In short
- Cooling load ≈ IT load plus a few per cent for lighting, UPS and distribution losses: 1 MW of IT is about 1 MW of heat.
- Rack density picks the method: room air up to ~10 kW, contained air to ~25 kW, rear doors to ~50 kW, direct-to-chip to ~150 kW, immersion beyond.
- The plant rejects the heat outdoors through chillers, dry coolers or cooling towers. Warmer water means fewer chiller hours.
- ASHRAE recommends 18–27 °C inlet air for most IT classes; the allowable range is wider.
- Judge the system by pPUE, WUE and the share of free-cooling hours.
The heat chain, from chip to outdoor air
Cooling is a chain of heat exchangers. Each link carries the heat one step further and costs a few kelvin of temperature difference:
- Chip to coolant: air across heat sinks, or liquid through cold plates.
- Room or rack to a water loop: a CRAH coil, a fan wall, a rear door or a CDU.
- Water loop to the plant: chilled water or warm facility water.
- Plant to outdoor air: a chiller with condenser, a dry cooler or an evaporative tower.
The fewer kelvin lost along the chain, the warmer the water can run, and the more hours a year the plant needs no compressor at all. That is the main lever on cooling energy.
Five ways to cool IT
| Method | Typical density | How it works | Watch for |
|---|---|---|---|
| Perimeter CRAH / CRAC | up to ~10 kW | Units on the room edge push cold air through a raised floor or into the room | Mixing of hot and cold air without containment |
| Contained air, fan walls, in-row | ~10–25 kW | Hot and cold aisles are separated; coils sit close to the racks | Air volume grows fast with density |
| Rear-door heat exchanger | ~20–50 kW | A water coil on the rack door cools the exhaust to room temperature | Water reaches the rack; door weight and access |
| Direct-to-chip cold plates | ~40–150 kW | Coolant flows through plates on GPUs and CPUs, fed by a CDU | 10–30 % of heat still goes to air |
| Immersion | ~100 kW + | Servers sit in a dielectric fluid, single- or two-phase | Different hardware and service model |
The ranges overlap on purpose. Containment, airflow management and supply temperature move the boundaries. The technology selector shows where your density sits.
Containment: the cheapest upgrade in an air-cooled hall
Without containment, hot exhaust curls over the racks and mixes with the supply. Operators then overcool the whole room to protect the warmest inlet. Closing either the hot or the cold aisle stops the mixing:
- Cold-aisle containment: roof and doors over the cold aisle; the rest of the room runs hot. Easy to retrofit on a raised floor.
- Hot-aisle containment: exhaust is ducted back to the coils; the room stays at supply temperature. Common in new slab-floor halls.
Blanking panels, brush grommets and sealed cable cut-outs matter as much as the doors. With mixing removed, supply air can move to the warm end of the ASHRAE range, and that increases free cooling.
The plant: chillers, dry coolers and towers
| Heat rejection | Strength | Trade-off |
|---|---|---|
| Air-cooled chiller | Works in any climate, any water temperature | Compressor energy; free-cooling coil only on cool days |
| Dry cooler | No compressor, no water use | Can only cool to outdoor dry-bulb plus its approach |
| Adiabatic dry cooler | Sprays or pads lower the air temperature on hot days | Some water use, water treatment |
| Cooling tower | Lowest temperatures for the energy; wet-bulb limited | High water use, legionella control |
With facility water at W32 and a 6 K dry-cooler approach, dry coolers carry the full load up to about 26 °C outdoor. Many European sites then keep only trim chillers for the hottest hours. The free-cooling chain shows the limit for each water class.
How warm can a data center run?
ASHRAE's thermal guidelines set a recommended inlet range of 18–27 °C for classes A1 to A4, and wider allowable ranges up to 45 °C for A4. The newer class H1 for high-density air-cooled IT recommends 18–22 °C. For liquid cooling, facility water classes run from W17 to W45 and W+.
Most halls still run cooler than they need to. Raising supply air by a few kelvin within the recommended range is usually the cheapest efficiency gain available. The full table is in the ASHRAE section.
How to judge a cooling system
| Metric | Formula | What it tells you |
|---|---|---|
| pPUE cooling | (IT + cooling energy) ÷ IT energy | Cooling overhead alone; 1.10 means 10 % on top of IT |
| WUE | site water (litres) ÷ IT energy (kWh) | Water cost of the cooling choice |
| Free-cooling hours | hours without compressors ÷ 8,760 | How much the climate and water class do for you |
| ERF | reused energy ÷ total energy | How much heat leaves as useful heat |
Cooling uses about 7 % of the electricity in efficient hyperscale sites and over 30 % in less efficient enterprise sites (IEA). Facility-wide PUE is covered on allinPUE.
Questions about data center cooling
What is the most common cooling method?
Air cooling with CRAH units or fan walls, usually with containment, still cools most halls. Liquid is growing fast because AI racks above about 50 kW leave little choice.
What is the difference between a CRAC and a CRAH?
A CRAC has its own compressor and refrigerant circuit. A CRAH only has a chilled-water coil and fans; the compressor, if any, sits in the central plant.
Does a data center need chillers?
Not always. With W32 or warmer water, dry coolers can carry the full load in much of Europe. Hot climates keep trim chillers or adiabatic assist for peak days.
How much heat does a rack produce?
Practically all of its electrical input: a 40 kW rack needs 40 kW of cooling, about 11.4 refrigeration tons or 136,000 BTU/h.
Sources
- ASHRAE TC 9.9, Thermal Guidelines for Data Processing Environments (5th edition summaries) and liquid cooling guidelines (W classes).
- IEA, Energy and AI: cooling's share of data center electricity.
- Density ranges reflect common industry practice; check the IT maker's requirements for design.