The present invention relates to burner assemblies, and particularly to gas-fired radiant burners. More particularly, the present invention relates to a radiant burner having a fast-heating and quick-cooling radiant surface for using a flame produced in the radiant burner efficiently to heat material passing over the radiant surface.
Radiant burners are typically used to dry, singe, or cure sheet material such as papers or textiles as it rolls past the burner on a line in a mill. It is also well known to dry paints, cure meats and synthetic resins, heat-treat various metals, and perform other types of industrial drying and heating operations using radiant burners.
Typically, a radiant burner includes a ceramic element or other refractory material that is heated by a flame fueled by a combustible mixture of air and gas to cause the ceramic element to emit radiant heat. It will be understood that radiant heat is heat transmitted by radiation as opposed to convection or conduction. In many applications, a radiant burner is preferred over an open-flame industrial burner because it can be operated to produce an intense heat that can be transmitted at a uniform rate over a wide area to a product moving past the radiant burner.
Conventional radiant burners are often slow to cool which can lead to significant product damage or even loss during a line shutdown. For example, if the paper in a line would stop rolling, the heat from a conventional radiant burner provided to dry the paper could ignite the paper causing a fire unless the radiant burner is able to cool quickly enough once turned off during a line stoppage or shutdown. Accordingly, there is a need in industry for a quick-cooling radiant burner.
There is always a need for a radiant burner that is able to maximize heat output while minimizing unwanted emissions of nitrogen oxides and carbon monoxide. Many proposed and enacted state and federal environmental regulations require that air-polluting emissions from industrial burners be reduced. During the development of an improved radiant burner in accordance with the present invention, it has been observed that it is possible to reduce the formation of thermally generated nitrogen oxide emissions by reducing the burner's flame temperature and carbon monoxide emissions by increasing the temperature of the burner's heat-radiating surface. A radiant burner designed to use fuel more efficiently to generate higher heat output at the heat-radiating surface using a lower flame temperature would lead to lower carbon monoxide and nitrogen oxide emissions and thus be a welcomed improvement over conventional radiant burners.
In the industry, it is not uncommon for hot radiant burners to be splashed with water during use. One problem known to users of industrial burners is that it often takes a long time for the heat-radiating surfaces of a conventional radiant burner to recover its surface temperature once it has been quenched with a liquid such as water. This temporary loss of surface temperature can lead to uneven product drying or curing, and thereby reduce product quality. A refractory-type radiant surface can be destroyed and fractured by splashed water. It would therefore be desirable to provide an improved radiant burner that heats quickly once quenched to minimize disruption to the surface temperature of its heat-radiating surface, and thereby exhibits good quench recovery.
According to the present invention, a radiant burner includes a plenum for receiving a combustible air and fuel mixture, a combustor unit, an ignition means, and a radiant member over the combustor unit. The combustor unit is formed to include an open-space combustion chamber having a top opening. The radiant member covers the top opening of the open-space combustion chamber to define an open flame-retention region in the open-space combustion chamber.
The combustor unit is also formed to include means for communicating the combustible air and fuel mixture from the plenum to the open-space combustion chamber. The ignition means ignites the combustible air and fuel mixture extant in the open-space combustion chamber to produce a flame underneath the radiant member. The radiant member includes a heat-receiving surface communicating with the underlying flame produced in the open-space combustion chamber and a heat-radiating surface emitting thermal radiation to heat a product positioned above the radiant member. By covering the top opening of the open-space combustion chamber, the radiant member operates to stabilize the flame the open flame-retention region provided in the open-space combustion chamber.
In preferred embodiments, the combustor unit is a unified block of ceramic fiber insulation material that is vacuum-formed in a mold to include a top-opening cavity defining the open-space combustion chamber and a plurality of apertures or conduits extending through the bottom of the block and coupling the plenum to the open-space combustion chamber to define the communicating means. The ceramic fiber insulation material has a low thermal conductivity and is formed to surround and underlie the flame produced in the open-space combustion chamber. Advantageously, the side walls of the block help to direct heat produced by the flame upwardly toward the heat-receiving surface of the radiant member. This maximizes radiant, convective, and conductive heat transfer from the flame to the radiant member. Also, the bottom wall containing the fuel-conducting apertures helps keep the temperature of the combustible air and fuel mixture in the underlying plenum below its ignition point.
Also in preferred embodiments, the radiant member is a flat, thin, woven, rigid sheet of porous ceramic material. Illustratively, the radiant member is a web of ceramic fibers coated with silicon carbide to enhance the thermal radiation emissivity of the radiant member. The radiant member is positioned over the top-opening of the open-space combustion chamber and is lightweight to heat up and cool down quickly.
This thin radiant member is characterized in use by a uniform surface temperature and radiant heat flux profile. It will be understood that radiant heat flux is the rate of emission or transmission of radiant energy. Because of these characteristics, the radiant member is able to remove a lot of heat from the combustion reaction taking place in the open-space combustion chamber and transmit that heat to the product to be heated. With the removal of this heat, the flame temperature is reduced and the formation of thermally generated nitrogen oxide emissions are reduced. Carbon monoxide emissions are low because the high surface temperature of the improved radiant member successfully burns any remaining species of carbon monoxide from the fuel lean combustion process. Also, in part because of its low mass, the improved radiant member is able to recover its surface temperature quickly after being splashed with water and thereby exhibits good quench recovery.
Because of the positioning of the radiant member over the flame in the open-space combustion chamber, the heat-receiving surface of the radiant member is heated by radiation, convection, and conduction. In turn, the heat-radiating surface emits thermal radiation to heat a product positioned above the radiant member. Advantageously, the porous character of the radiant member permits some heat flow therethrough to cause a product above the radiant member to be heated also by convection.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.