Many liquid cooling issues do not begin with a single piece of equipment. They begin with an unclear system boundary. Who defines supply and return temperatures, maintains pressure, manages fluid quality or integrates controls will affect selection, manufacturing and commissioning.
In a typical liquid-to-liquid architecture, three systems provide a useful common language: the Facility Water System (FWS), the Coolant Distribution Unit (CDU) and the Technology Cooling System (TCS). Names and configurations vary by project, but the interface logic remains useful.
System Boundary
Identify where heat and fluid cross each interface
The FWS brings facility-side cooling capacity to the CDU. The CDU exchanges heat between the loops and controls the TCS, which distributes coolant to manifolds, racks or cold-plate loops.
Facility Water System
Facility supply and return water connected to heat rejection, providing usable temperature, flow and pressure conditions to the CDU primary side.
Coolant Distribution Unit
Separates the FWS and TCS while using heat exchange, pumping, filtration, monitoring and controls to maintain secondary-side conditions.
Technology Cooling System
The IT-facing cooling loop that sends coolant to manifolds, racks and cold plates and returns the captured heat.
IT Equipment Interface
Manifolds, hoses, quick disconnects and cold plates whose pressure, flow and material requirements must align with the TCS.
FWS → CDU primary side | plate heat-exchange boundary | CDU secondary side → TCS → Manifold / Rack / Cold Plate
01 / CDU Scope
What a CDU typically does
A liquid-to-liquid CDU typically separates the TCS from the FWS through a heat exchanger and uses secondary pumps to regulate flow and pressure. Common configurations also include filtration, valves, temperature and pressure sensors, controls and communications.
“Typically” does not mean every CDU is configured the same way. Redundancy, heat-transfer capacity, fluid management, fill strategy, control logic and installation location must be confirmed for the project and manufacturer solution.
Thermal interface
Primary and secondary supply/return temperatures, design conditions, approach temperature and available heat-transfer capacity.
Hydraulic interface
Design flow, available differential pressure, terminal pressure drop, pump control range and bypass logic.
Control interface
Sensor points, alarms, communication protocols, supervisory-system integration and fail states.
02 / Interface Checklist
Seven interface groups to confirm early
Turning interfaces into reviewable parameters, documents and owners early helps reduce selection rework and site changes later.
Temperature and heat load
Design supply/return temperatures, target load, part-load conditions and available facility-side conditions.
Flow and pressure
Loop design flow, allowable working pressure, pressure-drop budget and control stability.
Fluid and materials
Coolant type, water-quality targets, material compatibility, filtration and maintenance sampling.
Redundancy and maintainability
Pump and power redundancy, isolation valves, online maintenance conditions and failure modes.
Leak and condensation control
Leak detection, containment and drainage, plus the relationship between supply temperature and dew point.
Monitoring and communications
BMS or DCIM point lists, protocols, alarm levels, trending and control permissions.
Commissioning and handover
Flushing, filling, venting, pressure testing, FAT/SAT boundaries and final documentation.
03 / Hybrid Cooling
Liquid cooling may not eliminate room air cooling
Not all IT heat is necessarily captured by liquid. Power supplies, storage or other components may still reject heat to the room, so the project must assess the liquid heat-capture ratio and residual air-side load.
CDU and TCS selection therefore cannot be separated from room thermal conditions, airflow organization and facility heat rejection.
04 / Project Handover
Carry the boundary into project documents
Maintain the same boundary information across system schematics, equipment data sheets, interface matrices, controls point lists and commissioning plans. When the design changes, technical parameters, supply scope and delivery milestones should change together.
For multi-supplier projects, a clear interface matrix often reduces coordination risk more effectively than isolated product catalogues.
Our role is to connect equipment, technical information and project milestones
Within an agreed scope, WBG can support equipment supply, technical selection discussions, supplier coordination, factory inspection and delivery follow-up. Project designers, equipment manufacturers and responsible parties must confirm final design parameters, approvals and system performance against project documents.
Source Notes
References
This article is based on publicly available technical material. Project-specific parameters and responsibilities must be confirmed by the project designer, equipment manufacturers and relevant parties.
- Open Compute Project30°C Coolant — A Durable Roadmap for the Future, Rev 1.0↗
- Open Compute ProjectLiquid-to-Liquid CDU Test Methodology and Performance Rating↗
- ASHRAEASHRAE Handbook — Data Centers and Telecommunication Facilities↗
- ASHRAEWater-Cooled Servers: Common Designs, Components, and Processes↗