From the facility water interface to the chip: six boundaries
Heat travels one way. Responsibility usually stops at each vendor's flange. This is the path as we draw it, with the maker of each boundary and the question we ask of it.
Definition
The whole thermal path (thermal path · 整条热通路) is the route heat takes in a liquid-cooled data centre, drawn as six boundaries numbered from the facility inward: B1 facility water interface, B2 heat rejection and reuse, B3 CDU and secondary loop, B4 rack manifold and quick disconnects, B5 hoses and fittings, B6 cold plate and chip. Each boundary has a maker, a test, an acceptance criterion and a record owner.
Boundaries are numbered from the facility inward, the direction a commissioning team walks and the direction water chemistry travels. Each entry names the party that usually makes the boundary and the one question its record should answer.
The list deliberately stops at six for the design conversation. For leak risk specifically we break the same path into ten leak points.
Facility water interface
Usually made by: Facility mechanical contractor
Where the building hands water to the cooling system. Supply and return temperature, flow, pressure and water chemistry are set here, and so is the metering boundary that any energy-reuse figure will be judged against.
→ Metered flow and both temperatures on this boundary, from day one?
Heat rejection and reuse
Usually made by: Dry cooler / tower / heat-pump vendor, or the heat buyer
Where the heat leaves the site: to air, to evaporation, or to a buyer. The return-water grade of the loop decides which of these are physically available. This is also where PUE, WUE and ERF stop agreeing with each other.
→ Which resource is actually being consumed, and on whose meter?
CDU and secondary loop
Usually made by: CDU manufacturer, integrator
The heat exchanger between facility water and the technology-cooling loop. Its approach temperature sets how warm the secondary loop must run for a given facility supply, and therefore how many free-cooling hours the site gets. Pump seals, gaskets and filter housings are enclosed leak boundaries.
→ Approach at rated load, with fouling allowance, recorded at commissioning?
Rack manifold and quick disconnects
Usually made by: Rack integrator, QD supplier
The highest count of removable joints in the system, and the ones disturbed every time a server is replaced. A disconnected QD is not automatically a sealed one. First-fill cleanliness is leak prevention, not housekeeping.
→ Leak test and cleanliness record after every disturbance, not just at handover?
Hoses, crimps and fittings
Usually made by: Hose assembler, rack integrator
A crimp is a controlled manufacturing process, not an assembly step. Once the sleeve is on, most defects are invisible. Elastomer selection decides whether a dry joint stays dry in year four.
→ Crimp process record and elastomer specification, not "compatible with water"?
Cold plate and chip
Usually made by: Cold-plate manufacturer, server OEM
Brazed seams, friction-stir welds, covers and ports, sitting directly upstream of the smallest flow passages in the system. Thermal performance here depends on everything upstream: flow, inlet temperature and fluid cleanliness.
→ Factory pressure-test record traceable to this serial number?
What the inner boundaries will cool next
Boundaries four to six were drawn around compute trays: manifolds and quick disconnects feeding servers, hoses into them, cold plates on processors. Five presentations at the OCP APAC Summit 2026 extended liquid cooling beyond compute trays. The boundaries stay the same; the rack manifold, the hoses and the cold plates will increasingly serve every component in the rack that connects to the technology-cooling loop, and each one brings its own joints, materials and test records.
- Optics. Front-panel pluggables have reached their thermal density limit; XPO is a liquid-cooled pluggable form factor with an integrated cold plate. Arista, XPO
- Switches. BMC management for liquid-cooled switches is now a SONiC work item. Microsoft and Nexthop AI, SONiC BMC for Liquid-Cooled Switches
- Storage. Liquid cooling for storage is expected from PCIe Gen7. Marvell, Silicon-Driven Storage
- Busbars. Air-cooled busbars scale poorly in size, weight and footprint as current rises; liquid-cooled busbars are the proposed answer. Microsoft, Busbar Cooling Challenges
- Power connectors and cables. Actively cooled power cables and connectors have been tested on a standard ORv3 busbar and power shelf. Molex, Actively-Cooled Power Cables
Sources are presentations at the OCP APAC Summit 2026. They describe directions and early tests, not shipping volumes.
Why the record has to travel with the heat
The point is not that factory joints never leak. It is that a boundary made under controlled conditions can be tested, recorded and released against defined acceptance criteria. A boundary made in a live hall at two in the morning deserves exactly the same evidence. When the records are held by six different parties in six different formats, the operator inherits a system nobody can defend in public.
Our job is to hold the line, within a scope that is written down. We design across the six boundaries, we specify what each one has to prove, and we assemble the records into one turnover package that follows the system into service.
Questions this page answers
How do you choose between cold-plate and immersion liquid cooling?
Start from the chip's stated inlet limit and the return-water grade you want to sell, then from the serviceability the operator can live with. Cold plates keep servers air-serviceable and return roughly 45–60 °C under OEM inlet limits; single-phase immersion removes the fan and hose-crimp leak points but changes every service procedure; two-phase can return above 60 °C but ties the site to the fluid's saturation temperature. The choice is a system decision across all six boundaries, not a rack-level one. Solutions by deployment type →
Where is reused heat measured?
At the data-centre boundary where the heat is handed off for use outside, not at the chip and not from the IT load. Commission Delegated Regulation (EU) 2024/1364 requires this for EU data centres with at least 500 kW installed IT demand; outside the EU it is still the only boundary a heat buyer will pay against. Return-water grade →
Who is responsible for each boundary of a liquid-cooling system?
Each of the six boundaries is usually made by a different party: the facility mechanical contractor, the heat-rejection or heat-buyer side, the CDU maker, the rack integrator and quick-disconnect supplier, the hose assembler, and the cold-plate maker or server OEM. Responsibility stops at each vendor's flange unless the contract names one party for the whole path and one turnover package for all ten leak points. The turnover package →