PID tuning for heating process

I'm not really experienced with PID so I need some help.
I have a 300l insulated pasteurizer with heat exchanger on the inner side wall and the bottom. The milk is heated to a desired set-point and stays there for 1,5 hour and it's not possible for the temperature to decrease since the tank polyurethane insulated. I also have immersion NTC sensor as feedback. At the moment I use on/off control but I have a small overshoot around +1,5 C.
I would like to decrease the overshoot to +0,5 C max so as far as I understand it's only possible if PID used and I need to turn on/off the contactor of the heating element.
The HVAC programmable controller has no auto-tuning and uses parallel PID algorithm.
The time to reach the set-point isn't very important but need to have as small error as possible and no overshoot.
Any tips would be appreciated...

Try taking a look at this thread:

http://www.plctalk.net/qanda/showthread.php?t=4619

It requires you to do a little research into your process (which is a good thing) but it also allows you to stick with on/off control.

Keith

I will have to skip SMC control for this project...
My controller doesn't support Ladder or ST, just FBD so it will be difficult to implement.
Also I can't use PWM but I have to convert PID block output into relay on/off action.

My question is if the aforementioned result is possible using just PID loop without cascade control as I can't measure the temperature of the water which is circulating in the jacket and is heating the milk in the tank.

How does your current on/off control work?

Always on until setpoint reached then turned off?

What is the brand and model of the controller that you are using? (you will get better answers)

Yes, typically it's on until the setpoint is reached.
I have also implemented a function where the user can set a differential in C so the heater is switched off before the temperature reaches the setpoint and the temperature is rising due to thermal dynamics.
The problem that sometimes the temperature can overshoot the setpoint and other times can't even reach it.
It depends mostly on how much milk there is in the pasteurizer. The overshoot with on/off control is bigger when there is 100l in the pasteurizer and smaller when there is 300l. It also depends on the power of the electric heater. 6kW and 12kW heaters for the same tank will have different behaviour. This why I'm thinking of using close loop control.

The controller is Carel, PCOXS refrigeration controller.

Think about what you are doing

When you change the heating element the physical system has changed (how fast you can heat the product)
When the quantity of milk is changed again you have changed the physical system. (Heat input required increases with more milk)

a simple PID will not cope unless you can change the parameters depending on the quantity of milk - ie change gain settings and Heating element selections after measuring the amount of milk.

The volume of milk could be different for each batch.
Changing the number of heating elements or PID parameters isn't an option unless the farmer is a controls engineer.

I will try to do some tests and check the results.