Free-cooling including modular coolant distribution unit

An integrated, incremental, liquid-liquid free-cooling system accommodating varying equipment rack heat loads in a data center comprising:

• at least a first passive rear door heat exchanger (RDHx) coolant loop interfacing with an RDHx;
• at least a second free cooling source coolant loop interfacing with a free cooling source;
• at least a third chilled water supply (CWS) coolant loop interfacing with a CWS;
• at least one bypass circuit for load control in at least one of said loops;
• a three-way heat exchanger interconnecting said first, second, and third loops, wherein at least one of said first, second, or third loops comprise automatic filling and bleeding, wherein said three-way heat exchanger simultaneously transfers heat between said first, second, and third loops;
• at least a first and a second proportional three-way valve controlling flow in said loops;
• an interlock between said first and second proportional three-way valves;
• at least a first and a second variable speed pump to generate said flow in said loops;
• at least one coolant distribution unit (CDU) providing cooling to said RDHx configured on an electronics rack;
• a control unit controlling said valves;
• and
• at least one of a coolant temperature sensor and air temperature and humidity sensor providing input to said control unit, wherein flow is based on CDU temperature set point and a temperature of said free cooling source and a temperature of said chilled water supply.