Patent Grant
8145548
U.S. Provisional Application 60/822,028 filed on Aug. 10, 2006 and pending patent application No. 12/345,939 “Wire Mesh Thermal Radiative Element and Use in a Radiative Oven” filed by De Luca on Dec. 30, 2008, both of which are hereby incorporated by reference in their entirety, describe an oven capable of cooking foods at accelerated times compared to conventional ovens.
Specifically, the oven described consists of a stored energy system of batteries, a switching system, a food holder, and a wire mesh heating element or radiative bulbs used to cook the food. Typical cook times (in seconds) for a system running about 20 KW of power are described below:
The radiant heat bulbs are central to the prior art as they produce the appropriate wavelength of infrared energy required (in the range of 1 to 3 nanometers) and the multiple bulbs provide the intensity. Typical bulbs include halogen based bulbs similar to those produced by companies such as Ushio, Sylvania, or Soneko with power density of approximately 100 w/in2. Although these bulbs are effective at reducing cook times, they have several primary draw backs which have to this point deterred the prior art from successful introduction in the marketplace. Specifically;
Another method for heating involves the use of Nichrome wire. Nichrome wire is commonly used in appliances such as hair dryers and toasters as well as used in embedded ceramic heaters. The wire has a high tensile strength and can easily operate at temperatures as high as 1250 degrees Celsius.
Nichrome has the following physical properties:
When considering the use of Nichrome within an oven it is important to consider not only the resistive characteristics but also the black body emission of the element when hot.
With regard to the general characterization of resistive elements,
The resistance is proportional to the length and resistivity, and inversely proportional to the area of the conductor.
L is the length of the conductor, A is its cross-sectional area, T is its temperature, T0 is a reference temperature (usually room temperature), ρ0 is the resistivity at T0, and α is the change in resistivity per unit of temperature as a percentage of ρ0. In the above expression, it is assumed that L and A remain unchanged within the temperature range. Also note that ρ0 and α are constants that depend on the conductor being considered. For Nichrome™, ρ0 is the resistivity at 20 degrees C. or 1.10×10−6 and α=0.0004. From above, the increase in radius of a resistive element by a factor of two will decrease the resistance by a factor of four; the converse is also true.
Regarding the power dissipated from a resistive element, where, I is the current and R is the resistance in ohms, v is the voltage across the element, from Ohm's law it can be seen that, since v=iR,
P=i2R
In the case of an element with a constant voltage electrical source, such as a battery, the current passing throught the element is a function of its resistance. Replacing R from above, and using ohms law,
P=v2/R=v2A/ρ0L Eq.2
In the case of a resistive element such as a nichrome wire the heat generated within the element quickly dissipates as radiation cooling the entire element.
Now, considering the blackbody characterization of the element:
Assuming the element behaves as a blackbody, the Stefan-Boltzmann equation characterizes the power dissipated as radiation:
W=σ·A ·T4 Eq.3
Further, the wavelength λ, for which the emission intensity is highest, is given by Wien's Law as:
Where,
σ is the Stefan-Boltzmann constant of 5.670×10−8W·m−2·K−4 and, b is the Wien's displacement constant of 2.897×10−3 m·K.
In an application such as a cooking oven, requiring a preferred operating wavelength of 2 microns (2×10E−6) for maximum efficiency, the temperature of the element based on Wein's Law should approach 1400 degrees K or 1127 degrees C. From the Stefan-Boltzmann equation, a small oven with two heating sides would have an operating surface area of approximately 4 ×0.25 m ×0.25 m or 0.25 m2. Thus, W should aproach 20,000 Watts for the oven.
In the case of creating a safe high power toaster or oven it is necessary for the system to operate at a low voltage of no more than 24 volts. Thus, using Eq. 2 with 20,000 W, the element will have a resistance of approximately 0.041 ohms, if 100% efficient at the operating temperature. Based on Eq. 1, a decrease in operating temperature to room temperature (from 1400 to 293 K) represents an approximate decrease in the resistivity of the element by about 1.44 times, and therefore an element whose resistance at room temperature is .0284 ohms is required.
Now, considering the relationship of the resistance of the element and the characterization of the element as a blackbody:
The ratio of the resistance of the heater to the black body radiative area of the same heater becomes the critical design constraint for the oven; herein termed the “De Luca Element Ratio.” The ideal oven for foods operating over a 0.25 square meter area at 2 micron wavelength has a De Luca Element Ratio (at room temperature), of 0.1137 ohms/m2 (0.0284 ohms/0.25 m2). The De Luca Element Ratio is dependant solely on the resistance of the material and the radiative surface area but is independent of the voltage the system is operated. In addition, for wire, the length of the wire will not change the ratio.
Table 1 lists the resistance per meter of several common nichrome wire sizes as well as the De Luca Element Ratio for these elements. It is important to note that all these wires have a De Luca Element Ratio far greater than the 0.1137 required for an oven operated at 1400 K, 24 V, and over 0.25 m2. Clearly the use of a single wire with a voltage placed from end-to-end in order to achieve the power requirement is not feasible. In contrast, a houshold pop-toaster, operated at 120V and 1500 W, over a smaller 0.338 m2 area at 500 K would require a De Luca Element Ratio of 35.5. Thus a 1 meter nichrome wire of 0.001 m radius with a 120 V placed across it would work appropriately.
Clearly a lower resistance or a higher surface area is required to achieve a De Luca Element Ratio of close to 0.1137.
One way to achieve the De Luca Ratio of 0.1137 would be to use a large element of 2 cm radius. The problem with this relates to the inherent heat capacity of the element. Note from Table 1 that to raise the temperature to 1400 K from room temperature would require 65.4 seconds and thus about 0.36 KWH of energy.
This calculation is derived from the equation relating heat energy to specific heat capacity, where the unit quantity is in terms of mass is:
ΔQ=mcΔT
where ΔQ is the heat energy put into or taken out of the element (where P×time=ΔQ), m is the mass of the element, c is the specific heat capacity, and ΔT is the temperature differential where the initial temperature is subtracted from the final temperature.
Thus, the time required to heat the element would be extraordinarily long and not achieve the goal of quick cooking times.
Another way for lowering the resistance is to place multiple resistors in parallel. Kirkoff's laws predict the cumulative result of resistors placed in parallel (
The following Table 2 lists the number of conductors for each of the elements in Table 1, as derived using equation 5, that would need to be placed in parallel in order to achieve a De Luca Element Ratio of 0.1137. Clearly placing and distributing these elements evenly across the surface would be extremely difficult and impossible for manufacture. Also note that the required time to heat the combined mass of the elements to 1400 K from room temperature at 20 KW for elements with a radius of greater than 0.0002 meters is too large with respect to an overall cooking time of several seconds.
In summary, the following invention allows for the creation of a high power oven by using a resistive mesh element. The heater element designed so as to allow for the desired wavelength output by modifying both the thickness of the mesh as well as the surface area from which heat radiates. The heater consisting of a single unit mesh that is easily assembled into the oven and having a low mass so as to allow for a very quick heat-up (on the order of less than a few seconds).
Specifically, the wire mesh cloth design calibrated to have the correct De Luca Element Ratio for a fast response (less than 2 sec) oven application operating at 1400 degrees K.
To date, the best mesh design for operating a quick response time oven is a nichrome wire mesh with strand diameter of 0.3 mm, and spacing between strands of 0.3 mm, and operating voltage of 24 V.
Although the stored energy high speed oven would appear to have significant commercial use, in practice, there are several key inherent obstacles that have inhibited the oven's success. Specifically,
The integration of a high speed oven with a vending machine system similar to that for beverages at first pass would appear to ease some of the inherent difficulties to commercialization of high speed stored energy ovens. Specifically,
Recently, conventional oven technology has been used in combination with vending systems for the sale of pizzas. Specifically, Wonderpizza of New Bedford, Mass. has developed a vending system as well as Tombstone Pizza, a division of Kraft Foods of Winnetka, Ill. Both systems are similar in size to commercial vending machines for sodas, on the order of 1 meter by 1 meter by 2 meters tall, and incorporate ovens. Several problems with the units exist though:
One vending system that is much more flexible than a conventional beverage vending machine is manufactured by Bartech Systems International of Millersville, Md. These units rely on an electronic communication system and infrared sensing technology to detect which items have been removed from the holding container (most generally the container being a small refrigerator sized unit). When an item is removed from the container, the sensor detects the missing item from the shelf or pocket and subsequently sends an electronic signal to a control module which may include a internet web based system. While this vending system works well for the sale of individual items removed from the unit, it does not provide the necessary elements for integration with a high speed cooking oven or secondary vending process associated with a high speed stored energy oven.
In considering the combination of a high speed stored energy oven incorporating batteries, such as that described in U.S. Provisional Application 60/822,028 filed on Aug. 10, 2006 and patent application “Wire Mesh Thermal Radiative Element and Use in a Radiative Oven” filed by De Luca on Dec. 29, 2008, with a vending machine system, several difficulties arise. Specifically:
It is therefore an object of the current invention to provide a novel food vending machine system incorporating a high speed oven stored energy that overcomes the obstacles of traditional vending machines. Specifically,
In summary, the invention consists of a high power stored energy oven coupled to a food storage container and an electronic control system to allow for control of the oven based on the food placed within the oven. The food storage container generally outfitted with a refrigeration unit to allow for chilling or freezing of foods and a sensor system to detect the placement or removal of a food or packaged food. Due to the weight and bulk of the energy storage system for the oven, it is generally located below the container, with high current bus bars extending between the oven and the energy storage system along the sides or back of the container.
The electronic control system communicating between the food storage container and the oven to allow for monitoring of the items removed from the container and sensing of the items to be cooked at the oven. Sensing technologies such as infrared, bar codes, vision cameras, radio frequency tags, and bar codes can be used with the container or oven to determine the item removed from them or placed within them. The oven cooking parameters including running voltage, cycle times, cycle profile, rack spacing, and fan speeds.
The invoicing and billing components of the vending system allowing for the incorporation of a user identification system by employing a coded id card fitted with a radio frequency chip, a magnetic strip, or a bar code and further synchronizing the system to a web portal through the internet. The billing system allowing the vending system service provider to charge a customer for either the food, or the use of oven, or both.
Preferred and best mode designs and forming techniques are hereafter described.
The invention will now be described in connection with the accompanying drawings in which:
In
When using batteries, the stored energy and switching system 4 may be very heavy and thus is most preferably placed at the bottom of the entire vending system 1 to insure that the unit is not top heavy.
In use, food items 101 which may be packaged are placed in storage container 3 upon shelving or trays 60. The container 3 may be further refrigerated, generally at temperatures ranging from −30 to +10 degrees Celsius. Sensor 22 will detect the items or their presence on the trays 60 and communicate to the central processing unit 40.
When desired, a user would most generally scan their identification card via a magnetic swipe 9 and remove item or items 101 from the food container 3. Upon removal from food container 3, registration that the item has been removed from container 3 is sent to the processor 40. Processor 40 may obtain the cooking information from its own memory system or through access to an off site database connected through the internet.
Once obtained from storage container 3 the food may be unwrapped and subsequently placed on tray 8 for cooking. Identification of the food item 101 on tray 8 may be done via sensor 10 which, most preferably, is a bar code scanner able to read a code placed on the packaging of food item 101. A vision system may also be used to detect the type of food placed on tray 8 through processor 40 and detector 10.
With confirmation of the item to be cooked within oven 2, the oven parameters are changed automatically, including running voltage, cycle times, cycle profile, the spacing between tray 8 and heating elements 7, and fan speeds. Start button 102 is subsequently pressed, sending a signal to controller 40 and control relays 20. The power originates from batteries 5 and the current passes through connectors 21 and bus bars 6 to allow for heating of the heater elements 7. The timing and pulsation width of the cycle controlled by the processor 40. When cooked, the food item is removed from oven 2 as detected by sensor 10 and the information is transmitted via processor 40 to the associated user account.
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