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REVOLUTIONARy VACUUM FURNACE TECHNOLOgy New Furnace Technology A high-temperature vacuum furnace has been built that had an internal thermal insulation system to reduce heat losses and an external vacuum vessel designed to safely dissipate all heat generated within the furnace to the surrounding air. The vacuum vessel wall remains at a sufficiently low-temperature to ensure structural integrity and to protect elastomer seals without the use of water cooling. (U.S. Pat. No. 5,987,053) A high-temperature vacuum furnace that does not require water cooling has many advantages. These include: • Operation with low electrical power inputs; • Simple construction; • Quick and easy installation by unskilled users; • Reduced operating expenses; • Long-life vacuum chamber; • Improved temperature uniformity in the work zone. Door Seal Sensor Penetration Window Elastomer Sight Window Assembly Hermetic Side Weld Hermetic Base Weld Bottom Plate Left Feedthrough Port In the case of small laboratory furnaces, other advantages were obtained. These include: • Elimination of cooling water; • Simplification of vacuum vessel construction; • Elimination of water manifolds, water hoses, water flow switches and water valves. The result was a high-temperature vacuum furnace weighing
approach is to provide an additional sensor penetration that can be used for inserting a thermocouple temperature sensor, or for observing the work zone with an optical or infrared pyrometer. In that case, a sight window assembly is sealed with a hermetic side weld to the chamber sidewall. The sight window assembly includes a viton window elastomer so that the window can easily be removed for cleaning or replacement. A vacuum port provides a means for removing air, introducing process gases and sampling the internal chamber atmosphere. Fibrous graphite high-temperature insulation surrounds the heating element and defines a useable work zone. A dense graphite retort is used to support the material being thermally treated and to separate the work zone from the heating element. The graphite retort also distributes heat created by the heating element and improves work zone temperature uniformity. High-temperature graphite insulation is suitable for use to 3000 K and can be used to insulate the furnace fully if desired. It can be surrounded by lower-temperature insulation to reduce fabrication costs. The lower-temperature insulation can be either a readily available ceramic fiber insulator rated to 1500 K or low-temperature carbon insulation with similar temperature capabilities. The combined thickness of the hightemperature and the low-temperature insulation should be approximately the same on the top, bottom and sides in order to maintain temperature uniformity within the work zone. Power (watts) 1500 750 Hold Power vacuum argon 0 0 1000 Temp (C) 2000 Power consumption of highly efficient laboratory size air-cooled vacuum furnace. Temperature gradients within the work zone are created by unequal heat flows through the different sides of the zone. In the ideal case, there would be no heat flows through the insulation, and the work zone would be isothermal. In actual furnaces, however, reducing the magnitude of the heat flows through the insulation, either by increasing the efficiency of the insulation or by eliminating water cooling, will result in a decrease in the magnitude of the temperature gradients within the work zone. Eliminating chamber water cooling results in a profound reduction in heat flow through the insulation because of the vast difference in heat transfer coefficient for heat transfer from the chamber to flowing water compared to the heat transfer coefficient for heat transfer from the chamber to ambient air. In operation, air is evacuated from the vessel through the vacuum port and electrical currents are passed through power feedthroughs to the power buss rods and from there to the heating element. The heating element is designed to have a higher total resistance than the power buss rods so that most of the heat is generated within the work zone. Af ter thermal equilibrium is reached, all the heat generated by the heating element passes through the thermal insulation or through insulation penetrations for accessories, e.g., the sight window assembly and power buss rods. Most of this heat is eventually transferred to the vacuum vessel components comprised of the chamber sidewall, bottom plate and top door. These components will rise in temperature until the heat output of the heating element is matched by the heat losses of the vacuum vessel components to the outside air. At this point, the temperature of the vacuum chamber will stabilize and rise no more. The chamber sidewall, bottom plate and top door transfer heat to the ambient air in the room by a combination of radiation and convection losses. The vacuum furnace will not require water cooling if the chamber and, hence, the chamber surface area is made large enough so that total heat losses match element heat generation before the vessel temperature reaches 500–600 K as previously described. A chamber can be designed for any size work zone that has enough surface area to dissipate all heat generated before this 500–600 K limit is reached. Summary This high-temperature, air-cooled vacuum furnace is a significant improvement over prior designs that required water cooling of the vacuum vessel. Water cooling used in the standard furnaces was effective in removing large amounts of heat from the furnace. This, in turn, required greater heat generation from the heating element to compensate. The elimination of water cooling reduces overall furnace heat losses dramatically, thereby allowing the furnace to reach the desired temperature with far less power consumption. This yields significant simplification of virtually all furnace subsystems. n Heating & Cooling Rates 0 1 2 3 Time (h) Heating and cooling rates for laboratory-size, air-cooled vacuum furnace equipped with 2-kW power supply. www.ceramicbulletin.org • February 2001 37 Temp (C) 2000 1000 0 Heating (argon) Cooling (helium)
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