Bell furnaces are removable retort type furnace which are lowered over the load and hearth. The inner retort is placed over the loaded hearth sealed at the bottom and provided with constant protective atmosphere. The outer heating shell is then lowered dover the assembly. A bell furnace with outer heating shell holds several retorts. In case of dense hearth loadings, a motor driven fan for circulating the atmosphere inside the retort provides more rapid uniform heating.
The work load is placed on an elevated refractory metal hearth that rests on an insulated base clad with an alloy plate material. A water cooled circumferential flange and vacuum gasket are located on the vacuum tight base cover
adjacent to the heated zone. A retort made with heavy walled heat resisting alloys covers the work load. A flange at the bottom of the retort fits on top of the base gasket to provide a vacuum tight enclosure. The bell shaped furnace equipped with internal electrical heating elements is lowered into position over the retort by a vertical hoist. The vacuum pumping system is connected through the insulated base, since this furnace cannot be heated or cooled rapidly even after rooming the bell shaped vessel. The production rates and number of thermal cycles within a given time period is limited. The hot retort will support the entire pressure of the external atmosphere for the wall being heavy. The operating temperature of these furnaces are limited to 925 °C.