Anwer given by 1969grad is not correct for this evaporator. This is a refrigerant evaporator and not a evaporative cooling device. The temperature drop through the unit depends on air flow and refrigerant flow rates. At 80F entering air temp you should be able to get 65F or a little lower...
I know that Marley suggests VFD for cooling towers but can only justify on the basis of savings in mechanical life of gear reducer assembly. Otherwise...I would investigate what the lowest temperature that the chiller can stand on the condenser water side and set an on/off thermostat to cycle...
