Upgrading A Vacuum Furnace Control System

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.

Example of electric heating modification which resulted in a spread reduction at 1050 deg from 10 deg C to 3 deg C. Click to enlarge.

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.

New Cabinet versus Retrofit Existing Cabinet

In some cases, upgrading an existing cabinet can cost more than replacing the entire cabinet due to time required on site tracing cables, testing and replacing failed/failing components and reproducing electrical drawings. New panels provide a fresh, documented solution. The footprint can also be reduced with modern, smaller instrumentation allowing older two- to three-door enclosures being replaced with single-door equivalents. In addition, new panels may significantly reduce unplanned downtime. Areas to address before deciding whether to replace the cabinet are:

• What is the condition of the motor starters, relays, transformers and other components within the control panel? As these components fail, each failure will result in some type of downtime, perhaps impacting product, potentially resulting in scrap or rework costs.

• What is the availability of a programmable logic controller (PLC), silicone-controlled rectifier (SCR), vacuum instrumentation and other components? Some of the components within a control panel may have become obsolete or have lead times exceeding four to six weeks. PLC failure is often the worst-case scenario with older control cabinets and results in extended downtime. If the PLC or other critical components are left in-situ then we recommend that the new documentation includes critical part lists with lead times stated.

• Are electrical schematics available for the control panel? If so, how accurate are they? Inaccurate schematics often extend unplanned downtime. Before beginning repairs, electricians must trace and re-label wires to understand how a furnace is wired.

Programmer/Controller Considerations

Standard controllers are far more capable than , gaining flexibility from historically basic ramp/soak profiles. Modern PLCs and controllers offer a wide range of custom features focused solely on vacuum heat treatment. When evaluating the existing controller, the following should be considered. Does it:

• Provide all of the functionality required by my customers and accreditation?
• Allow for a sufficient number of recipes for the number of parts the furnace will process?
• Provide appropriate operator visibility as to furnace operation, valve position and motor status?
• Allow for selectable load TC evaluations for guaranteed soaks?
• Generate and maintain an alarm history for all applicable alarms?
• Offer built-in PID tuning assistance with custom PIDs for various temperature ranges or recipes?
• Automatically provide vacuum and outgas interlocks?
• Include a built-in maintenance program?
• Allow for remote access for support?
• Is my current controller still offered by the manufacturer? Is it available “off the shelf,” or is it an obsolete item?

If any of those features are required but not attainable with existing hardware then we recommend the vacuum furnace PLC or controller is upgraded.

Built-In Maintenance Programs

Modern control systems also offer built-in maintenance programs. Simple maintenance programs record motor run times and valve and production cycles. Complex maintenance programs may utilise a database allowing specific users to track routine maintenance dates, downtime and costs. Custom reports and searches help sites move from reactive to predictive maintenance. Predictive maintenance alarms allow facilities to reduce unplanned downtime and optimise maintenance intervals.

Dew-Point Monitoring

Many heat treaters are required to record each furnace’s inert gas dew point. Common practices require an operator to manually sample gas into a handheld analyzer. This requires the operator’s time and necessitates a handwritten log. Combined with the trend chart, this creates two pieces of production documentation. Modern controls integrate dew-point sensors that can be trended 24/7, eliminating the need for paper logs. SCADA and process controllers allow the operator or recipe to define alarm thresholds, alerting the operator should the dew point rise above a specified temperature.

Part Traceability

Networked controllers can be connected to plant-wide SCADA software to effortlessly integrate production with a load-tracking database. Sophisticated databases allow management to:

• Minimise data entry and duplication with bar code systems.
• Instantly produce trend charts and custom reports from the plant’s entire history.
• Restrict which recipes furnaces can process.
• Maintain recipe revision history.
• Access data from multiple computers.
• Reduce audit time.

Summary

We hope this post has provided an insight into the additional value a control system upgrade can provide. Older furnaces are often ‘written off’ when the installation of a well designed control system upgrade could extend it’s working lifetime by many years. Please get in touch if you would like to arrange a survey of your vacuum furnace.

Many thanks to 

See images and specifications of our standard vacuum furnace control system here: www.supersystemseurope.com/product/systems/vacuum-furnace-control-systems