Patent Application
20080241776
Infrared (IR) energy is known to be able to cook certain types of foods faster than convection energy. Although IR is not as fast as microwave energy, IR energy is known to produce better cooking results than microwaves.
A problem with cooking using IR is that generating short-wavelength IR, which penetrates food deeper than long wavelength energy, typically requires a relatively large amount of energy because high temperatures are needed to adequately heat a surface to emit short-wavelength IR. Because short-wavelength IR almost always requires a very high temperature surface, generating short-wavelength IR therefore often requires additional time to generate. Another problem with IR cooking is that it is more difficult to control than convection heating.
An oven that is able to quickly, efficiently and controllably generate infrared energy for cooking different types of foods would be an improvement over the prior art.
A gas burner for an IR oven can quickly, efficiently and controllably generate short-wavelength IR as well as long-wavelength IR by combusting a gaseous fuel just below a low mass, low-specific heat burner screen until it emits IR. The gas supply is preferably cycled on and off, in order to allow the burner screen to absorb heat energy from combusting fuel until it reaches a desired temperature. The gas supply is then shut off to allow the burner screen to dissipate IR and cool, which testing shows will extend the burner screen's useful lifespan.
A fuel inlet pipe 20, with first and second opposing and open ends 22 and 24, extends through one of the sides 16A of the distribution chamber 12. As can be seen in
In
At a point 21 located approximately half-way between the first end 22 and second end 24 of the fuel inlet pipe 20, a discontinuity in the S1/S2 pipe diameters is formed by the termination of the pipe section S1 within S2. In other words, at the point identified by reference numeral 21, the inside diameter of the fuel distribution pipe 20 is stepped up or increased, causing a small but non-zero pressure drop at point 21. The discontinuity 21 is believed to create additional turbulence, which aids in the mixing of fuel and combustion air together. At the second or distal end 24 of the fuel inlet pipe 20, the fuel and combustion air leave the fuel inlet pipe 20, strikes the diverter 28 and from which it can evenly fill the distribution chamber 12.
As can be seen in
It is important that an oven be heated evenly and uniformly so that the oven's interior space can be fully utilized, especially so in a commercial oven, such as those used to cook pizza. In order to provide even and uniform heat, the fuel and air that leaves the second opening 24 fills the fuel distribution chamber 12 and flows upwardly into one or more wire mesh burner plates 32, that are placed over the open top 18 of the distribution chamber 12.
As shown in
Fuel and combustion air from the distribution chamber 12 enters open space within the wire mesh burner plates 32 where they mix together. As the fuel and air continue to flow into the burner plates 32, the fuel and air eventually flows out of the “top” of the burner plates 32 where it is ignited by a pilot flame (not shown), which is lit by an electric igniter controlled by a controller. The pilot light causes the fuel and air mixture leaving the top of the burner plates 32 to ignite and combust. The continued supply of fuel gas and combustion air from the distribution chamber 12 allows the combustion to continue, which in turn heats a wire mesh burner screen 36 spaced above the burner plates 32 and the burner plate assembly 30. A gasket 34 that surrounds the burner plates 32 (See
Infrared heat energy is quickly and controllably generated by the combustion of fuel gas below the wire burner screen 36, which preferably of a low mass and therefore quickly heated. The combustion of the fuel beats the wire burner screen 36 until it is hot enough to emit infrared. Once a desired IR emission is reached, the fuel gas is preferably shut off by a computer (not shown), after which IR will continue to be emitted as the burner screen 36 temperature drops. When IR emission drops to some empirically determined value, the burner can be re-lit by the controller (not shown) to re-heat the screen 36 and generate more IR. Since the burner screen 36 will be cooler when the burner 10 is re-lit, heat transfer efficiency from the combusting fuel to the screen 36 will be greater than when the burner screen 36 is continuously heated. By cycling the gas supply on and off, the energy transfer into the screen 36 can be improved over what it would be if the gas supply were simply left on during a cooking process. In addition, by cycling the wire screen 36 temperatures, the IR wavelength emitted from the wire screen 36 cyclically varies from relatively short-wavelength and deeply-penetrating visible IR emitted at high temperatures, to relatively long-wavelength, less-penetrating IR emitted at relatively low temperatures. By cycling the gas supply, the screen 36 can be made to emit IR across a continuously varying spectrum of wavelengths.
The fuel combustion that heats the burner screen 36 takes place above the burner plates 32 but below the burner plate screen 36, which is held in a spaced-apart relation above the burner plates by spacers as shown, with the preferred space being about one-half inch. The spacing between the burner plate screen 36 and the burner plate assembly 30 (or the individual burner plates 32) define a combustion space 38, the height of which is chosen to provide a space large enough to allow the fuel to fully combust below the burner plate screen 36 in order to maximize heat transfer into the burner plate screen 36.
As the height of the combustion space 38 decreases, some of the combustion process will occur above the burner plate screen 36, reducing heat transfer into the screen 38. Conversely, as the combustion space 38 increases, the combustion process will finish below the burner plate screen 38, allowing the combustion products to cool and external air to be drawn into the combustion space 38, thereby reducing heat transfer into the screen 38. Thus, there is an optimal spacing of the heat transfer screen 36 above the burner plates 32 that will maximize heat transfer for a given flow rate of fuel and combustion air into the burner 10. In a preferred embodiment, the burner plate screen 36 is about one-half inch above the burner plates 32, however, spacing as small as about one-quarter inch up to about one inch can also be used.
In one embodiment, the burner plate screen 36 is nichrome wire, however, alternate embodiments include using steel and stainless steel wire, with and without heat-tolerant coatings such as ceramic. In yet another embodiment, the burner plate screen 36 is made entirely of ceramic.
By combusting gas below a low-mass, low-specific heat screen, the screen 36 can be quickly heated to temperatures where the screen will emit short-wavelength and deep-penetrating infrared energy. By cycling the fuel supply on and off, the screen 36 is allowed to cool during gas-off time periods, during which time it will emit increasingly longer wavelength IR. Testing shows that rapid heating and cooling cycles also extends the screen's 36 life beyond the life it would have if the screen 36 were heated continuously.
The foregoing description is for illustration and not for limitation. The scope of the invention is defined by the following claims.
