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Elementary analysis of common electric heating elements in high temperature vacuum furnace
Jan 22, 2018

Elementary analysis of common electric heating elements in high temperature vacuum furnace

A. Refractory metal heating element


   Refractory metal heating elements in three shapes:


   (1) wire coil or the "zigzag" shaped electric heating element. This electric heating element and non-vacuum furnace used in the electric components no different, mainly made of molybdenum wire, the coil is generally a diameter of 2 to 2.5mm of the wire is made of molybdenum wire electric body temperature does not exceed 1700 Lt; 0 & gt; C. The "zigzag" heating elements are made of wire rods of larger cross-section than coil heaters. They require higher strength and are designed to have a vacuum of not less than 0.1 to 100 Pa.


   (2) fiber-like or rod-like heating element. This heating element for 2300 ℃ the following temperature, electric heating material is mainly tungsten, and sometimes with molybdenum (operating temperature below 1700 ℃). The ends of the rod are cast into the copper, forming a raised portion, whereby the rod can be secured. The upper end of each rod is cooled by water and is led out through the cover. The head is led out with a metal spring tube to keep the vacuum seal. The lower end of the rod is fixed on the water-cooled fan-shaped chassis. The general electric heater is made of tungsten rod, Rod and composition.


   The wire of the brazing electric heating element is a water-cooled copper tube, which is made of a wire material having a diameter of 5 mm. The fiber-like electric heater is composed of 12 tungsten brazes, which are arranged circumferentially to form a cylindrical heating space.


   The main drawback of the brazing and rod-like heating elements is that a large number of water-cooled wires in the furnace cause a large heat loss; they can not be divided into several heating zones to adjust the furnace temperature by height; normally only one heating zone furnace can use this heating element.


   (3) sheet metal heating elements. Molybdenum plate or tantalum plate for electric heating material. Due to mechanical processing and welding difficulties, generally do not tungsten plate. The electric heater has a cylindrical shape, and is composed of three sheets of 0.2 mm-thick crimp, and the lower part is fixed on a ring of 2 mm in thickness. Each of the upper parts of the metal plate is welded (or riveted) The strip guides the electric heater, which simultaneously holds the electric heater on a copper water-cooled wire. The heat conducting condition of such an electric heating element is preferred because the entire surface is involved in the heat exchange of the object to be heated. The disadvantage of such heating elements and solder-like or rod-like heating element is basically the same.


   B. Graphite heating elements


   In addition to refractory metals, graphite and refractory metals can be comparable to the vacuum furnace electric heater electric heating material. Graphite and refractory metals, compared to a good performance is that its resistance changes little with temperature. Graphite has a high resistance, can be larger in the electric body section, the use of dozens of volts of power. General graphite heating element for the vacuum of 0.1 a 100Pa conditions, the use of the limit temperature of 2200 ℃. Under the above vacuum conditions, the working life of graphite at 2300 ℃, only a few dozen hours, and 2400 to 2500 ℃, only a few hours. It is worth noting that the thermal conductivity of graphite at low temperature is good, and at high temperatures when the temperature dropped to a fraction of low temperature, it will cause the heating element center and outer surface temperature difference, resulting in mechanical stress, resulting in electric components damage. In the design of graphite heating elements should be calculated when the stress generated.


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