Quick answer: liquid cooling has arrived with its own alphabet. Here are the terms you’ll meet in a specification or design meeting, in plain English — from CDU to PG25 — with notes on what each means for the pipework.
CDU — Coolant Distribution Unit
The interface between the facility’s cooling plant and the IT hardware. Contains pumps, a plate heat exchanger, filtration and controls; conditions and circulates coolant on the technology side while keeping it hydraulically separate from the facility side. Sized from in-rack units (tens of kW) to row-scale megawatt-class machines. Piping angle: CDUs are dense stainless fabrications inside, and every one needs isolation valves and demountable connections outside.
TCS — Technology Cooling System
The “clean” loop between CDUs and the IT hardware, carrying conditioned coolant. Purity-critical, because it feeds cold-plate microchannels. Piping angle: this is where hygienic-grade stainless tube earns its keep.
FWS — Facility Water System
The building-scale loop connecting cooling plant (chillers, dry coolers, cooling towers, heat-reuse tie-ins) to the white space. Larger bores, conventional building-services engineering.
DTC / D2C — Direct-to-Chip cooling
Coolant circulated through cold plates mounted directly on processors. Single-phase (water/glycol stays liquid) is the dominant design; two-phase systems boil a dielectric fluid at the plate.
Cold plate
The liquid-cooled heat sink bolted to a CPU/GPU, internally channelled at sub-millimetre scale. The component everything upstream must keep clean.
Rack manifold
Vertical supply/return headers in the rack distributing coolant to individual servers, usually via quick disconnects (QDs).
RDHx — Rear Door Heat Exchanger
A liquid-cooled coil replacing the rack’s rear door, capturing exhaust heat before it enters the hall. A bridge between air and full liquid cooling.
Immersion cooling
Servers submerged in dielectric fluid in tanks; heat is rejected to a water loop via tank-side heat exchangers.
PG25
A nominal 25% propylene glycol / 75% water coolant with an inhibitor package — the common working fluid for single-phase DTC. More: our PG25 compatibility guide.
PUE — Power Usage Effectiveness
Total facility power ÷ IT power. Liquid cooling improves it by cutting fan and chiller energy; warm-water designs improve it further.
Warm-water cooling
Running TCS supply at 30–45 °C so heat can be rejected with dry coolers (no compressors) or reused — increasingly the default for AI facilities.
Ra / surface finish
The measured internal roughness of tube, in micrometres. Lower Ra = fewer sites for particles, corrosion and biofilm. Specified via standards like ASME BPE. Full explanation: why surface finish matters.
EN 10204 3.1
The material certificate type your commissioning pack wants: batch-specific test results from the manufacturer, traceable to the heat.
Specifying pipework against any of these? Start at our liquid cooling infrastructure for data centres page.
Talk to us about your project
Kevin Windsor — Product Manager, Data Centre Liquid Cooling
kw@neumo.co.uk · 01952 583 999