Quick answer
Quick definitions of the terms used in data-center liquid cooling, each linked to a fuller guide.
Key takeaways
- CDU, TCS and FWS describe the equipment and loops.
- PG25 and dielectric fluids are the two main coolant families.
- Each term links to a full guide.
General information. Confirm the exact fluid, equipment and qualification with your hardware or CDU supplier before specifying.
Terms A–Z
- Approach temperature
- Temperature difference across a heat exchanger between the two fluids. More →
- Biofilm
- Microbial growth on loop surfaces that can clog cold plates; controlled with biocides and cleanliness. More →
- CDU
- Coolant distribution unit: pumps and conditions the IT coolant loop and exchanges heat with facility water or air. More →
- Cold plate
- Metal block with internal channels mounted on a chip; coolant flowing through it removes heat. More →
- Dielectric fluid
- Electrically non-conductive fluid used to submerge hardware in immersion cooling. More →
- Direct-to-chip (DLC)
- Liquid cooling that brings coolant to cold plates on the hottest components. More →
- FWS
- Facility water system: the building loop that carries heat to chillers, dry coolers or towers. More →
- Inhibitor
- Additive that protects loop metals from corrosion; it depletes over time and must be tested. More →
- Manifold
- Rack-level pipework distributing coolant to each server. More →
- OCP
- Open Compute Project, which publishes liquid cooling and PG fluid guidelines. More →
- PFAS
- Per- and polyfluoroalkyl substances, a regulated chemical family that includes many fluorinated immersion fluids. More →
- PG25
- Inhibited propylene glycol at about 25% in water, common in direct-to-chip loops. More →
- Quick disconnect
- Dripless connector that lets a server be removed from a liquid loop. More →
- RDHx
- Rear-door heat exchanger: a water-cooled coil on the back of a rack. More →
- Single-phase immersion
- Immersion cooling where the dielectric fluid stays liquid and is pumped to a heat exchanger. More →
- TCS
- Technology cooling system: the closed loop from the CDU to the IT equipment. More →
- Two-phase immersion
- Immersion cooling where the fluid boils on the chips and condenses on a coil. More →
- W classes
- ASHRAE facility water classes named by maximum supply temperature (W17–W45, W+). More →