There is no universal coolant flow rate or inlet temperature for a liquid-cooled AI server. Use the server and cold-plate manufacturer’s operating envelope first, then size flow for the actual IT heat load, coolant properties and permitted temperature rise. Finally, verify that the CDU, pump, heat exchanger, manifolds, hoses, quick disconnects and facility heat-rejection system all operate within their own limits at the same design condition.
A practical design therefore starts with five values: required flow at the intended load, allowable supply-temperature range, maximum return temperature, permitted pressure or pressure drop, and approved coolant chemistry. Flow and temperature are coupled: changing the temperature rise changes the flow needed to remove the same heat, while changing inlet temperature changes thermal headroom and facility energy use.
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Why a quoted flow rate is meaningless without context
A statement such as “use 10 L/min” is incomplete unless it identifies the server platform, number of cold plates, coolant, heat load, supply temperature, expected temperature rise and hydraulic boundary. A rack with several high-power accelerators may need a very different flow from a single-node system, even when both are described as AI servers.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThe tightest applicable limit governs the complete technology-cooling system (TCS). That may be the cold plate’s minimum flow, a component’s maximum return temperature, a quick disconnect’s pressure rating, a CDU pump curve, a material-compatibility limit or a facility heat exchanger’s capacity. Open Compute Project (OCP) examples for OAI/OAM hardware are useful reference points, not universal requirements for other servers.
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Collect the server and cold-plate envelope
Before selecting a setpoint, obtain written values from the server or cold-plate supplier for:
- Approved coolant type, concentration and chemistry, including biocide or corrosion-inhibitor requirements.
- Permitted coolant inlet-temperature range and any minimum margin above room dew point.
- Required flow at idle, normal, maximum and transient loads.
- Maximum allowable pressure, pressure drop and differential pressure across the cold plate.
- Maximum coolant return temperature for the cold plate, server, hose, manifold and other wetted components.
- Filtration rating, cleanliness requirements, materials compatibility and leak-detection provisions.
Do not infer these values from a different generation of accelerator or from a demonstration rack. LBNL’s open-rack guidance likewise says to match each loop’s flow to IT requirements at the manifold supply temperature, check return temperature against every loop component and compare CDU capacity and heat-exchanger approach temperature with loop power and flow.
Calculate flow from heat load and temperature rise
The governing energy balance is:
Q = ṁ × cp × ΔT
Here, Q is heat removed, ṁ is coolant mass flow, cp is the coolant’s specific heat capacity at the operating temperature and concentration, and ΔT is the coolant temperature rise from supply to return. For a fixed heat load, a coolant with lower heat capacity or a smaller allowed temperature rise requires more mass flow. Volumetric flow also depends on coolant density, so convert using the supplier’s temperature-dependent properties rather than assuming water values.
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Illustrative sizing method
- Determine the maximum simultaneous IT heat load, including the CPU, accelerators, memory and any other liquid-cooled devices. Include a realistic transient or boost condition, not only an average utilization figure.
- Select a permissible coolant temperature rise from the server specification and the facility’s thermal strategy.
- Use the approved coolant’s density and heat capacity at the expected mean temperature.
- Calculate mass flow from the energy balance, convert it to volumetric flow and add the design margin required by the supplier or project standard.
- Check that the resulting flow is achievable through every parallel branch at the available CDU pressure, including filters, quick disconnects, hoses, manifolds and cold plates.
As an arithmetic illustration only, applying OCP’s PG25-based reference of 1.5 L/min per kW to a 100 kW loop gives 150 L/min. That result is not a universal requirement: it is valid only when the coolant basis, temperature rise, hardware and delivery arrangement match the stated OCP context.
OCP’s OAI/OAM figures: useful examples, not universal setpoints
OCP’s OAI System Liquid Cooling Guidelines (2023) provides platform-specific examples for OAM systems:
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| OCP example | Published value | How to interpret it |
|---|---|---|
| Flow per OAM cold plate | 1–2 L/min | Applies to the cited OAI/OAM platform and its specified coolant and hardware conditions. |
| Chassis-level flow | 4–16 L/min | Depends on loop-delivery arrangement and OAM power. |
| Recommended coolant temperature rise | 7.5–12°C | OCP associates this range with 1.25–2.0 L/min/kW for PG25-based coolant. |
| Typical design target | 10°C rise, or 1.5 L/min/kW | Use only with the stated PG25 basis and the applicable OAI/OAM design. |
OCP notes a trade-off: a higher temperature rise can improve power-usage effectiveness (PUE) by allowing warmer heat rejection, but it reduces cooling-performance margin. A lower rise increases margin and can simplify thermal control, at the cost of higher flow and potentially more pumping or facility-cooling energy. Do not transfer these figures to another coolant, cold plate, server platform or loop without recalculating and checking the manufacturer’s limits.
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Supply temperature determines how much thermal headroom remains between the coolant and the hottest server component. It also affects whether the facility can reject heat with dry coolers, cooling towers or other warm-water methods, and how much chiller work is required. The best target is the highest temperature that still meets the server’s thermal limits with adequate margin and remains safely above the room dew point.
| Observed supply-temperature grouping | Primary advantage | Primary compromise |
|---|---|---|
| 40–45°C | Emphasizes efficiency and environmental goals; can support warmer-water or dry-cooler heat rejection. | Provides less cooling capability and leaves less margin for load spikes, fouling or degraded heat transfer. |
| 30–37°C | Balances cooling capability and efficiency for many designs. | May still require mechanical cooling in warm ambient conditions, depending on the heat-rejection plant. |
| 15–25°C | Maximizes cooling capability and thermal headroom. | Usually increases reliance on chillers or evaporative cooling and can raise facility energy use. |
These three groups are observed approaches reported by OCP in 2023, not prescriptive setpoints. Ambient design conditions, server thermal resistance, control stability, CDU approach temperature and the desired transient margin can move a project outside any one grouping.
Understand the TCS upper bound
ASHRAE Technical Committee 9.9 defines the 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.” In practical terms, raising supply temperature may require more flow. If the available flow is already at the pump or cold-plate limit, a further increase can push IT equipment beyond its design temperature.
Evaluate the upper bound at maximum expected load, not at idle. Include heat-exchanger approach, filter loading, manifold imbalance and the loss of one pump or branch where the design is expected to continue operating after a failure.
Set the lower bound from dew point, not from a generic “cold is better” rule
Coolant below the surrounding air’s dew point can condense on pipes, fittings, cold plates or electronics-adjacent surfaces. Flow does not eliminate this risk. A larger pump may move heat faster, but it cannot make a surface warmer than the coolant entering it.
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ASHRAE’s white paper gives a 2°C buffer above room dew point for CDU secondary supply as protection against condensation and sensor uncertainty. Treat that as the cited guidance, then confirm the actual control setpoint and margin with the server and CDU manufacturers. The control system should use reliable room temperature and humidity sensors, alarm on a narrowing dew-point margin and prevent a low-temperature excursion during startup, valve changes or loss of facility heat.
Design the CDU and complete hydraulic loop
A CDU normally separates facility water from the TCS. Its heat exchanger transfers heat between the two domains, while a secondary pump circulates the approved coolant through racks and servers. The separation allows different water-quality controls and helps protect IT equipment from less-controlled facility water.
Verify the CDU at the same design condition
- Heat-exchanger capacity: Confirm that it can remove the full IT load at the selected secondary supply and return temperatures, facility-water temperature and approach temperature.
- Pump curve: Check that the pump can deliver the required flow at the total secondary pressure drop, including filters, manifolds, branches, hoses and cold plates. Check the curve at degraded conditions if a pump is expected to run at a higher speed after a failure.
- Control range: Ensure valves, sensors and the CDU controller can hold supply temperature while load changes rapidly.
- Fluid quality: Confirm filtration, fill procedure, sampling, corrosion control and the long-term compatibility of seals, metals, plastics and elastomers.
- Instrumentation: Provide calibrated flow, supply-temperature, return-temperature and pressure measurements at the CDU and at representative rack or manifold locations.
Balance manifolds and parallel branches
Parallel branches do not automatically share flow equally. The branch with the lowest resistance can starve a longer or more restrictive branch. LBNL’s open-rack specification gives these example checks:
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- 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 total assembly pressure drop with CDU pump capability.
These are design guidance from that specification, not universal code requirements. Use them as checks alongside the actual server supplier’s hydraulic limits. Measure branch flow after installation; a calculated balance is not proof of field balance.
Respect return-temperature and pressure limits
Return temperature is a system constraint, not merely an output to record. Compare it with the maximum rating of every wetted component, including cold plates, tubing, quick disconnects, manifold bodies, seals and the CDU heat exchanger. LBNL 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; its document cites secondary-loop operating pressure up to 60 psi as typical for its context. Neither range is a recommended operating point for every AI server. The lowest component maximum temperature and the actual pressure rating govern.
Compare candidate designs on the same basis
When two flow-and-temperature strategies both satisfy the server envelope, compare them at the same IT load, ambient condition and availability target. A nominal inlet-temperature comparison alone can hide a larger pump, a hotter return, a higher CDU approach or a smaller transient margin.
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| Decision axis | Questions to answer |
|---|---|
| Thermal margin | What is the coldest and hottest component temperature at maximum and transient load? |
| Flow and rise | What flow and temperature rise result from the specified coolant properties? |
| Hydraulics | What pressure drop occurs through the pump, filter, manifold, branches and cold plates? |
| CDU performance | Can the heat exchanger meet capacity and approach-temperature targets at design conditions? |
| Facility energy | How do pump power and chiller, dry-cooler or evaporative operation change? |
| Condensation protection | What dew-point margin is maintained, and what happens when sensors or controls detect a violation? |
| Flow balance | Are parallel branches within the permitted variation, and can they be balanced in the field? |
| Fluid management | Are chemistry, materials, filtration, sampling and maintenance responsibilities defined? |
| Reliability | What happens after loss of a pump, fan, sensor, branch or facility-water source? |
Commissioning and validation procedure
- Confirm the as-built envelope. Record the installed cold plates, coolant batch and concentration, pipe sizes, filters, valves, quick disconnects, manifold arrangement and CDU model. Reconcile them with the approved server and facility drawings.
- Flush and condition the loop. Follow the component manufacturer’s cleaning and fill procedure. Verify fluid chemistry, filtration and cleanliness before connecting production IT.
- Calibrate and locate sensors. Check flow, supply temperature, return temperature, pressure, room temperature and relative humidity. Place sensors where they represent the actual CDU and rack conditions rather than relying on a remote room average.
- Measure static and operating pressure. Record CDU differential pressure and branch pressures with pumps at the planned control speed. Confirm that no component exceeds its rated pressure.
- Balance every branch. Measure flow at each rack or manifold branch under operating conditions. Adjust balancing valves or investigate restrictions until measured distribution meets the project’s tolerance.
- Apply representative load. Test normal, maximum expected and transient workloads. Record supply and return temperatures, flow, pressure, IT power and alarms at a sampling interval that captures control response.
- Check facility heat rejection. Verify the facility-side inlet temperature, flow, heat-exchanger approach and CDU capacity while the IT load is high. Confirm that the selected temperature rise does not exceed the return limit of any component.
- Test condensation controls. Vary room conditions within the permitted test range or use an approved simulation. Confirm that the CDU maintains the specified dew-point margin and that low-temperature, humidity and sensor-fault alarms act as designed.
- Test failure modes. Exercise pump failover, loss of facility water, blocked-filter indication, sensor failure, communications loss and a branch isolation event where the design requires continued service. Verify safe throttling or shutdown and alarm reporting.
- Trend the accepted baseline. Store time-series data for supply and return temperature, flow, pressure, IT load, room dew point, pump speed, valve position and alarms. Use the baseline to detect fouling, imbalance, fluid degradation or changing facility conditions.
Common selection mistakes and corrections
Choosing a familiar flow number
Problem: A generic litres-per-minute target is copied from another rack.
Correction: Recalculate from heat load, coolant properties and permitted temperature rise, then verify the cold-plate minimum and the complete pressure-drop budget.
Lowering supply temperature without checking dew point
Problem: The design gains apparent thermal margin but creates condensation exposure during humid weather or control transients.
Correction: Control to a documented dew-point margin, using reliable humidity sensors and alarms, and prevent startup or valve actions from driving coolant below the safe lower bound.
Sizing only the CDU heat exchanger
Problem: The heat exchanger has enough nominal capacity, but the pump cannot deliver flow through the installed branches or the manifold is badly imbalanced.
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Correction: Check heat transfer and hydraulics together at the same supply, return, facility-water and load conditions.
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Using a warm return outside component ratings
Problem: A higher temperature rise appears efficient but exceeds the maximum temperature of a hose, seal, cold plate or CDU part.
Correction: Compare return temperature with every component’s documented limit; the lowest limit controls.
Trusting calculated flow after installation
Problem: Manufacturing tolerances, trapped air, fouled filters or different hose lengths produce unequal branch flow.
Correction: Measure and balance each branch under representative load, then retain the readings as the commissioning baseline.
What to monitor in operation
- Secondary supply and return temperature at the CDU and rack or manifold.
- Flow at the CDU and, where practical, at each parallel branch.
- Supply and return pressure and calculated differential pressure.
- IT power, accelerator utilization and thermal alarms.
- Room temperature, relative humidity and calculated dew point.
- CDU pump speed, valve position, filter differential pressure and heat-exchanger approach.
- Facility-side water temperature and flow, including dry-cooler or chiller operating state.
Trend these values together. A rising return temperature with stable IT power may indicate reduced flow or worsening heat transfer; a branch-flow change with stable CDU flow can indicate imbalance or restriction; a shrinking dew-point margin requires a control response even when server temperatures remain normal.
Practical decision rule
Select the highest coolant supply temperature and the lowest flow that remain inside the server’s documented envelope, preserve the required thermal and dew-point margin, and allow the CDU, manifolds, pump and facility heat rejection to operate with their required reliability margin. If any limit conflicts, do not average the limits: redesign the loop, change the operating point or obtain a revised specification from the responsible manufacturer.
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