Electric furnace heater

Abstract

Abstract A generally rectangular, self-supporting, unitary high temperature electrical resistance heater adapted for generally horizontal suspension within a high temperature furnace in which the heating element is composed of a flat resistor ribbon folded to form a series of continuous and substantially parallel beam-like heater segments disposed in grate-like fashion. The ribbon heater segments are retained in their parallel configuration by a plurality of refractory insulators disposed between and supported solely by the beam-like segments. The resulting furnace heater offers advantages of light weight, high radiation efficiency and minimum thermal inertia. FIELD OF THE INVENTION This invention relates in general to heating elements for exceedingly high temperature electrical furnaces, and more particularly to a generally rectangular self-supporting electrical resistance heater formed of a continuous elongated flat resistor ribbon folded to define a large number of parallel beam-like segments spaced and maintained in rigid form by refractory means supported on said ribbon. BACKGROUND OF THE INVENTION In electrical furnaces employed in the heat processing of materials and products at exceedingly high temperatures, for example, temperatures up to 1900° C, heating coils are typically supported by ceramic cores such as grooved plates or cylinders. One such configuration is shown in U.S. Pat. No. 3,846,621. The use of ceramic cores, however, imposes a significant limitation in the length of span of such members. Moreover, the ceramic cores are by their nature dense material, with low tensile strength and subject to spalling and fracture at high temperatures. These cores are ordinarily placed above the produce because if placed below the product the cores invariably collect metallic oxides which eventually short-circuit the lower element. Generally, due to the amount of ceramic necessary, and the density of such material, the weight of the ceramic constitutes approximately 75% of the heating assembly. As a result of such massive amount of ceramic, the system develops a high thermal inertia which requires considerable time when the furnace undergoes a required change of state such as cooling or heating. The function of the ceramic core in each of these prior heaters is to support and contain the electrical heating element. The core may be composed of a cylindrical rod or a circular or rectangular plate having a plurality of longitudinal re-entrant slots or grooves formed in the peripheral surface thereof and running the length of said surface. These grooves, due to the limitations imposed by the ceramic material, are necessarily of small diameter and will expose at the maximum one fifth the surface area of the electrical heating element itself. The shape of the heating element inserted into each groove is a function of the shape of its respective groove and thus normally helical in form. The ceramic core therefore effectively shades at least 80% of the direct radiation emitted by the coil to the product, thus providing a low standard of emissitivity. This low emissitivity in turn promotes a substantial differential in temperature between the product and the heating element, causing ineffiiciency and shorter heater life. There are many other problems with heater coils set in grooves. When using flat ceramic plate with a plurality of longitudinal parallel grooves formed in the plane of one surface, and placed above and below the materials and product being heated, the lower flat heater collects particles which must be cleaned or the heater will short-circuit. The use of large amounts of ceramic material, small restraining grooves and relatively thin heating wire combine to yield poor tensile strength, an inefficient level of thermal inertia and a large temperature differential between the heater and the product. Many other heater configurations have been used, some of which avoid the use of ceramic cores, as for example solid heater rods. The latter are limited however, in temperature range because at high temperatures the rod will sag and be unable to sustain their own weight for any appreciable length. Heaters have been devised which use, to some extent, self-supportive, resistor elements, but these have been limited chiefly to low temperature applications such as hot-air heaters in duct-work and the like. Examples of such prior devices are shown in U.S. Pats. Nos. 3,651,304; 3,673,387 and 3,798,417. These devices would however be wholly impractical at high temperatures in the region of 700°-900 ° C since the lack of support would cause the resistor elements to collapse entirely or at least sag to a non-useful configuration. In short, it has not previously been possible to provide an electrical high temperature furnace heater having a large area capable of being suspended without conventional heavy and relatively costly ceramic structures. SUMMARY OF THE INVENTION The present invention has a primary object the provision of a light weight high temperature electrical furnace heating element which is essentially self-supporting over a substantial area at operating temperatures where conventional elements would be incapable of support without auxilliary structures such as ceramic cores. Broadly speaking, the furnace heater of this invention comprises a generally rectangular structure formed of a flat continuous electrical resistor ribbon folded back-and-forth upon itself to provide a planar grate-like configuration of spaced longitudinal beam-like heater segments, the flat faces of the ribbon being generally vertical and parallel one to the other. A large number of light weight refractory elements such as ceramic spools are supported solely on the resistor ribbon segments and are arranged to space these by an appropriate distance while providing lateral support served to create a unitary large-area structure which will retain its shape at the high temperatures contemplated. The heater ribbon and the ceramic spacers are all readily and inexpensively assembled for insertion into the furnace where it rests on the folded edge portions of the ribbon. By virtue of the entirely open nature of the heater and the absence of enclosing ceramic cores, emissivity is increased and thermal inertia is reduced to a minimum. Apart from this increase in radiation efficiency, the open spaced vertical heater ribbons prevent accumulation of extraneous materials so that the heater functions equally above or below the product being processed. The invention offers numerous additional advantages over the prior art. Fracture of the heating element is virtually precluded assuring long life. The ceramic material used in the structure need not be more than 5-10 percent of the total weight, which minimizes thermal inertia and increases radiation efficiency. These features, in turn, combine to lower the energy requirements and enhance the operational efficiency when incorporated into a high temperature furnace.

Inventors

• 👤 Jacob Howard Beck

Bibliographic Details