I am trying to program a Temperature Reset Control in Allen Bradley ControlLogix.

I am want to maintain the outlet temperature of process through a heat exchanger by chaining the flow of coolant through the heat exchanger. The process fluid flow through the exchanger is variable depending upon downstream process demand. To be energy efficient, I am also trying to slow down the coolant chiller if the cooling demand is less for extended period.

Since the process load keeps on varying the chilling load keeps on varying on the coolant circuit, I can not actually create a table listing if current process temperature is X degrees, and the desired temperature is Y degrees, the chiller should run at Z% power.

The control must adapt to process load variations.

 

Hi there,

Think along the lines of a cascaded feed forward control configuration, ie once a flow change in the process is detected you want to increase or decrease the chiller power immediately and long before the actual temperature change is detected on the exchanger outlet, due to this increase or decrease in flow.

So for a start use a flow controller with a certain set point measuring the process flow after the exchanger. The outlet of this flow controller then goes to be the remote set point for the chiller outlet temperature controller.

So if the process flow increases the flow controller will sense that and decrease it's output which in turn should decrease the remote set point on the chiller temperature controller. The chiller will increase power or capacity in order to get to this lower set point therefore the coolant going to the exchanger will get colder to cool the process liquid quicker since the process liquid is now moving at a faster rate through the exchanger. By monitoring the process temperature separately and making adjustments to the speed and set points of the controllers you should be able to maintain a constant process temperature.