Liquid cooling from sources such as surface or potable city water can be used to remove heat from many processes. When the source water is too cold or contaminated to be directly circulated to certain devices or vessels, a water-to-water heat exchanger can be used. Series PWW water-to-water heat exchangers have two isolated water circuits: one on the supply side and one on the process side. The demand-side ciruit includes a coolant reservoir and a circulating pump. The process-side circuit includes an electronically actuated ball valve for modulating coolant flow rate in order to maintain the desired process or equipment temperature. Options include supply-side filters for dirty water and high pressure pumps for long process-side pipe runs.