LOL. Lots of vacuum furnaces are controlled by PLC... I used to work for a company that made them.
If the OEM is still around, call them and see what they're selling today. You may be able to buy a control system from them.
If not, sure you can control it with a PLC, very well. The harder part is the operator interface. PID functions are built into most PLCs, certainly into SLCs (I wouldn't recommend buying a PCL5 for this. Temperature control can often be done better by using separate loop controllers for each zone, and just having the PLC send the desired setpoints and ramp rates. Ramps, though, are often done by setting up the ramp in the PLC and then having it adjust the SP every few seconds... or minutes... so that the oven follows the desired curve. Nothing needed but an analog input for SP at the loop controller, which means you can use a pretty cheap controller. If you go fieldbus even better, but more expensive.
As Alaric says, vacuum instruments are often non-linear. With a PLC, that's simple - either program an equation into the PLC, which is a lot easier these days, or simply create a look up table. We used look up tables and interpolation in the 1990's; they have probably never been updated, knowing the guys I used to work with! Again, your OEM if still there has probably had all this for years. If not, it's straightforward to do but writing that first program and then debugging is expensive as always.
Yeah, if you have a vacuum controller then you need to replicate the "control" funtion in the PLC. Not very hard to do, once you figure out how to process the vacuum signal, and once you understand the processes you want to run. The outputs will be replaced by those on the PLC, and you have to write the logic that drives them.
Finally, though, once you have it running under PLC control how do you interact with it? If you have a nice silk screen panel with lights and switches, you can of course keep it. Or, you can write an HMI to go with the controller. Fun, but again expensive.
Good luck - it will work, but check your ROR and make sure that there's a good reason for spending the time and money. If you're adding capability, or decreasing OPR workload, then that may be the reason. If you're just replacing the controls because the existing ones are obsolete, well, you might want to consider just replacing all the loop controllers and vacuum instruments with their current equivalents, program them the same way the old ones are, and get another 10 years out of the new electronics.
