The temperature transmitter (TT) senses the distillate temperature exiting the condenser and transmits a corresponding 4-20 ma signal to the unit's PLC-based control system. The PLC distillate temperature logic (TIC) monitors this input signal and determines the error between the actual temperature exiting the condenser and the desired temperature. If the actual temperature is higher than the desired temperature, an output signal from the PLC will open the cooling water control valve to increase the cooling flowrate through the condenser. If the actual temperature is lower than the desired temperature, an output signal from the PLC will close the cooling water control valve to decrease the cooling water flowrate through the condenser. The output signal from the PLC will take into consideration the magnitude of the error, the length of time that the error has existed, and the rate at which the error is changing to provide the most accurate control possible.

The 4-20 ma output signal from the PLC is sent to the current to pressure transducer (TY), which converts the electrical signal to a corresponding 3-15 psig pneumatic signal. The pneumatic signal is sensed by the cooling water control valve positioner. The positioner also senses the position of the valve stem and maintains a valve position corresponding to the signal from the PLC, regardless of line pressure forces acting on the valve.