When choosing a data center liquid-cooling system, start with the servers and workload it must support, then verify that the cooling architecture fits the rack, facility loop, heat-rejection plant and service model. Compare equipment-specific temperature, flow and pressure requirements—not generic capacity claims—and assess how the system will be monitored, isolated and maintained. Liquid cooling is a connected IT-side and facility-side design, not a standalone piece of rack equipment.
What to look for in a data center liquid-cooling system
First map the equipment to be cooled: server and accelerator models, expected load, rack density, supported cooling interfaces and equipment-warranty boundaries. Then evaluate how each proposal captures heat, transfers it to the facility, handles failures and service, and fits the site’s heat-rejection options. A cooling system that meets a headline capacity figure may still be unsuitable if its operating envelope or connections do not match the IT equipment and facility.
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In a common liquid-cooled design, a coolant distribution unit (CDU) transfers heat between facility water and a technology cooling system (TCS) connected to the IT equipment. Depending on the design, the CDU may also circulate and control the technology-side coolant and keep the two water systems separate. ASHRAE’s Water-Cooled Servers: Common Designs, Components, and Processes and Chapter 20, “Data Centers and Telecommunication Facilities,” describe these roles and system arrangements.
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Which cooling architecture fits the workload?
Architectures differ in what they cool directly and how they connect to facility infrastructure. The appropriate choice depends on the supported equipment, the amount of heat remaining in the room, and the service and facility constraints—not on a universal ranking.
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| Architecture | How it captures heat | What to evaluate |
|---|---|---|
| CDU-mediated liquid cooling | A facility-side loop exchanges heat with a technology-side loop, which connects to the IT equipment. The CDU can circulate and control the technology-side coolant. | Verify both loops’ fluid and material requirements, CDU capacity and approach temperature at the specified conditions, and the equipment’s supply-temperature, flow and pressure-drop limits. ASHRAE Chapter 20 and its water-cooled server guidance describe this arrangement. |
| Direct-to-chip or direct-component cooling | Coolant is delivered to equipment and commonly to components through cold plates. | Confirm which components the design cools and which continue to reject heat to room air. Check server compatibility, connections, service access and the remaining room cooling requirement. ASHRAE Chapter 20 describes direct component cooling. |
| Hybrid cooling, including rear-door heat exchangers | Liquid cooling removes some heat at or near the rack while room air continues to handle the remainder. | Require a design-specific estimate of remaining room heat, airflow, water connections, capacity and service access. ASHRAE’s AI Data Center Energy Performance Framework describes rear-door heat exchangers as a hybrid option; it does not establish a universal retrofit result. |
| Immersion | IT components contact dielectric liquid directly. In single-phase systems the fluid stays liquid; in two-phase systems it evaporates and is condensed back. Systems can use enclosed chassis or open baths. | Check the exact server and component compatibility, fluid and materials compatibility, containment, fluid-level management, heat-exchanger or CDU interface, service procedures and territorial certification. These configurations are not interchangeable. OCP’s Immersion Requirements Rev. 2.0 defines the types and says equipment must comply with compulsory certification regulations where it is deployed. |
For immersion in particular, do not assume that standard servers or arbitrary combinations of tank, fluid and components are approved. Require the vendor to identify the supported hardware and applicable certification for the installation location.
How to verify the thermal and hydraulic match
Obtain the requirements for the exact IT configuration and design load, rather than copying a generic temperature or capacity into a specification. ASHRAE notes that flow and pressure requirements vary with manufacturer configuration, facility-water temperature and the heat dissipated to water.
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- IT-side operating envelope: Request the equipment’s allowable and recommended coolant supply-temperature range, flow rate and pressure drop. Confirm the limits for each server configuration in the proposal.
- CDU performance: Require capacity and approach temperature at the stated inlet conditions, flow rates and load. The approach temperature affects the relationship between facility-side and technology-side temperatures.
- Return conditions: Document expected return temperatures and the assumptions behind them at design load. Ensure the facility-side design is checked against those assumptions.
- Distribution: For high-density racks or complex piping, ask for documented flow-network analysis covering pressure drops, temperature rise and distribution through the facility and racks. ASHRAE Chapter 20 describes flow-network modeling for analyzing these interactions.
- Condensation controls: Specify the intended relationship between coolant temperature and room dew point, and how controls respond to changes in humidity or room conditions. ASHRAE flags condensation prevention for relevant water classes and describes CDU temperature regulation above room dew point.
Ask vendors to state the operating conditions behind every quoted capacity. A figure without its inlet temperatures, flow, pressure and load assumptions is not enough to establish that the system will meet the IT requirement.
What to check in coolant, materials and loop design
Coolant choice affects compatibility, heat transfer and ongoing maintenance. ASHRAE Chapter 20 cautions: “The selection of a coolant is a critical task, as one must investigate wetted material compatibility and equipment serviceability, as well as liquid maintenance and operational needs.” Treat the fluid specification as part of the equipment design, not as a consumable detail to decide later.
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- Request the approved fluid specification, wetted-material list and water-chemistry limits for each loop. Confirm that every component exposed to the fluid is within the approved compatibility envelope.
- Define sampling, treatment and filtration requirements, including the schedule and the party responsible for carrying them out.
- Document the fill, flush, replacement and disposal processes, along with commissioning ownership.
- Ask how coolant quality is monitored and handled if it changes over time. Additives can affect heat transfer and may change maintenance needs.
- For systems with separate facility and technology loops, record the fluid requirements on both sides and how the boundary is maintained.
How to assess facility fit and heat rejection
The liquid system must ultimately reject heat through the site’s available infrastructure. Compare the proposed connection to chillers, cooling towers, dry coolers or other heat-rejection equipment against local climate, water constraints, available footprint, required temperatures and any heat-reuse plan. Include the power and space needed for the full arrangement, not only the rack-side equipment.
Warm-water loops and dry coolers can support reduced chiller use in suitable conditions, but the outcome depends on the actual design and site. ASHRAE’s AI Data Center Energy Performance Framework discusses high-temperature loops and dry-cooler use conditionally, including footprint and high-ambient safeguards. Do not treat generalized savings or water-use claims as guaranteed project results; use site-specific models or measured operating data.
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Confirm applicable local codes and mandatory certifications for the installation territory. For immersion equipment, OCP’s Immersion Requirements Rev. 2.0 explicitly requires compliance with compulsory certification regulations in the deployment location.
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A liquid system is only maintainable if operators can detect abnormal conditions, isolate equipment and perform service without undermining the required cooling availability. Define the failure and maintenance scenarios the design must tolerate, then ask vendors to explain the response and remaining cooling capacity for each one.
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- Redundancy and ride-through: Specify the required redundancy, what load stays cooled during a failure or planned service, and any ride-through assumptions. Determine whether pump power backup is needed for the design and how spares will be managed.
- Isolation and replacement: Identify how major components can be isolated and replaced, and whether service can be performed without taking the system below its design reliability level. ASHRAE Chapter 20 recommends considering isolation and replacement of major components.
- Monitoring: Include temperature, pressure, flow and leak detection in the instrumentation plan. Establish how alarms and signals reach facility monitoring and which controls coordinate the IT-side and facility-side loops. These instrumentation and control areas are covered in ASHRAE’s AI Data Center Energy Performance Framework.
- Routine maintenance: Obtain procedures and intervals for valve exercise, filter or strainer cleaning, inspection and other required work. ASHRAE Chapter 20 describes valve exercise and filter or strainer cleaning as maintenance practices.
- Service responsibilities: Assign responsibility for commissioning, loop flushing, filling, sampling, routine treatment, fault response and return-to-service checks. Make sure the required technicians, procedures and consumables will be available at the site.
How to compare vendor proposals
Use a common request-for-proposal schedule so vendors answer against the same IT configuration, design load, facility conditions and service assumptions. Ask each bidder to supply evidence and assumptions, not just a product description.
- IT compatibility: List supported server and accelerator configurations, rack density, liquid connections and warranty boundaries. Identify which components are cooled and any remaining room-air load.
- Thermal envelope: Record supply and return temperatures, flow, pressure drop, heat-transfer capacity and CDU approach at the conditions specified for the project.
- Loop and fluid design: Document facility/TCS separation, coolant chemistry, wetted materials, connection scheme and provision for future expansion.
- Reliability and controls: Compare redundancy, failure behavior, ride-through assumptions, leak detection, alarms, monitoring integration and maintenance isolation.
- Service model: Specify access to IT equipment, technician procedures, filters and other consumables, response support and commissioning ownership.
- Facility fit: Compare heat-rejection method, climate assumptions, water constraints, footprint, power needs and any heat-reuse design.
- Lifecycle economics: Compare project-specific capital, operating, maintenance and downtime assumptions. The cited ASHRAE and OCP sources establish no universal cost or savings figure.
Before selection, require a written design basis tying the proposed system to the exact IT equipment and site conditions. If a bidder cannot state the applicable limits, assumptions, service plan and failure behavior for its proposal, the offer is not yet comparable with one that can.
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