Surface® Combustion was the first to recognize the advantages of vacuum processing to remove impurities with the advantages of heating and cooling under convection. The VacuDraw^® incorporates an initial pump down to 50 microns to remove air, water vapor, etc. from the chamber. This is followed by a backfill with pure nitrogen or nitrogen with slight amounts of hydrogen added. By using the initial vacuum purge, only one single charge of inert gas is required for the backfill. No continuous flow of gas is necessary which is a tremendous operating cost savings. After backfilling, the tempering process continues by recirculating the inert gas through a heating source using electric elements or indirect gas fired burners firing into radiant tubes. When the heating cycle is complete, the same charge of inert gas is recirculated through an internal or external cooling system. The goal in vacuum purged tempering is to cool the workload back down to ambient temperature to avoid workload discoloration and oxidation.

The construction of the VacuDraw temper furnace is revolutionary as well. A single-walled vacuum casing is used which eliminates the need for water cooling. To this casing, pyroblock insulation is attached in a true “hot wall construction” fashion. Sheathed electric heating elements or revolutionary single-ended recuperative burner technology (SER’s) are used as the heating source. Along with the rear-mounted, high-capacity convection fan, they provide for excellent temperature uniformity throughout the load.

Cooling systems can include an external recirculating blower working in combination with cooling coils. The heated inert gas is passed over these water-filled cooling tubes or coils. The heat from the process is transmitted through the finned radiators to the cooling water.

Aside from tempering, any type of clean, low-temperature processing such as stress relieving can be performed. The best applications are those under 1400°F. This is the transition point of the effectiveness of radiant heating versus convection heating. Below 1400°F, convection heating is the primary source of heat transfer. Many case studies have shown the true advantage of vacuum-purged tempering as the heating times with convection are much faster than with radiation. This has demonstrated that the VacuDraw is a cost-effective alternative to using a high-temperature vacuum furnace for the vacuum tempering process.

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