Air cooling vs Direct-to-chip liquid
Cooling is an equipment-and-facility envelope, not a universal rack-kW cliff. For the cited 132 kW nominal TDP: 115 kW liquid + 17 kW air NVL72-class profile, the OEM heat split requires direct-to-chip liquid plus residual room-air removal; other racks must be checked against component heat flux, airflow and inlet limits, liquid heat fraction, supported TCS/FWS or entering-water conditions, room rejection, climate, redundancy, serviceability, and the named refresh case.
| Axis | Air cooling | Direct-to-chip liquid |
|---|---|---|
| Cooling-service envelope | works where airflow, inlet temperature, pressure, containment, fan power, acoustics, and service access close for the named rack | required where the named rack's supported heat split and heat flux require liquid capture at source |
| PUE band | 1.4–1.6 legacy halls; industry weighted average stuck at 1.54 | 1.05-1.15 in temperate climates with warm-water loops |
| Coolant temperature | air inlet target within the named ASHRAE equipment class; heat rejection may be compressor-led or economized | select one supported TCS operating point; ASHRAE W-classes name FWS supply capability, while free-cooling hours depend on climate and approach temperatures |
| Water consumption | evaporative variants consume heavily; dry variants pay in PUE | dry, non-evaporative heat rejection drives WUE-site near zero; heat reuse (ERF) becomes feasible |
| Failure modes | fans, filters, containment and airflow; transient response depends on thermal mass and controls | leaks, fittings, coolant chemistry and loss of flow; response time must be proven for the named rack and loop |
| Retrofit story | n/a — it is the incumbent | CDUs + manifolds + floor loading + trades data centers haven't staffed before |
| Where it wins | profiles whose airflow and inlet envelope closes; often storage/CPU rows, network rooms, edge sites and legacy estates | profiles with an OEM liquid heat split or heat-flux/airflow limit that requires source capture |
Quantitative cells are the guide's canonical figures — each is date-stamped and sourced in the numbers register and derived in the chapters below.
How the decision falls
Use the named rack's supported cooling profile as the design basis. A profile with a declared liquid heat fraction commits the project to its TCS/CDU/FWS path and residual-air duty; a proven air envelope can remain simpler for other profiles. Preserve a viable future water path when the equipment roadmap warrants it.
What would flip it: A different equipment heat split, airflow envelope, water availability, climate, redundancy target, service model, or future-density case can change the selection.
Model this fork with your own numbers: Rack cooling feasibility calculator →
Full derivations, worked examples, and the numbers behind this matrix: Cooling-envelope selector and qualified modalities (Ch 5.1) · The thermal spine: chip to atmosphere (Ch 5.11) · PUE/WUE/ERF — the post-PUE metric stack (Ch 15.1)