Electric resistance heating devices – Heating devices – Tank or container type liquid heater
Reexamination Certificate
2000-05-16
2002-11-19
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Tank or container type liquid heater
C219S548000
Reexamination Certificate
active
06483990
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electric heating devices in general and in particular to heating elements useful in such devices.
BACKGROUND OF THE INVENTION
Heating devices employed for household, industrial or other use have typically a heating element which is embedded within the device and which then transfers heat by radiation, convection or conductance to an output surface of the device. The temperature at the output surface of the device is thus much lower than the working temperature of the heating element. There is usually a very big temperature drop between the temperature of the heating element which ranges from 100's to 1,000's° C., depending on the type of device, to a temperature at the output surface which may range from 60-90° C. for household heating devices to 120-300° C. for household cooking and baking devices.
Heating devices come in a large variety of form and shape. For example, one type of electric heating device has a bare heating element typically fashioned as either a band or a wire made from an alloy containing nickel and/or chromium and which typically reaches working temperatures ranging from about 400° C. to 1600° C. The heat generated by this type of heating elements is dissipated to the surrounding medium by mainly one or any combination of three heat transfer mechanisms, these being radiation, natural or free convection or forced convection (e.g. by the use of a ventilator). Such heating devices enjoy the advantages of being inexpensive, small, of a relatively low weight and having a long lifetime. However, they suffer from a drawback arising out of the high working temperature of their heating elements which poses a safety hazard.
In another type of electric heating device, commonly known as the electric radiator, the hazards associated with the bare heating element type of heating device are eliminated by submerging a heating element in a reservoir of oil or a similar liquid employed for transferring the heat generated by a heating element to the external walls of the radiator. Typically, the output temperature of a domestic heating radiator is about 70° C. whereas the working temperature of its heating element is 700° C. or above. Consequently, such a domestic heating radiator is typically equipped with heat-insulating means. As is well known, electric radiators suffer from the disadvantages that they are expensive, heavy, and relatively inefficient.
U.S. Pat. No. 2,600,486 discloses an electric heating element which comprises a flexible conducting metal sheet in which slits are cut so as to form an elongated relatively narrow torturous flow path for an electric current. A similar kind of heating device is also disclosed in U.S. Pat. No. 3,584,198. U.S. Pat. Nos. 3,525,850 and 4,551,614 disclose an electric heater comprising elongated heating elements in the form of corrugated matallic ribbons which are heated to a temperature ranging from about 1200° F. to 1800° F. (about 650-1000° C.). U.S. Pat. No. 2,719,213 discloses a heating device in the form of a flat panel which comprises an electric conductor arranged in a plane between two different non-conducting sheets or layers, and French Patent Specification 975,038 discloses a heating element in the form of arrogated plates. Another heating panel is disclosed in U.S. Pat. No. 3,244,858 wherein an electric heating wire is arranged in a plane to track a zig-zag path over both sides of a non-conducting planar core. U.S. Pat. No. 4,203,198 discloses a planar heating device employing a heating element arranged in a plane to track a tortorous path and sandwiched between two sheets of fiber glass. Another heating device with a heating element arranged in a plane between two insulating sheets is also known. U.S. Pat. No. 4,032,751 discloses a planar heating element utilizing electrically conducting carbonaccous pyropolymers. An electric planar heating device intended for use as an electrical bandage is described in U.S. Pat. No. 2,712,591, wherein an electrically conducting ribbon is embedded in a resilient strip of insulating material. A flexible circuit heater which can be used within clothing or the like is disclosed in U.S. Pat. No. 4,948,951, utilizing an electrically conductive strip made to track a torturous path within a flat flexible member. U.S. Pat. No. 4,665,308 discloses an electrical heating element that can be incorporated in the lining of an item of clothing which makes use of a ductile insulated metal wire fixed to a metal sheet.
GENERAL DESCRIPTION OF THE INVENTION
There is accordingly a need in the art to provide an electric heating device for heating an object to a required temperature that eliminates the need for thermo-insulating means or at least substantially reduces the requirements to such means.
It is an object of the invention to provide a novel heating device with as low as desired temperature gradient between the temperature of the outer surface of a heating element and the required temperature of the heated body.
It is an object in accordance with some embodiments of the invention to provide a heating device wherein the heating element constitutes the heat dissipating, output surface of the device.
It is an object in accordance with some other embodiments of the invention to provide a heating device wherein the heating element is embedded in or forms a structural element of a household object serving also a purpose other than heating.
It is flrhermore an object of the invention to provide domestic enclosure heating devices, electric cooking devices and therapeutic heating devices having characteristics in accordance with the above objects.
It is furthermore an object of the invention to provide a method for designing in constructing such heating devices.
Other objects of the invention will be clarified after reading the text below.
Generally speaking, the present invention provides a method and device capable of a so-called “low-gradient” heating. In practice, in most cases it is desired to have substantially low gradient between the surface temperature of a heating device and the temperature of the heating object, so as to reduce the danger of fire, as well as the danger of bums, when applying a heating device to the body for medical purposes. Additionally, such a low-gradient heating enables to reduce the requirements of electro- and thermo-insulation, and to increase the effectiveness of heating (i.e., achieving a desired effect at minimal energy). This means that in order to create a high-quality heating device, its heating element, intended for heating a given object, should be independently designed and calculated, taking into account not only the power required to obtain a desired temperature of the object, but also the fact that the surface temperature of the heating element should be as low as desired different from the required temperature of the heated object. This is a very complicated task, since the transfer of heat from the heating element towards the object is proportional to the temperature gradient (i.e., to the difference between the temperature of the heating element and the temperature of the heated object). Accordingly, the desired decrease of the temperature gradient should be compensated, and practically, the single solution for this is a maximal increase in the surface area of the heating element.
Thus, the main idea of the present invention consists of designing a heating element whose surface temperature and surface area are independently selected at the given power required for heating a given object to a required temperature.
It was found by the inventor, that the solution for the above task is based on directionally varying the ratio between an electric current passing through the heating element and voltages at its input and output circuits. For the purpose of independent control of the surface temperature and surface area of the heating element, materials for the heating element should be selected in accordance with their physical parameters, namely, the specific resistivity and dimensions of the heati
Bar-Keser Project Management Initiatives and Economic Consultant
Campbell Thor
Greer Burns & Crain Ltd.
Walberg Teresa
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