Industrial electric heating furnaces – Resistance furnace device – With heating element detail
Patent
1999-06-30
2000-09-26
Hoang, Tu Ba
Industrial electric heating furnaces
Resistance furnace device
With heating element detail
373119, 373130, H05B 366
Patent
active
061251348
DESCRIPTION:
BRIEF SUMMARY
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an electric furnace assembly, comprising at least one furnace wall and an elongated metallic resistor element being supported so as to extend adjacent to said furnace wall, said metallic resistor element being bent back and forth in its longitudinal direction so as to extend at a varying distance from said furnace wall between extreme positions located at relatively large and relatively small distances therefrom.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 AND 1.98
Such assemblies are generally known, e.g. as disclosed in the Swedish patent specification 389958 (Allmanna Svenska Elektriska AB), being designed for heat treatment of objects at high temperature, such as exceeding 1000.degree. C., and at high pressure, preferably exceeding 500 bar (50 MPa). In this known furnace, the elongated metallic resistor element is formed as a strip which is bent in a meander-like fashion, so that the flat resistor element is oriented with its flat side standing vertically, and the lower, longitudinal edge resting on the horizontal bottom surface of an annular ceramic body mounted at the inside of the cylindrical insulating wall of the furnace. Several annular support bodies with associated resistor elements are stacked one on top of the other so as to obtain a high power density and a uniform temperature within the furnace chamber.
In other previously known electric furnace assemblies, there are elongated resistor elements being embedded in the wall. Alternatively, coiled elements are supported by ceramic cores extending along the centre of the coil. See the KANTHAL.RTM. Handbook, Catalogue 1-a-5b-3, September 1994, illustrating various alloys, wire and strip elements and support systems for electric elements in furnaces.
Generally, in furnaces of this kind, the resistor elements are subject to thermal expansion and contraction during temperature changes. There is also a risk of permanent deformation, so that the dimensions of the resistor element change upon extensive use. Normally, the resistor elements are permanently elongated.
BRIEF SUMMARY OF THE INVENTION
The main object of the present invention is to overcome these problems and to provide an electric furnace assembly, wherein the resistor elements have a high degree of form stability, even after long use, and to allow for thermal expansion and contraction in a controlled way without causing any hazard of physical contact between any parts of the resistor element and the objects being treated in the furnace chamber.
Additional, secondary objects are to secure an efficient transport of thermal energy from the resistor element into the furnace chamber, to secure a long element life and to provide resistor elements which are chemically inert, which have a low thermal mass in order to allow for rapid temperature changes and which are inexpensive to manufacture and assemble, in particular resistor elements in the form of wire, strip or rod material with circular, oval or rectangular cross-section.
The main object stated above is achieved in that the resistor element is retained at fixed points by fixed holding members at some of said extreme positions, located at said relatively large distances from the furnace wall, so as to permit a controlled elongation of the resistor element while keeping the latter within a limited region adjacent to the furnace wall.
The resistor element is preferably bent back and forth in its longitudinal direction, e.g. in meander-like loops, and the element is retained by fixed holding members at the above-mentioned fixed points, viz. at the extreme positions located furthest away from the furnace wall. Accordingly, any thermal expansion and contraction will occur in a controlled way, the various loops being contracted or elongated like an accordion. Moreover, the supporting points are fixed and located in such a way that the loop-f
REFERENCES:
patent: 1597900 (1926-08-01), Keene
patent: 2035306 (1936-03-01), Fannin
patent: 2896004 (1959-07-01), Duffy et al.
patent: 4056678 (1977-11-01), Beall, III et al.
patent: 4142062 (1979-02-01), Wentworth
patent: 5497394 (1996-03-01), Jhawar et al.
patent: 5930285 (1999-07-01), Moller
Kanthal Handbook--Resistance Heating Alloys and Elements for Industrial Furnaces by Kanthal AB Sweden, 1994.
Jonsson Bo
Lewin Thomas
Magnusson Jonas
Hoang Tu Ba
Kanthal AB
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