BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to process heaters such as water heaters and boilers, and, more particularly, to improved performance and construction for gas fired process heaters. Particular areas of focus include burner, heat exchange, and heat exchange tube constructions and performance with particular emphasis on reducing or minimizing pollutant emissions.

Description of Related Art

Conventional gas fired process heaters commonly include a tank adapted to contain a body of liquid, e.g., water, a heat exchange tube in the liquid/water, and a burner producing hot combustion products directed into the heat exchange tube. The combustion products are typically vented or exhausted, e.g., vented or exhausted outside the room/building containing the process heater.

Various types of burners have been used for gaseous fuel combustion in process heaters. The attractiveness to manufactures and customers of particular or specific burners for use in or with process heaters typically involves three sometimes conflicting or contradicting factors or conditions, namely, cost, efficiency, and pollutant emissions (carbon monoxide CO and nitrogen oxides NOx). High efficiency burners are typically higher in cost and usually suffer from high emissions of either CO, NOx or both. Low NOx burners are usually high in CO emission and/or are not efficient as desired since high excess air is used for combustion to reduce NOx. High efficiency radiant burners can provide low NOx but suffer from high CO emissions.

SUMMARY OF THE INVENTION

The present invention contemplates a new and improved process heater construction which overcomes some or all of the above-identified problems as well as others. In accordance with a preferred embodiment there is provided a process heater of simpler construction which is economical to manufacture, economical to operate, burns fuel cleanly and answers governmental regulations.

Briefly stated, in accordance with one aspect of the invention, a process heater is provided having a tank adapted to contain a body of liquid, a heat exchange tube at least in part disposed in the liquid, a oxidant-fuel mixer, a radiant permeable matrix burner at the bottom and inside the heat exchange tube producing hot combustion products directed into the heat exchange tube, and a thermally insulated insert in the heat exchange tube above the burner. The combustion products can desirably be subsequently appropriately vented or exhausted.

A process heater in accordance with one preferred embodiment of the invention operates as follows: A body of liquid in the process heater tank is heated through a heat exchange tube in the tank and by thermal contact with the hot products of combustion resulting from the gaseous fuel flowing inside the heat exchange tube. To combust the gaseous fuel (e.g., natural gas), the fuel is mixed with combustion oxidant, e.g., air, by using a fuel injector or other mixing device. In a preferred embodiment, the air-fuel ratio is near or slightly above stoichiometric levels (e.g., 0-30% excess air). The air-fuel mixture enters a radiant burner and is combusted in a thin layer within a permeable matrix of the radiant burner and/or on the surface of the matrix. A large portion of heat from the combustion (˜30%) is transferred through infrared radiation from the matrix to the heat exchange tube and to the liquid. The high radiation intensity essentially increases heat transfer from the combustion products to liquid through the heat exchange tube, reduces combustion temperature and results in lower nitrogen oxides (NOx) and carbon monoxide (CO) emissions (e.g., ˜10 ppm or less at 3% oxygen (dry basis)) as compared to conventional non-radiant burners. A thermally insulated insert placed or disposed in the heat exchange tube above the burner prevents or reduces heat transfer from combustion products to the heat exchange tube, keeps the combustion products hot, thus essentially reducing carbon monoxide emissions to below 1-10 ppm as compared to combustion without such a thermally insulated insert. In addition, the radiant burner provides better utilization of heat from combustion compared to other existing process heater burners.

A general objective of the invention is to minimize or, preferably overcome, one or more of the problems or shortcoming of the prior art.

More particular or specific objectives of the invention include:

providing a process heater of improved operating characteristics which is inexpensive to manufacture on a production basis;

simplifying process heater design; and/or

improving the efficiency of utilization of allocated heat from a process heater burner.

It is yet another object of the present invention to provide conditions for essential reduction of carbon monoxide in the flue gas; and to provide a process heater which has essentially reduced emissions of carbon monoxides at ultra-low (˜8 ppm or less at 3% oxygen (dry basis)) NOx emissions.

As used herein, references to “high temperature” such as when referring to materials for use in construction of radiant permeable matrix burners are to be understood to generally refer to materials useful and functional at temperatures of 900° C. or greater. In view of higher material costs normally associated with higher temperature compatibility, temperatures of about 1400° C. form a general upper limit on such “high temperature” compatibility.

Further objects and advantages to the invention will be apparent from the following detailed description of preferred embodiments and from the accompanying drawings.

