MXPA99001025A - Heat retentive food servingware with temperature self-regulating phase change core - Google Patents

Abstract

A heat retentive, temperature self-regulating, food retaining apparatus (10) includes a body (12), heat retentive core (14) and magnetic induction heating element (16). The body (12) includes a substantially rigid, heatable, food-contacting wall (18) defining a cavity (24). The core (14) is positioned in the cavity (24), and in thermal contact with the wall (18) for selective heating of the wall (18). The core (14) includes a solid state phase change material for storing latent heat during a solid-to-solid phase transformation at a phase transformation temperature. A resilient material is in contact with the phase change material to permit expansion of the phase change material during a phase transformation. The heating element (16) is in thermal contact with the core (14) for heating the core (14) to a temperature above the phase transformation temperature to effect a phase transformation in the phase change material. The element (16) includes a ferromagnetic material responsive to a magnetic field for inducing an electric current in the element (16) to heat the element (16). The ferromagnetic material has a Curie temperature between the phase transformation temperature and the melting temperature of the phase change material. A food warming device (42) includes a heater (44) having a holder (46), a magnetic field generator (48), and a no load detector (50), and a food retaining apparatus (10) positioned on the holder (46).

Description

SERVER DI PRODUCTS ALSMENTICIOS THAT KEEPS THE HEAT WITH CEN RO OF CHANGE OF PHASE WITH AUTO REGULATION OF TEMPERATURE RELATED REQUEST This is a continuation in part of the application Serial No. 08 / 688,987, published July 31, 1996, and claims the benefit of provisional patent applications Nos. 60 / 035,815 and 60 / 044,074, published on 13 d ~ January, 1997 and April 24, 1997, respectively.

ANTICIDINTITIS OF THE INVENTION 1. Field of the Invention The present invention relates to food servers that retain heat. The preferred food or beverage server of the present invention comprises a heat holding center that includes a phase change material from solid to solid, a REF .: 29352 flexible material that allows the expansion of the material of phase change, and an element of heating inductor for the regulation of the temperature of the material of phase change. The food server of the invention is capable of self-regulating the temperature when heated by a magnetic induction cooking device. 2. Description of Previous Art.

Many preparers require the use of devices to keep the food warm before serving and during a meal. Such preparers include institutional food preparers and servers, restaurants, suppliers, individual consumers, etc. Institutional food servers such as hospitals, private senior homes, and other similar operations, commonly require a period of time between the preparation of the food and serving it may exceed thirty minutes.

Various server devices that retain heat to maintain the food until the food can be served are known in the prior art. Server devices that retain heat generally include a server base and an isolated dome for the base. The most common commercially used server base is designed to support standard saucers to hold the food. The examples of the previous art of. such server base are shown in U.S. Pat. No. 4,246,884 by Vandas, and are available from companies such as the Seco Products Corporation, and the Carter-Hoffman Corporation. The company Seco Product, for example, produces such products under the names "System 1" and "System 9".

The server base is typically comprised of a "semiconductor plate" of stainless steel or base with some type of heat storage material sealed therein, a lower coating of synthetic resin for insulation, and a single standard ceramic plate that is supports on the semiconductor plate. The common heat storage materials in the semiconductor plate include metals and wax.

The servers that retain the heat of the prior art are typically used in the following manner. First, the group of stainless steel semiconductor plates are pre-heated in a heated plate-type oven distributor. Simultaneously, the groups of plates of the separated individual plates are heated in the same or similar heated distributor. After sufficient heat has been stored in the semiconductor plates and stainless steel plates, the heat retaining servers are mounted during the processing of the food.

During such assembly, a worker carefully removes a hot semiconductor stainless steel plate using a large suction cup. The stainless steel semiconductor plate is placed on top of a lower plastic liner. Then, a single, heated plate is placed on top of the semiconductor plate. This assembly is then sent under a conveyor line where the food is placed on the plate. Finally, an isolated dome is coupled with the complete base to cover the food and finish the assembly of the server. The food enclosed within the server is kept warm by the heat passively released from the hot storage material and by the insulation effect of the dome and the lower coating.

The U.S. Patent No. 3,557,774 by Kreis, U.S. Pat. No. 3,837,330 by Lanigan et al., And U.S. Pat. No. 4,086,907 by Rochschild expose examples of server bases that have some type of metal or metal alloys as the material that retains heat. Each of the devices disclosed in these references includes variations in the structure of the server base to control the expansion of the metal and trap the expansion of the air within the server base. Although many commercial server bases with metallic heat storage material are currently in use, they do not keep the food hot enough for many institutional food service operations. For example, due to the storage of only the sensible heat, and the low specific heat, the high thermal conductivity and the high density of the metals, these server bases have to be extremely large or pre-heated at various temperatures to compensate for the performance of the server bases using phase change materials.

The U.S. Patent No. 3,148,676 By Truog et al., U.S. Pat. No. 3,734,077 by Murdough et al., And the Vandas reference disclose examples of wax center server bases that use solid-to-liquid phase change materials as a heat storage material. These references disclose carnauba or synthetic wax based on petroleum having a relatively high specific heat and a relatively low melting point, such as between about 170-270 ° F. The structural differences of the devices disclosed in these references include variations of expandable wall designs to avoid base breakage due to the melting / expanding of the wax and various means to improve the transfer of heat from the wax to the upper surface of the wax. the base of the server. Many of the wax center server bases are used by the current institutional food servers, which include the previously observed System 7 and System 9 devices manufactured by Seco Products-Corporation. Most servers that retain commercially available wax center heat keep food above 140 ° F for more than 30 minutes, some for more than an hour.

Despite the current widespread use of server bases that include solid to liquid heat retention centers among institutional food servers, there are several problems. For example, the pre-heating of the stainless steel bases is carried out within one or two hours in the commercially available heated base-type oven distributors, limiting the flexibility of the food service operation. Due to the conclusion of this time and the process of energy consumption, workers must take extreme caution in assembling the servers to avoid burns, as shown above.

Several alternative server designs in the prior art have faced these problems. The U.S. Patent No. 4,982,722 by Wyatt exposes a base of. server with upper and lower shell walls made of a material of low thermal conductivity, not metallic. An encapsulated heat center of solid to liquid phase change material is disposed in the cavity. This design proposes to solve the problem of potential burns when removing the base of the server from an oven-type heater. The required pre-heating time, however, is relatively long. U.S. Pat. No. 4,567,877 by Sepahpur does not face the problem of pre-heating time. The reference of Sepahrur exposes a server that retains the heat built with non-metallic materials that is designed to store heat by exposing wet sand encapsulated in its base by microwaves. However, the Sepahpur device does not face the problem of the vapor pressure encountered when the water changes to steam.

Even though the prior art pretends to solve the pre-heating and security problems mentioned above with the server bases, these and many other problems with the servers that. they retain heat of prior art remain if resolved. For example, servers that retain the heat of prior art are bulky. In the institutional service application, the large volume demands large transport carts for the release of multiple meals to patients, increasing equipment costs, and potentially causing undue efforts to the workers who deliver them. The servers that retain the heat of the prior art require special washing treatment and special supports for proper drying. Server bases that retain heat from the prior art also typically comprise multiple pieces that demand additional effort for man and time to assemble during food preparation and demand excessive space to store when not in use. In addition, the bases of the previous art server with times that keep the temperature for long periods, p. ex. With wax center bases, they could spill molten wax from their joints during normal use. This problem presents a security risk for institutional workers and waiters.

As a result of these disadvantages, restaurants in general resort to the pre-heating of individual standard ceramic plates and / or special metal plates in the cooking ovens. Restaurants also use infrared heaters to keep food warm before serving. These methods are relatively inefficient and time consuming. In addition, such methods result in only the outer layer of food being heated, allowing the food to cool and dehydrate significantly before being consumed by a customer. Other known service heating devices include electrically driven coffee shops, heating trays, and aluminum heat trays heated by candles, sterno or burners.

It is desirable to have a server that retains the heat that addresses the problems exposed to institutional food servers by the servers of the prior art. It is desirable that a new server not only be compatible with the present commercially available pre-heating equipment, but be capable of being pre-heated by new convenient methods to significantly increase the preparation time, reduce the required man strength, and decrease the security aspects. It is desirable that a new server that retains the heat and pre-heating methods are convenient, efficient and effective enough to open new markets for its use, eg. ex. restaurants, suppliers, individual consumers. Finally, it is desirable to provide a new server that retains the heat having structural characteristics especially the heat storage material, which is directly transferable anyway or another food server for use in all market segments.

To satisfy the above desires, a solid-to-solid phase change material should preferably be used. Many of these materials are known. For example, a large number of phase change materials from solid to solid were evaluated by the National Aeronautics and Space Administration (NASA) during 1960 as thermal capacitors to passively compensate for the temperature oscillations experienced by Earth's orbital satellites. See Hale et al., Phase Change Materials Handbook, NASA Report B72-1064 (August 1972).

Among the hundreds of phase change materials evaluated by NASA were a few materials that exhibited transformations from solid to solid with large enthalpies. It was thought that solid state phase change materials were not used in space applications, the extensive prior art research data quantify the thermal energy storage properties of a series of phase change materials of the solid state. Such phase change materials of the solid state have several potential advantages over the solid to solid phase change materials currently used in the servers that retain the heat of the prior art. These possible advantages include less stringent content requirements, greater design flexibility; and greater potential for efficient heat transfer to and from the phase change material.

The U.S. Patent No. 4,983,798 by Eckler et al., Teaches a heating device and food storage container using a type of solid to solid phase change material, discrete solid particles of polyol blends and pure polyols, as the storage medium of hot. The Eckler reference states that these polyols are lost at microwave frequencies, particularly at the 2450 MHz frequency of commercial microwave ovens. However, due to the low thermal conductivity of the polyols, a modest amount (220 g) of the pure polyol, or mixture of pure polyols, requires many hours in a conventional oven to store enough heat to trigger the phase transformation of solid. to solid through all the material. Another disadvantage is that the discrete particles have the ability to ensure good thermal contact with the surrounding and make it more difficult to eliminate air pockets that could cause expansion problems due to heating. Also, without understanding, the discrete polyol particles require a large volume to store sufficient amounts of energy. Finally, the discrete polyol particles will not adhere to other objects. Together, these problems prohibit discrete particles as described by Eckler's reference, to work as a center that retains effective heat from food servers.

A phase change material alone is not sufficient to satisfy the wishes listed above. An improved alternative method is needed to pre-heat a server that retains the heat that employs a phase change material from solid to solid. The preferred alternative heating method is magnetic induction heating. The induction-magnetic heating employs alternating magnetic fields such as those produced in an induction coil to induce an electric current in a body that includes ferromagnetic material located in the magnetic field. The current induced in the body creates "Foucault currents" which then cause the body to experience joule heating in direct relation to the energy, I2R, of the current through the body. The heating effect of joule heats the body so that the body can be used to increase the temperature of objects in contact with the body.

The use of magnetic induction as a means of preheating an improved server that retains heat allows an important feature not represented in the servers to retain heat from the prior art. Such a characteristic is the self-regulation of the temperature without the need for thermal contact between the server and the magnetic induction heating device. Many commercially available magnetic induction decoction ranges have temperature controls that allow temperature regulation of the bottom surface of a cooking utensil when the surface is in direct contact with the cooking range support surface. Typically, this is done via a feedback loop using a transducer attached to a bottom side of the magnetic induction cook. Employing a magnetic induction heating element within the server itself that acts as an impedance switch at a designated temperature, in conjunction with a current limiting switch inherent in current magnetic induction cooking devices, a new server that retains the heat it could be built in which the temperature regulates itself without direct heating of its lower surface.

The elements of heating by magnetic induction of self-regulation of temperature are known and have been used in boiler equipment and. electric welders. The following discussion highlights the theory beyond these elements of prior art. When a ferromagnetic metal reaches or exceeds a critical temperature, referred to as the Curie temperature, Tc, the relative magnetic permeability, μr, of the material falls rapidly from a value of between about 100 and 1000, depending on the metal or alloy, to a value of about 1. This change similar to the switch , reversible, automatic in relation to magnetic permeability directly affects the concentration of eddy current flow induced in a ferromagnetic heating element. The eddy current flow mainly along the surface of the element with the induced current density, j (x), decreases exponentially as a function of the distance of the element surface, x. This exponential relationship between the current density, j (x), and the surface distance of the heating element, x, is given by equation 1: where j0 in the current density at the surface of the element, and d is a property dependent on the composition of the material of the element known as depth of penetration. The greater the penetration depth of the particular heating element, the lower the concentration of the induced current is on the surface of the element. The penetration depth d, in units mks, is given by Equation 2: d = (2p /? μ) 1/2 where? is the angular frequency of the field applied in seconds-1, p is the electrical resistivity of the element in ohm-m, and μ is the magnetic permeability of the element. It is convenient to speak in terms of the relative permeability, μ, where μ, is the permeability normalized to the magnetic permeability of the vacuum, μv, where μv is equal to 4p x 10"7 b / Am. Thus, μr = μ / μv = μ / 4p x 10 ~ 7 Wb / Am. For non-magnetic materials, μr = 1.

It is now assumed that the frequency and magnitude of the current induced in the induction heating element are kept constant (by regulating the frequency and current in the primary winding of the magnetic induction heating device). Below the Curie temperature, the relative magnetic permeability, μr, of the heating element is relatively high. Therefore, the penetration depth of the element is small. Before the temperature of the heating element reaches the Curie temperature, the current flow induced through the element is highly concentrated in the surface region of the element. This high concentration provides a relatively small path for current flow, increasing the element's resistance. As a result, the joule heating speed is high and the heating element rapidly heats below the Curie temperature.

Once the element is heated above the Curie temperature, where the relative magnetic permeability of the element has dropped to 1, the induced current flowing through the heating element is allowed to be distributed inside the element. The lower concentration resulting from the current reduces the resistance. As a result, the joule heating rate of the heating element drops significantly, sufficient for the heating of the element to decrease. Because the ratio of the maximum heating rate to the minimum heating rate determines the range over which the heating element can adequately maintain the temperature setting, this ratio and the corresponding ratio, Rmax / Rminc are significant indications of the development of self-regulation of the temperature of the heating element.

The resistance of a heating element removes a unit width, a unit length, and a penetration depth thickness is: Rsuperficial p / d Substituting for d of Equation 2: Rsupe r ficial = (? Μ p / 2) 1/2 (4) ^ Surface is called surface resistivity and could be considered as the effective AC resistivity of a material. Because self-regulation of the most rigid temperature requires reaching the highest ratio of RmaX / Rmin, we find from Equation 5 that this medium reaches the highest ratio of: Unfortunately, magnetic induction cooking devices do not employ circuits to maintain the induced current with a load at almost constant levels as the magnetic permeability of the load drops precipitously, a premise due to which the described art elements depend. The term constant current refers to the following relationship: ? _ i / ?? (6) Fortunately, commercially available magnetic induction cooking devices employ circuits designed to prohibit excessively high flow currents through the inverter circuit and hence through the load. This type of circuit, typically called a "no load" or "abnormal load" condition detector, is designed to employ a feedback parameter that directly depends on the impedance of the load. This feedback parameter, whose direction and use does not require thermal contact with the load, and load sensing circuits are not used to interrupt a sustained current through the induction heating coil, thus interrupting the magnetic field and protecting the inverter abnormal load condition, when a situation of no load or a relatively low load is encountered. The U.S. Patents Nos. 3,978,307 by Amagami et al., And 4,115,676 by Huguchi et al., Incorporated herein by reference, exhibit circuits without charge. Food servers of the prior art, however, are not provided with heating elements configured to use non-charge sensing circuits to achieve self-regulation of temperature.

BRIEF DESCRIPTION OF THE INVENTION The food self-regulating food holding apparatus which retains the heat of the present invention faces the problems of the prior art discussed above. More particularly, the food holding apparatus includes an improved center which retains the heat, and a heating element configured to regulate the core temperature using non-charge sensing circuits of conventional magnetic induction heaters.

Generally speaking, the apparatus holding the food includes a medium that holds the food, a center that retains the heat operably coupled with the medium that retains the food and a heating element of magnetic induction. The center is provided to transfer heat to the medium that holds the food. The heating element is in thermal contact with the center to heat the center.

The medium holding the food includes a wall that puts food in contact, heatable, substantially rigid that defines a cavity, The center is positioned in the cavity and includes a matrix of a phase change material and a flexible material. The phase change material stores the latent heat during a phase transformation that occurs at a phase transformation temperature. The flexible material allows the expansion of the phase change material within the matrix during the phase transformation. The wall that puts the food in contact with the center cooperably provides an apparatus that retains the heat.

The phase change material is preferably a solid state phase change material that undergoes a phase change from solid to solid at a phase transformation temperature. Exemplary phase change materials include pentaerythritol (C5H? 204), pentaglycerin (C5H? 203), also called trimethylolethane, neopentyl glycol (C5H1202), neopentyl alcohol (C5H? 20), and neopentane (C5H? 2). These materials reversibly store large amounts of latent heat per unit mass, each at the unique constant transformation temperature well below their respective melting points. In addition, these transformation temperatures could be adjusted over a wide range of temperatures from 25 ° C to 188 ° C by selecting and mixing the different phase change materials of the solid state. See Murril et al., "Solid-Solid Phase Transitions Determined by Scanning Calorimetry", Thermochim. Minutes , 1 (1970) pp. 239-246 and 409-414, and in Thermochim. Minutes , 3 (1972) pp. 311-315; Chandra et al., "Adjustment of solid-solid Phase Transition Temperature of Polyalcohols by the Use of Dopants", Advances in X-Ray Ana lysis, 29 (1986) p. 305-313; and Font et al., "Calorimetric Study of the Mixtures PE / NPG and PG / NPG", Solar Energy Ma teria l s 15 (1987) p. 299-310.

Although the phase change materials of the solid state are paramagnetic and can not be heated directly by magnetic induction, they could be heated by placing materials in thermal contact with a ferromagnetic heating element. Therefore, the center which retains the preferred heat of the food server of this invention, comprises an appropriate material that retains the heat in thermal contact with some form of ferromagnetic heating element, producing a server that retains the improved heat which can heat by magnetic induction. The solid state phase change material should not be in direct physical contact with a metal to prevent degradation of the heating capacity of the polyol crystals after a limited number of cycles.

Therefore the heating element includes the ferromagnetic material sensitive to a magnetic field to induce an electric current in the element by the joule heating of the element. The ferromagnetic material has a Curie temperature between the phase transformation temperature of the phase change material. As a result, the element is configured to heat the center to a temperature above the phase transformation temperature of the phase change material. Once above this temperature, the phase change material is able to release the stored energy to maintain the wall contacting the food of the medium holding the hot food for extended periods.

Magnetic induction as a heating method has several advantages over microwave heating. For example, because the frequency range of the radiation is much lower, the radiation hazards are much smaller. This allows more design flexibility in the heating devices than larger heat numbers of food servers that retain heat containing a solid state heat storage material in a short amount of time. Another advantage is that ferromagnetic materials have been shown to be efficient heat generators at exposure to alternating magnetic fields in the same frequency range (20 kHz-s 50 kHz) as is currently used in magnetic induction cooking devices commercially available. As a result, the electronics necessary for the magnetic induction heating of the ferromagnetic heating elements is relatively inexpensive and readily available.

Another advantage of using magnetic induction as a heating method for an improved server that retains heat is that self-regulation of the server temperature is possible. For example, the ferromagnetic material is preferably designed to autoregulate indefinitely near a temperature only above the phase change temperature of the phase change material of the solid state but well below the melting temperature thereof. Self-regulation of temperature allows a device to be heated with magnetic induction for an indefinite period of time without fear of losing thermal control. Such a security feature allows for flexibility in the use of magnetic induction heating devices for servers and related food servers. Self-regulation of temperature also allows the device to double as a device that maintains the temperature and the server that retains the heat. Restaurants, for example, could place the food server that retains the heat at the top of the cooking by magnetic induction or another magnetic induction device to have the food held by the server at a relatively constant temperature for an indefinite period before to serve it to customers. Once served, the material that retains the heat keeps the food warm throughout the food.

The present inventive apparatus holding the food could also be used with an improved magnetic induction heater to heat several pieces of food at the same time. For example, a group of plates of such an apparatus that holds the food acts as an electromagnetic center consisting of ferromagnetic material, increasing the magnetic flux of the applied field. The magnetic flux within the center increases as a multiple proportion to the relative permeability of the core material. In addition, the resulting magnetic field is focused in and through the entire extent of the center. This principle can also be applied to improve the development of this invention. By homogeneously mixing a mild ferrite powder in the polyol mixture of the center which retains the heat of this invention, a group of plates of the food holding apparatus is intended to be a ferrite center. As a result, the magnetic field created by an induction coil could be focused through several devices in a group of plates, providing heat generation in more than one device at a time.

DESCRIPTION OF THE FIGURES Fig. 1 is a cross-sectional view of an apparatus that holds the heat retaining food having a self-regulating temperature center constructed in accordance with a preferred embodiment of the present invention.

Fig. 2 is a plan view of a heating element of the apparatus of Fig. 1.

Fig. 3 is a cross-sectional view of a member of the heating element of Fig. 2.

Fig. 4 is an alternative embodiment of the apparatus of the present invention.

Fig. 5 is an alternative embodiment of the heating element of the apparatus of Fig. 1.

Fig. 6 is a schematic illustration of a self-regulating food temperature heating device of the present invention.

Fig. 6A is a flow chart of a conventional non-charge detection circuit.

Fig. 6B is a flow diagram of an alternative non-charge detection circuit.

Fig. 7 is a partial section elevational view of a coffee maker constructed in accordance with an alternative embodiment of the present invention.

Fig. 8 is a plan view of a heating element constructed in accordance with an alternative embodiment of the present invention.

Fig. 9 is a sectional view of the heating element of Fig. 8 taken along line 9-9.

Fig. 10 is a sectional view of a container including the heating element of Fig. 8.

Fig. 11 is a perspective view of a cylindrical heating element constructed in accordance with an alternative embodiment of the present invention.

Fig. 12 is a sectional view of a semiconductor plate which retains heat.

Fig. 13 is a sectional view of a container constructed in accordance with an alternative embodiment of the present invention.

Fig. 14 is a partial section elevational view of a food heating device constructed in accordance with an alternative embodiment of the present invention.

Fig. 15 is a sectional view of a coffee cup constructed according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES An apparatus that holds the self-regulating temperature food, which retains the heat 10 constructed in accordance with a preferred embodiment of the present invention is illustrated in FIG. 1. The apparatus holding the food 10 broadly includes a body 12, a heat retention center 14, and a magnetic induction heating element 16 coupled in the center 14.

The body 12 is provided as a means for holding the food and includes a wall that contacts the food, generally rigid 18, and an annular flange portion 20. The wall 18 includes a wall portion extending downwardly. and defines a cavity 24 configured to receive the center 14. The body 12 is constructed of a vitrified ceramic material that has been glazed. Of course, glass, plastic materials, or any other suitable material could also be used. Body 12 has a heat resistance of at least 100 ° C (212 ° F) and is essentially transparent to electromagnetic energy in the RF and microwave frequency ranges.

In the preferred form illustrated, the apparatus holding the food 10 is formed similar to a conventional dish, and is compatible with the commercially available insulated domes. Therefore, the body 12 is generally circular with an outside diameter that adapts to the inside diameter of the dome to be used. Such domes typically have an inside diameter of between approximately 73/4"-9". Wall 18 could present a decorative style or design.

It should be noted that any manner of the server body could be replaced by the previously described body 12 provided that the server body comprises an open cavity for receiving the heat storage composition of this invention. Any form or type of server that retains the heat will still contain all the advantages of this invention. Other contemplated types of servers that retain heat include bowls, trays, cups, bread dishes, all manner of specialized serving dishes, beverage containers, etc.

The center that retains the heat 14 is comprised of a matrix of heat storage composition of solid state phase change material, ferrite material, a fire retardant additive, and a flexible epoxy binder. The heating element 16 engages in the center 14 for selective heating of the center 14.

The phase change material solid state polyalkylenes C2-C4 is advantageously selected from the group consisting of polyhydroxy compounds (p. G., Polyhydric alcohols (polyols) and glycols), and. The polyhydroxy compounds specimens include trimethylol ethane, also known as pentaglycerol, pentaerythritol, neopentyl glycerol, trimethylol propane, monoamine pentaerythritol, diaminopentaeritritol, acetic tris (hydroxymethyl), high density polyethylene, cross-linked (HDPE) or a mixture of such compounds. The C2-C4 polyalkylene is preferably a cross-linked high density polyethylene.

The solid state phase change material provides sensible heat storage in addition to reversible latent heat storage through the transformations of the crystalline phase, solid to solid. Phase change materials store large amounts of latent thermal energy close to a phase transition temperature. The latent thermal energy is emitted in a narrow temperature band centered approximately at a temperature slightly lower than the transition temperature. Table 1 is taken from "Solid State Phase Transitions in Pentaerythritol and Related Polyhydric Alcohols", Solar Energy Materials, 13 (1986) p. 134, by Benson et al., And shows the thermal properties for some of the polyols mentioned above.

Table 1 The amount of latent thermal energy stored by the neopentyl glycol and trimethyl ethane is compatible with the energy stored by waxes available finer currently used in servers which retain heat commercially available, about 160 kJ / kg. The amount of latent thermal energy stored by pentaerythritol is significantly higher. However, phase change materials of the solid state have other more significant advantages over conventional waxes. An advantage is that the crystalline phase transition temperature of the solid phase change materials of the invention can be adjusted over the temperature range of between about 7-200 ° C (45-392 ° F) selected from certain materials of phase change mentioned above, alone or in appropriate mixtures, depending on the specific transition temperature desired. Examples of the appropriate mixtures and their resulting phase transition temperatures can be found in Advances in X-Ray Analysis, Vol. 29, 1986, p. 305-313, entitled "Adjustment of Solid-Solid Phase Transition Temperature of Polyalcohols by the Use of Dopants", by D. Chandra et al., Which is incorporated herein by reference.

Another advantage of solid phase phase change materials over waxes is that the latent heat is stored in a solid to solid transition instead of a solid to liquid transition. This advantage is a multiple advantage. First, the content becomes less critical and therefore easier. Because during normal use there will be no molten material inside the food server, the spillage of dangerously hot fluids is avoided. Furthermore, due to thermal expansion during a transition from solid phase to crystalline solid is minimal compared to the expansion experienced by a wax during a solid transition to liquid, less space is required for expansion and vessel designs could be used more simple Finally, permanently increasing thermal conductivity of a phase change material from solid to solid such as a polyol is much easier than for a solid to liquid phase change material. Additions of particulates once they are homogeneously dispersed in a phase change material through all the transformations solid to solid. In a solid to liquid transformation, additions of particulates tend to settle in a phase change material under the influence of gravity.

Trimethylol ethane is the preferred polyol for use in food servers that retain the heat of this invention. The phase transformation temperature of trimethylol ethane of about 81 ° C (178 ° F) is ideal for storing the latent heat in commercially available heated base type oven distributors. This material releases its stored latent heat at a temperature below 81 ° C (178 ° F) but much lower than that required to maintain the food at temperatures above 60 ° C (140 ° F) for extended periods of time. In addition, trimethylol ethane, which is extremely low in toxicity, has been approved for use in contact with food by the FDA, and is readily available at a relatively low cost.

The addition of ferrite powder to the center that retains heat serves two main purposes.

First, the ferrite powder increases the magnetic flux density within the center that retains heat for a given magnetic field resistance produced by an induction coil. Second, the powder increases the thermal conductivity of the center that retains heat, allowing it to be transferred through the polyol material that stores heat more quickly.

The ferrite powder increases the magnetic flux density of the apparatus holding the food 10 so that the apparatus 10 is heated more quickly and more efficiently by a magnetic induction heating coil. The ferrite can allow several adjacent or adjacent dishes to be heated simultaneously by a single induction coil.

In addition, ferrite also generally increases the low thermal conductivity of the polyol material. Ferrites, which are ceramic materials, in a completely oxidized state, should not degrade the heat storage capacity of polyol crystals. Previous art that has shown the addition of ferrites to polyols is not known to allow them to improve their thermal conductivity while at the same time allowing more efficient heating by magnetic induction.

The ferrite powder preferably has high initial permeability, high microwave losses (particularly 2450 MHz), and low loss for the RF magnetic induction frequency used to heat the food server. Many of the commercially available ferrites that have been used over the years as center materials for transformers and other electrical equipment establish this profile. Such commercial uses require high magnification of the magnetic flux density while having small energy losses at low frequencies due to the production of eddy currents. It is known that ferrites can possess any range of properties forming compounds with zinc, manganese, cobalt, nickel, lithium, iron or copper as disclosed in two publications: Ferries, by J. Smit and H.P.J. Wijn, John Wiley and Sons, New York, 1959, page 1, etc. and Ferries: A Revi ew of Ma teria l s and Appl i ca tions, by F.E. Riches, Mills and Boon Limited, London 1972, page 9, etc. Therefore, selection of the appropriate ferrite powder to provide high initial permeability, low RF losses, high microwave frequency losses, and relatively high thermal conductivity will be apparent to one skilled in the art. For each different type of food servers of this invention, a single ferrite or ferrite combination may be appropriate. Several ferrites of manganese zinc, nickel zinc and copper zinc with acceptable properties are available from Steward, Inc., of Tennessee. A zinc manganese ferrite, designated Stewart 35 moment, has shown adequate development in the tests.

The fire retardant additive is preferably selected from the group consisting of alpha-alumina trihydrate, phosphate esters, chlorinated hydrocarbons, brominated hydrocarbons, antimony trioxide, borates, phosphorus-containing polyols and brominated bisphenol A. The additive is added to the mixture of polyol / ferrite powder before mixing with the flexible epoxy binder during the processing of the center.

Alpha-alumina trihydrate is the most preferred retardant additive. When alpha-alumina trihydrate is exposed to fire, the hydrate decomposes endothermically, releasing most of its chemically bound water, and acts as a heat sink to absorb heat from the fire. Various properties of the alpha-alumina trihydrate are advantageous for use in this invention. Being a ceramic, it can be obtained in powder form with the particle size below 10 micrometers. The dimensioned micrometric particles allow homogeneous mixing with the polyol and ferrite in powder form. Alpha-alumina trihydrate is also readily available, relatively inexpensive, safe to handle, and has a "generally recognized as safe" (GRAS) rating from the FDA. The finely ground alpha alumina trihydrate, for example, is used as a constituent in toothpastes.

The flexible epoxy binder serves as a binder for the heat retaining composition, an encapsulation for the phase change material of the solid state, an adhesive for maintaining a thermal contact between the heat retaining center 14 and the body 12, a thermal expansion equalizer (which allows the expansion of the phase change material of the solid state within the decomposition matrix during phase transformation) and a low release of polyol energy to the body 12. In addition, the flexible epoxy binder is capable to maintain its properties at continuous operating temperatures up to 177 ° C (350 ° F) and peak temperatures of 204 ° C (400 ° F).

As a binder, the flexible epoxy maintains the thermal contact between the ferrite and the polyol. As an encapsulation, the flexible epoxy coats each particle of the phase change material of the solid state, acting to keep such particles from contacting the heating element 16. Such contact would eventually degrade the development of heat storage of the polyol. The binder acting as a safety encapsulation should gain overheating of the heat retaining apparatus 10 resulting in the phase change material of the solid state becoming partially or completely molten. As an adhesive and a thermal expansion equalizer, the binder ensures a durable bond between the center 14 and the body 12 allowing the expansion of the phase change material during a phase transformation of between about 5-15% of the volume of the material of phase change before the transformation. As an insulator, the binder ensures a steady, slow heat conduction of the encapsulated polyol to the wall which puts the food 18 in contact with the wall that puts the food 18 in contact.

The preferred flexible epoxy binder is a mixture of three resins and two curing agents. The resins include bisphenol A resin, such as the Dow D.E.R. resin. 383, novolak epoxy resin, such as Dow D.E.N. 431, 'and a flexible epoxy resin additive, such as an aliphatic diepoxide. The Dow D.E.R. resin 732 is an appropriate aliphatic diepoxide. The curing agent includes the cycloaliphatic amine, such as Ancamine 1770 available from Air Products and Chemicals, Inc., and N- (2-hydroxyethyl) diethylene triamine, such as Ancamine T also available from Air Products and Chemicals, Inc. Many Mixture ratios of these three resins and two curing agents could be employed for the products of this invention, depending on the desired control temperature.

A preferred resin mixture for low temperature application includes 56% by weight of bisphenol A resin, 14% by weight of novolak epoxy resin and 30% by weight of flexible epoxy resin additive. The flexible epoxy resin additive could be lowered to 25% or increased to 40% by weight while maintaining the same ratio of bisphenol A to novolak. The optimum parts by weight of the curing agent per hundred parts of this epoxy resin mixture is approximately 11 phr of Ancamine T and 5 phr of Ancamine 1770.

Another preferred resin mixture for applications at higher temperatures include 70% by weight novolak epoxy resin, 10% by weight bisphenol A epoxy resin and 20% by weight flexible epoxy resin additive. The flexible epoxy-resin additive could be lowered to 10% or increased to 30% while maintaining the same ratio of epoxy resin from novolak to bisphenol A. The optimum parts by weight of the curing agent for hundreds of parts of this mixture of Epoxy resin is approximately 12 phr of Ancamine T and 5 phr of Ancamina 1700.

The heating element 16 of this invention has several preferred features. The element 16 is self-regulating at a temperature that is above the phase change temperature, but below the melting temperature of the phase change material of the solid state at the center 14. The element 16 is also self-regulating when heated by commercially available magnetic induction cooking devices that do not employ circuits to maintain the induced current within the heating element 16 at almost constant levels. The element 16 transfers heat evenly to substantially the entire core 14. In addition, the element 16 should occupy a minimum space within the center 14.

The heating element 16 of the present invention is self-regulating temperature when heated by commercially available magnetic induction cooking devices that do not employ circuits to maintain the induced current within the heating element at almost constant levels. As noted above, such prior art devices typically employ circuits designed to prohibit excessively high flow currents through the inverter circuit, and hence through the load.

The heating element 16 of the present invention is designed to have an impedance when heated above the Curie temperature, whose magnitude, Zmin, is below that triggered by the non-charge circuits of the commercially available magnetic induction device. to interrupt his generation of magnetic field. For the later discussion, the magnitude of the impedance of. load that triggers the non-charge detection circuits will be referred to as Zdetecor1- The heating element 16 also has an impedance when at a temperature lower than the Curie temperature whose magnitude, Zmax, is significantly greater than Zdetector to reach a heating rate meaningful Because the heating element 16 does not change its geometry (the slight metallic expansions can be ignored) during the transitions through the Curie temperature, any of the changes in the impedance, Z, of the element 16 are proportional to the changes in the resistance, R, of the element 16. Therefore, according to Equation 4, the impedance Z, of the element 16 is proportional to the equation (? μp / 2) 1/2. Assuming that the angular frequency,?, Of the element 16 remains relatively constant as the transitions of the element 16 through the Curie temperature, the maximum impedance Zmax, of the heating element occurs only before the Curie temperature, and obeys the following ratio of proportionality: Zmax Oí (Ur, T < Tc * Pt < Tc) (7) Similarly, the minimum impedance, Zm? N, of the heating element occurs only after the Curie temperature, and obeys the following ratio of proportionality: Zmax a (μr, t> tc * Pt > tc) (8) Because the value of Zdetector could vary slightly from a commercially available magnetic induction cooking device to another, the heating element 16 is constructed of materials to allow a relatively wide difference between Zm? N and Zmax. This allows Zm? N to be designed under the Zdetector while allowing Zmax to be sufficiently high to achieve acceptable heating rates and efficiencies for the commercially available and virtual cooking devices.

In summary, the principle of the self-regulating temperature heating element 16 is Actually, the impedance of the external load (heating element 16) could not be directly "detected or recorded", but the influence of the impedance due to the development of the circuit is reflected in a parameter that is directly "detected". The exact "detected" parameter by the different "no load" detection systems referred to in this discussion to interrupt the flow of current in the inverter circuit that produces the alternating magnetic field could differ (some direction the amount of current flow through of the induction coil, some sense that the voltage drops through a resistor in the detection circuit, some detection in a variation of the oscillation frequency, 'still other parameters). However, each commercially available "no load" detection system finally reacts to a threshold value of the load impedance, which we will later call Zdetector, is de-energized below the current through the induction coil. Therefore, the entire discussion of the interaction between the heating elements of this invention and the commercially available magnetic induction heating devices employing "no load" detection circuits will result in an "impedance sensing means" and this threshold load impedance. that at a regulation temperature very close to its Curie temperature, the impedance of the element 16 falls to a level so that the circuits of the non-charge detection system of a commercially available cooking device de-energize the current flow through of the induction heating coil, whereby the magnetic field production is eliminated and thus interrupting the joule heating of the element 16. As soon as the temperature of the heating element 16 falls below the regulation temperature, the impedance of the element 16 increases to a level far below that required for the "circuit of the detection system of" does not charge "to re-energize the interrupter elements of the inverter, whereby the magnetic field change is re-emitted. , joule heating is re-established.Due to this heating / cooling cycle, the heating element self-regulates close to the regulation temperature.

Referring now to FIG. 6A, a flow chart is illustrated which corresponds to the actions of the conventional non-charge detection circuit. In the numerical reference 1000, a magnetic field is generated. Subsequently, the impedance of the element 16, Zmed dac is detected in the numbering 1002. Zme lda is then compared with Zdetected numbering 1004, and if Zmedi a is less than Zdetected representative of the temperature of the element 16 which is higher than the Curie temperature. , the magnetic field is interrupted, numbering 1006. After the magnetic field is interrupted, the field is periodically regenerated so that the impedance of element 16 can be detected again. The field will be interrupted again if Zmed? Da remains below Z etectadac representative of a fall in the temperature of the element below the Curie temperature, the magnetic field will remain generated. This series of detection and comparison is repeated continuously while the cooking device is used.

Experimentation has shown that the temperature at which the heating element 16 self-regulates could be adjusted by altering the distance between the heating element 16 and the source of the magnetic field. The effective load impedance that the heating element 16 presents to the magnetic induction circuit is dependent on the distance between the heating element 16 and the induction decay coil. As a result, Zmax, and in this way the difference between Zmax and Zdetec aa > is inversely proportional to the distance between element 16 and the magnetic field source. Because the impedance of the element 16 falls to Zm? N during a given finite temperature range, the temperature at which the impedance of the element 16 falls below Zdetected could be adjusted throughout the range by adjusting the distance between the element 16 and the magnetic field source.

An alternative method of detecting the impedance of the heating element 16, Z, and determining when to interrupt the magnetic field is illustrated in FIG. 6B. The alternative method is configured to eliminate the dependence of the self-regulating temperature on the distance between the heating element 16 and the magnetic source. In this alternative method, two comparisons are made in determining whether to interrupt the magnetic field. The first comparison, number 2004, is similar to the comparison shown in the method of Fig. 6a, the measured impedance, Z measured, is compared to a predetermined impetiancy level, Zi. If Zme ida is less than Zi, the circuit will interrupt the magnetic field and cause periodic measurements of the impedance of the heating element 16 to be made. As long as Z is greater than Zi, a second comparison is made.

The second comparison, numerical reference 2008, is based on the absolute value of the change in the impedance, I? Zl, between the current and the immediate measured measured impedances, measured and Zpassed-respectively. It is observed that during the first round of measurements, Zpasada will not be assigned value, therefore, the magnetic field will always be interrupted after the initial measurement and comparison. After the second measurement of the impedance of element 16, the field will be interrupted if I? Z1 is greater than a second value of the preselected impedance, Z2. As long as I zl remains less than Z2, the impedance of the heating element 16 will be measured again, as shown in the flow chart, Fig. 6B.

The second comparison effectively eliminates the dependence of the self-regulating temperature on the distance between the heating element 16 and the magnetic induction heating coil because the absolute value of a change rate of the impedance of the heating element 16 between Zmax and Zm? N, IdZ / dtl, is not linear. The tests show that IdZ / dtl increases as the temperature of the heating element increases towards the temperature corresponding to Zm? N. Therefore, by selecting a particular value of I? Zl, called Z2, during the specific time interval during which the second comparison is made, a particular temperature (within a small temperature range), corresponding to the IdZ value / dtl becomes the self-regulating temperature, with respect to the corresponding value of the Z-measured temperature • The reason that the first comparison is still needed is that the second comparison alone will not interrupt the magnetic field (after two measurements) if the heating element will be placed or does not load much below its Curie temperature due to the magnetic induction cooking device.

Several of the materials could be used to construct the heating element 16 to achieve the preferred characteristics. For example, the element 16 could be constructed from a single single pure ferromagnetic metal or a simple ferromagnetic alloy having a relative magnetic permeability that falls significantly at temperatures above the Curie temperature. The relation Pt < tc / Pt > tc is sufficiently close to 1, therefore, the difference between Zmax and Zmin depends on the difference between μr, t < tc and μr, t > tc- Because μr, t < tc has values that fall in the range of 100 to 1000 for most ferromagnetic metals, and μr, t > tc is approximately equal to one, the difference is significant.

The ferromagnetic material is preferably composed of a nickel alloy with either aluminum, zinc or copper. As shown in Figure 1, taken from "Magnetic Properties of Metals-d-elements, alloys, and compounds", editor H.P »J. Wijn, Springer-Verlag, Berlin, 1991, nickel alloyed with copper shows a linear relationship between ferromagnetic Curie temperature and composition percentages. This linear relationship and the miscibility of nickel and copper in one and the other makes an attractive nickel-copper alloy for use as the material of the heating element 16. By choosing the percentages of nickel and copper, it is possible to select the temperature of the nickel and copper. Appropriate curie for various types of food servers that retain heat.

Graph 1: Ni-Cu alloy. Variation of ferromagnetic Curie temperature Tc with the composition.

With reference to Equations 7 and 8, a greater difference between Zmax and Zmin can be achieved when the electrical resistivity, p, and the magnetic permeability, μ, of the heating element are made to fall dramatically only after the Curie temperature. The features are produced when the heating element 16 is constructed of a substrate 26 of non-magnetic material and a layer 28 surrounding the substrate 26 of the ferromagnetic material, illustrated in Fig. 3. The non-magnetic material has high thermal and electrical conductivity. , while the ferromagnetic material has low electrical conductivity. As the induced currents are dispersed in the body of the heating element at temperatures above the Curie temperature of the ferromagnetic material, the cross-sectional area of the current flow path is increased, and the current path is dispersed In the most highly conductive material of the center, therefore, the impedance of the heating element at temperatures above the Curie temperature, Zmin, becomes lower due to a fall in relative magnetic permeability and a drop in the electrical resistivity.

Of course, it is necessary to keep the max value high enough to reach the large desirable difference between Zmax and Zmin observed above. By providing a ferromagnetic coating layer of approximately 1.5 1.8 depth of penetration in thickness, Zmax remains essentially that of a heating element constructed exclusively of the same ferromagnetic material. Therefore, a relatively large difference between Zmax and Zmin is achieved. This greater difference not only allows Zmin to be designed below Zdetectaa for virtually all commercially available cooking devices, but allows Zmax to be even below Zdetectada, thus allowing higher speeds and heating efficiencies than with just one metallic heating element.

For the server that retains the heat of this invention, a heating element with a copper or aluminum core and a nickel-copper alloy coating is particularly practical. One method to achieve the desired alloy coating is via electrodeposition. The exact percentages of nickel and copper of the desired alloy coating are achieved in an electroplating process of the copper or aluminum center. Electrodeposition of alloys is discussed in detail in Electrodeposition of Alloys: Principles and Praxis, Volume 1 of 2, by Abner Brennar, Academic Press, New York, 1963, pp. 1 sec Ed. Incorporated by reference.

The nickel to copper ratio in the alloy coating is adjusted mainly by changing the nickel to copper ratio in the electroplating bath. The thickness of the nickel-copper electroplating is manipulated by adjusting the electroplating time.

The preferred element 16 of the food holding apparatus 10 includes a substrate 26 constructed of aluminum, and a layer 28 surrounding the substrate 26 of a ferromagnetic alloy, as illustrated in Fig. 3. The alloy is composed of approximately 78 nickel and 22 percent copper, producing a Curie temperature of approximately 100 ° C (212 ° F), a temperature above the phase change temperature of trimethylol ethane, 81 ° C, but 'well below its melting temperature of 197 ° C. The electrodeposition of the coated layer 28 has an advantage in that only complete circuit paths selected from the inductor heating element could be coated, reducing the cost of the heating element.

The relatively thinner layer 28 could also be attached to the relatively thicker sheet of the copper or aluminum substrate. The thin sheet of the desired nickel-copper alloy could be produced by melting the constituent metals together and then forming the sheet as described in this disclosure. Various methods or bonding agents resistant to electro-conduction temperature and thermal conduction capable of withstanding differences in thermal expansion rates of the substrate and the coating are known in the prior art.

In an alternative form, the food retaining apparatus 10 includes an element 16 constructed exclusively of an alloy of approximately 78 percent nickel and 22 percent copper. The alloy Curie temperature is about 100 ° C (212 ° F), a temperature above the phase change temperature of the preferred phase change material, trimethylol ethane, 81 ° C (178 ° F), but well below the melt temperature of 197 ° C (387 ° F) of the phase change material. The inclusion of copper improves the thermal conductivity of nickel, thus affecting the most efficient transfer of heat through the entire heating element and through the entire center that retains heat.

The proper proportions of the pure metals are melted together to form alloy ingots. These ingots are then converted into a strip form which the heating elements could be manufactured, as discussed in more detail below. The advantage of this metal method is the ease of manufacture after the ingots have been produced. A disadvantage of this method is the higher cost and bulkiness of the element constructed of such an alloy. For example, to obtain the total benefit of the difference between Zmax and Zmir? of a homogeneous nickel / copper alloy strip, it should be at least one depth of penetration thickness in each temperature range, that is, at temperatures below and above its Curie temperature. At temperatures below its Curie temperature, the penetration depth d of a nickel / copper alloy of high percentage of nickel, assuming that μr = 100, p = 8 x 10"8 ohm-m, at a frequency of 20 kHz , typically the lower end of frequencies used by most commercial magnetic induction cooking devices, is approximately 0.004 in. However, at temperatures above the Curie temperature of the alloy, the penetration depth increases to approximately 0.038. inches under the same conditions.The latter value of the penetration depth needs a relatively bulky heating element.The costs of the material of the heating element, of course, increases with volume.

For some points of the server of this invention where the added volume and cost can be tolerated, a heating element made of a single pure ferromagnetic metal will be economically and mechanically feasible. However, for most server points, the material used to make the heating element preferably consists of a coating center of non-magnetic material with high thermal and electrical conductivity with a thin surface layer of a low conductivity ferromagnetic material. electric The coating designs described above offer the advantages of reduced cost and bulk in relation to the heating element constructed entirely of a single ferromagnetic alloy. For example, only a relatively thin surface layer of nickel-copper alloy needs to be electroplated or bonded (approximately 1.5 to 1.8 penetration depth) onto the much thinner copper or aluminum substrate (approximately 1 penetration depth). For a nickel / copper alloy with a high percentage of electroplated nickel on a strip of pure copper, the penetration depth d of a nickel / copper alloy with a high percentage of nickel (assuming that μr = 100, and p = 8 x 10" 8 ohm-m) at a frequency of 20 kHz (typically the lower end of the frequencies used by most magnetic induction cooking devices) is approximately 0.004 inches, while the penetration depth d of pure copper to the same Thus, a strip of the alloy coating heating element approximately 0.025 inches thick could develop a single alloy heating element strip approximately 0.038 inches thick. the cost of copper or pure aluminum is lower than that of a high percentage of nickel-nickel-copper alloy, the heating material of Alloy coating also has a material price advantage over its simple metal counterpart.

With reference to Fig. 2, the heating element 16 constructed in accordance with a preferred form is illustrated. The shape of the element 16 allows the element 16 to conduct the heat to the center 14 homogeneously. The preferred form of element 16 is. that of a die of an expanded metal sheet cut into the shape required to substantially engage the body cavity 12. The expanded metal sheet begins as the ordinary metal sheet or tape. It is cut and lengthened simultaneously by means of forming tools which determine the pattern of the number of openings. The dimensions of the strand, width and thickness, the overall thickness of the expanded sheet metal, and the weight per square inch are controlled variables. The Exmet Corporation of Naugatuck, Connecticut, produces expanded metals for virtually any specification. A square foot of ordinary metal sheets results in two or three square feet of the expanded metal die.

An overall thickness of approximately 0.100 inches could result from an ordinary sheet metal thickness of 0.005 inches. This ability to create large global thicknesses of very thin metal sheet alloys allows the heating element 16 to transfer its heat uniformly to the center 14 while taking the minimum volume of the center 14.

The size, shape, and number of openings per square inch of element 16 are important specifications. The heating element 16 has the shape of a circular disc. The diameter of the element 16 is slightly smaller than the internal diameter of the body cavity 12. The thickness of the original sheet of the element metal is approximately 0.015 to 0.020 inches in thickness. The overall expanded thickness of the heating element 16 is slightly smaller than the overall thickness of the center which retains the heat 14.

Referring again to Fig. 1, the food holding apparatus includes a center that retains the cover 30. The cover 30 is provided to encapsulate the center 14 within the cavity 24, and presents an aesthetic, waterproof surface. , durable for the base of the apparatus 10. The cover is constructed of a flexible synthetic resin configured to adhere to the body 12 and allow slight expansions and contractions of the center to maintain integrity through types of consecutive heating / cooling cycles. Of course, for the plastic body 12, the cover 30 could be constructed of the same plastic material as the body 12 and then adhered or welded to the body 12.

The cover 30 is preferably constructed of the same flexible epoxy mixture described above for use in the center 14. A fire retardant selected from the sample shown above, in fine powder form, could be partitioned with the epoxide mixture. A pigment of choice is added for aesthetic purposes. The preferred thickness of the cover 30 is approximately 0.0625 inches. The cover 30 could alternatively be constructed of the material including thermoset plastics, such as urea-formaldehyde or phenolic resins, or thermoplastic resins with properties comparable to the flexible epoxy blend described in this disclosure.

The apparatus holding the food 10 is constructed in the following manner. The body 12 is provided and lowered inwards so that the heating element 16 can be placed in the cavity 24 of the body 12. The element 16 is positioned to rest in general on the smooth surface of the cavity 24. Several drops of silicone adhesive , such as RTV 102, are then placed on the heating element to adhere the element 16 to the body 12. After the adhesive has cured, the heating element is in the proper position and the heat retaining composition is ready. to be placed inside the cavity 24 of the body 12.

Then, the composition is mixed to form the center. First, the preferred polyol, the ferrite and the fire retardant, which are in a dry state, are mixed together to produce a homogeneous mixture. The approximate percentages by weight of the polyol, ferrite and fire retardant for the optimal performance of the individual plate that retains the heat are as follows: Polyol 67% Ferrite Powder 17% Fire Retardant 17% Alternatively, the ferrite powder and / or the fire retardant could be removed from the dry mix, in which case its respective percentage by weight would be replaced by the polyol. The total mass of the dry constituents of the composition that retains the heat used in a food server part that retains the heat of this invention depends on the size, geometry and the desired heat storage capacity of the server.

The flexible epoxy components are then mixed thoroughly, and added to the dried constituents mixed homogeneously under high stress. The proper ratio of the flexible epoxy resin to the dry constituents is such that all the polyol particles can be completely humidified by the resin, thus providing the desired encapsulation. It has been found that the optimum percentages by weight of the dry and wet ingredients are approximately: dry ingredients 67% flexible epoxy resin 33% The height of the mixture above the surface of the heating element 16 (away from the flat surface of the cavity 24) should be kept down so that the joule heating of the heating element 16 during the magnetic induction heating can be transferred more easily to all parts of center 14 formed by the mixture. If the desired thickness of the center 14 is significantly greater than the thinnest heating element 16 available, tests have shown that a layer or layers of expanded copper or aluminum metal mesh could be attached to the heating element 16 (on the surface side). of the cavity 24 near the body 12) to provide excellent thermal conductivity while self-regulating the temperature of the center 14 is not prohibited.

After the mixture has been poured, the apparatus 10 is cured in the oven for about 1 hour to about 93 ° C (200 ° F) and about 1 hour to about 121 ° C (250 ° F). The curing apparatus 10 allows the mixture to establish and form the center 14.

The center that holds the cover is then poured into place in the cavity 24 of the body 12 that covers the center 14. Curing is carried out to remove the air below the surface level of the cover 30. The thickness of the cover 30 covering the surface of the heat retaining composition should be chosen to provide a durable cover for the base of the apparatus 10. The preferred thickness of this layer is approximately 0.0625 inches.

In use, the apparatus holding the food 10 is preheated whether it is placed in a convection oven at approximately 121 ° C (250 ° F) for at least one hour, or on top of induction cooking magnetic during an indeterminate amount of time. The food is then placed on the upper surface of the server to maintain the hot food for a substantially longer period of time than prior art devices. An isolated cover placed on the food prolongs the maintenance time.

With reference to Fig. 4, an alternative embodiment of the apparatus holding the food 10 is illustrated. The alternative embodiment includes a plastic sponge sheet 32 positioned below the center 14 and above the cover 30. The sheet 32 decreases the losses of heat through the base of the apparatus 10.

The plastic sponge material is preferably a medium density closed cell silicone plastic sponge sheet. Other sponge materials with high heat resistance and good degree of flammability such as neoprene or nitrile could also be used. The sheet is approximately 0.0625 inches thick. The plastic sponge cut by die 24 could be purchased from Lamatek, Inc. of New Jersey.

In mounting the alternative embodiment of the apparatus 10, the sheet 32 is cut by the die in the shape of the center 14 and placed in the center 14 prior to curing the center so that the sheet 32 adheres to the sticky mixture. Care is taken to avoid the formation of air pack between the plastic sponge 24 and the mixture. The mixture is then cured in the oven, and the cover 30 is poured and cured.

Another embodiment of the apparatus retaining the food 10 includes the heating element 34 illustrated in FIG. 5. The alternative element 34 is relatively thin compared to the preferred element 16, and is used in the application requiring a lower profile. The alternative element 34 could be constructed of a simple ferromagnetic alloy or a non-magnetic substrate having the ferromagnetic layer.

The alternative element 34 is in the form of a single, flat, annular spiral coil with a central terminal end 36 ohmically connected to an outer terminal end 38 by a flat belt 40. The belt 40 is electrically isolated from all other points of the flat spiral coil. The insulation is made by insulating the coil with a thin layer of enamel epoxy resin, temperature resistant paint or other suitable material. Preferably, an adhesive is used to isolate the tape 40 and to attach the tape 40 to the coil, such as a ceramic adhesive available from Aremco Products, Inc., Ossining, NY, or a high temperature epoxy resin filled with thermally conductive materials such as alumina. The spiral coil is cut with a die of a sheet of conductive material. Several almost identical coils could advantageously be made from the same sheet of conductive material, reducing material costs.

In addition, the element 34 could include a switch between the end 38 and the tape 40 for opening and closing the electrical circuit created by the coil and the tape 40. As a result, the switch could be used to selectively activate and deactivate the element 34.

With reference to Fig. 6, a self-regulating temperature heating food device 42 is illustrated. The device 42 widely includes a magnetic induction heater 44 and the food holding apparatus 10 described above is positioned on the heater 44. The heater includes a fastener 46 for holding the apparatus 10, a magnetic field generator 48 and a non-charge and abnormal charge detector 50. The generator 48 provides a means for generating a magnetic field through space by above the fastener 46. The non-charge detector 50 provides a means for detecting the impedance of a body placed on the fastener 46 in the magnetic field, and for interrupting the magnetic field when the detected impedance is less than a predetermined value. The operation of the detector 50 and the interaction with the apparatus holding the food 10 are described below.

There are numerous advantages of the food heating device 42 over the prior art heating / holding devices. The energy efficiency of the device 42 will be greater than that of the devices of the prior art because the energy is consumed only when the apparatus holding the food 10, or other inductor heating, is placed on the holder 46. In addition, the element of auto heating will regulate the temperature of the total center 14 and in this way the apparatus that holds the food 10 indefinitely on the heater 44. The user need not worry about the thermal leakage of the apparatus holding the food 10 because it could leave indefinitely in the heater, allowing great flexibility of use.

Also, the center that retains the heat 14 will keep the food hot for an extended period of time after the apparatus 10 has been removed from the heater.

In an alternative configuration, the food heating device 42 includes a metal closet, insulated to accept a column or several columns, from the apparatus holding the vertically coupled food 10. The heater 44 is positioned inside the cabinet. A lid is provided to close the cabinet. The magnetic generator of the alternative food heating device includes a magnetic field coil, such as those used in detection devices that are currently employed in the industry to harden metals by magnetic induction. These coils are in the form of a solenoid of sufficient length to create an almost uniform magnetic field within the center of the solenoid. The intensity of this magnetic field is increased within the center of the tube by the inherent magnetization of the ferrite material in the individual plates centered within the induction coil. Electromagnetic protection is provided to reduce the emissions of this device through the metal cabinet or other magnetic protection methods known in the art.

The coil is driven by a worm wheel that drives the length of the cabinet, which is introduced to the apparatus that holds the food 10 once with energy by magnetic induction. This device is capable of heating the stack of individual plates that retain relatively efficient heat and quickly compares with the time required of 1 or 2 hours by the furnace type heated base distributors currently used in most hospitals. In addition, the apparatuses holding the food 10 are hot only in their central regions, adjacent to the center 14, leaving the flange portion 20 cold to the touch. As a result, the unloading and handling of the dishes is relatively safe than in the prior art.

In a further alternative configuration, the device that heats the food 42 includes a metal or plastic closure cabinet, insulated to accept a column, or several columns of the apparatus that holds the vertically stacked food 10, each positioned on its own heater 44 A door is provided for closing the cabinet.

In another alternative configuration, the food heating device includes a conveyor belt for transporting a plurality of food heating apparatuses 10 between the inlet and outlet positions. The magnetic field generators and non-charge detectors are positioned along the conveyor so that the apparatus holding the food 10 can be brought to an operating temperature. The device could be designed to accept a plurality of apparatuses 10 either vertically or horizontally stacked.

Referring now to FIG. 7, an apparatus that retains the self-regulating temperature food 100 is shown in the form of a coffeemaker. The apparatus 100 broadly includes a top and bottom portion 102 of the coffee maker insertable insertable with the top portion 102. The insert attachment allows separation of the top portion 102 and the bottom portion 104 for cleaning. Of course, the upper portion 102 and the lower portion 104 could be adhesively bonded alternately to one another. The lower portion 104 includes a solid sheet heating element 106 for heating the contents of the apparatus 100.

The heating element 106 is thermally insulated from the non-metallic outer wall of the lower portion 104 via insulation foam, an air hole, a vacuum space, or any other thermal insulation means known in the prior art. The upper portion 102 of the coffee machine as shown is insulated with double clear plastic walls that have an air gap between them. Upon further isolation of the contents of the apparatus 100, lower input energy of the heater by magnetic induction 44 is required to maintain the contents at a set temperature. Experimentation with a prototype apparatus 100 whose solid sheet heating element 106 was formed from a single alloy of 73% nickel and 27% copper, was carried out in a magnetic induction cooking device Sunpentown International Model SR- 1330 The experiments showed that the regulation of the temperature occurred at 190 ± 2 ° F, with respect to the amount of coffee within the apparatus 100. The Sunpentown SR-1330 cooking device interrupted its magnetic field output at the average of approximately 67% of time. Thus, the cooking was actively heating the container only 33% of the time to maintain a set temperature.

Experimentation also showed that by raising the apparatus 100 about 1/32 inch above the cooking surface, a set temperature of 181 ° F was achieved. This decrease in set temperature was possible until the vessel was about 1/8 inch above the cooking surface, at this time the set temperature was 155 ° F. The set temperature / height ratio appears to be linear with a slope of approximately (9 ° F) / (height increase of 1/32 inch). Any additional height increase prevented the starter from cooking, and thus avoiding any heating of the entire container. This ability to adjust the set temperature of the vessel by raising it above the cooking surface (and therefore the magnetic induction coil) allows the design of the coffee maker including a height adjuster 108 to allow the user to select the exact set temperature desired . The height adjuster 108 is simply a curved cover 110 that serves as a fastener for the coffeemaker. The curved cover 110 is rotated by the user to raise the coffee maker or decrease it above a factory set height to decrease or raise the coffee setting temperature. Because the adjusted temperature / height ratio appears to be linear, the height adjuster of the cover is easily calibrated and adjusted at the factory for a selected set temperature.

An alternative heating element 150 is configured to be positioned within the device that holds the food and is shown in Figs. 8 and 9. The heating element 150 is generally disk-shaped and includes the structure defining a plurality of openings 152 through the element 150. The element 150 could alternatively be of other shapes and sizes. In addition, a plurality of dimples 154 are formed in the element 150. The dimples 154 act to lift the floor disc and the device holding the food so that food within the device is allowed to flow through the openings 152.

By providing the element 150, devices that hold food that are not otherwise designed for heating and adjusting the temperature of the food through the use of magnetic induction could easily be converted by food heaters by magnetic induction. In addition, the devices that keep the food thermally insulated could be designed to be devices that maintain self-regulation of extremely efficient energy temperature. Referring now to Fig. 10, a table vessel 158 has been converted to a thermally insulated, self-regulating device 156. The heating element 150 is positioned within the vessel 158 to allow heating by magnetic induction of the contents of the vessel 158. The device 156 includes the vessel 158 and a ferromagnetic core 160 spaced from the vessel 158.

The ferromagnetic core 158 is preferably constructed of a plastic material such as polycarbonate. While the space between the shell 158 and the ferromagnetic core 160 provides thermal insulation between the contents of the vessel 156 and the ferromagnetic core 160, the additional insulation could be obtained by coating the inner surface of the ferromagnetic core 160 with a material of previous art insulation. One such material is a low emissivity coating, such as that found in the film used to thermally insulate office windows. Experimentation using such a film available from 3M Corporation has shown that heat losses could be reduced by approximately 25%.

An advantage of using the heating element 150 in a device such as 156 is that the element could conveniently be removed and washed periodically. This convenient cleaning ability is especially important for water tanks where mineral deposits are stored all the time in conventional heating elements. In addition, due to the magnetic field frequencies-employed in the magnetic induction cooking devices, typically in the range of 20-50 Khz, the ultrasonic vibrations induced in the element 150 act to resist the accumulation of mineral deposits, such as silt and corrosion.

A cylindrical heating element 200 is shown in Fig. 11. The element 200 includes open upper and lower ends and the wall structure defining a plurality of openings 202 therethrough. The heating element 200 is configured to be used in the. overheating of cold or frozen foods. For example, prior to cooling the food, the food is placed in an appropriate container, such as a poly bag, and the heating element 200 is placed inside the food. The food is then reheated simply by positioning it so that the heating element 200 is within the magnetic field to induce a current in the element 200, heating the element 200 and thereby retaining the food. One advantage of overheating the food in such a way is that it could be done in the same way as for storage. In addition, there is no overheating of the food due to the self-regulating characteristic of the element 200 temperature.

A heat retaining capsule 250 is illustrated in Fig. 12. The capsule includes an encapsulation shell 252, a core that retains the heat 254 positioned within the shell 252, and a heating element 256 coupled to the core 254. The core 254 and the heating element 256 are comparable with the core 14 and the element 16, respectively, of the preferred embodiment. As a result, the capsule 250 provides a self-contained unit that is capable of storing the latent heat to heat the area surrounding the capsule 250. The capsule, or a plurality of capsules, could be heated via the food heating device 42. described in the exhibition. Such a capsule is particularly useful when inserted into a cart, of insulated food, such as that manufactured by Cambro Manufacturing Company. Tests have shown that a capsule 250 prototype containing 500 g of polyol can increase the temperature adjustment capacity of a Cambro 400MPC 'isolated by more than 50%.

A food heating container 300 employing several of the features of the present invention is shown in Fig. 13. The container 300 includes a lid 301, a body 302 and a liner 304 inserted within the body 302. A heating element 306 is provided between the body 302 and the liner 304 for magnetic induction heating of the contents of the container 300. The heating element 306 is similar to the element 34 described above in this disclosure except that the element 306 is configured to surround the liner 304. By providing the heating element 306 which is configured to surround most of the contents of the container 300, the contents could be heated more uniformly and effectively. An insulation material 308 is additionally provided between the body 302 and the liner 304 to isolate the container 300. The insulation material 308 could be constructed from the matrix that retains the heat described above, but could also be foam or any other suitable insulator .

As discussed above, the temperature at which self-regulation occurs could be adjusted by varying the distance between the magnetic induction heating element and the magnetic field source. Alternatively, the variation in self-regulating temperature could be achieved by incorporating a plurality of heating elements, each constructed of material having unique Curie temperatures.

Fig. 14 illustrates a food container 350 in the form of a beverage vase including first and second heating elements 352, 354. The heating elements 352, 354 are similar, to the element 34 described above in this discussion except that the elements 352, 354 are configured to surround the drinking vase. A switch 356 engages with each of the elements 352, 354 to selectively open and close the circuits defined by the elements 352, 354. As a result, a user could selectively open the circuit of the first element 352 to have the container 350 heated to the self-regulating temperature of the second element 354, and vice versa. Therefore, switch 356 provides a self-regulating temperature adjustment means.

Referring now to FIG. 15, a cup of coffee or express 400 is shown constructed in accordance with an alternative embodiment of the present invention. The cup 400 includes a body 410 constructed of ceramic material. The body 410 defines a lower cavity 420. The heat conducting material 430, such as alumina powder, or the matrix retaining the heat of this invention, is positioned within the cavity 420, and a heating element 440 is positioned within the material 430. A lower wall 450 is provided for the encapsulation of the material 430 and the element 440 within the cavity 420. A pair of openings 460 are formed in the wall 450 and could be sealed by an adhesive, such as Ceramabond 569, available at Aremco Products, Inc. of Ossining, NY.

The 400 cup is constructed in a multi-stage process. First, the body 410 is formed by presenting the cavity 420. Then, the cup 400 is inverted, and the element 440 is positioned to encapsulate the heating element 440 while allowing the flow of air between the cavity 420 and the ambient air. . At this point, the cup 400 is heated, cooled and heated again. After the cup 400 has cooled, the cavity 420 is filled with the material 430. As noted above, the material 430 is preferably an alumina powder that exhibits sufficient heat conductivity while preventing excessive expansion during heating of the element 440 to prevent rupture of the cup 400. For applications using a material 430 that undergoes significant expansion during heating thereof, a foam layer could be positioned below the material 430 to allow the expansion of the material 430 without the rupture of the cup 400. A phase change material, such as the one described above, could be substituted for the material 430.

Once the material 430 has been positioned within the cavity 420 an adhesive is injected into the openings 460 to seal the openings 460. Due to the curing of the adhesive, the cup 400 is ready for use.

In use, the cup 400 could be heated by magnetic induction before being filled with coffee so that the coffee is not cooled by the contact of the body 410. For the coffees such as the express, in which the taste of the coffee is directly related- with the coffee temperature, the cup 400 could be advantageously used to inhibit a desired reduction in coffee temperature. Alternatively, the cup 400 could be heated by magnetic induction as it is filled with espresso, thus regulating the temperature of the express until the cup is filled and removed.

The present invention has been described with reference to the illustrated embodiments. It is noted that substitutions and changes and equivalents employed could be made without departing from the scope of the invention as set forth in the claims.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates.

Having described the invention as above, the content of the following is claimed as property.

Claims (34)

1. The self-regulating temperature heating apparatus, characterized in that it comprises: a magnetic induction heater including a magnetic field generator for generating a magnetic field, and an impedance detector for detecting a load impedance parameter of a magnetically coupled load in the magnetic field; Y a self-regulating temperature device in position for magnetic coupling with the magnetic field, the device includes a ferromagnetic induction heating element, the response of the element to the magnetic field for induction heating of the element at a predetermined temperature; the impedance detector which is operable to periodically detect the load impedance parameter while the device remains in said position, and in response thereto, altering the strength of the magnetic field at a reduced level when the load impedance parameter is below or above a selected value correlated with the predetermined temperature.

2. The apparatus of claim 1, characterized in that the device can be moved to a position outside the magnetic field.

3. The apparatus of claim 1, characterized in that it includes an adjuster for adjusting the distance between the device and the magnetic field generator to change the predetermined temperature.

4. The apparatus of claim 1, characterized in that the device includes the structure of the operable wall to maintain the food, the structure of the wall that is in thermal contact with the element.

5. The apparatus of claim 1, characterized in that the reduced level of the magnetic field resistance is zero.

6. The apparatus of claim 1, characterized in that the impedance detector operable to maintain the strength of the magnetic field at the reduced level as long as the load impedance parameter is below the selected value.

7. The apparatus of claim 1, characterized in that the load impedance parameter is a parameter of an induction heating circuit that depends on the impedance of the load.

8. The apparatus of claim 1, characterized in that the load impedance parameter is the absolute value of the change rate of the induction heating circuit parameter that depends on the load impedance, the impedance detector is operable to periodically determine the value absolute of the rate of change of the load impedance of the element and compare the absolute value with a predetermined change rate of the circuit parameter and compare the absolute value with the selected value, and reduce the resistance of the magnetic field to the reduced level when the absolute value is greater than the selected value.

9. The apparatus of claim 1, characterized in that the device includes the material that retains the phase change heat of the solid state in thermal contact with the element.

10. The apparatus of claim 1, characterized in that it includes a flexible binder in contact with the material.

11. The apparatus of claim 1, characterized in that the predetermined temperature is above the Curie temperature of the element.

12. The apparatus of claim 1, characterized in that the element formed is nickel-copper alloy.

13. The apparatus of claim 1, characterized in that the selected value is adjustable to change the predetermined temperature.

14. A method for controlling the temperature of a device for regulating the temperature, characterized in that it comprises the steps of: place the device in a position for magnetic coupling of the device and a magnetic field generated by a magnetic induction heater, the heater has a magnetic field generator to generate the magnetic field and an impedance detector to detect a load impedance parameter of a load magnetically coupled in the magnetic field, the device includes a magnetic induction heating element sensitive to the magnetic field by induction heating of the element at a predetermined temperature; Y operate the impedance detector to periodically detect the load impedance parameter while the device remains in position, and at the same time, reduce the resistance of the magnetic field to a reduced level when the load impedance parameter is below a value selected correlated to the predetermined temperature and increase the resistance of the magnetic field of the magnetic field when the load impedance parameter is above the selected value.

15. The method of claim 14, characterized in that it includes the step of adjusting the distance between the device and the magnetic field generator to change the predetermined temperature.

16. The method of claim 14, characterized in that it includes the step of adjusting the selected value to change the temperature, predetermined.

17. The method of claim 14, characterized in that the device includes the structure of the wall operable to maintain the food, the structure of the wall that is in thermal contact with the element.

18. The method of claim 14, characterized in that it includes the step of reducing the resistance of the magnetic field to zero as the reduced level.

19. The method of claim 14, characterized in that it includes the step of maintaining the strength of the magnetic field at the reduced level provided that the load impedance parameter is below the selected value.

20. The method of claim 14, characterized in that the predetermined temperature which. is above the Curie temperature of the element.

21. The method of claim 14, characterized in that the parameter that is the absolute value of the rate of change of a heating circuit parameter by induction that depends on the load impedance, the impedance detector is operable to periodically determine the absolute value of the change rate of the circuit parameter and compare the absolute value with the selected value, and reduce the resistance of the magnetic field to the reduced level when the absolute value is greater than the predetermined rate of change.

22. The method of claim 14, characterized in that the device includes the material that retains the phase change heat of the solid state in thermal contact with the element.

23. The method of claim 22, characterized in that it includes a flexible binder in contact with the material.

24. The method of claim 14, characterized in that the element is formed of nickel-copper alloy.

25. A matrix that retains heat, characterized in that it comprises the respective amounts of a phase change material of the solid state for storing the latent heat during the phase transformation from solid to solid, and the flexible material to allow the expansion of the change material. of phase within the matrix during the phase transformation, the phase change and the flexible materials are intimately intermixed to form the matrix, the flexible material comprises a flexible epoxy binder.

26. The matrix as set forth in claim 25, characterized in that the phase change material is selected from the group consisting of polyhydroxy compounds and the C2-C4 polyalkylene.

27. The matrix as set forth in claim 24, characterized in that the phase change material is selected from the group consisting of pentaerythritol, trimethylol ethane, neopentyl glycol, trimethylolpropane, monoaminopentaerythritol, diaminopentaerythritol, tris (hydroxymethyl) acetic acid, and polyethylene. high crosslink density.

28. The matrix as set forth in claim 25, characterized in that the phase change material undergoing transformation from solid to solid phase at a phase transformation temperature., the matrix further includes a magnetic induction heating element in thermal contact with the matrix to heat the matrix to a temperature above the phase transformation temperature, the element includes the ferromagnetic material sensitive to a magnetic field to induce a current The electric element in the element to heat the element, the ferromagnetic material-has a Curie temperature greater than the phase transformation temperature.

29. The matrix as set forth in claim 28, characterized in that the element is formed of nickel-copper alloy.

30. An apparatus that retains heat, characterized in that it comprises: wall structure retaining the shape substantially by heat adapted to be positioned adjacent the substance to be heated; a center that retains the heat coupled with the wall structure for selective heating of the wall structure, the center includes a phase change material of the solid state to store the latent heat during a phase transformation and a flexible epoxy binder that allows the expansion of the phase change material, transformation occurs at a phase change transformation temperature; Y an element of heating by magnetic induction in thermal contact with the center to heat the center to a temperature above the phase transformation temperature, the element includes the ferromagnetic material sensitive to a magnetic field to induce an electric current in the element for heating said element, the ferromagnetic material has a Curie temperature between the phase transformation temperature and the melting temperature of the phase change material.

31. The apparatus of claim 30, characterized in that the wall structure is adapted to maintain the food.

32. The apparatus as stated in the. claim 30, characterized in that the phase change material is selected from the group consisting of pentaerythritol, trimethylol ethane, neopentyl glycol, trimethylol propane, monoaminopentaerythritol, diaminopentaerythritol, tris (hydroxymethyl) acetic acid, and the C2-C polyalkylene, and polyethylene high crosslink density.

33. The apparatus as set forth in claim 30, characterized in that the binder encapsulates the phase change material.

34. The apparatus as set forth in claim 30, characterized in that the ferromagnetic material is formed of a nickel-copper alloy. An apparatus for holding the food, self-regulating temperature, which retains the heat (10) includes a body (12), the center that retains the heat (14) and the element of heating by magnetic induction (16). The body (12) includes a wall that puts food in contact, heatable, substantially rigid (18). The center (14) includes a phase change material of the solid state for storing the latent heat during a phase transformation from solid to solid at a phase transformation temperature. A flexible material is in contact with the phase change material to allow expansion of the change material during a phase transformation. The heating element (16) is in thermal contact with the center (14) for heating the center (14) to a temperature above the phase transformation temperature to effect a phase transformation in the phase change material . The element (16) includes a ferromagnetic material sensitive to a magnetic field to induce an electric current in the element (16) to heat the element (16). The ferromagnetic material has a Curie temperature between the phase transformation temperature and the melting temperature of the phase change material. A food heating device (42) includes a heater (44) having a fastener (46), a magnetic field generator (48) and a non-charge detector (50), and an apparatus that holds the food (10). ) positioned on the fastener (46).