Coolant distribution unit (CDU)

A coolant distribution unit, or CDU, is a pump and heat exchanger that moves heat from a liquid-cooled rack's coolant into facility water, keeping the fluids separate. ASHRAE calls the server-side loop the technology cooling system, or TCS, and the building-side loop the facility water system, or FWS. CDUs come in in-rack, in-row and perimeter designs. Liquid-to-liquid units hand heat to facility water, while liquid-to-air units reject it into room air and need no facility water.

Approach temperature is the gap between the coolant a CDU supplies to the servers and the facility water entering it. ASHRAE Standard 127, amended in February 2026, rates liquid-to-liquid CDUs with 26 °C facility water and 30 °C coolant to the servers, a 4 °C approach. Vertiv rates its liquid-to-liquid CoolChip CDUs at a 4 °C approach, including a 121 kW in-rack unit and in-row units of 600 kW and 1,350 kW. Its 70 kW liquid-to-air model is rated at an 11 °C approach. A smaller approach lets facility water run closer to the coolant temperature the servers need.

Compare CDU ratings only at the same approach temperature. Ask whether the filter was installed during rating, since filter pressure loss lowers the pump head left to push coolant through the racks.

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Last verified October 7, 2026.