Dew Point Liquid Cooling Sets a Floor 2°C Above the Room

ASHRAE TC 9.9 puts the lower bound of a TCS supply temperature at least 3.6°F (2°C) above the room dew point, according to its 2019 white paper on water-cooled servers. TCS here means the technology cooling system, the loop that reaches the server, which a coolant distribution unit (CDU) separates from the facility water system (FWS). The claim under test is that colder coolant is simply better. It holds only down to a floor that the room decides, and the sections below put numbers on that floor.

The coolant lower bound follows the room dew point plus 2°C, not the 2°C class minimum

The 2019 white paper defines two limits for a TCS loop. The upper bound is the highest supply temperature that still gives the attached equipment enough cooling at a given flow rate. The lower bound is the minimum supply temperature that may safely be provided, and the paper says it may be dynamic because it depends on the dew point in the cooling space.

The CDU carries that job. It monitors the ambient dew point and lifts the secondary loop supply to at least 3.6°F (2°C) above the room dew point, a margin the paper says also covers some sensor uncertainty. Preventing condensation within the IT equipment by regulating TCS water above room dew point appears in the paper’s list of CDU functions, beside coolant chemistry separation and flexible coolant temperature.

Without a CDU, the same paper asks the data center to keep facility water at least 3.6°F (2°C) above the room ambient dew point at all times, and it notes that many FWS supply chilled water colder than the air dew point. The listed mitigations are telemetry of supply water temperature and server inlet air dew point, proportional or binary control valves, and building management alarms when supply water dips within 3.6°F (2°C) of the incoming air dew point. The paper calls condensation on a cold plate inside a server potentially catastrophic.

Source: ASHRAE TC 9.9, Water-Cooled Servers: Common Designs, Components, and Processes (2019), sections on CDU and non-CDU implementations.

The 2021 reference card names its liquid cooling classes by facility water supply temperature, from 17°C for W17 to above 45°C for W+. A footnote sets the minimum water temperature for every class at 2°C. Nothing in that table refers to the room.

A reader who takes 2°C as a usable setpoint is comparing it with the wrong number. The same card caps the recommended air envelope at a 15°C dew point and allows higher dew points class by class, so the water classes and the dew point limits describe two different things, and Table 1 lines them up.

Chilled water makes the mismatch practical. An Electronics Cooling summary of the ASHRAE liquid cooling book says a chilled water system usually provides 50°F (10°C) water to the datacom equipment center. That is 5 K under the 15°C dew point limit of the recommended envelope, so chilled water reaching a server directly would run below the room dew point even when the room is inside its recommended limits.

Source: ASHRAE TC 9.9, 2021 Equipment Thermal Guidelines for Data Processing Environments reference card, Tables 2.1, 2.2 and 3.1.

A 15°C dew point sets a 17°C floor, and each higher dew point class lifts it one for one

Dew point follows from dry-bulb temperature and relative humidity. The Magnus form fitted by Alduchov and Eskridge gives saturation vapor pressure over water as e(t) = 6.1094 × exp(17.625 × t / (243.04 + t)) in hPa, with t in °C. Dew point is the t at which e(t) equals the actual vapor pressure, so for a relative humidity RH at dry-bulb temperature t, the dew point is Td = 243.04 × γ / (17.625 − γ), where γ = ln(RH / 100 × e(t) / 6.1094).

Take the warm corner of the recommended envelope, 27°C dry bulb at the 15°C dew point limit. e(27) = 35.59 hPa and e(15) = 17.02 hPa, so RH = 17.02 / 35.59 = 47.8%. At 27°C the dew point cap binds long before the 70% rh cap, because the reference card says the upper moisture limit is the lower absolute humidity of the two stated values. The same room at 50% rh has a dew point of 15.7°C, already past the cap. These are calculated values from fitted coefficients, not measurements.

The floor is the dew point plus the paper’s 2°C, so a room at a 15°C dew point gives 15 + 2 = 17°C. Repeating that for each dew point limit on the card gives the rest of the table.

Table 1. Coolant floor by room dew point limit, and headroom under each liquid cooling class (°C; headroom in K)

* Floor = maximum dew point + 2 K buffer. Headroom = class value minus floor; negative means the class supply temperature is below the floor. Class values taken as upper supply temperatures (minimum 2°C for all classes).

Source: ASHRAE TC 9.9, “2021 Equipment Thermal Guidelines for Data Processing Environments” reference card, Tables 2.1, 2.2, 3.1 (2021, 2024); “Water-Cooled Servers” white paper (2019). Floors calculated here.

Read across the first row. A W17 facility is specified at a 17°C facility water supply, so a room held at the recommended dew point limit leaves it zero headroom, and every higher dew point class puts that supply below the floor unless the water temperature is lifted. The white paper names the tools for that lift: flow rate reduction, bypass loops or auxiliary heat input at the CDU. For how that boundary works in hardware, see what a CDU does at the loop boundary.

The far end checks against an independent document. The OCP liquid distribution guide states that the highest dew point limit supported by ASHRAE TC 9.9 is 28°C for Class C data centers, and that the lowest temperature in its elevated-loop discussion is 30°C, at which condensation protection would not be a concern. That matches the last row of the table, although the OCP text does not say it derived 30°C from a 2°C margin.

Source: Alduchov and Eskridge, Improved Magnus’ Form Approximation of Saturation Vapor Pressure (NOAA National Climatic Data Center, via OSTI, 1997); ASHRAE 2021 reference card; OCP, Data Center Liquid Distribution Guidance and Reference Designs, Revision 0.

A fixed 17°C setpoint meets the 2°C rule at a 15°C dew point and loses it everywhere above

The 2008 Electronics Cooling article describes two ways to keep a TCS loop above dew point. For the TCS loop, it says, the minimum liquid supply temperature must remain above the dew point at all times. The simplest method is a fixed control point above 63°F (17°C), the maximum dew point specification for a Class 1 environment. The more sophisticated method holds a fixed offset above the continuously measured dew point.

The article summarizes ASHRAE’s first liquid cooling book, and the book’s own wording is not quoted here. It says above 17°C, not at it, so the 17°C cases below are the bare threshold and any setpoint over it adds that much margin. What the article supports is the shape of both methods. The 2019 white paper supplies the offset: 2°C.

The margin of a fixed setpoint is the setpoint minus the dew point. At a 15°C dew point, 17 − 15 = 2°C, which meets the white paper rule.

At a 17°C dew point the margin is zero, and the white paper’s alarm condition, supply water within 2°C of the incoming air dew point, would already be active. At 21°C, the A2 allowable limit, it is −4°C.

I count a fixed 17°C setpoint as a shortcut for rooms that never leave the recommended envelope. The measured dew point version, with a stated buffer, is the one that stays valid when the room drifts toward an allowable limit, which is the situation the allowable ranges exist to permit.

A 2°C buffer absorbs about 5.8 points of humidity sensor error and no more

The white paper gives no sensor accuracy figure, so the size of the buffer can only be expressed as a sensitivity. Use the same warm corner, 27°C dry bulb, true RH 47.8%, true dew point 15.0°C. A hygrometer reading 5 points low (42.8%) yields a calculated dew point of 13.3°C, 1.7°C under the truth. Reading 7 points low (40.8%) yields 12.6°C, 2.4°C under. The 2°C buffer is used up at a reading about 5.8 points low.

Direction matters here. A reading too low understates the dew point and pulls the setpoint toward the wet side. A reading too high errs toward warmer coolant, at the cost of cooling margin. The 5.8-point figure belongs to the 27°C case and shifts with the dry-bulb temperature of the room.

The sensor is not the only draw on the buffer. The OCP guide says a CDU that keeps the TCS above dew point minimizes the need for condensation protection during normal operation, then adds that in high dew point environments the loop may pass below dew point during transitions and maintenance, so alerts to the building management system should be considered. It also records that liquid cooled facilities historically relied on drip trays and leak detection, and that many now operate above dew point. For the detection side of that history, leak detection and where valves take over covers the same hardware from the leak direction.

Source: ASHRAE TC 9.9, Water-Cooled Servers (2019); OCP, Data Center Liquid Distribution Guidance and Reference Designs, Revision 0, sections on CDU planning and reducing probability risk.

Colder coolant helps only down to the room’s dew point plus 2°C, and a fixed setpoint fails first

The condensation data center operators have to prevent forms on the surfaces the water touches, so the supply temperature at those surfaces is what matters, and the rule above ties that temperature to the room. Table 2 applies the rule to the cases that appeared in the sections above, sorted by margin.

Table 2. Verdict by case: supply temperature against room dew point (°C; margin in K)

* Margin = supply temperature minus room dew point; the white paper lower bound requires at least +2 K. The 10°C chilled water figure is a typical supply, not a requirement.

Source: ASHRAE TC 9.9, “Water-Cooled Servers” (2019); “2021 Equipment Thermal Guidelines” reference card; OCP, “Data Center Liquid Distribution Guidance and Reference Designs, Revision 0”; Electronics Cooling, “Liquid Cooling for Datacom Equipment Centers” (August 2008). Margins calculated here.

Above the floor, a colder supply lowers coolant temperature everywhere in the loop by the same amount at a fixed flow and load. Below it, condensation on the surfaces the water touches becomes possible. The number to design around is therefore the highest dew point the room is allowed to reach, plus 2°C: 17°C for a room held to the recommended limit, 30°C for a room allowed to 28°C. Whether a given facility can hold that margin with its own sensors, valves and loop volumes needs qualified engineering review.

References

• ASHRAE TC 9.9, Water-Cooled Servers: Common Designs, Components, and Processes / ASHRAE / 2019 (errata of 10/10/2019 incorporated) / https://www.ashrae.org/file%20library/technical%20resources/bookstore/whitepaper_tc099-watercooledservers.pdf
• ASHRAE TC 9.9, 2021 Equipment Thermal Guidelines for Data Processing Environments, Reference Card (from Thermal Guidelines for Data Processing Environments, Fifth Edition) / ASHRAE / 2021, 2024 / https://www.ashrae.org/file%20library/technical%20resources/bookstore/supplemental%20files/therm-gdlns-5th-r-e-refcard.pdf
• Liquid Cooling for Datacom Equipment Centers / Electronics Cooling / August 2008 / https://www.electronics-cooling.com/2008/08/liquid-cooling-for-datacom-equipment-centers/
• Data Center Liquid Distribution Guidance and Reference Designs, Revision 0 / Open Compute Project, Advanced Cooling Facilities subproject / undated / https://ocp-all.groups.io/g/OCP-Cooling-Environments/attachment/11/3/OCP%20ACF%20Reference%20Design%20Guidance.pdf
• Alduchov and Eskridge, Improved Magnus’ Form Approximation of Saturation Vapor Pressure / NOAA National Climatic Data Center, via OSTI / November 1997 / https://www.osti.gov/biblio/548871