As was discussed last month, the heart of any vacuum furnace is its hot zone. If properly designed, constructed and maintained it will help ensure that your furnace performs in an optimal manner. One of the most important aspects of the hot zone is its insulation and the choice of materials used in its construction. The focus here will be on graphite-lined hot zones.
A popular alternative to all metal lined hot zones utilizes graphite-based insulation. A typical design for a maximum operating temperature of 1315ºC (2400ºF) consists of a 50 mm (2 in.) thick graphite material, typically in the form of (felt) blanket or board (Fig. 1). It is not uncommon to have two or more layers of material. For those familiar with high-temperature atmosphere furnaces, this insulation thickness may seem surprising considering the process temperatures inside the furnace. However, the thermal characteristics of graphite materials make it possible. In addition, the outer shell of the furnace is typically water cooled and the fact that most processes run either under vacuum or in a partial pressure atmosphere further reduce heat transfer. If it were not for the cooling jacket, the exterior temperature could be in the range of 150 – 260°C (300 – 500° F).