Building a furnace with more than one thermocouple for accurate temperature measurement

Question

I am a mechanical engineering student. I am in the process of making a special purpose heating equipment. It could be considered as a 3 phase electrical resistance furnace.

I want the temperature in the chamber to be uniform, so I want to place three K-type thermocouples to feed to a temperature controller-contactor set up to control the temperature.

I have previously built an electrical resistance furnace of 2 kW but with a single thermocouple and controller-contactor setup.

How can I have more than one thermocouple to control the temperature in the machine?

Edit 1. The purpose is to heat metal. Based on my calculation it would consume 36 kW of electricity. Based on the comments and answers, I am planning to split the heating coils into three separate sections with independent thermocouples and controller setup, where each section would be wired as an independent 3 phase heater.

Would this design work?

Answer 1

For uniformity, you should STIR the air inside the furnace.

Measuring with 3 thermocouples does nothing to ensure uniformity.

I've worked with people who design/test silicon temperature sensors. The oil baths are always STIRRED.

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Be wary of abruptly switching 36,000 watts. The energy stored in the power lines will cause ARCS at any contact points on relays, or high (destructive) junction voltages on FETS or bipolars or SCRs.

Answer 2

I do suggest you to not use separate controllers for each phase, rather turn all three on/off heaters at the same time using three phase contactor and 3ph protecting circuit breaker. Imbalanced high loads on 3ph network may cause several problems.

Answer 3

I am planning to split the heating coils into three separate sections with independent thermocouples and controller setup, where each section would be wired as an independent 3 phase heater.

Would this design work?

Probably.

You will have (mostly radiant?) heat transfer between the heaters and your workpiece, and conductive heat transfer within the work piece. This will lead to cross-talk between the zones: When you try to heat up the "left side" of the work piece some of that heat will end up in the "right side". That means that while your PID controller for the "right side" is trying to do one thing, the heat from the "left side" might be making something else happen.

This could lead to a chaotic mess, but most likely as long as you tune your PIDs to be overly damped and don't try to make any fast changes to the temperature you'll be fine.

Though be warned: optimizing the heater geometry to heat your workpiece uniformly is a whole separate topic. (Assuming primarily radiant heat transfer.)

Answer 4

Just a note about the 3 phase elements. Ones like silicon carbide can change their resistance as they age. I would suggest connecting the elements in star, with the star point isolated from the neutral. That way, the potential of the star point can change, to give the higher resistance element a bit more voltage & the others a bit less, to reduce the imbalance of power. If you are using a thyristor control, you will need to tell the supplier what element connection it is (neutral connected, or no neutral). The SCR timing is set different for each.