Abstract:
A multi-fuel boiler having multiple, independent draft or combustion air sources and controls. A conventional oil/gas burner and nozzle assembly is fitted to an injector tube and isolated from a thermal transfer medium. A butterfly valve is supported in the injector tube and a motorized linkage directs valve operation. When the burner is non-operational, the valve prevents secondary combustion air from passing through the oil/gas burner to effect combustion in the firebox.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to wood-fueled boilers that are capable of using multiple fuels (e.g. wood, fuel oil and/or natural or propane gas) and, in particular, to a boiler outfitted with a gas/oil burner assembly having an independently controlled, air control valve mounted between the firebox and burner nozzle. 
     Wood and biomass fueled boilers necessarily include air controls to vary the draft or amount of combustion air admitted to the firebox. Uncontrolled drafts are to be avoided, if optimized control is to be obtained over combustion. 
     Where the boiler is operable with multiple fuel sources, separate nozzle and burner assemblies are typically coupled to the firebox to admit heated combustion gases. The burner is typically controlled independently to assure continuing boiler operation, even if the wood fire is allowed to burnout. Operational problems can arise, however, from extraneous and uncontrolled airflow through the burner assembly that is not coordinated with the draft control to the wood fire. 
     The present improved boiler was developed to accommodate the foregoing need and appropriately control and/or isolate airflow through an oil/gas burner assembly relative to the firebox. An injector tube that contains an oil/gas nozzle and through which combustion gasses flow includes a butterfly valve to control airflow through the tube. The valve is normally closed, unless the burner is operational, to isolate the firebox from the ambient environment and unregulated air. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the invention to provide a multi-fuel boiler with a combustion air-controlled oil/gas burner assembly. 
     It is further object of the invention to provide a boiler, primarily fueled by wood or other solid fuels, with a secondary burner assembly fueled by oil, gas and other non-solid fuels and having a separately controlled, independent source of combustion air that can be isolated from the firebox. 
     It is a further object of the invention to provide an injector tube that contains a fuel nozzle and a butterfly valve and which valve operates to isolate combustion air at the secondary burner assembly from the firebox when the burner isn&#39;t operating (i.e. wood fire only) to control extraneous airflow to the firebox. 
     It is further object of the invention to provide multiple modes of secondary burner operation to backup, supplement or replace wood combustion and during which the valve is open, for example, a) automatic secondary fuel backup, b) timed “startup”—secondary fuel burner fired for timed period at startup of wood fire, and c) oil/gas only. In the latter mode, the wood fire damper is closed. 
     The foregoing objects, advantages and distinctions of the invention are obtained in a presently preferred, butterfly controlled burner assembly. The assembly provides a conventional oil/gas burner and nozzle assembly that is fitted to an injector tube. A butterfly valve is supported in the injector tube, forward of the nozzle, and a motorized linkage directs valve operation. When the burner is non-operational, the valve isolates the oil/gas burner from the boiler to prevent drafts that can interfere with the regulation of the primary combustion air to the firebox. 
    
    
     Still other objects, advantages, distinctions and constructions of the invention will become more apparent from the following description with respect to the appended drawings. Similar components and assemblies are referred to in the various drawings with similar alphanumeric reference characters. The description should not be literally construed in limitation of the invention. Rather, the invention should be interpreted within the broad scope of the further appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective drawing shown in partial cutaway to a dual fuel fired boiler of the invention. 
     FIG. 2 is a perspective drawing of the water-cooled oil/gas injector assembly. 
     FIG. 3 is a perspective drawing shown in exploded assembly to the oil/gas injector assembly. 
     FIG. 4 is a perspective drawing shown in exploded assembly to the butterfly control linkage of the oil/gas injector assembly. 
     FIG. 5 is a schematic diagram of the electro-mechanical control for the boiler. 
    
    
     Similar structure throughout the drawings is referred to with the same alphanumeric reference numerals and/or characters. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a view is shown in partial section to a wood-fueled boiler  2 . The boiler  2  includes an aesthetically pleasing external enclosure  4  that surrounds an internal firebox  6  and adjoining liquid heat exchange chamber  8 . Convoluted channels  10  are formed in the top wall  12  of the firebox  6  to increase the thermal transfer surface area. A channel  14 , immediately adjacent an exhaust space  16  and that opens to the chimney  18 , depends substantially beneath the other channels  10  to screen the exhaust space and promote secondary combustion before the combustion gases are exhausted. 
     A hinged door  20  provides access to the firebox  6 . A draft control  22 , which is coupled to an aqua stat or other temperature sensitive controller, reference FIG. 5, regulates the draft and flow of combustion air to the firebox  6  to optimize combustion relative to the temperature of the thermal transfer media. A water/glycol mixture is typically used as the thermal transfer media and piped underground to the heated structure. 
     The principal fuel of the boiler  2  is wood, coal or other solid biomass material. The boiler  2 , however, is adapted to permit any variety of alternative fuels that may be available in different regions. This also allows the user to reduce operating costs and provides user convenience, such as during burn down or startup periods. 
     An oil/gas burner assembly  24  is separately mounted to the enclosure  4  to accommodate such fuels. The assembly  24  extends through the heat exchange chamber  8  and into the firebox  6 . The assembly  24  can accommodate fuel oil, propane or natural gas. Combustion air to the assembly  24  is independently and separately drawn from the ambient environment surrounding the enclosure  4  via vents  25 . The assembly  24  is also capable of being isolated from the ambient environment and segregated from effecting the draft control  22 , when not in use. 
     The burner assembly  24  is operable in the several modes, which include a) wood only, b) automatic dual fuel backup, which depending upon the temperature of the firebox provides heat even if the user does not refuel the firebox  6 , b) clean start or timed “startup” which causes the secondary fuel burner to be fired for a timed period at startup of a wood fire to immediately create secondary combustion, and c) oil/gas only. In any of the modes when the present burner assembly  24  is operational, the burner assembly  24  can provide up to 250,000 BTU&#39;s with propane/natural gas and up to 420,000 BTU&#39;s with oil. The particular burner rating for each boiler  2 , however, depends upon the burner and fuel type. 
     With additional attention to FIG. 2, the secondary fuel is appropriately admitted to the burner assembly  24  and the flame from combustion is directed through an injector tube  26 , past a butterfly valve  28  and into the firebox  6 . In each of the foregoing oil/gas modes, the valve  28  is held open to fully expose the bore of the injector tube  26  and the damper  22  is independently controlled to promote and/or maintain wood/solid fuel combustion. In a “wood only” mode, the valve  28  is closed and the damper  22  solely regulates the flow of combustion air to the firebox  6 . Otherwise, leakage of air through the burner assembly  24  can interfere with and produce inefficiencies with the solid fuel combustion. 
     FIGS. 3 and 4 depict the detailed construction of the burner assembly  24  and the drive control to the valve  28 . FIG. 5 depicts the electro-mechanical control schematic to the operation of the draft control  22  and burner assembly  24 . The assembly  24  includes a conventional pressurized burner  30  that is compatible with a desired secondary fuel, such as fuel oil, propane gas or natural gas, waste oil, bio-diesel, vegetable oil to name a few of the many available non-solid fuels. A nozzle and orifice (not shown) is enclosed within a surrounding burner tube  34  along with an adjoining igniter (not shown) to control combustion with the benefit of air admitted from the vents  25 . 
     A mounting plate  36  supports the burner  30  to a flange  40  attached to the injector tube  26 . The plate  36  and flange  40 , in turn, are secured to a flange  42  that is welded to the end of a waterjacket tube  38 . The tube  38  projects through the heat exchange chamber  8  and into the firebox  6 . The flange  42  is secured with fasteners  43  to the external walls of the boiler  2  and sealed to prevent leaking from the heat exchange chamber  8 . A liquid thermal transfer medium surrounds the tube  38 . 
     A forward, eccentric-shaped plate  44  is welded to the tubes  26  and  38  and generally supports the tubes  26  and  38  in concentric relation to each other. The butterfly valve  28  pivots in the tube  26  at an axle  46  that spans the bore of the tube  26  and extends at one side into the space between the tubes  26  and  38 . A sprocket  48  mounts to the axle  46  and a chain linkage  50  extends to another sprocket  52  adjacent a bracket  53 . A spring tensioner  54  is mounted along the length of the chain  50  to control chain tension and prevent chain breakage. 
     An axle  56  is secured to the sprocket  52  at the bracket  53 . A damper position switch  58  sensitive to the rotation of said valve  28  is separately secured along the axle  56  and a motor  60  is coupled to the end of the axle  56 . The motor  60  is separately attached to the flange  40 . 
     A four-position switch  62  operates in concert with a timer  64  and temperature sensitive switches  66  (e.g. snap disks), shown at FIG. 5, and the switch  58  to control the operation of the motor  60  and rotation of the axle  56  and thereby the valve  28  during the foregoing modes. The valve  28 , in turn, controls the flow of secondary combustion air to the firebox  6 . The timer  64  operates during the “clean start” mode in concert with one of the switches  66  to provide a period of time during which the burner is operational to induce a draft in the firebox  6  or until the switch  66  opens. During the “backup” mode, the second switch  66  causes the burner  24  to operate at temperatures below the preset temperature of the switch (e.g. a nominal 155° F.). 
     A door switch is also provided at the door  20  to terminate or prevent operation of the burner assembly  24 , if the door  20  is open. 
     While the invention has been described with respect to a number of preferred constructions and considered improvements or alternatives thereto, still other constructions may be suggested to those skilled in the art. It is also to be appreciated that selected ones of the foregoing features can be used singularly or can be arranged in different combinations to provide a variety of improved boiler assemblies. A variety of alternative air control valve structures, drive linkages and/or burner controls can be used in lieu of those described. The foregoing description should therefore be construed to include all those embodiments within the spirit and scope of the following claims.