In the lower temperature range – below 1550°F (850°C) – the radiant heat transfer is low and can be increased by convection-assisted heating. For this purpose, after evacuation, the furnace is backfilled with an inert gas up to an operating pressure of 1-2 bar, and a built-in convection fan circulates the gas around the heating elements and the load. In this way, the time to heat different loads (especially those with large cross-section parts) to moderate temperatures, for example 1000°F (550°C), can be reduced by as much as 30-40%. At the same time, the temperature uniformity during convection-assisted heating is much better, resulting in less distortion of the heat-treated part.
The following media (listed in order of increasing intensity of heat transfer) are used for the cooling of components in vacuum furnaces:
- Sub-atmospheric cooling with a static or agitated inert gas (typically Ar or N2)
- Pressurization (up to 20 bar or more) cooling with a highly agitated, recirculated gas (Ar, N2, He, H2 or mixtures of these gases)
- Oil – still or agitated
With multi-chamber furnaces, such as a vacuum furnace with an integral oil quench, an additional cooling medium, namely oil, is also available. These oils are specially formulated (evaporation-resistant) for vacuum operation.
One variation worth noting is the plasma or ion furnace. Plasma furnaces exist in all styles – horizontal in single or multiple-chamber configurations and vertical designs such as bell furnaces and bottom loaders. The basic differences between these designs and conventional vacuum furnaces are the electrical isolation of the load from the furnace vessel via load support isolators; the plasma current feed-through; the high-voltage generator, which creates the plasma; and the gas dosage and distribution system. Plasma furnaces also utilize conventional vacuum-furnace chamber and pumping systems.
Depending on the specific application, they are either low-temperature furnaces (1400°F/750°C) for plasma (ion) nitriding or high-temperature furnaces up to 2400°F (1100°C) for plasma (ion) carburizing. Low-temperature furnaces for plasma nitriding are constructed as cold-wall or hot-wall furnaces. High-temperature furnaces are usually cold-wall furnaces with water-cooled double walls. They can be equipped either with a high-pressure gas quench system or an integrated oil quench tank.
The generator needed to create a plasma glow discharge inside a plasma furnace has to be a high-voltage dc generator (up to 1,000 volts). There are currently two types of generators in use: one type has continuous-current outputs and the other has pulsed-current output.