provided with teeth  100  of equal width interrupted by a series of substantially identical recesses  102  therebetween. The fins are suitably fastened (usually by welding) to plate  50  at the intersection of each tooth  100  with the plenum or chamber side of plate  50 . It is essential that fins  56  and  58  be in excellent heat transfer relationship in plate  50 . The surface of plate  52  is made to match the surface of plate  50  and each of the fins  58  which are provided with tabs  104  are plug welded to plate  52 . In the construction shown in this application plate  50  is provided with a peripheral lip  106  (see FIG. 5) which is press fined or welded into shell  12 . Similarly, plate  52  is provided with a peripheral lip which is press fitted or welded into lip  106  of plate  50 . 
     The importance of heat exchanger  88  to the overall performance of stove  10  cannot be over emphasized. It is important that invitiated air leaving heat exchanger  88  at exhaust ports  90  has acquired sufficient heat during passage through heat exchanger  88  to achieve a temperature as close as possible to the temperature existing in combustion chamber  82 . Heat exchanger  88  is especially designed so that air entering port  87  in the lower region of plate  50  is allowed to steadily decrease in velocity as it rises in the chamber  17  until the mid-point of travel in the heat exchanger is reached. The heat exchanger air is now steadily accelerated during the last half of the passage through heat exchanger  88  until port  90  and port  92  are reached. 
     The slowing down of the air travelling through heat exchanger  88  allows the air to absorb a substantial amount of heat from the large central area of plate  50  and fins  56  and  58  so that the air exiting from ports  90  and  92  has acquired the maximum available amount of heat during passage through heat exchanger  88  to promote easy combustion of any unburned combustible gases or hydrocarbons encountered in the combustion chamber  82 . It is essential that the air exiting from ports  90  and  92  has been heated to the highest possible temperature to facilitate the burning of any unburned hydrocarbons and other combustible gases which are emitted or pyrolized from the burning fuel. Typically the temperature of the heated air leaving ports  90  and  92  in an established fire in stove  10  would be from about 500 to 950° F. 
     The slowing down of the air travelling through plenum  88  allows the air to absorb a substantial amount of heat from the large central area of plate  50  and fins  56  and  58  so that the air exiting from ports  90  and  92  has acquired the maximum available amount of heat during passage through plenum  88  to promote easy combustion of any unburned combustible gases or hydrocarbons encountered in the combustion chamber  82 . It is essential that the air exiting from ports  90  and  92  has been heated to the highest possible temperature to facilitate the burning of any unburned hydrocarbons and other combustible gases which are emitted or pyrolized from the burning fuel. Typically the temperature of the heated air leaving ports  90  and  92  in an established fire in stove  10  would be from about 500 to 950° F. 
     It is important that the surfaces of stove  10  which are exposed to the hot burning gasses produced during combustion are protected with a suitable barrier of a protective material. Although some metallic coatings are commercially available i.e. aluminized steel, the stove of this invention has an interior surface coating of a suitable glass material. This material must have an expansion coefficient which nearly matches the steel surface on which it is to be deposited in order to prevent cracking, crazing and peeling; the glass coating should also possess good heat transfer characteristics. The glass which has proved to be an excellent coating for this purpose is a high temperature glass having a content of titanium approaching 8%. It is essential that the interior surface of the combustion chamber etc. be coated with the above glass composition or an acceptable substitute. It is usually not necessary to coat the interior of duct  86  or the interior surfaces of plenum  88  with the glass material but these surfaces may in some instances be coated with a glass coating to preserve the surface integrity of these components if desired. Similarly heat transfer fins  56  and  58  may be glass coated (if desired) before final assembly of the rear closure member  16 . 
     It will be found that the glass coated combustion chamber shell  12  yields heat in the shortest possible time when compared to heavy prior art stoves. Because there are no bricks or heavy castings used in the construction of the combustion chamber of this stove, the stove has a minimum thermal mass, thus enabling fast heat production from start-up. 
     The shape of stove  10  has been chosen to be as nearly cylindrical as is possible in order to achieve ease of manufacturing. Other shapes such as elliptical and polygonal are entirely possible. It is difficult to fabricate the rear closure member  16  to include a plenum  88  in an external configurative shape which is not circular. The construction of rear closure member  16  has been chosen to be light and robust (fins  58  fasten plates  50  and  52  together in an assembly) so that no banging or “oil canning” occurs during heating up or cooling down operations. 
     It may be found that in some jurisdictions the emission standards are somewhat relaxed from 40 U.S.C. Part 60. In these instances some of the components of stove  10  may be omitted. For instance insulating disc  60  (in the rear closure assembly  16 ) may be omitted (which slightly reduces the operating temperature of heat exchanger  88 ) as well as heat shield  64  in rear closure  16  in order to simplify the stove construction. 
     As well, the duct  84  located on top of baffle may be omitted from some models in countries where emission requirements are not as stringent as the U.S. The supply of hot “tertiary” air at the front of the combustion chamber is present to meet stiff environmental standards for present and future and to assure that any combustible products which have escaped combustion by the primary and secondary circulated air are exposed to the hot “tertiary” air to promote in one last combustion attempt before such gases are released up the flue. 
     Baffle  76  is an essential element of this construction in order to cause the hot gases to linger in the combustion chamber for a longer duration than would occur in the absence of baffle  76 . Baffle  76  may be attached to the combustion chamber shell in a number of ways, but it has been found that the baffle may be held in place by four (preferably) projecting abutments  77  from the surface of shell  12  which hold baffle  76  in place. Baffle  76  is provided with four recesses such as  75  shown in FIG. 6 which permit baffle to be installed in stove  10 . Recesses  75  are lined up with projections  77  and baffle  76  is bowed upwardly by pushing upwardly in the centre of baffle to position baffle  76  above abutments  77 . As soon as baffle  76  is bowed upwardly between the projecting abutments  77  the necessary clearance between the baffle and the surface of combustion chamber shell  12  is obtained the baffle may be slid rearwardly to its “home” position (against plate  50 ). Baffle  76  is then allowed to relax to an intermediate position which spring loads the baffle against and between the projections  77 . Because the baffle is still bowed in an upwardly convex shape, any dimensional changes occurring in baffle  76  during start up or shut down do not produce annoying clicks and bangs due to expansion and contraction of the baffle  76  or the shell  12  in which the projections  77  holding the baffle  76  in place are formed. The curved shape of the baffle  76  assures that any distortions of the baffle which occur will proceed in a predictable manner. 
     To those skilled in the art, changes and alterations will become immediately apparent once the basic design is disclosed. For instance, FIG. 11 shows an alternative embodiment in the stove  110  Illustrated, in which the preheat heat exchanger  86  shown in FIG. 5 is replaced by a series of tubular ducts  112  in the combustion chamber on which fuel to be burned is placed. The tubes  112  function as efficiently as the preheat heat exchanger  88  (produced by floor  78  and shell  12 ) in performing a heat transfer to air passing through the tubes. It will be obvious to one skilled in the art that other methods of directing the secondary air are possible which still achieve the required heat absorption 
     The disclosure has been relatively silent regarding the presence of heat shielding applied to the stove for applications where safety is a concern. Because of a variation in safety laws, a variety of shielding devices for the stove are possible. Side panels  34  and rear shield  64  have been included in this description but certainly other heat shields i.e. belly shield may be included for various heating applications as the situation demands. In most instances the presence or absence of heat shields such as  34  and  64  have little effect on the overall stove efficiency or the E.P.A. rating, but the shields do affect the temperature of surrounding walls and objects in the immediate area of the stove. 
     In summary, a long life lightweight stove has been disclosed which is easy to fabricate, transport and install. 
     Much of the success of this stove is due to the protection provided to the steel enclosure by the protective coating. Aluminized steel provides a measure of protection and is available commercially. However, a continuous layer of a self-cleaning high temperature glass on the interior surface of the combustion chamber is the preferred coating for this application. 
     Heat exchanger  88  formed between plates  50  and  52  have the general shape of a Belville washer and a real advantage is gained by the production of an enlarged curved surface area of plate  50  facing the burning fuel (when compared to plate  50  if it was flat). The fins  56  and  58  must be curved to match the curving interior surface of heat exchanger  88 . The teeth and recesses of the fins  56  and  58  may be varied in width to slightly increase the resistance to air flow in the centre of the heat exchanger  88 , thus forcing the moving air to spread out across the chamber  17 . 
     Door  36  of stove  10  has been illustrated with a fire viewing glass  40  installed therein. It will be obvious that door  36  may be a solid door. 
     The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.