In our previous discussion, we looked at the reasons vacuum technology is important to the thermal processing industry and discussed a few of the most common applications for vacuum furnaces. Here, we will talk about the major components of a batch vacuum furnace and briefly consider continuous (aka semi-continuous) vacuum furnaces. Batch vacuum furnaces are made up of various subsystems (Fig. 1): vacuum pumps; a hot zone complete with a heating source, insulation, hearth/load support structure; vessel or shell with structural support system; power components; and instrumentation.
The purpose of vacuum pumps and the related components is to remove the air from the heating chamber. Although the vacuum pumping system does not remove all the air, the goal is to remove enough of the remaining air so that any oxygen present will not react with the material being processed in the furnace. There are two categories of pumps employed on typical vacuum furnaces: a primary pump with or without an optional booster pump (aka blower) and an optional secondary diffusion pump (Fig. 2).
For most batch vacuum systems, a combination of a mechanical pump (i.e., wet or dry pump) and booster pump will reduce the pressure to roughly 10-3 mbar (millibar) which is considered a medium to high vacuum. For reference, 1 bar is atmospheric pressure, whereas a millibar is 1 thousandth (0.001) of current atmospheric pressure. Therefore, 10-3 mbar is 1 millionth (0.000001) of atmospheric pressure. The primary pump is used for the initial pump-down from atmospheric pressure via the opening of a roughing valve. When the efficiency of the primary pump or primary pump plus booster pump drops off, the roughing valve is then closed and the poppet valve is opened to the secondary (diffusion) pump. The secondary pump is designed to reduce the pressure to very high or ultra-high vacuum, as low as 10-10 mbar.