Industrial electric heating furnaces – Resistance furnace device
Reexamination Certificate
2000-08-25
2001-10-23
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Resistance furnace device
C373S117000, C373S128000, C219S552000
Reexamination Certificate
active
06307874
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electric furnaces for the heat treating of metals, and, in particular, to the current carrying heating element in such furnaces.
BACKGROUND OF THE INVENTION
Certain electric heat treating vacuum furnaces utilize heating elements, formed of bands or strips of molybdenum, as a source of radiant heat. These elements are typically supported by insulated hangers or standoffs, and may be arrayed in a substantially circular arrangement in the hot zone of the furnace, or in straight sections arranged transversely across the hot zone at the ends thereof. The molybdenum heating elements are connected in either a parallel or series circuit configuration to the electrical power terminals which penetrate the vacuum vessel.
As electrical current is introduced into, and subsequently removed from, the heating elements, they become heated to high temperatures and cooled from those temperatures, respectively. This heating and cooling of the element subjects it to high stresses from expansion and contraction. Such thermal cycling accelerates warping, bending, cracking, and overall distortion of the molybdenum heating elements and reduces their useful life. Furthermore, the potential for damage to the hot zone and contamination of the load in the furnace is increased if a heating element fails catastrophically.
In view of the foregoing problems, it would be highly advantageous to have a means of reducing the adverse effects of the stress on the heating elements that result from thermal cycling.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention there is provided a novel heating element for a vacuum heat treating furnace. The heating element according to the present invention includes a first heating element portion formed of a thin form of conductive, refractory metal or alloy and an expansion loop formed in or attached to the first heating element portion. The expansion loop is positioned and arranged to absorb expansion or contraction resulting from thermal cycling.
In accordance with another aspect of the present invention, there is provided a hot zone for a vacuum heat treating furnace. The hot zone, according to the present invention, includes a sidewall formed of thermally insulating material and having an interior surface. The hot zone also has a heating element disposed around the interior surface of said sidewall. The heating element has a first heating element portion formed of a thin form of a conductive, refractory metal or alloy, and an expansion loop formed in or attached to said first heating element portion. The expansion loop is positioned and arranged to absorb expansion or contraction resulting from thermal cycling.
In accordance with a further aspect of the present invention, there is provided a vacuum heat treating furnace. The vacuum furnace according to the present invention includes a pressure vessel and a hot zone disposed within the pressure vessel. The hot zone includes a sidewall formed of thermally insulating material and having an interior surface. A heating element is disposed around the interior surface of said sidewall. The heating element has a first heating element portion formed of a thin form of a conductive, refractory metal or alloy, and an expansion loop formed in or attached to said first heating element portion. The expansion loop is positioned and arranged to absorb expansion or contraction resulting from thermal cycling.
REFERENCES:
patent: 1624345 (1927-04-01), Keene
patent: 2356237 (1944-08-01), Geller
patent: 3274374 (1966-09-01), Matheson et al.
patent: 4238636 (1980-12-01), Burstrom
patent: 4499369 (1985-02-01), Gibb
patent: 5497394 (1996-03-01), Jhawar et al.
patent: 6023487 (2000-02-01), Jones
Dann Dorfman Herrell and Skillman, P.C.
Hoang Tu Ba
Ipsen International, Inc.
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