Temperature Ramping

Hi
I have to raise the temperature over a fixed time (plant steam in coil) ...and then hold it at temperature and the cool (cooling water in same coil)

So it is something like...Heat to 80C over 25 minutes, hold at 80+/-1 then cool to 30C over 1 hour

My question is the ramping... I am thinking of simply calculating the PID Setpoint over time, so it ramps up..The PLC is a Siemens S7-300 type and I usually use FB41 for PID control

For the cooling I am thinking of opening the valve slightly for the first x seconds because I am concerned the process may damage the hot coil

Cheers

Opening the the cooling valve slightly may be all you need to cool down at the desired rate so use a limit on how much the cooling valve can be open at first.

Unless you need to cool below ambient or need to cool quickly you may not need to use cooling water. 30C sounds like it is above ambient and you are ramping down very slowly. You may want to try ramping down without using cooling water to see if the plant will cool down fast enough. If not then you should probably limit how much the cooling valve can be opened until you get an idea how open the cooling water valve must be to achieve the ramp down rate.

Take start temp (as measured).
Take final temp (user SP)
Take time (user SP)
Convert time to cycles (FB 41 in OB35 @ 100ms would be 10 cycles a second)
(final-start) / cycles gives you ramp per cycle
Increase SP every cycle.
Same for cooling, just the other way.
you might want to make a controller that can go negative, you could then use the negative value to open cooling (if needed)