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Thursday, May 24, 2018
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Control on Temperature In To or Out From Heating Process?
System with water as heating medium "small" water volume circulated with electrical heater and a larger amount of metal being heated to desired temperature.

Hi,

This might be a bit on the basic side. But we are having some discussions of how to best control our heating process.

We have a small heating system with a quite heavy metallic piece being heated by circulating water. The water is heated with a electric heater.

The question is; is it best to control the system based on the temperature before the metal piece or the temperature after the metal piece? I.e. TI 01 or TI 02?

The ones preferring controling the TI 01 say the control system will be much faster. The ones preferring controlling the TI 02 say the control system will be more rigid and less prone to becoming unstable.

Is there a clear "correct answer"?

Thank you!
//Erik

Erik,

I am not sure there is a clear "correct answer," but I would suggest using both TI 01 and TI 02. I am assuming the desire is to maintain the metal mass at some specified temperature without being able to directly measure its temperature. I would guess that you would have some upper limit on the TI 01 temperature so I would override based on that limit. I would guess that the TI 02 temperature gives a better indication of the actual temperature of the metal mass so I would use that as the main control point.

Controlling the voltage to the heater would give you a better overall control than simple on-off control.

Thank you for your quick input. Your assumptions are correct. Usually we'd like 70 or 80 C in the metal.

I was worried it wasn't as clear cut to answer.

Yes if we rebuild a variable output would be beneficial.

//Erik

I agree with the previous poster T02 tells you what the temperature of the Mass is, assuming you have a good thermal transfer. T01 won't be much hotter than T02 depending on the rate of circulation.

The normal way to control Electric Heat is Time Duration not Voltage,
very easy to do using Solid State Relay with a time proportional output from control system.