A. Refractory metal electrothermal elements
There are three shapes of refractory metal electrothermal elements:
(1) Coils or zigzag electrothermal elements made of wire. This kind of electrothermal element is not different from the electrothermal element used in non-vacuum furnace. It is mainly made of molybdenum wire. The coil is generally made of wire with a diameter of 22.5mm. The working temperature of the molybdenum wire electrothermal body does not exceed 1700 C. The zigzag electrothermal elements are made of wires with larger cross-section than coil electrothermal bodies. They require higher strength and require no less than 0.1-100Pa vacuum in the furnace when they are designed.
(2) Fibrous or rod-shaped electrothermal elements. This kind of electrothermal element is used at temperatures below 2300 C. The main material of the electrothermal body is tungsten, and sometimes molybdenum (working temperature is below 1700 C). The two ends of the rod are cast into the copper to form a protruding part, which enables the rod to be fixed. The upper end of each rod is cooled by water through the furnace cover, and the head part is sealed by a metal spring tube. The lower end of the rod is fixed on the water-cooled sector chassis. The electric heater made of tungsten rods is composed of nine rods arranged along the circumference.
The conductor of brazed electrothermal element is water-cooled copper pipe. Brazing is made of wire with diameter of 5 mm. The filamentous electric heater is composed of 12 tungsten brazes, which form a cylindrical heating space arranged along the circumference.
The main drawbacks of brazed and rod-shaped electrothermal elements are that a large number of water-cooled wires in the furnace cause great heat loss; they can not be divided into several heating zones to adjust furnace temperature according to height; generally, only one heating zone can be used in the furnace.
(3) Metal sheet electrothermal elements. Molybdenum or tantalum plates are used as electrothermal materials. Tungsten plate is generally not used because of the difficulties in machining and welding. The electric heater is cylindrical in shape, consisting of three thin plates with 0.2 mm edges. The lower part is fixed on a ring with 2 mm thickness. The upper part of each metal plate is welded (or riveted) with thin strips. The current is transferred from thin strips to the electric heater. These thin strips fix the electric heater on a copper water-cooled wire at the same time. This kind of electrothermal element has the best thermal conductivity because the whole surface participates in the heat exchange of the heated object. The disadvantages of this kind of electrothermal element are basically the same as those of brazed or rod electrothermal elements.
B. Graphite electrothermal elements
In addition to refractory metals, graphite is an electrically heated material comparable to refractory metals for electric heaters in vacuum furnaces. A good property of graphite compared with refractory metals is that its resistance varies little with temperature. Graphite has a high resistance and can use tens of volts of power supply when the section of the electrothermal body is large. Generally, graphite electrothermal elements are used in the vacuum of 0.1-100 Pa, and their service limit temperature is 2200 C. Under the above vacuum conditions, the working life of graphite is only tens of hours at 2300 C, but only a few hours at 2400-2500 C. It is worth pointing out that graphite has good thermal conductivity at low temperature, but at high temperature it decreases to a fraction of that at low temperature, so it will cause the temperature difference between the center and the outer surface of the electrothermal element, which will produce mechanical stress and lead to the damage of the electrothermal element. When designing graphite electrothermal elements, the stress produced by them should be calculated.
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