FIELD OF THE INVENTION

The present invention relates to heating devices and, more particularly, to a high turndown boiler, and related system and method for controlling a boiler.

BACKGROUND OF THE INVENTION

Burners which combust gas or other fuel are widely known. Gas burners, incorporated for example into indirect heating devices, utilize the combustion of a gas or similar fuel (e.g., propane, natural gas, or fuel oil) for heating a work substance, oftentimes a flowable substance such as air or water. In operation, natural gas or other fuel is controllably forced through a nozzle or jet portion of the burner, where it is intermixed (most typically) with air from a blower, forming a gas spray or aerosol for enhancing combustion. In premix burners, some or all of the air required for combustion is mixed with some or all of the fuel prior to burning. To start the burner, a pilot is ignited, which in turn is used to ignite the main burner on demand.

In the case of an indirect heater, the combustion product (heated air/plasma) is directed into a heat exchanger, where the energy produced by the combustion process is transferred to the working substance to be heated. The combustion exhaust is then moved to an exhaust exit, possibly after one or more recirculation steps or the like to further recapture heat from the combustion product. A cylindrical housing is often employed to cover most or all of the components.

For a gas heating device, the amount of fuel burned per unit time (e.g., liters or btu per hour) is referred to as the firing rate. Simple heating devices are configured to run at a single firing rate, with the heater being cycled on and off in cases where it is desired to achieve an average output that is less than the maximum possible output. If a heating device is capable of steady state operation at two or more firing rates within acceptable combustion parameters (e.g., combustion byproducts are kept to below a desired level, according to ANSI safety and performance standards or the like), this is referred to in the industry as “turndown.” In other words, while keeping within acceptable operational parameters, it is possible to “turn down” the heating device from the maximum possible firing rate to one or more lower firing rates. The ratio of the highest firing rate to the lowest firing rate in a heating device, at steady state operation and keeping within acceptable operational parameters, is referred to as the “turndown ratio” of the heating device.

High turndown ratios are desirable for achieving greater levels of efficiency in a heating device. For example, although it is possible to vary the average actual heat output of a single firing rate heating device by cycling the device between on and off operational modes, this can result in low levels of combustion efficiency, higher levels of fuel use per unit heat output, and a greater level of undesirable combustion byproducts. Among other reasons, this is because the conditions in the combustion chamber vary widely over time as the combustion process is turned on and off. When combustion is ongoing, the gas spray produced by the burner is consistent, and the temperature in the combustion chamber is high, factors that favor efficient operation. However, when combustion is turned off or restarted, this results in temperature variations in the combustion chamber, and variances in the quality of the gas spray input, factors that inhibit efficient operation.

At higher fire rates (i.e., low turndown), the velocity of the air/fuel mixture is sufficient to cool the burner, resulting in a high tolerance to air/fuel variation. However, as input is reduced (increased turndown), the fuel mixture combusts closer to the burner surface. It is necessary to keep the burner temperature from becoming excessive, as this can result in burner sag, reduced life and potential flashback. The consequence is that increased air must be introduced (a higher air/fuel ratio), which limits the maximum turndown ratio of the heating device. Control of the air fuel ratio becomes even more stringent as the turndown is increased, where the velocity of air through the burner is lower. It is this acceptable temperature band width that determines the required oxygen at any specific turndown. The higher the turndown, the narrower the temperature band width.

One of the limiting factors in achieving higher turndown ratios is the loss of control of the air/fuel mixture at low flow rates. Failing to achieve the theoretically ideal fuel/air mixture can result in emissions of carbon monoxide, aliphatic aldehydes, nitrous oxides, and other contaminants that are judged to be harmful. Producing those contaminants will cause a burner design to fail ANSI (American National Standards Institute) safety and performance standard tests.

In view of the above, there is a need for a boiler system having a high turndown ratio, and a related system and method for controlling a boiler to achieve a high turndown ratio.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a high turndown boiler.

It is another object of the present invention to provide a system and method for controlling a boiler to achieve high turndown.

These and other objects are achieved by the present invention.

According to one embodiment of the present invention, a heating device includes a combustion chamber, a housing in fluid communication with the combustion chamber, a burner disposed in the housing, a blower assembly connected to the housing for directing air into the interior of the housing, a valve assembly connected to the housing for controlling a flow of fuel into the burner, an optical color sensor for sensing a color profile of a surface of the burner, and a control unit configured to control the valve assembly in dependence upon the color profile of the surface of the burner.

In another embodiment, a method of controlling a gas burner includes the steps of determining a target color profile of a burner surface, the target color profile corresponding to a target temperature of the burner surface during combustion, sensing an actual color profile of the burner surface, and adjusting an amount of fuel provided to the burner in dependence upon the actual color profile.

