Patent Application
20220074596
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.
The invention enabling the use of an etched or stamped metal plate or ribbon as further described in co-pending U.S. provisional patent application No. 62/730,893 filed Sep. 13, 2018, later filed as PCT/US2019/050805 on Sep. 12, 2019 entitled “Heater Element Incorporating Primary Conductor for Use in a High-Speed Oven” (“the '805 PCT application”), the entirety of which is incorporated by reference herein, 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 0.25 m×0.25 m 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.
The invention further incorporating a novel tensioning and clamping method for the heater element. The tensioning and clamping mechanism enabling a quick change during use as well as proper registration during placement and alignment during use.
The use of heater mesh is fully described by De Luca in U.S. Pat. No. 8,498,526B2 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 0.25 m×0.25 m 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 U.S. Pat. No. 8,498,526B2 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.
The co-pending '805 PCT application 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.
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 the '805 PCT application 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″×8.25″ planar heating element NP25 operated in two power regimes 2000 W and 1500 W. 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.
Similarly, in example 2, the planar heating element NP-16 measuring 5″×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.
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.
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 15000-5000 W is typically needed for an 8.5×5″ planar element.
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 10× reduction in the tension only increases life by about 50% in the planar element.
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.
As described in co-pending the '805 PCT application (which designates the US), 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,
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.
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 A1 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 A1 (a higher grade material for use in this application).
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.
Prior to the herein described invention, the following plot shows cycle life for various flat heating elements produced per the '805 PCT application, or U.S. Pat. No. 8,498,526B2 “Wire Mesh Thermal Radiative and Use in a Radiative Oven”. All elements are approximately 5″×8.25″ in size and vary in geometric mesh cut patterns to adjust for the appropriate voltage and current.
In addition, the following table lists details of the testing for these various elements as well as their corresponding DER per U.S. Pat. No. 8,498,526B2.
oven
D 30% Open
×
30%
A1 Wire Mesh
thick 30%
thick 30%
indicates data missing or illegible when filed
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.
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 μ0/2πr
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:
μ0=4π×10−7 Tm/A
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.
Placing a planar heating element within a heating cavity and insuring that the heater is properly connected to the electrical and tensioning system can be difficult. Replacing the element quickly is necessary within the context of use within a quick serve or drive through restaurant. In some cases the element may not latch correctly and may slip off during the normal expansion and contraction that occurs during use. In addition, if proper force in not applied at the electrical connection ends, the high electrical current may arc and increase the temperature at the connection which eventually leads to oxidation and thermal degradation including melting.
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.
It is further an objective of the following invention that the heater element described above could be made per the description of co-pending the '805 PCT application (which designates the US) and as such does not require a separate welding step for manufacturing.
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 1500 W for a 5″×8.5″ sized element.
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.
It is another objective of the following invention to provide for a heater element that has a DER of less than 2 ohms/m2.
It is a further objective of the following invention that the heater element provided be easy to register and to place within the oven or holder and that it is properly tensioned during use.
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 the '805 PCT application (which designates the US) 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 0-48V and have a have a low electrical resistance of less than 2 ohms so as to deliver at least 1500 W for a 5″×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 U.S. Pat. No. 8,498,526B2 “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.
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.
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.
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.
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.
In
In one preferable embodiment, the stepped down at 45, 46, 47, and 48 of
In order to place element 1 within in holder box 800 so as to secure a simultaneous electrical voltage application and a mechanical tensioning, the element ends 302 and 301 are placed under secondary conductor bars (or clamps) 73 and 72 respectively. The secondary conductor bars 73, 72 may be biased to a position where they engage the element ends 16, 17 when provided therein, and when not provided the clamps 73, 72 engage a horizontal surface of the holding box 800. Clamps 73, 72 may be each further connected to the positive and negative electrical circuit that powers the element 1. These clamps 73, 72 may have a positive actuation locking mechanism, a spring forcing mechanism, or any other mechanism intended to provide positive connection and pressure to insure a proper electrical connection. Engagement portions of the clamps 73 and 72 may be nickel plated so as to prevent wear and insure a strong electrical contact with minimal resistance. In some embodiments, each of clamps 73, 72 include a peg that is configured to extend within the corresponding tensioning hole that is provided at the respective end 16, 17 of the element to result in mechanical and electrical connection between the clamps 73, 72 and the element 1.
In an embodiment depicted in
In another embodiment depicted in
In some embodiments, the hole 19z may be a round hole, while in other embodiments, as best shown in
In some embodiments, the end 7 of the element may include two or more holes 19w, which may be round holes or shaped as in the hole depicted in
In some embodiments, the pegs 819a and the respective holes 19a that engage the pegs 819a may be provided to ensure that the element 1 can only fit onto the pegs 819a in one specific orientation, such as to avoid installing the element 1 upside down or backwards. For example, as depicted in
The embodiments described above and depicted in
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
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.
The process of manufacturing a roll 90 such as that in
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.
This application claims priority from U.S. Provisional Application No. 62/801,750, filed on Feb. 6, 2019, the entirety of which is fully incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/016972 | 2/6/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62801750 | Feb 2019 | US |
