Patent Application
20110198341
This invention relates to a thin, lightweight, electrically-heated film that can be cut into any shape and produce a constant watt-density, regardless of size when a low voltage is applied anywhere on its opposite faces.
Presently, most heated surface devices utilize a heated linear element, like an electrically-heated wire element, or a pipe element flowing with heated fluid, that is distributed over the surface to be heated by winding it in a serpentine pattern and terminating the ends permanently in one location. The element must then be held in place by a surrounding matrix material or a bracket apparatus. A surface heated by such a device has hot areas, where the element is, and cooler areas in the spaces between segments of the element, which results in uneven heating and requires that the device be pre-engineered to fit a given space and to produce the desired heat flow characteristics. For example, if it is desired that a one-meter-diameter circular area be heated at 50 watts per square meter, a specific serpentine pattern must be designed for the element to fill the circular area and an element must be selected with specific thermal properties, depending on its length and the spacings within the pattern. A permanent location for the terminal ends of the element must then also be decided upon. Thus, such heaters are necessarily custom-engineered and manufactured for each particular application, rendering them time-consuming, bulky, heavy and expensive to produce and only useful for a specific application.
In the case of the electrical version of such an existing device, as a desired area to be heated becomes larger, a longer element is required, increasing the total resistance of the element and requiring correspondingly higher voltages to produce the same heating characteristics. The voltage required to power larger and larger areas eventually reaches hazardous levels, particularly if the heater is within human contact.
In the case of the piped fluid version of such an existing system, as the heated fluid travels through the pipe, it loses heat energy all along its path becoming cooler and cooler in temperature. This results in uneven heating and eventually the fluid can cool so much as to become ineffectual, requiring intermediate re-heating.
The present invention is directed to overcoming one or more of the problems set forth above.
The present invention comprises a plurality of layers of electrically-conductive material, the common faces of which are in electrical contact with one another. An interior layer is made of a material that possesses a high electrical resistivity. On each of the outer faces of this interior layer is applied or affixed an outer layer of material that possesses a low electrical resistivity. The layers are permanently attached across their mating surfaces so that they are in continuous electrical contact over the entire interface. This results in a thin, laminated film comprising the low-resistivity layers with a high-resistivity layer sandwiched between them.
When an electrical voltage is applied across the film (positive voltage to any spot on one outer layer face and negative voltage to any spot on the opposite outer layer face), a flow of electricity occurs from face to face through the interior, high-resistivity layer material. Through face-to-face current flow, the middle layer produces resistive heat evenly throughout. Due to the way a resistive material behaves when conducting from face to face, the watt density (e.g. watts per square meter) for a given applied voltage is constant, regardless of the area covered by the film. This effect can take place at a very low, harmless voltage.
The consequences of the present invention's behavior make it extremely useful for a wide variety of applications. An advantage of the present invention is that it can be easily and inexpensively mass-produced and may be made extremely thin and lightweight. So the film may be cut with ordinary scissors, or any other cutting method applied to thin materials, into any shape desired without the risk of severing an interior element. Any cut shape will still maintain even warmth at every point as long as the shape is contiguous. Even if an application requires non-contiguous parts, the individual parts can simply be electrically interconnected in parallel with wires and the whole will perform the same as one contiguous shape. The film can be produced in pre-engineered bulk rolls or sheets which can be used for a given application simply by cutting the film to shape and applying voltage at any point on the opposite surfaces of the heated film and produce the rated heating characteristics.
Another advantage of the present invention is that is exhibits constant watt-density over its entire surface regardless of size and at a constant, low, harmless voltage. So very large areas can be heated without the need for higher, hazardous voltages. This also allows the present invention to be powered with batteries, solar panels, fuel cells, or any other low-voltage power source.
Another advantage of the present invention is that it can be sewn like fabric without degrading its performance. Possible uses for this could be its use in or as heated clothing, curtains, tents, furniture, carpet, or any other heated application involving fabric or thin foils or films.
Yet another advantage of the present invention is that power can be applied at any point on its opposite surfaces. So there is no need to permanently locate terminal ends for a given piece of heating film, since they can be located anywhere.
The present invention is also very rugged. It can sustain considerable damage and still function perfectly.
For a better understanding of the present invention, reference may be made to the accompanying drawings in which:
Referring to
To fully understand the operational characteristics of the present invention, it is necessary to explain the phenomenon of face-to-face conduction. In
From classical physics we know that, at a given ambient temperature, the resistance to electrical current flow in a solid object is
where R=total resistance of the wire segment,
But electrical conduction has different implications when passed face to face through a flat sheet conductor. Such a flat conductor 5 is shown in
From classical physics we know that the rate of heat dissipation P of a resistive material depends on its resistance R and the voltage applied to it V, by the relation
where P=heat dissipation rate (e.g. “watts”).
which is the total heat dissipation that emanates from the flat conductor 5.
The distributed heat dissipation per unit area, or “watt density” (e.g. “watts per square meter”) would be equal to the heat dissipation rate P divided by the total area that heat radiates from. Since heat can radiate from both sides of the sheet, the total area would be 2A (neglecting the minuscule amount of heat dissipation that would emanate from the thin edges of the sheet). If both sides of equation (4) are divided by the total surface area 2A we are left with the relation
And since the resistivity ρ of a material is constant, we can say that for a sheet material of given thickness t and resistivity ρ, the amount of heat that flows in each unit area of film depends only on the voltage V and is independent of the area A. This means that if a voltage is applied to the two faces, the film will radiate the same heat over its entire surface, regardless of size. It makes no difference if the sheet has a surface area of one square meter or 1000 square meters: it will still heat with the same watt density at a given voltage. Only the current draw will increase. This is because as the sheet conductor becomes larger, its total resistance becomes smaller, as opposed to the behavior of a wire conductor.
It is necessary that the outer layers 1 of the heating film 3 be made from a material possessing very low electrical resistivity in order that the interior, high-resistivity layer 2 has a uniform electrical voltage potential applied across its interface surfaces 4. In this way, electrical voltage can be applied at any point on the surface of the outer layers 1 and this electrical voltage will remain the same at any other point over the rest of the surface, with very little or no degradation of electrical potential as the distance from the voltage application point increases, due to internal resistances in the outer layers.
Since the conducting area A is very large in comparison to the conducting thickness t the operating voltage can be quite low and the resistivity of the interior, high-resistivity layer can be very high. Its resistivity falls within the common range for standard, inexpensive, graphite-filled semiconductive plastic film sheets that are routinely used for static dissipation applications. A preferred construction of the heated film would be to apply ordinary copper or aluminum foil, with a conductive adhesive backing, directly to both faces of graphite-filled semiconductive plastic film.
While several specific uses for the present invention have been mentioned above, there is no limit to its possible uses. It is intended as a bulk product for use in any application requiring heat. It can also be used where it is desired to replace an existing heating material with one that is thinner, lighter, easier to use, less expensive, more rugged, and/or safer.
