I am from beverage industry...my problem is related to the equipment pasteurizer. We have to maintain certain flow rate at specific temperature range (96-99 C) through it.

For that we use PID controller which also co-ordinates the steam valve according to the heat required at that flow rate.

Up to flow rate 410L/min to 420L/min, controller works well but beyond 420L/min of flow rate the pasteurizer is unable to maintain the temperature required, and there is huge fluctuation in temperature.
the pasteurizer is ideally designed for 450L/min of flow rate.

help plzz!!!!!

 

It seems that your process gain, from steam valve position (reference) to beverage temperature, varies significantly between below 420L/min and above 420L/min. You may need to change the PID control parameters, e.g. through gain scheduling, to compensate for the process gain changes.

You do not mention how far open your steam valve is when the beverage flow is above 420L/min. If it is more than approximately 80% open, it would not control very well and you may need a larger valve (but not too large, because that would likely lead to poor control at lower flow rates).