Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2003-04-25
2004-03-30
Hoang, Tu Ba (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S517000, C219S529000, C219S494000
Reexamination Certificate
active
06713733
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to soft and flexible electrical heaters, and particularly to heating elements, which have soft and strong metal or carbon containing electrically conductive textile threads/fibers.
2. Description of the Prior Art
Heating elements have extremely wide applications in consumer household products and in, construction, industrial application, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications. Numerous types of thin and flexible heating elements have been proposed. For example, a heating element proposed by Ohgushi (U.S. Pat. No. 4,983,814) is based on a proprietary electro conductive fibrous heating element produced by coating an electrically nonconductive core fiber with electro conductive polyurethane resin containing the carbonatious particles dispersed therein. Ohgushi's manufacturing process appears to be complex; it utilizes solvents, cyanides and other toxic substances. The resulting heating element has a temperature limit of 100° C. and results in a pliable but not soft heating element. In addition, polyurethane, used in Ohgushi's invention, when heated to high temperature, will decompose, releasing very toxic substances, such as products of isocyanides. As a consequence, such heating element must be hermetically sealed in order to prevent human exposure to toxic off gassing. Ohgushi claims temperature self-limiting quality for his invention; however “activation” of this feature results in the destruction of the heater. He proposes the use of the low melting point non-conductive polymer core for his conductive fabric-heating element, which should melt prior to melting of the conductive layer, which uses the polyurethane binder with the melting point of 100° C. Thus, the heating element of Ohgushi's invention operates as a Thermal Cut Off (TCO) unit, having low temperature of self-destruction, which limits its application.
U.S. Pat. No. 5,861,610 to John Weiss describes a heating wire, which is formed with a first conductor for heat generation and a second conductor for sensing. The first and second conductors are wound separately as coaxial spirals with an insulation material electrically isolating the two conductors. The two spirals are counter-wound with respect to one another to insure that the second turns cross, albeit on separate planes, several times per inch. The described construction results in a temperature sensing system, which can detect only the average change of resistance in the sensing wire due to elevation of the temperature in the heated product. Therefore, in the event of overheating of a very small surface area of the blanket or pad (for example, several square inches), the sensor may fail to detect a minor change of electrical resistance (due to operating resistance tolerance) along the heating element. In addition, such heating cable does not have inherent Thermal-Cut-Off (TCO) capabilities in the event of malfunction of the controller. The absence of the localized hot spot detection and the use of breakable metal wires make this heating element vulnerable to failure and not sufficiently safe for foldable products, such as heating pads and heating blankets.
Thrash (U.S. Pat. No. 5,801,914) describes an electrical safety circuit that utilizes two parallel conductors connected to a positive temperature coefficient material (PTC) and sacrificial fuse filament. Such sacrificial filament is connected to a separate switching circuit, which terminates electrical continuity of the PTC heating element in the event of fire hazard. The main disadvantages of this design are that (a) the switching circuit deactivates power only after arcing/fire has already started and burned the sensor fiber filament, thus producing a fire hazard to a heating product; and (b) the addition of a sensing sacrificial filament enlarges the overall thickness of conventional PTC cables, which already feature stiffness and bulkiness.
Gerrard (U.S. Pat. No. 6,310,332) describes an elongated heating element for an electric blanket comprising a first conductor means to provide heat for the blanket and extending the length of the element, a second conductor means extending the length of the element, and a meltdown layer between the first and second conductor means which is selected, designed and constructed or otherwise formed so as to display a negative temperature coefficient (NTC), and including electronic controller set to detect a change in the resistance of the meltdown layer to provide a means of changing the power supply to the first conductor means (providing heat to the blanket), to prevent destruction of the melt down layer. The element further includes a meltdown detection circuit for detecting meltdown of the meltdown layer and for terminating power to the first conductor means in the event that the control means fails and the meltdown layer heats up to a predetermined degree. The disadvantage of this construction is that the final safety of the blanket relies on a complex NTC/meltdown detection system located in the controller. In the event the controller fails, or significantly delays detection of NTC layer meltdown, then a severe scorching of the heating product, or fire hazard, can occur.
In the event a blanket user bypasses the controller by energizing the blanket directly from the power outlet, the heating element will not provide any overheat or fire hazard protection because the Gerrard's heating element does not have inherent Thermal-Cut-Off (TCO) properties. The heating element utilizes winding of breakable metal wires, which makes construction thicker and more obtrusive for flexible heating products, such as heating pads and blankets.
Another disadvantage of the Gerrard's invention is that its control system utilizes a half-wave power cycle for heating and another half-wave power cycle for meltdown stroke detection in order to provide proper heating output and meltdown protection. Therefore, the heating wire has to be twice thicker than comparable systems utilizing a full-wave power output. This feature becomes especially challenging for 120V and other lower voltage heating systems, compared to traditional European 240V systems. An increase in the thickness of heating wire leads to: (a) increase in the cost of heating conductor; (b) increase in the overall size of the heating element and (b) possibility of breaking the heating wires due to their reduced flexibility.
The present invention seeks to overcome the drawbacks of the prior art and describes the fabrication of a heater comprising metal fibers, metal wires, metal coated, carbon containing or carbon coated threads/fibers, which is economical to manufacture, does not pose environmental hazards, results in a soft, flexible, strong, thin, and light heating element core, suitable for even small and complex assemblies, such as hand wear. Significant advantages of the proposed invention are that it (a) provides for fabrication of heaters of various shapes and sizes with predetermined electrical characteristics; (b) allows for a durable heater, resistant to kinks and abrasion, and (c) with its electro-physical properties it is almost unaffected by abuses such as pressure, severe folding, small perforations, punctures and crushing. A preferred embodiment of the invention consists of utilizing electrically conductive textile threads/fibers having an inherent Thermal Cut Off (TCO) function to prevent overheating and/or fire hazard. The preferred system utilizes a NTC sensing layer for hot spot detection, which does not require having low-temperature meltdown characteristics. Because the proposed conductive fibers are extremely flexible, the coaxial winding process is not required in the heating element manufacturing, which makes the heaters extremely thin, light and durable. The heaters described in this invention may also comprise a continuous temperature PTC sensor to precisely control heating power output in the h
Kochman Dmitry
Kochman Eric
Hoang Tu Ba
Liniak Berenato & White
Thermosoft International Corporation
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