Does anyone know what Ammonia Flow Control Units do ?

Flow control is accomplished normally with a PID loop that opens a valve based on the process variable. Example: Process variable is 50 degrees F, that's the current temperature of the item you want to cool. The CV or control variable is the temperature you want it to be or known as set-point The PID loop then opens or closes the valve to let more coolant or ammonia through. PID stands for Proportional, Integral and derivative. It is a controller that reads the PV, CV and makes adjustments based in time to a descrete device or a valve in this case. Adjustments to the PID can be made to speed up or slow down the changes in PV. Abrupt oscillations are very bad in a PID loop and need to be tuned out.