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Specify a rack-level direct liquid cooling system by defining the facility-to-technology loop boundary, then requiring the selected IT, CDU and distribution vendors to document the actual rack heat-removal duty, flow, pressure differential, allowable supply temperature, fluid limits and service requirements. Do not copy generic flow, pressure or temperature values into the design: they depend on the exact equipment, facility supply conditions and heat load.
Define the system boundary and design basis
Start by identifying what the liquid loop cools and what it does not. Direct-to-chip cooling may remove only part of a server’s heat; remaining components and other room loads can still require air cooling. Record both the heat assigned to liquid and the residual air-side load for the selected IT configuration.
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Show the two loops and their interface
A common arrangement uses a cooling distribution unit (CDU) to interface the facility water system (FWS) with the technology cooling system (TCS). The TCS typically includes supply and return distribution, manifolds, server branches, hoses, valves, quick disconnects, sensors and controls. Require a labeled schematic showing those paths, the CDU heat-exchanger boundary, and any bypass or isolation provisions. ASHRAE describes this arrangement and its components in its data-center cooling guidance.
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A CDU may be rack-mounted or external/floor-standing and may serve one rack or multiple racks. Compare the options against served rack count, distribution routing, service access, loop ownership, and the project’s thermal and hydraulic requirements. ASHRAE describes both arrangements but does not identify one as universally preferable.
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Assign ownership for design, operation, maintenance and acceptance of each loop and interface. The FWS is often building-owned, while responsibility for the TCS may involve the CDU vendor, IT provider or engineering firm. State who is responsible for fluid quality and for responding to alarms in each loop.
Specify thermal and hydraulic duty from the chosen IT
Record the rack’s expected liquid heat load and operating configurations, separating normal operation from the design condition. Require the IT supplier to provide flow and pressure-differential requirements for the exact equipment configuration at stated TCS inlet conditions and heat load. Require the CDU and distribution designer to demonstrate that the complete circuit can meet those requirements at the design condition.
ASHRAE notes that required flow and pressure drop vary with equipment configuration, facility supply temperature and rack heat dissipation. A generic flow-per-rack or pressure target is therefore not a defensible substitute for vendor-confirmed values.
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Select a facility liquid-cooling class, then check the IT-side temperature
ASHRAE’s current Handbook chapter identifies W-class facility liquid-cooling classes by maximum facility supply-liquid temperature:
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| ASHRAE class | Maximum facility supply-liquid temperature |
|---|---|
| W1 | 17°C |
| W2 | 27°C |
| W3 | 32°C |
| W4 | 45°C |
| W5 | Above 45°C |
These are facility-class descriptors, not universal rack inlet setpoints. The temperature delivered to IT depends in part on the CDU heat-exchanger approach: account for that temperature difference when checking whether the TCS can meet the selected equipment’s allowable supply temperature. ASHRAE states in Chapter 20. Data Centers and Telecommunication Facilities that the facility owner or designer must account for the planned CDU’s approach temperature to ensure the proper temperature reaches IT equipment. Confirm the applicable handbook edition and the selected hardware’s limits during design.
Keep fluid and water-quality requirements specific to each loop
Specify the fluid and water-quality limits separately for the FWS and TCS, along with monitoring, treatment, maintenance and acceptance responsibilities for each. Because the CDU commonly separates the loops and their owners may differ, a requirement written for one loop should not automatically be applied to the other; ASHRAE warns that misapplying quality requirements can create reliability risks.
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Before selecting hoses, seals, fittings or treatment, obtain the approved coolant composition and material-compatibility requirements from the manufacturers of the chosen servers and CDU. There is no single fluid recipe established for every product. Include the vendors’ limits and instructions in the project documents rather than assuming that nominally similar coolants or materials are interchangeable.
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Evaluate rack liquid temperatures against local room dew point and the operating envelope of the selected equipment. ASHRAE identifies condensation prevention for certain facility classes and says liquid circulating within a liquid-cooled rack should remain above dew point. Specify the project’s prevention method and control sequence using the actual environmental conditions and permitted liquid-temperature range.
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List the required temperature, pressure and flow measurements, where each is taken, and which control functions or alarms use them. Monitoring and control equipment are part of the liquid-cooling system, but alarm thresholds and shutdown or recovery behavior are not universal. Obtain those values and sequences from the selected IT and CDU documentation, and state how the system responds to out-of-range readings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Specify service access, commissioning and handover
Design for isolation and component access
Show how a server or rack can be isolated and accessed, and specify required service clearances. Include quick disconnects where the selected hardware uses them. ASHRAE describes quick disconnects as supporting server or rack disconnection and reconnection while other servers remain in operation. Confirm connector, coolant, pressure, flow and hardware compatibility with the relevant suppliers; do not select fittings by nominal size alone.
Make the supplier’s deliverables and acceptance checks project-specific
Require connection schedules, loop schematics, operating limits, flushing and cleanliness instructions, coolant-fill procedures, maintenance intervals and commissioning criteria. Numerical limits must come from the selected equipment and the project requirements.
At commissioning, verify the installed system at the expected operating point. The project acceptance plan should address delivered flow and pressure, supply and return temperatures, CDU approach, control response, leak-alarm behavior and representative service operations. These checks follow from the equipment-dependent duty and the components in the system; no single acceptance protocol or universal pass value is established for all rack designs.
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