Quick answer
A coolant distribution unit (CDU) sits between the building’s water and the IT equipment. It pumps a clean, controlled coolant (often PG25 or treated water) through the technology cooling loop to the cold plates, and passes the heat to the facility water loop (liquid-to-liquid) or to room air (liquid-to-air), keeping the two loops separate.
Key takeaways
- A CDU separates the facility water system (FWS) from the technology cooling system (TCS).
- Liquid-to-liquid CDUs give the most cooling capacity; liquid-to-air CDUs avoid facility water piping.
- The CDU controls flow, pressure, temperature and filtration of the coolant that reaches the chips.
General information. Confirm the exact fluid, equipment and qualification with your hardware or CDU supplier before specifying.
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What a CDU does
In direct-to-chip cooling, coolant flows through cold plates mounted on GPUs and CPUs. That coolant must be clean, at the right temperature and pressure, and chemically controlled. Building chilled or condenser water is not suitable to send straight to servers, so a CDU creates a separate, closed loop for the IT equipment. Inside the CDU, a heat exchanger moves heat from that loop to the facility side, while pumps, filters, sensors and controls keep the IT loop within its limits.
Types of CDU
| Type | How heat leaves | Typical use |
|---|---|---|
| Liquid-to-liquid (L2L) | To facility water through a heat exchanger | High-density AI and HPC rows; needs facility water piping |
| Liquid-to-air (L2A) | To room air through a coil and fans | Adding liquid-cooled racks to an air-cooled room without facility water |
| In-rack | Unit sits inside the rack | One rack, smaller loads |
| In-row / row-based | Floor-standing unit serving several racks | Rows of high-density racks |
| Centralised / facility-scale | Large units in a plant room | Multi-megawatt deployments |
Vertiv notes that liquid-to-air CDUs are quicker and cheaper to install and take less space, while liquid-to-liquid units give the best cooling performance but need pipes and pumps to the facility water.
Sizing and redundancy
- Capacity is quoted in kW or MW of heat removed, at a stated facility water temperature and approach. Check the conditions behind any headline number.
- Approach temperature: the IT-loop supply runs some degrees above the facility water supply, so the facility water class (for example ASHRAE W32 or W45) affects what the CDU can deliver. See facility water vs technology cooling loop.
- Redundancy: many units offer N+1 pumps; critical sites may add redundant CDUs.
- Flow and pressure must match the cold plates and manifolds specified by the server maker.
The coolant inside the CDU loop
Most direct-to-chip loops use an inhibited propylene glycol mix such as PG25 or a treated water fluid specified by the hardware maker. The CDU usually provides filtration, a reservoir or expansion tank, and ports for sampling and top-up. Keeping the fluid in specification is an operational task: see CDU coolant maintenance.
Questions to ask a CDU supplier
- Which coolants are approved for the unit and the cold plates, and who must supply them?
- Rated capacity at your facility water temperature, and the resulting IT-loop supply temperature.
- Pump redundancy, filtration level, leak detection and controls integration.
- Commissioning, fluid fill and flushing procedure, and who is responsible for fluid testing.
- Service coverage and spare parts in your region.
See the CDU suppliers list for manufacturers.
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Get the checklist →Frequently asked questions
What does CDU stand for in data centers?
Coolant distribution unit (sometimes cooling distribution unit): the equipment that circulates and conditions coolant for liquid-cooled IT equipment.
Do I need a CDU for direct-to-chip cooling?
In almost all cases, yes. The CDU separates the IT coolant loop from facility water and controls flow, temperature and filtration.
What coolant goes in a CDU?
Usually an inhibited propylene glycol mix such as PG25 or a treated water fluid approved by the server and CDU makers.
How we source this page
- Facts come from manufacturer pages, datasheets, owner's manuals and standards bodies, linked under Sources.
- We do not test products, and no brand pays to be listed or ranked. Advertising and affiliate links are labelled and never change the data.
- Anything we could not find is marked "not published" rather than guessed. Spotted an error? Tell us.