Quick answer
Most direct-to-chip liquid cooling for AI servers uses a water/propylene-glycol mix, typically PG25 (25% PG with inhibitors), in a sealed secondary loop driven by a CDU. Immersion cooling instead submerges hardware in a dielectric fluid (single-phase or two-phase). Fluid choice depends on hardware vendor approvals, qualifications such as NVIDIA DSX Ready, and the maintenance program.
Key takeaways
- Direct-to-chip: water/glycol (often PG25) in a sealed secondary loop.
- Immersion: non-conductive dielectric fluids.
- A CDU separates the technology loop from facility water.
- Hardware and CDU vendor approval comes first.
- Fluid maintenance (concentration, inhibitors, filtration) is ongoing.
How data-center liquid cooling loops work
A facility water loop rejects heat to chillers, dry coolers or towers. A coolant distribution unit (CDU) with a heat exchanger transfers heat from the technology cooling system (TCS), the secondary loop that runs through cold plates on GPUs and CPUs. The TCS fluid is what "data-center coolant" usually means.
Coolant types used
| Type | Where | Examples (see profiles) |
|---|---|---|
| PG25 (water + 25% propylene glycol + inhibitors) | Direct-to-chip secondary loops | BASF, Shell, Castrol, Dow, Valvoline, Dynalene, PFX |
| Other glycol/water OAT fluids | Direct-to-chip | Arteco ZITREC EC (MPG/MEG/water-based) |
| Single-phase dielectric | Immersion tanks | Shell, Engineered Fluids, Submer, Valvoline DE1 |
| Two-phase dielectric | Two-phase immersion | Chemours Opteon 2P50 (developmental) |
Qualifications and standards
Public qualifications include NVIDIA DSX Ready coolant qualifications, the Open Compute Project (OCP) PG25 guideline and marketplace listings, and immersion certification programs such as Intel's. See the qualified coolants list. A qualification doesn't replace your hardware vendor's approval.
Choosing a fluid: checklist
- On your cold-plate and CDU vendors' approved lists?
- Ready-to-use (PG25) vs concentrate?
- Inhibitor chemistry and wetted-material compatibility
- Maintenance limits and sampling program
- Supply and service in your region
Explore
- What is PG25?
- Direct-to-chip vs immersion
- CDU coolant maintenance
- NVIDIA DSX Ready coolants
- DC coolant comparator
- Coolant suppliers directory
In-row vs in-rack CDUs
CDUs come in different forms. In-rack units sit inside a single rack and serve its cold plates. In-row or floor-standing units serve several racks through a manifold network. Liquid-to-air units reject heat into the room air (useful for retrofits without facility water), and liquid-to-liquid units transfer heat to facility water. The fluid choice is usually tied to the CDU vendor's and server vendor's approvals.
Commissioning a new loop
- Clean and flush the new pipework per the CDU and fluid supplier guidance.
- Fill with the approved, pre-diluted fluid, avoiding contamination.
- Vent the air and verify flow and pressure.
- Take baseline samples (concentration, pH, conductivity, appearance).
- Set up the monitoring and sampling schedule.
ASHRAE W classes for facility water
ASHRAE renamed its liquid-cooling water classes so the upper temperature limit is part of the name. Warmer classes allow more hours of chiller-free cooling, and with W45 and W+ waste heat can be reused, for example in district heating, but they need IT equipment and CDUs designed for warmer inlet fluid.
| ASHRAE class | Facility water supply temperature (upper limit) | Typical heat rejection (per ASHRAE) |
|---|---|---|
| W17 | 17 °C | Chiller / cooling tower |
| W27 | 27 °C | Cooling tower, water-side economizer |
| W32 | 32 °C | Cooling tower |
| W40 | 40 °C | Cooling tower or dry cooler |
| W45 | 45 °C | Cooling tower or dry cooler; district heating possible |
| W+ | >45 °C | District heating |
All classes carry a 2 °C minimum. W classes describe the facility water supply (FWS), not the fluid temperature at the IT equipment, which is typically several degrees warmer after the CDU approach.
Comparing data-center fluids?
Open comparator →Standards and guidance you should know
| Document | Publisher | What it covers |
|---|---|---|
| Liquid Cooling Guidelines for Datacom Equipment Centers | ASHRAE TC9.9 | FWS and TCS design, water quality (chapters 5 and 6) |
| Water-Cooled Servers white paper | ASHRAE TC9.9 | Common designs, components, water quality |
| TC9.9 Technical Bulletins | ASHRAE TC9.9 | TCS cleaning, commissioning and monitoring |
| PG25 Guidelines Rev 2.0 | Open Compute Project | PG25 in the TCS: materials, cleaning, filtration, monitoring |
| PG25 Base Specification v1.0.0 | Open Compute Project | Baseline production requirements for PG25 fluid suppliers |
For the suppliers making these fluids, see the supplier directory.
ASHRAE facility water temperature classes
ASHRAE renamed its classes to include the upper supply-temperature limit. Warmer classes extend free cooling and enable heat reuse, provided the IT hardware supports the resulting inlet temperatures.
| ASHRAE class | Facility water supply temperature (upper limit) | Typical heat rejection (per ASHRAE) |
|---|---|---|
| W17 | 17 °C | Chiller / cooling tower |
| W27 | 27 °C | Cooling tower, water-side economizer |
| W32 | 32 °C | Cooling tower |
| W40 | 40 °C | Cooling tower or dry cooler |
| W45 | 45 °C | Cooling tower or dry cooler; district heating possible |
| W+ | >45 °C | District heating |
All classes carry a 2 °C minimum. W classes describe the facility water supply (FWS), not the fluid temperature at the IT equipment, which is typically several degrees warmer after the CDU approach.
Where each loop's water-quality rules live
Facility and IT-side loops have different water-quality guidance. The table and explanation are on our CDU coolant maintenance page, alongside the monitoring parameters ASHRAE bulletins list.
Who owns what in a liquid-cooled hall
| Element | Typical owner |
|---|---|
| Facility water plant (chillers, towers, dry coolers) | Facilities / building owner |
| CDU and secondary piping | CDU vendor, integrator, or operator |
| Cold plates and server manifolds | IT hardware vendor |
| TCS fluid | Specified with the hardware/CDU vendors; supplied by the fluid maker |
| Monitoring and sampling | Operator, often with the fluid supplier |
Further reading
- What is PG25?
- CDU coolant maintenance
- Direct-to-chip vs immersion
- NVIDIA DSX Ready coolants
- DC coolant comparator
- Coolant suppliers directory
More questions answered
Is liquid cooling only for AI?
It's widely used for high-density compute, including AI and HPC.
Reader questions
Do liquid-cooled racks still need air cooling?
Often for remaining components, depending on design.
Key point to remember
Start with hardware approvals, then fluid and monitoring.
In practice
In practice, document everything.
Frequently asked questions
What coolant is used in data centers?
Direct-to-chip systems typically use PG25 water/glycol. Immersion uses dielectric fluids.
What is a CDU?
A coolant distribution unit that pumps the secondary loop and exchanges heat with facility water.
Related guides
Sources
- Arteco: offices
- BASF news release, Oct 2026
- Data Centre Magazine: NVIDIA qualifies Shell DLC fluid
- Castrol newsroom
- Valvoline Global: Beyond launch
- Dow: DOWFROST LC 25 product page
- ASHRAE TC9.9 white paper: Water-Cooled Servers: Common Designs, Components, and Processes
- ASHRAE TC9.9 white paper: Emergence and Expansion of Liquid Cooling in Mainstream Data Centers
- OCP: Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks (Rev 2.0, Sep 2026)
- OCP: Base Specification for Single-Phase PG25 fluids v1.0.0