Quick answer
Liquid-cooled data centers run two loops. The facility water system (FWS) carries heat to chillers, dry coolers or cooling towers. The technology cooling system (TCS) is the clean, closed loop that reaches the cold plates. A CDU links them through a heat exchanger. ASHRAE classifies facility water by maximum supply temperature (W17, W27, W32, W40, W45, W+).
Key takeaways
- The TCS coolant (often PG25) never mixes with facility water.
- ASHRAE W classes name the highest facility water supply temperature equipment must tolerate.
- The CDU approach means the TCS runs some degrees warmer than the FWS supply.
General information. Confirm the exact fluid, equipment and qualification with your hardware or CDU supplier before specifying.
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The two loops
| Facility water system (FWS) | Technology cooling system (TCS) | |
|---|---|---|
| Where | Building plant to the CDU | CDU to the cold plates in the racks |
| Fluid | Treated water or glycol, building-grade | Tightly specified coolant, e.g. PG25 or treated water |
| Cleanliness | Building water treatment standards | Fine filtration, strict chemistry and microbiology control |
| Rejects heat to | Chillers, dry coolers, cooling towers | The CDU heat exchanger |
ASHRAE W classes
ASHRAE TC 9.9 introduced liquid cooling classes in 2011. The fifth edition of its Thermal Guidelines renamed them by the maximum facility water supply temperature: W17, W27, W32, W40, W45 and W+ (above 45 °C). Warmer classes allow more free cooling and heat reuse but leave less margin for the chips.
Because the CDU heat exchanger needs a temperature difference (the approach), the TCS supply is warmer than the FWS supply by several degrees. Check that the hardware is rated for the resulting coolant temperature, not only for the facility water class.
Coolant on each side
- TCS: follow the server and CDU makers’ fluid specification, for example OCP PG25 guidance. See what is PG25.
- FWS: building water treatment, sometimes glycol where freezing is possible in dry coolers.
Related: what is a CDU · CDU coolant maintenance.
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What is the difference between FWS and TCS?
FWS is the facility water loop to the building plant; TCS is the closed coolant loop from the CDU to the IT equipment.
What are ASHRAE W classes?
Facility water classes named by maximum supply temperature: W17, W27, W32, W40, W45 and W+ (fifth edition).
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