There is often a tough decision when a vacuum furnace reaches a certain age. Do you scrap it and replace it with a state-of-the-art new furnace? Or do you commit time and resources to upgrade the existing furnace’s control system? Sometimes the driver for this decision may dictate that a furnace is replaced, but there is often a false assumption that an ‘old furnace can not be taught new tricks’. In reality, mechanically, the furnace may be in reasonable condition. The real source of the issue is unsupported, unreliable, out-of -date, non-compliant, and in the worst case, unsafe control technology. Increasingly demanding customers, combined with standards that require more traceability, force heat treaters to ask the question: do we buy a new vacuum furnace or upgrade our existing vacuum furnace control system?
A seemingly complex vacuum furnace actually consists of relatively few components: the shell, hot zone, heating systems, cooling systems, diffusion pump, vacuum pumping system, pipework and valves. Degrading hot zones, cooling jackets, failing diffusion/vacuum pumps can be cleaned, repaired, rebuilt or replaced.
Vacuum Furnace Uniformity
Many vacuum furnaces struggle with passing temperature uniformity survey (TUS), especially where uniformity requirements are tight and strictly enforced in the aerospace sector. Even with routine hot-zone maintenance, this may be inevitable. TUS failures are commonly cited as a reason to discard a furnace. However, even minor modifications to the existing control and heating system can result in drastic improvements.
Heat chambers traditionally have three to five distinct zones with the same number of variable reactance transformers (VRTs). Each VRT’s output is proportional to a command signal and generates from a single silicone-controlled rectifier (SCR), split into three to five distinct signals. Each is manually trimmed via adjustable rheostats. Optimal rheostat settings often vary between temperature ranges which limits the overall uniformity capabilities of the furnace. Huge improvements are achieved by installing an independent SCR per VRT and trimming, through an SSi 9220 for example, the outputs from the controller to the SCR during tuning.
SSi controllers can scale each output at specified temperatures, which improves uniformity at the required set points. It also means we can center the load TC delta around the set point which is a simple but effective way to quickly improve uniformity, since it is deviation from set point that determines total uniformity, not simply the spread between hot and cold TCs.