There is many types of balancing control valve.
You might want to get someone in the field to explain you about this. Suppliers / AC contractor/ Engineers, etc.
But simple answer for your simple question,
What is important for AHU / FCU is the chilled water flowrate required for the cooling.
So one way to control it by control the differential pressure to give correct flowrate required. CHW flowrate will modulate according to the heat load required in the A/C space.
To install at the return side of the CHW pipe (leaving CHW pipe, after the AHU / FCU).
ps:
I have my first post in eng-tips. Yay