Opinions: Tank Heating Control

Bullzi said:

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• Fluid: Bio-Diesel
• The Heaters are placed about 8" above the bottom of the tank in 2.5" pipe.
• The Temp Transmitter is about 12" above the heaters.

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You may want to consider level instrumentation to disable the heaters if fluid level falls below the temperature sensor.

I would have it done by now assuming the design is right and the heaters can generate enough heat or doesn't lose heat faster than it is generated.
I would fill the tank half way and turn on one heater. When it reached a steady state I would turn on the second heater. When steady state is reached or a high temperature limit I would turn the second heater off and let the temperature reach a steady state.
I would record all of this of course. I would then use my python auto tuning program I posted a couple of months back to calculate the system model for a half full tank.
I would then repeat the procedure with a full tank. I would then use the two models to estimate the system gain, time constant etc when the level is at some level beside full or half full. A linear interpolation should be more than good enough.

There are formulas for calculating the PID gains based on the open loop gain, time constants and dead time. These PID gains can be updated as the level changes so the control will always be close to perfect. If there is an ambient temperature sensor it is possible to add a small output bias to compensate for when the ambient temperature drops.
http://deltamotion.com/peter/Mathcad/SOPDT/Mathcad - Sysid SOPDT.pdf
See page 9/10 for the formulas for updating the controller gains. Notice the document is 10 years old.

Thanks for all the advice everyone,
I did confirm that the tank will be insulated. I hope it is done before I start up the system otherwise it will make the tuning very difficult.

I do have a Level Transmitter in the tank feed to the PLC. So I can turn off the heaters when the level gets too low. Also there is a Type J Thermocouple in each heater that is tied to a Transmitter back to the PLC. This is provided by the heater manufacture and is used to detect a heater overheating situation The SSR's are feed through a Contactor so I can open up the Contactor if the temp in the tank gets above a set point. If all hell breaks loose and the heaters themselves over heat I can then fire the Shunt Trip on the main breaker and kill the whole system.

Thanks again for the help!!

Peter Nachtwey said:

I could make an "Advanced Control" topic where "we're going to engineer the heck out of this"

It would be gross overkill but educational.

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Another "yes please!" from me, Peter, if that is any help