Patent Publication Number: US-2010112500-A1

Title: Apparatus and method for a modulating burner controller

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention describes a control system to modulate fired combustion burners based on a fuel flow feedback loop with corresponding modulating combustion air flow to provide precise control of the fuel/air ratio. In particular, the present invention relates specifically to the control of burners and includes provisions for systems and techniques for a wide variety of applications. 
     2. Discussion of the Related Art 
     West, U.S. Pat. No. 4,645,450, discloses a system and a process for controlling the flow of air and fuel to a burner. West discloses a pair of differential pressure sensors connected across 1) the air conduit and the burner, and 2) the fuel conduit and the burner. West discloses a butterfly valve in the blower conduit after the blower. 
     Tesar et al., U.S. Pat. No. 6,213,758, discloses a burner air/fuel ratio regulation method and apparatus for a web dryer. Tesar et al. discloses monitoring the differential air pressure between the air chamber and the burner enclosure. Tesar et al. discloses regulation of the air flow of a combustion blower with a variable speed drive controlled motor rather than with a damper to achieve faster and more accurate burner modulation with less electrical energy. 
     Yoshihiko, Japanese Patent Application Publication 11-12419, discloses a combustor and its control method during ignition. Yoshihiko discloses keeping the gas proportional valve at a predetermined value from the time gas is fed until burner ignition to address a temporal reduction of the gas flow rate at the ignition. Yoshihiko does not address modulation over a broad range of operating parameters. 
     Kazou et al., Japanese Patent Application Publication 06-174381, discloses equipment for controlling an atmosphere in a furnace. Kazou et al. discloses a proportional valve on both the air and the gas supply pipelines to make the air/gas ratio fixed over the output range. Kazou et a. disclosures a gas pressure setting proportional valve upstream of the first gas proportional valve that changes gas pressure in accordance with an output. 
     The art has long recognized the benefits of modulation and turndown for combustion devices. Although the foregoing disclosures provide advances in the art, there is a need and a desire for a modulating combustion apparatus and method having low cost and high turndown capabilities. There is also a need and a desire for a modulating combustion apparatus and method able to operate outside of the range of a low cost sensor. There is also a need and a desire for a modulating combustion apparatus and method able to stably turndown a combustion air device. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cost effective control methodology for variable output burner applications through the use of a series of controllable variable output components and utilizing feedback sensors for precision closed loop control. In some embodiments, the invention includes a modulating combustion apparatus and method having low cost and high turndown capabilities. In some embodiments, the invention also includes a modulating combustion apparatus and method able to operate outside of the range of a low cost sensor. In some embodiments, the invention also includes a modulating combustion apparatus and method able to stably turndown a combustion air device. 
     In a first embodiment, the invention includes a combustion apparatus for use in fired variable demand applications. The apparatus includes a fuel valve modulating a fuel flow, and a fuel sensor with a range measuring the fuel flow and sending a fuel output. The apparatus also includes a combustion air device modulating an air flow, and an air sensor measuring the air flow and sending an air output. The apparatus also includes a controller connected to the fuel valve, the fuel sensor, the combustion air device, and the air sensor. The controller modulates the fuel valve based on the fuel output using an extrapolation algorithm when the fuel output extends outside of the range of the fuel sensor, and the controller modulates the combustion air device based on the air output. The controller simultaneously and/or sequentially modulates the fuel flow and the air flow over an extended fuel/air ratio and provides continuous modulation during a single burn cycle. 
     In a second embodiment, the invention includes a combustion apparatus for use in fired variable demand applications. The apparatus includes a fuel valve modulating a fuel flow, and a fuel sensor with a range measuring the fuel flow and sending a fuel output. The apparatus also includes a variable speed driver modulating an air flow of a combustion air device, and a damper modulating the air flow of the combustion air device. The apparatus also includes an air sensor measuring the air flow and sending an air output, and a controller connected to the fuel valve, the fuel sensor, the variable speed driver, the damper, and the air sensor. The controller modulates the fuel valve based on the fuel output, and the controller modulates the variable speed driver and the damper based on the air output. The controller simultaneously and/or sequentially modulates the fuel flow and the air flow over an extended fuel/air ratio and provides continuous modulation during a single burn cycle. 
     In a third embodiment, the invention relates to a method of operating a combustion apparatus for use in fired variable demand applications. The method includes the step of measuring a fuel flow with a fuel sensor having a range and a fuel output, and the step of measuring an air flow with an air sensor having an air output. The method also includes the step of modulating the fuel flow with a fuel valve and a controller based on the fuel output, and the step of modulating the air flow with a combustion air device and the controller based on the air output. The method also includes the step of calculating the air flow or the fuel flow when the fuel output extends outside of the range of the fuel sensor with an extrapolation algorithm, and the step of maintaining simultaneously and/or sequentially the fuel flow and the air flow over an extended fuel/air ratio and to provide continuous modulation during a single burn cycle with the controller. 
     In a fourth embodiment, the invention relates to a method of operating a combustion apparatus for use in fired variable demand applications. The method includes the step of measuring a fuel flow with a fuel sensor having a range and a fuel output, and the step of measuring an air flow with an air sensor having an air output. The method also includes the step of modulating the fuel flow with a fuel valve and a controller based on the fuel output, and the step of modulating the air flow with a damper and a variable speed driver of a combustion air device and the controller based on the air output. The method also includes the step of maintaining simultaneously and/or sequentially the fuel flow and the air flow over an extended fuel/air ratio and to provide continuous modulation during a single burn cycle with the controller. 
    
    
     
       BRIEF DISCUSSION OF THE DRAWINGS 
       These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings, wherein: 
         FIG. 1  shows a diagram of a modulating burner controller, in some embodiments; 
         FIG. 2  shows a diagram of burner controller components for modulating fuel and air feedback loops, in some embodiments; 
         FIG. 3  show a diagram of a fuel/air mixture curve with a fuel/air ratio having operating end points for a range of a fuel flow sensor and an extrapolation for an extended fuel flow sensor range, in some embodiments; 
         FIG. 4  shows a diagram of an air/fuel mixture curve with the air/fuel ratio having operating end points for a range of a fuel flow sensor and an extrapolation for an extended fuel flow sensor range, in some embodiments; 
         FIG. 5  shows a diagram of a burner controller using an extrapolation to extend a modulating fuel valve range and with fuel and air feedback loops, in some embodiments; 
         FIG. 6  shows a diagram of a burner controller using modulating combustion air inlet vane control, in some embodiments; 
         FIG. 7  shows a diagram of a burner controller using a communication port to stage sequential burners, in some embodiments; 
         FIG. 8  shows a diagram of a burner controller using a flue emission sensor, in some embodiments; and 
         FIG. 9  shows a schematic of a modulating combustion apparatus, in some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In some embodiments, the invention relates to burners or combustion devices, such as furnaces, heaters, forced warm air applications, water heaters, boilers, power burners, kilns, ovens, process furnaces, gas turbines, internal combustion engines, and/or the like. The apparatus and the method can provide control of a burner beyond merely supplying fuel and providing air for combustion at a fixed flow rate while igniting the mixture. Desirably, the apparatus and the method allow for efficient operation over a wide or broad range of operation or firing. 
     Furnaces may include any suitable size and/or configuration. Residential furnaces may include any suitable capacity, such as less than about 41 kilowatts (about 140,000 british thermal units per hour). Commercial furnaces may include any suitable capacity, such as greater than about 58 kilowatts (about 200,000 british thermal units per hour). Additional furnaces or units may be staged, such as to add or supply additional capacity. 
     Power burners may include any suitable combustion chamber. Power burner applications may include process applications, cooking ovens, glass melting, glass blowing, industrial processes, and/or the like. 
     Burners may include fixed (single) operation, two-stage (levels) operation, three-stage (levels), and/or more. Fuel valves can modulate or vary by three positions or more to supply fuel to a burner, for example. Modulating fuel valves and two stage combustion air blowers offer limited turndown capacities. However, cost effective modulation and control over a wide range of capacities or outputs is not presently available in the industry. Desirably, it may be appropriate to operate a burner in a very rich (excess fuel) condition for a short time to facilitate a smooth and consistent light-off (ignition), but operate the burner at a leaner (lower fuel/air ratio) there after. 
     Modulating or controllable burners can be used in conjunction with a combustion air blower or device. The combustion air blower may be before the burner in a forced draft configuration or after the burner in an induced draft configuration. Balanced devices or combination devices may include both a forced draft and induced draft configuration. The combustion air device may include any suitable motive force apparatus, such as a fan, an axial fan, a radial fan, a variable pitch fan, a blower, an axial blower, a radial blower, a squirrel cage blower, a compressor, an ejector, and/or the like. 
     The combustion air device may include a damper or suitable flow control device, such as an inlet vane damper, an inlet damper, an inlet suction valve, a butterfly valve, a discharge damper, a discharge valve, and/or the like. Combinations of inlet and discharge dampers are within the scope of this invention, as well as combinations of dampers and variable speed drivers. 
     In some embodiments, the combustion air blower includes an inducer blower to create or make a negative pressure in a combustion chamber, such as to bring or draw air into the combustion chamber and create or make a draft to remove products of combustion. In other embodiments, the blower forms part of a power burner to create or make a positive pressure in a combustion chamber. 
     In some embodiments, the method may include a technique to control the flow of combustion air into a power burner application. The technique may allow accurate stable control of the flow for moderate to high turn-down applications, for example. 
     Known control of the combustion air flow in power burner applications has traditionally been accomplished in one of two ways. The first known method is to employ a damper assembly in the combustion air flow path to decrease the flow during reduced rate operation. This known damper method is commonly used because of low implementation costs and straight-forward mechanical design methods (mechanical linkages), such as without complicated electronic controls. However, the stability of these known systems at higher turn-down ratios is sacrificed for the simplicity of design, causing significant commissioning time at start-up and frequent scheduled maintenance. 
     The second known method for controlling combustion air flow has been with variable speed blower motors utilizing three phase variable frequency drive systems having elaborate feed-back circuits to directly monitor the combustion process through the use of oxygen sensors and the like. These systems may achieve stable control at higher turn-down ratios, but are prohibitively expensive and used only on very large burner systems with high maintenance costs. 
     In some embodiments, this invention may include a less-expensive (reduced capital costs, and/or reduced operating costs) method for controlling a power burner or other combustion device. The apparatus may include a variable speed combustion air blower using a flow-indicating feed-back, such as pressure drop across the blower. The apparatus may include turndown ratios of about 3 to 1 with a relatively high control pressures, such as with the use of affordable (low cost) sensors for the control feedback. For higher turn-down systems of 10:1, 20:1, 30:1, or more, a combination of control may improve operation. 
     Given a burner system with a relatively large turn-down range of 30:1, a combination of mechanical dampers and variable speed motor technologies can be optimized for affordable, stable control of the combustion air flow over the modulated range. Utilizing air flow measurement sensors, such as a pressure transducer sensing the pressure drop across the combustion air blower, the mechanism of controlling the combustion air flow to the desired level to optimize combustion efficiency, flame stability, combustion emissions or other required parameters, can be divided between restricting the air flow with a damper in the flow stream and/or reducing the blower motor speed. These methods can be optimized to the blower system characteristics and/or fan curves, for example. 
     During modulation of the burner in the upper portion of the modulated range or envelope, the combustion process can be much more forgiving of variations of the combustion air flow and stability of the air flow, for example. This portion of the modulation process may be suited for control by the restricting damper in the combustion air flow stream. The combustion blower motor can be efficiently maintained at full speed while the damper can be adjusted to maintain the desired combustion air flow. 
     As the requirement for additional turn-down increases, the damper cannot be closed further towards the completely closed position without encountering decreased stability due to the flow resolution at these operating conditions. With the damper closed, the blower may experience internal recirculation, starved flow, and/or other less stable operation. Desirably, air flow modulation switches from the damper at some point and continues with the reduction of combustion air flow to the burner using the variable speed capabilities of the combustion air device drive. With the restriction the damper has already placed on the system, modulation of the blower motor may result in the deep turn-down operation to support the high turn-down capabilities of the burner. In the alternative, the blower may slow down and the damper may open, such as for a net reduction in air flow. 
     Other methods or apparatus may include holes or apertures in the damper at or around the damper sealing surface, such as to allow complete closure of the damper actuator while allowing a designed air flow to by-pass the damper blade, such as at least about 0.1 percent flow, at least about 0.5 percent flow, at least about 1 percent flow, and/or the like. 
     Efficient burner operation may include operating with a close to stoichiometric amount of fuel and oxygen. Stoichiometric broadly refers to the amount of an element or compound needed to complete a balanced chemical reaction or equation. Stoichiometric combustion broadly refers to the proper ratio of fuel to oxygen for complete combustion with out excess oxygen, such as 1 mole of methane (CH 4 ) with 2 moles of oxygen (O 2 ) yield 1 mole of carbon dioxide (CO 2 ) and 2 moles of water (H 2 O). Stoichiometric combustion can efficiently use all of the available fuel without warming up excess oxygen or air. Desirably, stoichiometric combustion includes combustion of the fuel, such as generally without particulate matter or soot. 
     Complete combustion broadly refers to carbon in the fuel being oxidized to substantially carbon dioxide, such as with reduced, minimal, or no carbon monoxide. If the amounts of fuel and combustion air are known, the actual combustion conditions, relative to stoichiometric, maybe defined. Some applications operate in conditions more appropriately within a lean setting while others may operate more appropriately at a rich setting when compared to stoichiometric. The use of catalytic combustion processes is within the scope of this invention. 
     There are several factors that may affect efficient combustion. Low or fluctuating fuel line pressure can cause unwanted variation of the fuel flow rates. Also altitude can have an effect on burner performance, such as at higher altitudes burners receive air that is less dense with less oxygen. Known controllers without adjustment are derated (lower thermal output) when used at higher elevations than base or nominal altitude, such as sea level. Other factors affecting combustion efficiency may include ambient air temperature, humidity, barometric pressure, and/or the like. Factors affecting combustion efficiency can very from time of day, change in season, and/or the like. 
     Known technology can physically vary the supplies of fuel and combustion air in finite increments, such as through the use of complex mechanical systems or mechanical jackshafts. Mechanical jackshafts are difficult to maintain calibration and need frequent maintenance. Other known applications may modulate or vary fuel flow over a wide supply range for a wide range of burner capacity (firing rates) through a single burner assembly, but prohibitively use high fuel pressures and/or very expensive sensors. Known sensors for modulating fuel flows can be more than about 7 times as costly or more compared to the sensors in some embodiments. 
     In some embodiments, modulating the fuel/air mixture may greatly increase a system overall efficiency. Known two-stage systems capable of operating at a high and a low firing rate provide limited scope and range of operation due to their inability to precisely control the fuel and air mixture at two levels only. The known two-stage systems have a wide excess-air safety margin which reduces efficiency. 
     A continuously modulating appliance or apparatus can use close control of the fuel/air ratio at all output levels, in some embodiments. Modulating broadly refers to changing, regulating, proportioning, and/or the like, such as in response to a change or a need. Continuously modulating broadly refers to adjusting or changing a value in increments or steps of less than about 5 percent of a range or span, less than about 2 percent of a range or a span, less than about 1 percent of a range or a span, less than about 0.5 percent of a range or a span, less than about 0.1 percent of a range or a span, and/or the like. In some embodiments, modulating may exclude discrete or stepwise changes, such as going from pilot light about 2 percent capacity to a minimum burner firing of 30 percent capacity with no stable firing regime in between. Full modulation broadly refers to covering a complete span or range, such as from pilot to full firing. 
     Various known techniques to directly measure the fuel flow and air flow rates may independently determine a fuel air mixture, but such measurement systems use expensive and complex sensors which are prohibitive for many burner applications. Another known technique to increase the efficiency includes variable speed blower motors for tempered air movement, but variable speed blower motors alone do not allow the burner to vary its output since other components must also be driven to safely modulate the combustion application. Many variable speed motors are expensive. 
     Generally, for more modulation and control capability placed into a burner controller, the greater the cost to supply and maintain control and sensing to achieve the desired burner efficiencies. In some embodiments, the invention provides a control system for a modulating burner system at a reasonable cost and performance level. In additional embodiments may include inexpensive variable speed motor technology as described in U.S. Pat. No. 6,864,659 for a control of a shaded pole or standard permanent split capacitor (PSC) AC induction motor. The entire teachings of U.S. Pat. No. 6,864,659, U.S. Pat. No. 5,590,642, and U.S. Pat. No. 7,293,718 are of common ownership with this specification, and are incorporated herein by reference in their entirety. 
     In some embodiments, a variable burner output application includes a system with a controller to vary or modulate controlled elements of an appliance and corresponding sensors to assure safe and efficient operation at all firing rates of a modulating output fuel valve or fuel valves and a combustion air blower or device. Controllers broadly refer to devices having comparative or logic capabilities, such as single loop controllers, programmable logic controllers (PLCs), distributed control systems (DCS), personal computers, work stations, microprocessors, central processing units, digital computers, analog computers, hardwired relay boards, hardwired circuits, and/or the like. Controllers may include suitable software or programming, such as machine code, compiled executable programs, ladder logic, and/or the like. 
     In additional embodiments, the system may include a fully modulating fuel valve and a variable speed combustion air blower with corresponding sensors to produce a modulating combustion fuel/air mixture. Additional variable components and feedback sensors are within the scope of this invention. 
     Modulating broadly refers to adjusting to or keeping in proper measure or proportion. Modulating may be over or within a certain span or range, such as 0 percent to 100 percent, about 20 percent to 100 percent, any other suitable range, and/or the like, for example. Modulating may include discrete, finite, or stepwise increments, such as one burner at full firing or two burners at full firing. Any suitable number of modulating steps may be used, such as at least about 2, at least about 3, at least about 5, at least about 10, at least about 20, and/or the like. The steps may be generally equally spaced and/or may be varied, such as smaller increments at or near a lower operating regime and larger increments at or near higher operating regime. 
     In addition, modulating may include continuous or infinite spans, such as any spot or location within a span or range. Fully modulating broadly refers to provide continuous modulation. 
     In some embodiments, a controller may respond to an input command signal to initiate or adjust burner operation. The input command may originate from a suitable input/output sensor or control unit, such as from an On/Off thermostat, a temperature sensor, a thermocouple, a boiler pressure sensor a pressure switch, an analog control input, an intelligent proportional control device, and/or the like. 
     A firing demand or heat call can determine a fuel/air mixture, herein sometimes collectively referred to as a “firing rate”, from a variable, or modulating, element controlling such conditions. For example, the controller may set a variable, or modulating, fuel valve to the desired setting and deliver an assumed amount of fuel flow. Then, based on an algorithm defined firing rate for the burner, the controller adjusts the airflow from the combustion air device to match the assumed fuel flow. 
     A set of variable operating conditions can be derived by calculation of the required air flow for the assumed fuel flow or by accessing a lookup table so as to achieve a desired stoichiometry or ratio. A speed of the combustion air device may be economically and reliably monitored by an air flow sensor. The variable speed driver or motor of the combustion air device may be adjusted until obtaining or reaching the correct air flow, for example. 
     In some embodiments, the system trims or fine tunes the fuel/air mixture based on the measured fuel flow from the fuel flow sensor. Desirably, the measured fuel flow from the fuel flow sensor feedback allows or provides accurate position of the fuel valve corresponding to the fuel flow. The combustion air can then adjust to meet the defined fuel/air ratio for the intended firing rate. When a different burner output is commanded, the modulated fuel valve as well as the speed of the combustion air device driver may be altered and then re-trimmed to achieve the correct fuel/air ratio at the new firing rate, for example. 
     The fuel valves may include any suitable device, such as electronic combination valves, pneumatic valves, valve trains, globe valves, plug valves, ball valves, diaphragm valves, pinch valves, butterfly valves, shut off valves, stop cocks, solenoids, and/or the like. Actuators, positioners, and/or other suitable linkages may be included with the fuel valve. Valves may be used with gas phase fuels, vapor phase fuels, or liquid phase fuels including slurries, suspensions and pneumatically conveyed streams, for example. For solid fuels, an equivalent to modulating fuel valves include a variable rate feeders or delivery devices, such as rotary feeders, screw augers, vibratory feeders, pellet dispensers, conveyors, chutes, hoppers, and/or the like. 
     A variable input signal from a separate controller or control system may connect to the burner controller through the communication port to modulate the fuel/air supply. In some embodiments, the apparatus may include a suitable burner, such as powered burners, induced draft in-shot burners, partially mixed burners, fully pre-mixed burners, and/or the like. In-shot burners may be used in residential furnace applications or commercial furnace applications, for example. Pre-mixed burners may provide improved emission characteristics, for example. 
     Referencing  FIGS. 1 ,  3 ,  4  a controller  1 , a fuel valve  10  and a combustion air blower  11 , such as may be employed in a control burner application, is shown in some embodiments.  FIG. 1  shows the components of the modulating burner control platform.  FIGS. 3 and 4  show the combustion mixture ratios for the fuel/air mixture curve and the air/fuel mixture curve as they correspond to various burner applications, in some embodiments and as further described below. 
       FIG. 2  illustrates a block diagram of a controller  1  for the primary application of modulation of fuel flow and air flow based on both fuel and air flow feedback, in some embodiments. Components of the burner control application may include a controller  1  with modulating outputs  4  to a fuel valve  10  and a modulating output  5  to a combustion air blower  11 , a modulating fuel valve  10 , a combustion air blower  11 , a fuel flow feedback sensor  3 , and an air flow feedback sensor  6 . 
     The controller  1  may receive a command for operation of an appliance or a furnace from a sensing element, such as a simple On/Off thermostat, or a separate command controller. A command algorithm or program residing in the controller  1  may then be used by the controller  1  to determine the firing rate of the variable, or modulating, fuel valve  10  and the corresponding airflow from the variable speed combustion air device  11 , to efficiently operate the burner, as further discussed below. 
     The input signal to the modulating fuel valve  10  ( FIG. 2 ) may be set in accordance with an appropriate value calculated by memory, from a lookup table value, or it may be an arithmetic component of the controller central processing unit  2  (CPU). The modulating fuel valve  10  can be initially positioned at the percent of the operating range to produce the desired burner capacity. The speed of the combustion air blower motor  11  can then be adjusted until attaining or reaching the correct air flow so as to achieve the correct fuel/air stoichiometry operating point. The controller  1  may then further trim the fuel/air ratio by adjusting the fuel flow, the air flow, or a combination of both, through the output to the fuel valve  10  and the combustion air blower  11 , respectively, as further explained below. 
     The controller  1 , in addition, may perform the following standard functions of the exemplary fuel burner control including safety start-up checks, the burner ignition sequencing, the safety routines, the monitoring of control limits, the monitoring of the fuel flow sensor  3  for controlling the firing rate, providing and monitoring the air flow sensor  6  in order to maintain optimum burner performance, and/or the like. 
     In some embodiments, the invention may include simultaneously modulating, or varying, both fuel flow and combustion air flow based on a commanded operating point and continuously adjusting the flow based on fuel flow sensor feedback. The fuel valve  10  can be initially driven to the calculated position to achieve the desired burner capacity. The separate fuel flow sensor  3  and the air flow sensor  6  inputs to the controller  1  to provide the basis for developing incremental adjustments to the precise position of the fuel valve  10 . The burner output can be further adjusted to the desired operating point on the fuel/air mixture curve by modulation of the variable speed combustion air blower  11  to match the fuel/air mixture curve of  FIG. 3 , for example. Similarly, the invention includes a process for initially setting the air flow and then adjusting the fuel flow to achieve the corresponding operating point on the air/fuel mixture curve in  FIG. 4 . 
     A modulating fuel valve  10  or valves may respond to either an analog or digital input signal where it can be desirable to apply a variable input signal to the fuel valve  10 . The fuel valves  10  may be modulated through a wide operating range. Variable fuel/air burner systems may allow operation of a fully modulated burner using any suitable method of modulation. The controlled operating range of the modulating devices can be defined as 0 percent to 100 percent and can be controllable in minimal 1 percent increments. Turn down ratios or rangeability may include any suitable amount, such as at least about 2:1, about 3:1, about 4:1, about 5:1, about 10:1, about 12:1, about 15:1, about 18:1, about 20:1, about 25:1, about 30:1, about 40:1, about 50:1, and/or the like. In some embodiments, modulation may exclude periodic turning on and off, such as providing continuous circulation of air through a furnace. 
     The modulating fuel valve of  FIG. 2  can be less expensive and permits finer tuning when used in conjunction with a self-calibrating controller using fuel sensor feedback. The relationship between desired air and fuel flow to assure proper stoichiometry can be performed with a lookup table or algorithm equation, such as within the controller. 
     Fuel for burners or combustors broadly may include hydrogen, natural gas, methane, ethane, propane, butane, liquefied petroleum gas, carbon monoxide, gasoline, diesel fuel, jet fuel, fuel oil, bunker fuel, methanol, ethanol, butanol, biodiesel, wood, wood pellets, coal, municipal solid waste, sanitary sewage, and/or any other suitable combustible material from renewable or nonrenewable sources. 
     Fuel supply pressures may include any suitable range, such as about 13 centimeters of water column gauge (about 5 inches of water column gauge), to several bar of pressure or more depending upon the application. Supply pressure may influence turndown capabilities, such as dividing about 13 centimeters into 30 increments for a 30:1 turndown uses a sensor that detects at low as about 4.33 millimeters of water column, for example. In the alternative, about 13 centimeters of water column in 100 increments includes sensors and/or techniques for measuring about 1.3 millimeters of water column. 
       FIG. 2  can be defined for the performance of the modulating fuel valve with a fuel pressure sensor and an air flow pressure sensor as the prescribe flow feedback sensors in an actual application, in some embodiments. 
     A fuel pressure sensor  3  ( FIG. 2 ) located in the fuel burner manifold can be used as a mechanism of providing a feedback loop control for the modulating fuel valve. The fuel pressure corresponding to fuel flow may be automatically sensed by the controller to adjust the position of the fuel valve to the desired flow position. The fuel valve may be a ball valve and a compatible actuator to achieve the desired valve positing accuracy and commanded by the controller output signal. 
     An air pressure sensor  6  in  FIG. 2  can be also used as a mechanism of providing feedback loop control of the air flow through the combustion air blower  11 . The motor speed can be automatically increased or decreased until achieving the desired air pressure. The air pressure sensor  6 , when used in this manner, may be able to measure the combustion mass airflow and compensate for air side variations, such as varying vent lengths, flow blockages, plugged inlet filters, altitude, and/or the like. 
       FIG. 2  can be further defined for the performance of the modulating fuel valve with another fuel flow sensor as the prescribe feedback sensors in other actual applications. Sensors broadly refer to devices for measuring or monitoring at least one attribute, characteristic, or quality. Desirably, sensors include an output in some manner proportional (directly, indirectly, or otherwise) corresponding to the measured attribute. Sensors may include a useful or rated span, such as a linear output range. Output broadly refers to a signal or impulse, such as corresponding to a measured attribute of a sensor. Outputs or signals may include pneumatic, hydraulic, electronic, analog, digital, and/or the like. 
     In some embodiments, these alternate fuel flow sensors may include pressure sensors, mass flow sensors, volumetric flow sensors, positive displacement sensors, and/or the like. In some embodiments, the fuel flow sensor includes pressure switches, pressure sensors, differential pressure sensors, anemometers, turbine meters, vortex shedding meters, orifice plates, venturi, nozzles, pitot tubes, coriolis meters, and/or the like. Sensors may include suitable mounting equipment and signal equipment, such as pipes, tubing, wiring, fiber optics, radio frequency transmitters, and/or the like. 
     In some embodiments, the variable driver of the combustion air device may be controlled inexpensively and efficiently through a wide speed range in order to provide the correct airflow for the combustion process. Additionally, the more expensive brushless DC motors can be used to provide variable speed control for combustion air. The variable speed drive may include any suitable device, such as an inerter driven (variable frequency drive) AC motor, brushless DC motor, PSC motor, steam turbine, hydraulic turbine, steam engine, combustion engine, gas turbine, microtubine, mechanically regulated constant speed devices (transmissions, gearboxes, belts, continuous variable transmission, and/or the like), and/or the like. 
     These modulating and feedback control concepts can be applied to 2-stage combustion and 3-stage combustion air as well as full modulation operation, for example. 
     In some embodiments, to avoid using a more costly modulating thermostat as an input to describe the desired control point, aspects of the present invention may provide a software based command thermostat algorithm, or routine, which translates the incoming command signal from a simple low cost thermostat into an output signal proportional to the system demand. The thermostat may include a bimetallic device and/or the like. The controller  1  can use this command algorithm to increase or decrease the firing rates, such as the amount of fuel supplied directly for the modulating valve and indirectly for the fuel pressure feedback. Duty cycle, or on time, of the fuel supply and speed of the combustion air movement, desired from the combustion air blower  11  may also be determined by the thermostat algorithm. 
     The controller may respond to a call for heat by requesting a predetermined firing rate, such as a fuel output from the appliance. Based on the desired output, the controller may also determine the airflow required from the inducer blower. The input signals to the modulated fuel valve  10  ( FIG. 2 ) may be set in accordance with the appropriate firing rate value so as to achieve the correct stoichiometry (rich, lean, and/or stoichiometric). Fuel flow accuracy can be achieved with a ball valve and a compatible actuator. The speed of the inducer blower fan may be adjusted until the correct pressure may be attained or satisfied. When a different burner output may be commanded, the speed of the combustion air blower motor as well as the modulated fuel valve setting may be altered to achieve the correct stoichiometry or ratio at the new firing rate 
     The burner control algorithm can be configured to provide over all maximum burner efficiency, to maintain target efficiency over the entire operating range, and/or to maximize turn down. Maximum burner efficiency may include any suitable number or percent oxidized fuel, such as combusting at least about 90 percent of the fuel, at least about 95 percent of the fuel, at least about 98 percent of the fuel, at least about 99 percent of the fuel, at least about 99.9 percent of the fuel, and/or the like. Maintaining target efficiency over the entire operating range may include any suitable efficiency of combusting the fuel, such as at least about 30 percent of the fuel, at least about 50 percent of the fuel, at least about 75 percent of the fuel, at least about 85 percent of the fuel, at least about 90 percent of the fuel, at least about 95 percent of the fuel, and/or the like. Maximizing turn down may include any suitable amount or percent of the firing span or range, such as at least about 50 percent of the span, at least about 75 percent of the span, at least about 85 percent of the span, at least about 90 percent of the span, at least about 95 percent of the span, at least about 98 percent of the span, at least about 99 percent of the span, at least about 99.5 percent of the span, and/or the like. 
     Other applications of the burner controller may be to optimize the burner combustion setting to minimize carbon monoxide (less than a few parts per million, molar basis), excess oxygen (less than a few percent, molar basis), nitrogen oxides (less than a few parts per million, molar basis), or some other combustion characteristic. Stack temperature may also be a feedback variable. 
     The controller of this invention can be configured as in  FIG. 5  to provide extended efficient operation of the burner system with a fuel valve operating control point above and/or below the manufactures published accuracy point on the fuel flow sensor detection range as shown in  FIGS. 3 and 4 , such as outside the range of the sensor. The controller may use a fuzzy logic algorithm, a linear extension of the fuel/air curve, a math formula using selected points on the fuel flow curve, and/or the like to extrapolate or extend the low end of the fuel sensor operating point below the point of minimum accuracy of the sensor. 
     Possible math formulas may include where the extrapolation algorithm has a mathematical function derived from selected non-linear points near an extreme end of the measured fuel—sensor curve which provide a better extension of the desired stoichiometry characteristics. The formula may include any suitable curve fitting technique including linear functions, power functions, trigonometric functions, exponential functions, geometric functions, hypergeometric functions, and/or the like. The relationships for the extrapolation techniques of this invention may be applied to below and/or above the range of the sensor. 
     With the extrapolation of the fuel sensor curve beyond the range, the fuel ball valve can be driven to the extended position for the required fuel flow, for example. Desirably, the modulating range of the combustion air blower may be compatible with the fuel valve extended positioning. The extrapolation can be extended to the point of fuel valve and burner operation until the flame goes out or is “snuffed” out. Optionally, the controller can recognize the snuff out condition and shift the fuel valve operating point upward until continuous burn can be maintained. The new minimum fuel operating points for the given fuel valve and burner configuration will be redefined or spanned as the 0 percent to 100 percent modulating range and may be controlled in 1 percent increments, for example. 
     In some embodiments, the increments of this invention over the range may include at least about 50 percent, at least about 33 percent, at least about 25 percent, at least about 20 percent, at least about 15 percent, at least about 10 percent, at least about 5 percent, at least about 2 percent, at least about 1 percent, at least about 0.5 percent, at least about 0.1 percent, and/or the like. 
     The fuel/air mixture curves of  FIGS. 3 and 4  can also include extrapolation to provide either an air rich combustion curve or a fuel rich combustion curve. The excess air for a lean combustion curve may include any suitable amount above stoichiometric, such as at least about 0.05 percent, at least about 0.1 percent, at least about 0.5 percent, at least about 1.0 percent, at least about 1.5 percent, at least about 2 percent, at least about 3 percent, at least about 5 percent, and/or the like. The excess fuel for a rich combustion curve may include any suitable amount above stoichiometric, such as at least about 0.05 percent, at least about 0.1 percent, at least about 0.5 percent, at least about 1.0 percent, at least about 1.5 percent, at least about 2 percent, at least about 3 percent, at least about 5 percent, and/or the like. 
     In some embodiments, the burner controller can also be configured according to  FIG. 6  to provide burner operation of very large burner assemblies with correspondingly large combustion air blower assemblies. These large blowers may be difficult to accurately control air flow near the ends of their operating range or envelope, such as either open or closed. The burner controller can optimize the burner capacity at these extended operating end points using the same extrapolation techniques for the air flow sensor as described above for the fuel flow sensor and provide an auxiliary output  7  to modulate a combustion air blower inlet vane  15  to fine tune the air flow to match the fuel/air curve in  FIG. 3  or  4 . 
     The controller may also be applicable to very large burner assemblies consisting of a modulating burner in combination with multiple fixed capacity sequentially staged burners. Control of the staged burners can be configured in the controller and sequenced into to the continuous modulation of the total burner assembly through a communication method as shown in  FIG. 7 . Various communication serial or parallel methods may be used, such as RS485, RS232, and/or the like. 
     Desirably, the controller provides modulation of the primary variable capacity burner unit. The control algorithm can be expanded to modulate the primary burner up to full capacity in response to the command from the media sensing unit. If the heat or firing call exceeds the capacity of the primary unit to satisfy the request, the controller can turn on the next burner stage by the communication link, such as to run or operate the next burner at full burner capacity. The controller in  FIG. 7  can then reduce the modulation rate on the primary modulating burner back to a burner output that can satisfy the call. This process may be repeated with each burner stage as necessary to satisfy the call (adding or reducing a number of fixed capacity burners). The staging sequence can work with one or more additional burner units, such as a sequentially staged burner configuration. The use of multiple burners may also include two or more modulating burners communicating with each other and in communication to one or more staged burners to provide continuous modulation over the entire operating range of the system. Multiple burner applications can also include systems using several simple on/off burners and one or two modulating burners to achieve wide modulation ranges. 
     Due to environmental restrictions on burner emissions or to operate the burner at specific emission level (regulatory compliance), the controller in  FIG. 8  can be configured with a flue emission sensor. The controller algorithm can be programmed to monitor the flue fuel emission to further adjust the burner operating point to meet fuel emission requirements or to set operating limits for the discharge. The flue emission sensors may include a temperature sensor, a carbon monoxide sensor, an oxygen sensor, a nitrogen oxides sensor, gas chromatograph, mass spectrometer, and/or the like. 
     The sensed or measured emission value can be incorporated into the burner control loop to adjust the burner capacity to the required operating performance of the burner. For example the firing call may result in adjusting the fuel valve to a position and the air device driver at a speed, but a carbon monoxide sensor may read too high. The controller may increase the speed of the air device drive to supply additional oxygen to reduce the carbon monoxide. If needed the fuel valve may adjust or increase to compensate for the additional air in a feedback loop. Feedback and feed-forward loops are within the scope of this invention. 
     As schematically shown in  FIG. 9  and in some embodiments, the invention includes a modulating combustion apparatus  20 , such as a gas forced warm air furnace. The apparatus  20  includes a burner  21 , a housing  24 , an on/off fuel valve  23 , a modulating fuel valve  10 , and a fuel sensor  4 . The apparatus  20  also includes an air sensor  22  and a combustion air device  11 . Desirably, in furnace applications, the modulating controller provides increased comfort, such as for persons within the room or building. The temperature within a room may vary less than about 5 degrees Celsius, less than about 4 degrees Celsius, less than about 3 degrees Celsius, less than about 2 degrees Celsius, less than about 1 degree Celsius, less than about 0.5 degrees Celsius, less than about 0.1 degrees Celsius, and/or the like. 
     In some embodiments, the invention may include a combustion apparatus for use in fired variable demand applications. The apparatus may include a fuel valve modulating a fuel flow, and a fuel sensor with a range measuring the fuel flow and sending a fuel output. The apparatus may also include a combustion air device modulating an air flow, and an air sensor measuring the air flow and sending an air output. The apparatus may also include a controller connected to the fuel valve, the fuel sensor, the combustion air device, and the air sensor. The controller may modulate the fuel valve based on the fuel output using an extrapolation algorithm when the fuel output extends outside of the range of the fuel sensor, and the controller may modulate the combustion air device based on the air output. The controller simultaneously and/or sequentially may modulate the fuel flow and the air flow over an extended fuel/air ratio and may provide continuous modulation during a single burn cycle. 
     Simultaneously broadly refers to two or more items at the same time or substantially the same time, such as both the fuel and the air moving in five percent increments in response to a change in heat load (increase or decrease). Sequentially broadly refers to one item first and then the other item or substantially a first item and substantially a second item, such as moving the fuel first followed by the air each in five percent increments in response to a change in heat load (increase or decrease), the air first followed by the fuel each in five percent increments in response to a change in heat load (increase or decrease), and/or the like. 
     Single burn cycle broadly refers to one stage combustion where at least a substantial amount of the fuel burns or oxidizes. A dual burn cycle or multiple burn cycle may include partial oxidization of the fuel in a first stage lacking enough oxygen for complete combustion and then a second stage with additional oxygen and complete combustion, for example. In some embodiments, this invention may include multiple stage burn cycles, such as with over fire air. 
     As discussed above, the extrapolation algorithm may include fuzzy logic derived from a fuel sensor curve over the range, linear extension derived from a fuel sensor curve over the range, a mathematical function derived from selected points on a fuel sensor curve over the range, and/or the like. In some embodiments, the controller utilizes the extrapolation algorithm to provide a predetermined rate leaner or with excess air than a stoichiometric ratio of air to fuel. In the alternative, the controller utilizes the extrapolation algorithm to provide a predetermined rate richer or with excess fuel than a stoichiometric ratio of air to fuel. 
     In some embodiments, the fuel sensor includes a pressure sensor, a mass flow sensor, and/or a volumetric flow sensor, such as an anemometer, a turbine, an orifice, a venturi, and/or a nozzle. Desirably, but not necessarily, the combustion air device includes full modulation operation full modulation operation between a minimum of the extrapolation algorithm and a full system capacity, such as between about 2 percent and 100 percent, between about 5 percent and about 100 percent, between about 10 percent and about 100 percent, between about 12 percent and 100 percent, between about 15 percent and about 100 percent, and/or the like. 
     The controller may maximize burner efficiency, maintain a target efficiency over an entire operating range, maximize turn down, and/or the like, as discussed above. Optionally, the controller minimizes carbon monoxide, excess oxygen, and/or nitrogen oxides. 
     Desirably, the controller learns from a flame-out due to low combustion fuel (unstable regime for the burner) and modifies the extrapolation algorithm for future use upward, to the right, and/or the like to prevent additional flame-outs, such as resetting the zero flow of the span. 
     In some embodiments, the apparatus may include a first burner with a capacity, and one or more additional burners each with a capacity and with the first burner forming a sequentially staged burner system. The controller may communicate with the first burner and the one or more additional burners. When a heating demand or call exceeds the capacity of the first burner, the controller may activate the one or more additional burners, as described above. Desirably, but not necessarily, the controller uses a sequential algorithm modulating the first burner to provide continuous modulation operation over a system range, such as over a broader range but still with fine adjustment by the modulating burner. 
     In the alternative, the controller may use a sequential algorithm modulating the first burner and the one or more additional burners to provide continuous modulation operation over a system range. Optionally, the apparatus may include a second sequentially staged burner system to provide a broader system range, such as a greater range than with a single sequentially staged burner system. The sequentially staged burners may be in different furnace units or banks of furnace units, for example. 
     In some embodiments, the apparatus may include a flue gas sensor indicating a flue gas characteristic, such as a temperature sensor, a carbon monoxide sensor, an oxygen sensor, nitrogen oxide sensor, and/or the like, as discussed above. 
     In some embodiments, the invention may include a combustion apparatus for use in fired variable demand applications. The apparatus may include a fuel valve modulating a fuel flow, and a fuel sensor with a range measuring the fuel flow and sending a fuel output. The apparatus may also include a variable speed driver modulating an air flow of a combustion air device, and a damper modulating the air flow of the combustion air device. The apparatus may also include an air sensor measuring the air flow and sending an air output, and a controller connected to the fuel valve, the fuel sensor, the variable speed driver, the damper, and the air sensor. The controller may modulate the fuel valve based on the fuel output, and the controller may modulate the variable speed driver and the damper based on the air output. The controller may simultaneously and/or sequentially modulate the fuel flow and the air flow over an extended fuel/air ratio and may provide continuous modulation during a single burn cycle. 
     The combined damper and variable speed driver may provide greater turn down, finer control, and/or a more stable flame pattern, as discussed above. Desirably, but not necessarily, the combined damper and the variable speed driver may be combined with the extrapolation algorithm, such as to provide or extend the operating envelope even further for the apparatus. 
     The fuel sensor may include any of the characteristics and qualities discussed above with respect to any other embodiments. The modulation of the apparatus and the controller may include any of the characteristics and qualities discussed above with respect to any other embodiments. 
     In some embodiments, the invention may include a method of operating a combustion apparatus for use in fired variable demand applications. The method may include the step of measuring a fuel flow with a fuel sensor having a range and a fuel output, and the step of measuring an air flow with an air sensor having an air output. The method may also include the step of modulating the fuel flow with a fuel valve and a controller based on the fuel output, and the step of modulating the air flow with a combustion air device and the controller based on the air output. The method may also include the step of calculating the air flow or the fuel flow when the fuel output extends outside of the range of the fuel sensor with an extrapolation algorithm, and the step of maintaining simultaneously and/or sequentially the fuel flow and the air flow over an extended fuel/air ratio and to provide continuous modulation during a single burn cycle with the controller. 
     Calculating broadly refers to any suitable logic based operation to derive, estimate, and/or arrive at the desired output, such as discussed above. Maintaining broadly refers to executing a control loop structures such as a feedback loop. The feedback loop may include a set point and a measured output. The set point may be adjusted based on the measured output, for example. 
     The extrapolation algorithm of the method may include any of the characteristics and qualities discussed above with respect to any other embodiments. 
     In some embodiments, the maintaining comprises a stoichiometric ratio, a lean ratio, and/or a rich ratio, as discussed above. Optionally, the controller may operate different fuel/air ratios in different operating modes, such as start up, stand by, minimum firing, mid-level firing, maximum firing, and/or the like. 
     The modulating of the method may include any of the characteristics and qualities discussed above with respect to any other embodiments. The method may also include minimizing or reducing carbon monoxide, excess oxygen, nitrogen oxides, and/or the like, as discussed above. 
     In some embodiments, the invention may include a method of operating a combustion apparatus for use in fired variable demand applications. The method may include the step of measuring a fuel flow with a fuel sensor having a range and a fuel output, and the step of measuring an air flow with an air sensor having an air output. The method may also include the step of modulating the fuel flow with a fuel valve and a controller based on the fuel output, and the step of modulating the air flow with a damper and a variable speed driver of a combustion air device and the controller based on the air output. The method may also include the step of maintaining simultaneously and/or sequentially the fuel flow and the air flow over an extended fuel/air ratio and to provide continuous modulation during a single burn cycle with the controller. 
     The modulating the damper and the variable speed driver may be done in any suitable manner including simultaneously and/or sequentially, such as to avoid unstable operations or excessive “hunting” as the system moves between points or settings. The controller of the method may include any of the characteristics and qualities discussed above with respect to any other embodiments. Similarly, the fuel/air ratio of the method may include any of the characteristics and qualities discussed above with respect to any other embodiments. The modulating and the reduced emission control of the method may include any of the characteristics and qualities discussed above with respect to any other embodiments. 
     A system has been shown whereby a controller and an economical sensing and control systems provides an inexpensive means for operating a modulating burner assembly through the use of a variable output components. It will be appreciated that details of the foregoing embodiments, given for purposes of illustration, are not to be construed as limiting the scope of this invention. Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications and/or combinations are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications and/or combinations are intended to be included within the scope of this invention, which is defined in the following claims and all equivalents thereto. Further, it is recognized that many embodiments may be conceived that do not achieve all of the advantages of some embodiments, particularly of the preferred embodiments, yet the absence of a particular advantage shall not be construed to necessarily mean that such an embodiment is outside the scope of the present invention.