Multi Planar Heater Element for Use in a High-Speed Oven

FIELD

[0001] The present disclosure teaches a radiative heater for use in a high speed oven formed from two or more planar heater elements stacked closely to form an effective single element and allowing for extended life through the minimization of concentrated eddy currents in both elements. Compared to a single planar element, the multi-planar heater element creates a modified magnetic field that helps to diffuse the current evenly and minimizes any concentrated currents that greatly reduce the usable life of the element by creating current concentrations resulting in local heat spots or pockets.

[0002] The invention enabling the use of an etched or stamped metal plate or ribbon as further described in co-pending US provisional patent application number 62/730,893 filed September 13, 2013 and entitled“Stepped Heater Element for Use in A High-Speed Oven” to operate at high power levels at a significant increase in life (observed at 3 to 75 times). The element may be formed from a single material stock or mesh with two more sections of different thickness and density adjusted to optimally deliver heat to an item to be cooked. The heater element is suitable for use at low voltages with a De Luca Element ratio of less than 2 (when compared to a 0.25m x 0.25m flat area and with a resistance measured across the oven length) and further allowing for heat ramp up to a maximum temperature in less than 3 seconds. In some embodiments, the heater element includes ends with a lower electrical resistance to allow connectivity of elements in series and further insure that the ends do not over heat as well as to facilitate the proper clamping and tensioning of the element.

BACKGROUND

[0003] The use of heater mesh is fully described by De Luca in U.S. Patent number 8498526B2 as a means to safely deliver high power at a low voltage to an oven cavity. Typical means described by De Luca for delivering a high power output at a wavelength of 1-3 microns (which is most ideal for cooking food items such as toast) involves use of an element which when forming an oven of 0.25m x 0.25m with a top and bottom element in parallel has the typical characteristic of having a ratio of its resistance to a black body radiative surface area of less than 2 ohms/m2. As further described by De Luca in US patent 8498526B2 the ability to quickly increase the temperature of the element is important to facilitate high speed cooking, avoid energy consumption when the oven is not in use, allow for“instant” use, and further to be able to cycle the heater on and off so as to prevent excessive heating. The ability to cycle the heater is required for the process of being able to cook with a high power radiative heater and a recipe for an item can typically include 3-15 on-off cycles.

[0004] Co-pending US provisional patent application number 62/730,893 describes a heater formed from a single planar sheet of metal and includes a step used to decrease the thickness of the metal in the heater area and thus increase the speed at which the element heats. The element can be formed with holes in a mesh pattern so as to increase the black body radiative area and also increase the resistance of the metal. While the use of a flat sheet mesh versus a wire mesh has significant manufacturing advantages at high power levels with significant cycling (i.e., generally greater than 30 watts per square inch of flat cooking surface and greater than 1000 on-off cycles), it has been observed that the heater elements have a usable life of less than 1000-5000 cycles before failure. In comparison, round wire mesh operated at similar power levels may have an operational life of 10-15,000 cycles.

[0005] Life expectancy data for heater materials typically operate in a constant on mode and the primary deterioration is associated with oxidation of the element when hot. As an example, planar mesh formed per Co-pending US provisional patent application“Stepped Heater Element for Use in A High-Speed Oven” may have a life of far greater than 100,000 seconds when left on at 2500 watts but when cycled on for 5 seconds, off for 5 seconds at the same wattage, the element will only last 5-15,000 seconds. As an example, the following chart shows the results of 3 different tests of a single layer 5” x 8.25” planar heating element NP25 operated in two power regimes 2000W and 1500W. The planar heating element NP25 lasted a total of 8220 seconds on when cycled 5 seconds on 5 seconds off versus 100,000+ seconds on when cycled only 28 times during a continuous test at two different power levels.

Example 1 Cycles Seconds On Voltage Power

NP25-K 1644 8220 25.60 2112

NP25-K 28 100800+ 25.60 2099

NP25-K 28 100800+ 20.8 1352

[0006] Similarly, in example 2, the planar heating element NP-16 measuring 5” x 8.25” lasted a total of 11,000 seconds on when cycled on and off every 5 seconds but continued past 100,000 seconds when cycled only 28 times during a continuous test. Example 2 Cycles Seconds On Voltage Power

NP16-304 2200 11000 20.8 3120

NP-16-304 28 100800 20.8 2870

[0007] In examples 1 and 2 above, the materials used respectively were a Kanthal (an iron based material) and a 304 stainless. In both cases it was clear that the cycling of the element was responsible for the early failure versus a result of the material itself.

[0008] One obvious solution to the above limitation on life through cycling would be to operate at a lower voltage value and pass less electrical current through the element. Typical life curves for material operating in the radiative regime of 800-1400 degrees C decrease exponentially as the temperature and associated power increase. In the case of using a high speed oven though, this is not a solution as temperature ramp up needed of the element is typically 100-500 degrees C per second, and thus is not an option as 15000-5000W is typically needed for an 8.5 x 5” planar element.

[0009] Another obvious option for increasing the life of the planar element involves modifying the tensioning system to reduce tension. Crack propagation in materials is associated with the stress in the material and therefore it would seem logical that an increase in stress would accelerate potential crack propagation and failure. While reducing the spring force has some effect, example 3 below clearly shows that even a lOx reduction in the tension only increases life by about 50% in the planar element.

Example 3

Life Cycles Power Spring Voltage

NP26-K 1000 2558 19 lbs/in 20.8

NP26-K 1527 2374 2 x 1.67 lbs/in 22.4

[0010] In further considering the tremendous life discrepancy associated with a constant on mode versus cycling, another obvious observation could be made regarding the propagation of cracks with the cooling and heating of the element. It could be concluded that by keeping the average temperature over the test more consistently high, the material would undergo less elongation change during the test and therefore the life would be increased. In example 4, the cycling of the 5 seconds on 5 seconds off test was changed to 5 seconds on, 2 seconds off in hopes of seeing an improvement in the performance. No such improvement was seen.

Life

Example 4 Cycles Power Cycle Voltage

NP24-K 2400 2612 5secON 5secOFF 28.7

NP24-K 1684 2622 5secON 2secOFF 28.5

[0011] As described in co-pending US patent application“Stepped Heater Element for Use in A High Speed Oven”, the compact U shape element formed from a single planar metal allows for tensioning from a non-current applying side and power delivery from fixed ends. During use though, concentrated heat patterns are observed to develop at the union end between the legs of the“U” as the current wraps around from one terminal to another and failure occurs at the juncture of the union end and the mesh. These concentrations of heat are observed as glowing hot spots on the union section metal and they tend to increase in size and depth with the number of cycles the mesh is operated. Specifically, within said application, FIGs 3, 4, 9, and 10 show a“U” element with the union end and indications of the overheated area. Further, the formation of connections within the union area of the“U” element with equivalent resistance paths is described which has been shown to help decrease the concentration of power and heat within the union. Though this tends to decrease the formation of hot spots within the union, the extension in life provided appears to be only minimally significant as compared to achieving the same life of a wire mesh of 10,000 cycles. As shown in example 5, accounting for power decrease, only a minimal increase, if any, was achieved through the application of even pathways at the union end.

Example 5 Life Cycles Power Union Description Voltage

NP18-304 2200 2870 Solid Back 20.8

Modified Union

NP24-304 3655 2184 Path 20.8

Modified Union

NP25-304 2883 1789 Path 20.8 [0012] When using the above mentioned preferred“U” element design, the failure mode of the element appears as an overheating of a single filament at the juncture of the long segments and the union. When a single filament along the current path fails, a cascade effect occurs as the electrical current is concentrated in fewer and fewer of the strands until the element no longer operates. Attempting to cool this area using a tube to blow air would be an obvious option, yet doing so provided minimal or no increase in life as shown in Example 6.

Example 6 Life Cycles Power ETnion Description Voltage

NP17-304 1500 3328 No Cooling 20.8

NP17-304 1496 3328 Air Cooled 20.8

NP17-304 1519 3328 Air Cooled 20.8

[0013] Another and final obvious construction to try to increase the life of a planar heating element used in cycling applications such as a“U” element, would be to increase the thickness of the element and further avoid the use of a step which may cause a stress

concentration. While increasing the thickness of the element will inherently increase the elements mass and therefore the time required to heat up, it could be presumed that doing so would also increase the strength of the element and reduce the potential for crack propagation due to cycling. In example 7, several elements are compared including two made of a single thickness of 0.015” Kanthal Al and one made of a 0.004” thick Kanthal D. Despite the variance associated with tensioning force and power levels, there is little to no increase in the life of the element compared to other planar elements for the thicker elements despite their being made of Kanthal Al (a higher grade material for use in this application).

Example 7 Life Cycles Power Description Voltage

0.015" Thick l9lb/in

NP26-K 1000 2558.4 spring 20.8

0.015" Thick 2 x

NP26-K 1527 2374.4 l.67lbs/in 22.4

0.004" Thick Gravity< 5

NP04-K U 4118 2764.5 lbs 28.5 [0014] While there has been some increase in life associated with applying no tensioning with the exception of gravity to the element (NP04-K above achieved a life of about 4118 cycles) and in some cases not using a“U” but instead applying the voltage across the entire width as a single element evenly (in this case a straight NP04-K achieved 6000 cycles before partial degradation), no tests using flat sheet have shown the same life as a wire mesh.

[001] Prior to the herein described invention, the FIG. 9 plot shows cycle life for various flat heating elements produced per co-pending US Provision Application“Stepped Heater Element for Use in A High Speed Oven”, or US patent #8498526B2“Wire Mesh Thermal Radiative and Use in a Radiative Oven”. All elements are approximately 5” x 8.25” in size and vary in geometric mesh cut patterns to adjust for the appropriate voltage and current.

[0015] In addition, FIG. 10 lists details of the testing for these various elements as well as their corresponding DER per US patent #8498526B2

[0016] It is important to note that the DER values which represent the ability of the element to quickly get hot and radiate power are all well below a threshold value of 2.

[0017] It is also important to consider the magnetic fields produced as a result of the high current in the element and the induced electrical currents as the heater is switched. An electric current passing through a wire can be characterized by amperes law:

B = I x mq / 2pG

[0018] Where, B is the magnetic field in Tesla produced by the current I, at a distance r, and with the permeability of free space equal to:

mo = 4p x 10 ⁷ Tm/A

[0019] As an example, a wire carrying 110 amps would produce a magnetic field of 2.2 gauss at approximately 0.1 meters. In addition, the magnetic field created when a current is pulsed on or off creates a greater magnetic field that is described by Faraday’s law which states that the induced current is proportional to the rate of change of the magnetic field. When using a single layer element, the induced current and magnetic fields produced can force the current to flow in specific areas that therefore concentrate heat and lead to deterioration of the element. Magnetic fields in the range of 0-40 gauss have been measured on single layer elements further described above in non-shielded areas.

[0020] It is therefore a primary objective of the following invention to provide for a planar heater element that can be used in a high speed oven and can operate at over 1500 watts and can be cycled on and off at a 5 sec on 5 sec off rate with a life of greater than 15,000 cycles.

[0021] It is further an objective of the following invention that the heater element described above could be made per the description of co-pending US patent application“Stepped Heater Element for Use in A High Speed Oven” and as such does not require a separate welding step for manufacturing.

[0022] It is another objective of the current invention that the heating element be useful in a safe high speed oven and be operational at a low voltage of 0-48V and have a have a low electrical resistance of less than 2 ohms so as to deliver at least 1500W for a 5” x 8.5” sized element.

[0023] It is another objective of the current invention that the heating element be able to achieve a ramp up heating rate of at least 100 degrees C per second.

[0024] It is another objective of the following invention to provide for a heater element that has a DER of less than 2 ohms/m2.

SUMMARY

[0025] The present teachings provide embodiments of a novel bi-planar heater element, and features thereof, which offer various benefits. The invention provides for a bi-planar heater element that can be used in a high speed oven and can operate at over 1500 watts and can be cycled on and off at a 5 sec on 5 sec off rate with a life of greater than 15,000 cycles. One element herein described having been cycled over 74,000 times at 2500 watts. The heater made by overlaying two similar elements and forming a common path for current flow. Each of the elements inducing a magnetic field in the other during operation such that the electrical eddy currents and current concentrations normally present in a single layer are moved more evenly throughout the element and thus increase the life of the element. The element further being capable of being manufactured from a singular piece of sheet metal that could be made per the description of co-pending US patent application“Stepped Heater Element for Use in A High Speed Oven” and as such does not require a separate welding step for manufacturing. The high wattage heater further capable of being safely operated within a high speed oven and at a low voltage of 0-48 V and have a have a low electrical resistance of less than 2 ohms so as to deliver at least 1500W for a 5” x 8.5” sized element at low voltage. The element being formed by a material thin enough to be powered and achieve a ramp up heating rate of at least 100 degrees C per second and to be cycled on and off for optimum cooking recipes. As such, the invention provides for a heater element that has a DER of less than 2 ohms/m2 as further defined by US patent #8498526B2“Wire Mesh Thermal Radiative and Use in a Radiative Oven”. In some embodiments, the bi-planar heater element has ends that are increased in thickness and density so as to provide more material which acts as a primary conductor as further described in co-pending US patent application“Stepped Heater Element for Use in A High Speed Oven”. In a preferred embodiment, the element is formed using an etching process (such as EDM or chemical etching) that creates two or more distinct thicknesses in the element so as to lower the resistance of the mesh at the integrated primary conductor areas and then folded on itself to create the two heating layers. The manufacturing process further enabling elements to be formed with quasi-identical segments that allows for ease of tensioning and registration within a secondary conductor and use with higher voltage. The manufacturing process also allowing for formation of a roll of elements located end to end such that a continuous element is created from a single original sheet which can be formed into a bi-layer heating element at the time of use. Additional coatings can be applied to the element during the manufacturing process which can be done in a continuous automated fashion.

[0026] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE FIGURES

[0027] The accompanying drawings, which are included to provide a further

understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention.

[0028] FIG. 1 is an isometric view of a flat mesh heating element made from a single flat sheet and foldable so as to create two parallel planar sections for carrying a high current further spaced together so as to induce mutual magnetic fields during use that distribute the current evenly and allow for cycling above 15,000 times.

[0029] FIG. 2 is an isometric view of the heating element in FIG.1 folded so as to create a multi-planar element.

[0030] FIG. 3 is an isometric view of the heating element of FIG. 1 and FIG. 2 folded completely to form a multi-planar heating element. [0031] FIG. 4 is an isometric closeup view of the connection paths of the union section of the multi -planar heating element of FIG. 1, FIG. 2, and FIG. 3.

[0032] FIG 5 is an isometric view of a tensioning system used to hold the mesh of FIG.

1, FIG. 2, and FIG. 3.

[0033] FIGS 6a and 6b are isometric views of a roll of sequentially formed elements such as that in FIG. lc so as to create a continuous string of elements.

[0034] FIG 7 is an isometric view of the manufacturing process used to make the element of FIGs. 1-6 further including a coating process.

[0035] FIG. 8 is a diagram illustrating the relative placement of the multi-planar heating element on a plot of the life during cycling versus the wattage and further compared to past developed heating elements with DER values less than 2 for use in high speed ovens.

[0036] FIG. 9 plot shows cycle life for various prior art flat heating elements.

[0037] FIG. 10 lists details of the testing for various prior art heating elements of FIG. 9.

[0038] Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience.

DESCRIPTION

[0039] The present teachings disclose a novel heating element having a DER of less than 2 ohms/m2 an ability to be powered at over 1500 watts, capable of increasing repeatedly in temperature at a rate of at least 100 degree C per second, and be capable of being cycled more than 15,000 times on and off every 5 seconds. The following details the specifications of two such bi-layer elements and the cycling life achieved when cycled 5 seconds on / 5 seconds off.

As can be seen from the table, the first element cycled over 74,378 times before complete failure and the second cycled over 50,000 times.

0.004 K-Diamond Cut 50% Bi Metal W/Cut Even 0.015”

Back

Max Life Cycles 74378 cycles

Voltage 20.80 volts

Single Element Resistance 0.17 ohms Single Element Watts 2496 watts Single Element Radiative Area 0.05 m2

Extrapolated Element Radiative Area for 0.25m x 0.25m

oven 0.21 m2

Extrapolated Resistance Radiative Area for 0.25m x 0.25m

oven 0.02 ohm

DER 0.1 ohm/m2

Extrapolated Power/DER Ratio 98106 watts-m2/ohm

0.004 K-Diamond 50% Bi Metal W/Filled Even 0.015”

Back

Max Life Cycles 50000+ cycles

Voltage 23.20 volts

Single Element Resistance 0.19 ohms

Single Element Watts 2853 watts

Single Element Radiative Area 0.05 m2

Extrapolated Element Radiative Area for 0.25m x 0.25m

oven 0.21 m2

Extrapolated Resistance Radiative Area for 0.25mx0.25m

oven 0.02 ohm

DER 0.11 ohm/m2

Extrapolated Power/DER Ratio 103072 watts-m2/ohm

[0040] FIG. la is an isometric view of the novel heating element 1 in a preferred embodiment formed from a single sheet of heating material 2. These materials include Kanthal alloys, stainless steel alloys, nickel chromium alloys, and other ferrous and non-ferrous metals used for heating elements. Mesh areas 4 and 24 formed through etching, stamping, or other machine process on both halves 3 and 5 along centerline 6 such that the resistance of the element is matched with the driving voltage and current required. In some cases, mesh areas 4 and 24 may be solid, thinned, cut, and otherwise modified. Heating element 1 having a DER of less than 2 ohms/m2 an appropriate resistance which may be less than 2 ohms. Halves 3 and 5 having union ends 7 and 8 respectively and formed with equal resistive paths 9 to mitigate the formation of hot spots during operation in areas 10, 11, 12, and 13. In some cases meshed areas 14 and 15 further thinned down in thickness compared to ends 16 and 17 and union areas 7 and 8 such that the regions can be heated quickly to an optimum wavelength for radiative cooking. As an example, meshed areas 14 and 15 may have a thickness of 0.002”-0.0l5” while union ends 16 and 17 may be 0.0l5”-0. l00” thick.

[0041] In FIG. 2, halves 3 and 5 are folded along centerline 6 so as to mate union ends 7 and 8, ends 17 and 18, and meshed areas 4 and 24 as well as the tensioning holes 18, 21, and 19 on half 5 to the corresponding holes on half 3.

[0042] FIG. 3 illustrates element 1 now completely folded at centerline 6 to form element 30 with edges 40, 41, and 42 and mated areas 3 and 5. In some cases, welding the two halves 3 and 5 in regions 31, 32, 33, and/or 34 may help to insure proper current distribution when the element is powered from ends 16 and 17.

[0043] In one preferable embodiment, the stepped down at 45, 46, 47, and 48 of FIG. 3 and FIG. 4 allow for a flat surface for mating in region 5 between halves 3 and 5. The closer mating of the surfaces allowing for the induced magnetic fields during operation to affect the current flow to thereby avoid current concentrations.

[0044] FIG. 5 illustrates a holding box 800 for element 1 and 30 with springs 71 attached to the mated union ends 7 and 8 through holes 19. Secondary conductor bars (as further described in co-pending provisional application“Stepped Heater Element for Use in A High Speed Oven”) 72 and 73 carry a voltage potential that passes electrical current through the two “legs” 74 and 75 of area 3 and 5 of element 1 and 30 through ends 16 and 17. The electrical current may be of various forms, including dc or ac, stepped, triangular, square waves, pulse modulated, or in multiple phases. The holding box which may become part of an oven further including a reflective surface 80 and side walls 81. Monitoring the temperature of said surface or surfaces 80 may be done when they are formed into an oven cavity which may itself be monitored. A predetermined cycle or continuously adjusted cycling based on input to the control system from a sensor and operation of the element may be performed to control the output wavelengths of the heater to optimize performance in an application such as cooking, baking, searing, curing, or other heating. The heater may also be submerged in liquids for heating.

[0045] One of the observations made of the novel bi-element is the reduction of the magnetic field in areas 300, 400, 301, and 302 in direction 401. In one trial, a single layer region was used for the union area 7 testing in the holding fixture 800 of FIG. 5 and it was found that the magnetic field at 300 and 400 in direction 401 was reduced from about 39 gauss to 9.5 gauss (at 0.1 meters).

[0046] While it is difficult to fully characterize the eddy currents induced in the multiple layer heating element, the change of the magnetic field versus a single element and the presumed associated redirection of the electrical current can be considered a significant factor for the increased life.

[0047] FIG. 6a illustrates a continuous roll 90 of elements 1 and 30 joined sequentially to form a roll with the potential of being operated many millions of cycles. US Patent application US151183967 describes a continuous mesh system yet does not describe integrated primary and secondary conductor bars. Co-pending provisional application“Stepped Heater Element for Use in A High Speed Oven” describes primary conductors that are integrated within the continuous mesh yet does not include the two or more layers that form the primary mesh or a post folding process to create a bi-element as herein described per this invention. FIG. 6b is an alternative roll form being a single layer having elements 1 and 30 of FIGs. 1 through 5 formed sequentially but then folded manually or in an automated fashion at the time of use.

[0048] The process of manufacturing a roll 90 such as that in FIG. 6a and FIG. 6b is further shown in FIG. 7. The process for making the two halves of element 1 and 30 through etching, stamping, pressing or thinning, or other machine process from blank roll stocks 100 and 101 is done within system or systems 590 and secondary process such as coating done at 591. A single roll 100 may also be used and rather than folding the element 1 per FIGs. 1, 2, and 3, two parallel single sheets may be formed along edge 107 of Fig 3 and then folded along the edge to create element 30. Other symmetrical folding or manufacturing processes may be employed to create an element with multiple layers per the specifications of this patent and further each of these elements may be parted singularly or in multiples before, during, or after use.

[0049] FIG. 8 is a diagram illustrating the relative placement of the multi-planar heating element on a plot of the life during cycling versus the wattage and further compared to past developed heating elements with DER values less than 2 for use in high speed ovens. As can be noted from the graph, the multi-planar elements provide very significant benefit.

[0050] The examples presented herein are intended to illustrate potential and specific implementations. It can be appreciated that the examples are intended primarily for purposes of illustration for those skilled in the art. The diagrams depicted herein are provided by way of example. There can be variations to these diagrams or the operations described herein without departing from the spirit of the invention. For instance, in certain cases, method steps or operations can be performed in differing order, or operations can be added, deleted or modified.