WIRE MESH THERMAL RADIATIVE ELEMENT AND USE IN A RADIATIVE OVEN

Abstract

A high speed cooking apparatus employing a low voltage high current system for heating foods employing a novel wire mesh heating element. The system herein described providing the benefits of high speed cooking like that further described by U.S. Provisional Application 60/822,028 filed on Aug. 10, 2006, but yet providing significant cost benefit and simplicity over said system.

Claims

1-48. (canceled)

49. A radiant heater comprising: a heating element having a radiative black body area; and a power supply to supply power to the heating element, wherein the radiative black body area is converted to be equivalent to 0.25 m.sup.2 and a ratio of a resistance of the heating element to the equivalent area is less than 2 ohms/m.sup.2.

50. The radiant heater of claim 48, wherein the radiative black body area is over 0.25 m.sup.2.

51. The radiant heater of claim 48, wherein the radiative black body area is less than 0.25 m.sup.2.

52. The radiant heater of claim 48, wherein the ratio is approximately 0.1137 ohms/m.sup.2.

53. The radiant heater of claim 48, wherein the heating element comprises a wire.

54. The radiant heater of claim 48, wherein the heating element has a specific heat capacity of less than 0.36 KWH of energy for raising the temperature of the heat element from room temperature to about 1400 K, wherein an equation relating a heat energy to the specific heat capacity, where the unit quantity is in terms of mass is:
Q=mcT where Q is the heat energy put into or taken out of the element (where Ptime=Q), m is the mass of the element, c is the specific heat capacity, and T is the temperature differential where the initial temperature is subtracted from the final temperature.

55. The radiant heater of claim 48, further comprising a relay for cycling the power to the heating element, and a control circuit for controlling the relay.

56. The radiant heater of claim 48, wherein the heating element comprises multiple heating elements, the multiple heating elements share a first bus and a second bus, the first bus is in electrical communication with a positive voltage of the power supply and the second bus is in electrical communication with a negative voltage of the power supply.

57. The radiant heater of claim 48, further comprising a control circuit configured to monitor a condition of a load by measuring one or more of: a color of the load, a moisture level of a surface of the load, and a moisture level of air in the oven.

58. The radiant heater of claim 48, further comprising a heating cavity configured to receive a load to be heated, wherein the heating element is sized and positioned to heat the load.

59. The radiant heater of claim 48, wherein the power supply comprises a low voltage high current system.

60. The radiant heater of claim 48, wherein the heating element has a combined resistance of less than 10 ohms.

61. The radiant heater of claim 48, wherein the heating element comprises a wire having a thickness less than or equal to 1 mm.

62. The radiant heater of claim 48, wherein the heating element comprises a wire mesh cloth.

63. A heating method comprising: heating a heating element; and discharging current from a power supply through the heating element, wherein the radiative black body area is converted to be equivalent to 0.25 m.sup.2 and a ratio of a resistance of the heating element to the equivalent area of the heating element is less than 2 ohms/m.sup.2.

64. The heating method of claim 63, wherein the radiative black body area is over 0.25 m.sup.2.

65. The heating method of claim 63, wherein the radiative black body area is less than 0.25 m.sup.2.

66. The heating method of claim 63, further comprising cycling the power to the heating element.

67. The heating method of claim 63, further comprising monitoring a condition of a load by measuring one or more of: a color of the load, a moisture level of a surface of the load, and a moisture level of air in the oven.

68. The heating method of claim 63, wherein the heating element has a combined resistance of less than 10 ohms.

Description

DRAWINGS

[0045] The invention will now be further described in connection with the following graphs and photographs.

[0046] FIG. 1 is a graph illustrating the radiative area of a mesh element as a function of the center to center spacing of the mesh strands.

[0047] FIG. 2 is a graph illustrating the electrical resistance of a mesh element as a function of the radius of the strand and the mesh spacing.

[0048] FIG. 3 is a graph illustrating the ramp up time of a two sided 125 mm250 mm mesh element oven as a function of the radius of the strand and the mesh spacing and power drain of 20 KW.

[0049] FIG. 4 is a composite graph of FIGS. 1 and 2, indicating the regions applicable for high speed oven cooking with a De Luca Element Ratio close to 0.11 ohms/m2.

[0050] FIG. 5 is a photograph of a small 24V oven built using the mesh system.

[0051] FIG. 6 is a photograph of a 0.3 mm0.3 mm mesh using 0.3 mm diameter nichrome wire which operates well at 24V across a 200 mm oven.

DESCRIPTION OF DRAWINGS

[0052] In considering the best mesh design, it is important to evaluate the blackbody radiative area as well as the resistance of the element as a function of the following: [0053] 1) The number of strands per unit area of the mesh [0054] 2) The radius of the mesh strands [0055] 3) The mesh strand material [0056] 4) The potential for radiation occlusion between strands.

[0057] FIG. 1 describes the blackbody area as a function of the number of strands and the strand spacing of the mesh. Interestingly, the surface area is independent of the radius of the wire strand if the spacing is made a function of the radius.

[0058] Using equation 5 from above, the resistance of the mesh can be calculated for a specific wire strand radius. FIG. 2 illustrates the electrical resistance of a nichrome mesh element as a function of the radius of the strand and the mesh spacing. Limitation in Equation 5 become apparent as the number of strands becomes very high and the resistance becomes very low; thus atomic effects associated with random movement of electrons in the metal at room temperature form a minimum resistive threshold.

[0059] Using nichrome as the strand material in the mesh and operating the system at 20 KW, the ramp up time to achieve an operating temperature of 1400 degrees K is a function of the strand radius and the mesh spacing (note that a nominal mesh size of two times 125 mm250 mm is used). FIG. 3 illustrates the region below which a ramp up of less than 2 seconds is achievable (note that wire radius above 0.5 mm are not shown due to the long required ramp up times).

[0060] FIG. 4 is a composite graph of FIGS. 1 and 2, indicating the regions applicable for high speed oven cooking with a De Luca Element Ratio close to 0.11 ohms/m2.

[0061] FIG. 5 is a photograph of oven 3 with top and bottom wire mesh elements 1 and 2 each 125 mm230 mm and operated at 24V. Each wire mesh (1 and 2) has 766-125 mm long filaments woven across 416-230 mm long elements, each element 0.3 mm in diameter. A 24 V battery source is placed across the length of the 766 elements at bus bars 4 and 5. The wire surface area for a single strand of 0.14 mm diameter wire is 0.000440 m2/m. Thus, a total surface area (for combined top and bottom elements) can be calculated as:

Total Blackbody Radiating Area=20.000440(4160.23+7660.125)=0.168 m2

[0062] The resistance across bus bars 4 and 5 as well as 6 and 7 was measured at 0.04+/0.01 ohms. (Note that bars 4 and 6 as well as 5 and 7 are connected by cross bars 8 and 9 respectively.) Thus calculating the De Luca Element Ratio for the elements gives:

0.02 ohms+/0.01 ohms/0.168m2=0.119+/0.06 ohms/m2

which is within experimental error to the desired value for the De Luca Element Ratio providing the most optimal cook time. These experimental values also match closely to the expected values shown in FIG. 4.

[0063] Panels 10 and 11 are reflectors used to help focus the radiation towards the item placed in area 12.

[0064] FIG. 6 is close up photograph of the wire mesh 1 from FIG. 5, Mesh 1 is a 0.3 mm0.3 mm mesh (2R) using 0.14 mm diameter nichrome wire and operates well at 24V. Caliper 20 has a spacing between ends 21 and 22 of 2.0 mm for reference, bounding approximately 7 strands (spacing of 0.3 mm between strands).