A vacuum tube furnace belongs to the group of hot wall vacuum furnaces where the tube is heated from the outside by heating elements exposed to ambient air. Depending on the tube's material and size, the maximum temperature in vacuum varies. On the ends of the work tube, the tube must be adapted to vacuum flanges. Depending on the material and size some vacuum tube furnaces require water cooling, some work without cooling water.
Of course, vacuum itself is a possible atmosphere for vacuum tube furnaces. Depending on the application, the vacuum level can be rough, fine, high, or even ultra-high vacuum. Additionally, as a vacuum furnace allows complete removal of the atmosphere, it is also possible to work with various gases or mixtures, such as Argon, Nitrogen, Hydrogen, Carbon Monoxide, Helium etc. For non-metallic tubes, even corrosive gases, such as Ammonia or Silane among many others become possible.
The combination of maximum size and temperature required for the application decides whether a vacuum tube furnace is sufficient or a cold wall vacuum chamber furnace is required. For vacuum temperatures of 1450°C, the maximum inner diameter of the tube is restricted to 88 mm. In this case the work tube is a consumable with limited lifetime and restricted to low temperature ramps. Larger sizes or higher temperatures require a vacuum chamber furnace.
