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There is no universal coolant flow rate or inlet temperature for an AI server. Use the server and cold-plate manufacturer’s operating envelope first, then size flow from the actual heat load, coolant properties, and chosen temperature rise. Finally, verify that the CDU, pump, heat exchanger, manifolds, hoses, fittings, controls, and facility heat-rejection system all operate within their limits at the same design condition.
A practical design therefore starts with four linked decisions: how much heat the loop must remove, how many degrees the coolant may rise, which coolant is approved, and how much thermal and dew-point margin the facility requires.
Start with the server’s approved operating envelope
Before selecting a pump setpoint or CDU, obtain the limits for the exact server, cold plate, and deployment configuration. Ask the supplier for:
- Approved coolant chemistry, concentration, additives, and materials compatibility.
- Permitted coolant inlet-temperature range at each relevant workload.
- Required minimum and maximum flow, including whether the value applies per cold plate, server, chassis, or rack.
- Allowable pressure, pressure drop, and differential pressure across the equipment.
- Maximum return temperature for the cold plate, server, hoses, quick disconnects, manifold, and other wetted parts.
- Filtration, particle-size, cleanliness, and flushing requirements.
- Flow, temperature, and alarm behavior during startup, shutdown, partial load, and loss-of-flow events.
The tightest applicable limit governs the loop. A cold plate may tolerate a higher return temperature than a quick disconnect, or a pump may provide enough flow but exceed a component’s pressure rating. Treat the complete path as one thermal-hydraulic system rather than choosing a number from a generic “AI server” table.
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LBNL’s open-rack specification makes the same design point: match each loop’s flow to the IT requirement at the manifold supply temperature, check return temperature against every loop component, and compare CDU capacity and heat-exchanger approach temperature with the loop’s power and flow. Its values are design guidance for that specification, not universal code requirements.
Calculate flow from heat load and allowed temperature rise
Flow and temperature rise are coupled. For a loop removing a heat load Q, the required mass flow is:
mass flow = Q ÷ (specific heat capacity × coolant temperature rise)
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Volumetric flow is the mass-flow result divided by the coolant density. In practice, use the manufacturer’s property data at the expected operating temperature and concentration. Density, heat capacity, viscosity, and pressure drop all change with fluid composition and temperature, so a flow figure without its coolant basis is incomplete.
What a larger or smaller temperature rise changes
- Larger rise: less flow and potentially lower pump energy, but a hotter return, less thermal margin at the cold plate, and greater stress on components with lower return-temperature limits.
- Smaller rise: more flow and usually more pumping power, but a cooler return and more cooling capability at the server.
- Higher coolant temperature: can improve chiller efficiency or enable dry-cooler operation, but reduces the temperature difference available between the coolant and the silicon.
- Lower coolant temperature: increases cooling capability, but may increase facility energy use and condensation risk.
Do the calculation at maximum expected IT load, not only at an average utilization level. Include sustained accelerator power, CPU and memory heat, network equipment, storage, and any design allowance for workload transients.
Illustrative calculation using the OCP PG25 basis
Open Compute Project’s 2023 OAI System Liquid Cooling Guidelines give a typical target of a 10°C coolant rise, stated as 1.5 L/min per kW for the specified PG25-based coolant. On that stated basis, a 100 kW loop would start at approximately 150 L/min before accounting for the supplier’s exact fluid properties, distribution losses, control margin, and equipment limits. This is an arithmetic example, not a universal sizing rule. Recalculate it when the fluid, temperature, heat load, or platform changes.
Use OCP’s OAM figures only as platform-specific examples
The OAI/OAM guidance published by Open Compute Project in 2023 provides useful reference points, but it does not define every liquid-cooled AI server:
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| OCP example | Published figure | Qualification |
|---|---|---|
| Flow per OAM cold plate | 1–2 L/min | Applies to the OAM guidance and its specified platform context. |
| Chassis flow | 4–16 L/min | Depends on loop-delivery arrangement and OAM power. |
| Recommended coolant temperature rise | 7.5–12°C | Presented for the stated PG25-based coolant context. |
| Equivalent flow range | 1.25–2.0 L/min/kW | Same PG25 basis; do not transfer to another coolant without recalculation. |
| Typical design target | 10°C rise, or 1.5 L/min/kW | OCP’s stated target for that guidance, not a general AI-server requirement. |
OCP also notes a system trade-off: a higher temperature rise can improve facility efficiency and PUE, while a lower rise can preserve cooling performance at the cost of more flow. Confirm whether the server supplier expects flow per cold plate, per board, per chassis, or per rack before translating any figure into a CDU setpoint.
Choose the inlet temperature as a facility-and-server trade-off
Choose the highest supply temperature that still meets the server’s thermal limits with adequate margin and keeps the facility’s heat-rejection equipment within its design envelope. Higher temperatures can reduce or eliminate mechanical chilling in favorable climates; lower temperatures provide more thermal headroom but usually demand more refrigeration or evaporative cooling.
| Observed supply-temperature grouping | Primary objective | Engineering trade-off |
|---|---|---|
| 40–45°C | Emphasize efficiency and environmental goals | Less cooling capability at the cold plate; requires servers, controls, and heat rejection designed for warm water. |
| 30–37°C | Balance cooling capability and efficiency | Moderate facility energy demand and thermal margin; still must satisfy the specific server envelope. |
| 15–25°C | Maximize cooling capability | More reliance on chillers or evaporative cooling and greater condensation-control burden. |
These groupings are approaches observed among hyperscale users in OCP’s 2023 guidance, not prescribed setpoints. The correct value depends on server thermal capability, ambient conditions, heat-rejection equipment, desired transient margin, and the room dew point.
Interpret the TCS upper and lower bounds correctly
ASHRAE defines the technology cooling system (TCS) upper bound as “The maximum TCS water supply temperature that can be sustained at a given flow rate to provide sufficient cooling capability to the attached ITE.” Above that point, additional flow may be needed or IT equipment may exceed its design temperature. Therefore, increasing flow and increasing supply temperature cannot be evaluated independently.
The lower bound is set partly by condensation. A supply temperature that is safe in a dry room may be unsafe after a humidity excursion or in a different season. The CDU should maintain secondary supply at least 2°C above room dew point as a buffer against condensation and sensor uncertainty, consistent with the ASHRAE white paper’s guidance. Confirm the exact control setpoint, sensor location, and margin with the server and CDU manufacturers.
Do not generalize warm-water reference architectures
ASHRAE’s AI data-center framework describes a reference architecture with facility inlet water up to 45°C and rack return water up to 65°C, with dry coolers as one possible heat-rejection method. Those values describe an architecture, not an operating promise for every AI server, climate, or component. The framework also notes disagreement about the practical use and long-term durability of warm-water cooling. Use such figures to frame feasibility discussions, then apply the actual server, CDU, and facility limits.
Protect against condensation before lowering supply temperature
Condensation is controlled by comparing coolant temperature with the room’s dew point, not by increasing flow. More flow can remove heat, but it does not prevent a cold surface from falling below the surrounding air’s dew point.
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- Short press the button to change the mode: Flow Mode: real-time water flow rate; Consumption Mode: last-time water usage; Average Mode: daily average water usage of a week (count from next day); Total Mode: total water usage.
- Long press the button at flow mode to switch the unit (US gallon to liter). Reset data when long press the button at other modes. The LCD screen goes to sleep automatically if there is no operation in 1 minute.
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- Measure room temperature and relative humidity at representative locations near the rack, not only at a remote building sensor.
- Calculate or obtain the room dew point from those measurements.
- Set the CDU’s secondary-supply lower limit at least 2°C above that dew point, unless the equipment manufacturer specifies a larger margin.
- Place sensors where the coldest credible surface and the actual supply water are represented.
- Define a response for sensor failure, rising humidity, loss of airflow, and chilled-water control faults.
- Verify that insulation, drip management, leak detection, and shutdown interlocks remain effective during a transition to a warmer or colder operating mode.
Test the control sequence during commissioning. A system that is safe at steady state can still condense during startup if cold facility water enters a warm, humid room before the CDU establishes its secondary setpoint.
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A CDU normally separates facility water from the secondary TCS loop, transfers heat through a heat exchanger, circulates the approved coolant, and controls temperature. Separating the two water-quality domains helps protect IT equipment from less-controlled facility water, but it adds heat-exchanger approach temperature, pump, control, and filtration requirements.
Check the heat exchanger at the design point
- Calculate IT heat load and secondary-loop flow at the selected supply and return temperatures.
- Verify CDU heat-exchanger capacity at the actual facility-water temperature, not a nameplate condition that uses colder water or a different flow.
- Include the heat-exchanger approach temperature in the available thermal margin.
- Confirm facility-side flow, pressure, water chemistry, and maximum allowable return temperature.
- Check performance during the hottest ambient condition and with the expected fouling or maintenance allowance.
Check pump, branches, and fittings
Compare the CDU pump curve with total secondary-loop pressure drop at the required flow. Include cold plates, rack manifolds, hoses, quick disconnects, filters, control valves, elevation changes, and any redundant path. A pump that reaches the nominal flow on a short test loop may not reach it after all branches are connected.
Parallel branches must be balanced. A low-resistance branch can take excess flow while a restrictive branch starves, even when the rack’s aggregate flow looks correct. Use balancing valves or engineered restrictors where required, and measure each branch during commissioning.
Apply LBNL manifold guidance as guidance, not law
LBNL’s open-rack specification gives several useful design checks:
- Keep manifold velocity below 1.0 m/s.
- Keep supply-and-return manifold pressure drop below 1/40 of the individual IT-loop pressure drop.
- Limit pressure-drop variation between loops to 10% or provide balancing valves.
- Compare the total assembly pressure drop with CDU pump capability.
The same document describes secondary-fluid temperatures from 2°C to 60°C or more as possible across different configurations, while warning that higher temperatures require compatibility checks. It gives secondary-loop operating pressure up to 60 psi as typical for its context. Neither range is a recommended operating point for a particular server; the lowest component maximum temperature and the lowest pressure rating remain decisive.
Verify coolant chemistry, materials, and cleanliness
Use only the fluid approved for the cold plate and the rest of the loop. A fluid that is thermally suitable can still attack elastomers, solder, coatings, aluminum, copper, stainless steel, or pump seals. Confirm concentration, corrosion inhibitors, biocide policy, conductivity requirements, freeze protection, and replacement intervals.
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Define ownership for filling, flushing, filtration, sampling, and contamination response. Filters can protect cold plates but also add pressure drop and require service. Establish differential-pressure alarms and a maintenance procedure before the rack is energized.
Commission with measurements, not nameplate assumptions
- Document the design condition. Record IT load, coolant type and concentration, target supply temperature, target return temperature, expected temperature rise, facility-water condition, room dew point, and allowable pressure limits.
- Flush and inspect the loop. Confirm cleanliness, filter installation, valve positions, materials compatibility, leak detection, and isolation boundaries.
- Prove sensor calibration. Check supply and return temperature, flow, pressure, room temperature, and humidity sensors against traceable references or the project’s approved calibration process.
- Measure static and operating pressure. Record pressure at the CDU, manifold, and representative branches with pumps running and with the expected valves in their normal positions.
- Balance parallel branches. Measure each server or cold-plate branch, adjust balancing devices, and verify that pressure-drop variation meets the project criterion.
- Apply representative load. Test sustained accelerator and CPU load, memory-heavy work, network activity, and expected transient behavior. Record supply temperature, return temperature, flow, pressure, IT power, and room dew point together.
- Test control limits. Raise facility-water temperature or reduce heat-rejection capacity within the approved test plan to verify CDU control, high-return-temperature alarms, and thermal protection.
- Test failure responses. Simulate loss of a pump, loss of facility flow, blocked-filter indication, sensor failure, and a branch isolation event. Confirm that alarms reach the operations system and that the server response is safe.
- Trend after handover. Retain time-series data for supply and return temperature, flow, pressure, IT load, room dew point, valve position, pump speed, alarms, and CDU approach temperature.
Compare feasible designs on the same basis
When two designs meet the server envelope, compare them at the same IT load, ambient condition, and reliability requirement. A lower nominal inlet temperature is not automatically better if it requires substantially more chiller energy or creates a smaller dew-point margin.
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| Comparison axis | Question to answer |
|---|---|
| Thermal margin | What is the coldest and hottest component temperature at maximum sustained and transient load? |
| Flow and rise | What flow and temperature rise result from the approved coolant properties? |
| Pressure | Can the CDU pump maintain every branch flow without exceeding component ratings? |
| CDU capacity | Does heat-exchanger capacity and approach temperature remain adequate at facility design conditions? |
| Facility energy | What pump, chiller, evaporative, or dry-cooler energy is required? |
| Condensation control | What dew-point margin remains during humidity excursions and startup? |
| Balance | Are branch flows within the allowed variation, and can they be adjusted in the field? |
| Fluid management | Are chemistry, filtration, sampling, compatibility, and maintenance responsibilities defined? |
| Reliability | What happens after pump, sensor, valve, facility-flow, or heat-rejection failure? |
Common selection mistakes and their fixes
Using a single “AI server” flow number
Problem: The number may refer to a specific OAM cold plate, coolant, heat load, and delivery arrangement.
Fix: Identify the platform, fluid, temperature rise, and measurement location, then recalculate from the actual heat load.
Choosing temperature from facility preference alone
Problem: A warm-water target may exceed a cold plate, hose, connector, or CDU limit.
Fix: Check the complete component envelope and the heat exchanger’s approach temperature at the intended facility condition.
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Fix: Control supply temperature against measured dew point with at least the specified 2°C buffer and test fault behavior.
Checking aggregate rack flow but not branch balance
Problem: A restrictive branch can be under-cooled while another branch receives excess flow.
Fix: Measure and balance every parallel loop; use the manifold pressure-drop and variation criteria adopted by the project.
Trusting nameplate CDU capacity
Problem: Capacity may be rated with colder facility water, a different flow, or a smaller approach temperature than the site provides.
Fix: Obtain a performance curve or certified operating point for the actual facility and secondary-loop conditions.
What to trend in normal operation
Operations should trend, alarm, and retain at least:
- Secondary supply and return temperature.
- Facility-side supply and return temperature.
- Total CDU flow and branch flow where instrumentation is available.
- CDU and manifold pressure, differential pressure, and pump speed.
- IT power and workload or accelerator utilization.
- Room temperature, relative humidity, and calculated dew point.
- Heat-exchanger approach temperature and valve position.
- Filter differential pressure, leak detection, and all high-temperature, low-flow, and sensor-fault alarms.
Correlating these signals lets operators distinguish rising IT load from a blocked filter, an unbalanced branch, a failing pump, a facility-water problem, or a humidity-driven temperature limit.
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Choose flow by solving the heat-load and temperature-rise relationship with the approved coolant properties, then verify the result against the server’s required flow and pressure envelope. Choose the highest supply temperature that preserves component and silicon margin, supports the facility’s heat-rejection strategy, and remains safely above room dew point. Use OCP’s 1–2 L/min per OAM cold plate, 4–16 L/min chassis examples, 7.5–12°C rise range, and 1.5 L/min/kW PG25 target only as clearly labeled platform references. The final setpoint is the one demonstrated by measured flow, temperature, pressure, balance, alarms, and failure tests across the complete CDU-to-cold-plate loop.
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