Sintering Furnaces

Vacuum Furnace Solution #2 – Cold Wall

There have been notable applications of externally heated, thick wall, large metallic tubes in vacuum atmospheres approaching 1000° C. However since the mid twentieth century, most high temperature vacuum furnace designs have relied on an internally heated cold wall design. For a cold wall design, the heating element is placed inside rather than outside the vacuum vessel. Thermal insulation packs are placed between the heating element and the vacuum vessel. This strategy reduces both heat load to the vessel and furnace power consumption. This flexible approach allows vacuum furnaces to operate up to 2000-2500° C temperatures (3000° C with inert gas) and have several meters of workzone diameter.

The vacuum vessel now must contain the heating elements and insulation packs. Because of this, the vessel must be much larger than the small tubes employed in furnaces described in Vacuum Furnace Solution #1. Since the larger vacuum vessel size has considerably more surface area, it has more crushing force applied. Unfortunately, with increasing temperature the steel or aluminum used in nearly all cold wall furnace designs loses strength exponentially. The vessel must be kept near room temperature at all times.

In nearly all commercial products employing this approach, the vacuum vessel is water cooled to achieve this goal. The standard approach to avoid hot spots and effectively distribute cooling capacity is to employ a double wall vessel design, with cooling water circulating between the inner and outer vessels. An alternate approach for moderate sized vessels is to braze tubes for water cooling to a single wall vessel. The cold wall furnace design also allows widespread use of elastomer seals on multiple ports and doors, avoiding the inconvenient access at the end of long tubes required on hot wall tube designs.

The following figure shows some of the features of the RD-G cold wall vacuum furnace. It depicts a 2200° C laboratory furnace that uses the innovative Webb air cooling design in place of water cooling. For more details about this design, click the figure.

RD-G Vacuum Furnace Cross-Section

RD-G Cold Wall Vacuum Furnace Design