Vacuum furnaces can be either hot wall (Fig. 1) or cold wall (Fig. 2) designs. There are notable differences between the two, and each has specific advantages and limitations. In this and our next article, we will examine these differences and describe which applications are best suited for each.
Early in the history of vacuum furnaces, hot wall designs were the only ones available. These furnaces, just as the ones today, utilize a retort (aka muffle) into which the load is placed. The retort is then sealed. A pump evacuates the retort and the process runs either under vacuum or at a specific (negative or positive) pressure once the initial vacuum level is reached.