Auxuro

Every watt of IT becomes a watt of heat.

A working reference for mechanical engineers designing and operating data centres in New Zealand and Australia: sizing formulas, calculators, cooling schematics, thermal envelopes and the industry's largest sites.

Pipe colours throughout follow one convention:

Supply / cold Return / hot Condenser / facility water

Quick heat load

–Room heat load, kW
–Tons of refrigeration
–kW per rack
–Cooling units installed
–Facility power, kW
–Annual energy, MWh
Section 1

Heat load basics

Almost all electrical power delivered to IT equipment ends up as sensible heat in the room or in the coolant. Latent load is negligible because data halls have few people and little moisture, so the sensible heat ratio is close to 1.0. Start with the IT load, add heat from electrical losses, lighting and the building envelope, then size for redundancy.

Q = ṁ · cp · ΔT

The one equation behind every cooling calculation: heat = mass flow × specific heat × temperature rise.

Air: Q [kW] ≈ 1.21 × V [m³/s] × ΔT [K]

ρ = 1.2 kg/m³, cp = 1.006 kJ/kg·K at sea level. Imperial: CFM ≈ 3.16 × W ÷ ΔT[°F].

Water: Q [kW] ≈ 4.18 × V [L/s] × ΔT [K]

At a 10 K rise this is about 1.43 L/min per kW. For PG25 glycol use roughly 3.95.

PUE = Facility energy ÷ IT energy

ISO/IEC 30134-2. 1.0 is ideal; modern hyperscale sites run about 1.1–1.2, older enterprise rooms 1.6–2.0.

WUE = Site water [L] ÷ IT energy [kWh]

From The Green Grid. Evaporative plant trades water for lower electricity; dry coolers do the opposite.

1 TR = 3.517 kW = 12,000 BTU/h

1 kW = 3,412 BTU/h. COP = kW cooling ÷ kW input; kW/TR = 3.517 ÷ COP.

Why liquid wins at high density

Per unit volume, water carries about 3,500 times more heat than air. Removing 120 kW with air at a 10 K rise needs about 9.9 m³/s (21,000 CFM) through one rack; the same heat in water at 10 K needs about 2.9 L/s.

Section 2

Calculators

All calculations run in your browser and give concept-stage figures. Confirm final selections against manufacturer data.

Airflow for a heat load

–m³/s
–m³/h
–CFM

Liquid flow and pipe size

–L/s
–L/min
–US GPM
–Min. ID, mm
–Steel pipe
–Actual m/s

IDs approximate for Schedule 40 steel. Check pressure drop separately.

PUE, energy and cost

–Current MWh/yr
–Target MWh/yr
–Saving NZD/yr
–Overhead cut %

Water use (WUE)

–ML per year
–m³ per day
–Olympic pools/yr

Unit converter

Chiller efficiency

–Compressor kW
–kW/TR
–Compressor MWh/yr

CDU sizing (direct-to-chip)

–Heat to liquid, kW
–TCS flow, L/s
–L/min per rack
–CDUs installed
–Residual air, kW
–Residual airflow, m³/s

Liquid capture share and CDU sizes are indicative; rack-scale AI systems often exceed 85%. Size against the OEM facility spec and the CDU vendor's approach temperature.

Section 3

How air cooling works

Cold air reaches the front of the racks (cold aisle), servers pull it through and exhaust it hot at the back (hot aisle), and cooling units return it to the coil. Containment stops hot and cold air mixing, which lets you raise supply temperature and ΔT, cutting fan energy and extending free cooling.

Raised floor data hall with hot aisle containment and CRAH units Ceiling return plenum Raised floor supply plenum CRAH coil + EC fans Rack Rack Rack Hot aisle (contained) Cold aisle Cold aisle ≈22–27 °C ≈35–40 °C CHW from plant
Figure 1. Raised-floor hall with hot aisle containment. CRAHs push cold air through the floor plenum and perforated tiles; hot exhaust returns via the ceiling plenum.

Air cooling systems compared

SystemHow it worksTypical densityNotes
CRACSelf-contained DX unit with its own compressor≤ ~8 kW/rackSmall rooms and edge sites
CRAHChilled water coil + fans, no compressor~5–15 kW/rackStandard for larger halls
Fan wallEC fan and coil array in a gallery beside the hall~10–30 kW/rackCommon in hyperscale; slab floor
In-rowCoil units between racks, short air path~15–30 kW/rackGood for retrofitting dense pods
RDHxWater coil on the rack's rear door~20–50 kW/rackHandles residual air in hybrid racks
Indirect evaporativeAir-to-air HX with evaporative assist~5–20 kW/rackVery low PUE in mild climates
Section 4

Chilled water plant and free cooling

A central plant produces chilled water (CHW) for CRAHs or fan walls. The chiller moves heat into condenser water (CW), rejected outside through cooling towers or dry coolers. When outdoor conditions allow, a plate heat exchanger — the waterside economiser — cools the CHW directly and compressors switch off.

Chilled water plant with cooling tower, chiller, waterside economiser and CRAH Cooling tower or dry cooler Plate HX waterside economiser Chiller condenser evaporator compressor CRAH / fan wall serves data hall CW pump CW ≈25 / 31 °C CHW pump CHWR 26 °C CHWS 18 °C Series economiser: return water is pre-cooled in the plate HX, then trimmed by the chiller.Typical warm-water temperatures shown; legacy plants run 7 / 12 °C.
Figure 2. Simplified chilled water plant with series waterside economiser. Valves, bypass and redundancy omitted.

Why warmer water saves energy

Raising chilled water temperature is the biggest efficiency lever. Moving from 7/12 °C to around 18/24 °C can improve chiller efficiency by roughly 30–40% and multiply free cooling hours. New Zealand and southern Australia have mild climates with low wet-bulb temperatures for much of the year, so warm-water designs perform especially well there.

  • Redundancy: N+1 chillers, pumps and towers minimum for Tier III; dual distribution paths for concurrent maintainability.
  • Thermal storage buffer tank to ride through chiller restart (often 5–15 minutes).
  • Variable primary flow with VSD pumps; design for high ΔT.
  • Controls: CHW reset, economiser changeover, chiller staging, BMS/DCIM integration.
  • Heat recovery: 25–40 °C return water can feed heat pumps for nearby buildings.
Section 5

Liquid cooling

An NVIDIA GB200 NVL72 rack is planned at roughly 120 kW, and the GB300 NVL72 reference goes up to about 142 kW — far beyond what air can remove. These racks ship liquid-cooled only.

Direct-to-chip (cold plate)

Cold plates sit on GPUs, CPUs and switches. A Coolant Distribution Unit (CDU) separates the Technology Cooling System (TCS) loop in the white space from the Facility Water System (FWS). Cold plates typically capture 70–80% of server heat on hybrid designs and more on rack-scale AI systems; the rest still needs air.

Direct-to-chip liquid cooling with CDU, technology cooling system and facility water system Dry cooler / cooling tower FWS: facility water e.g. 30 → 40 °C CDU plate HX P filter TCS supply≈25–35 °C TCS return ≈ +10 K AI rack ≈ 120 kW Cold plates on GPUs and CPUs (manifolds + QDs) air
Figure 3. Direct-to-chip liquid cooling with CDU, TCS and FWS loops.
ParameterPlanning valueComment
Rack powerGB200 ≈120 kW; GB300 ≤ ≈142 kWUse OEM facility spec
Coolant flow≈1.5 L/min per kW @ 10 K≈170–195 L/min per 120–130 kW rack
TCS supply≈18–32 °CTrend towards warmer
CDUIn-rack, sidecar (≈60–80 kW) or row (≈1–2 MW+)N+1 pumps common
Rack weight≈1.4 t+Check slab loading

Immersion cooling

Servers are submerged in dielectric fluid. Single-phase fluid is pumped through a heat exchanger; two-phase fluid boils at the chip and condenses on a coil. Immersion removes nearly all heat to liquid but needs specialised tanks and servicing, and some two-phase fluids face PFAS restrictions.

Single-phase immersion tank with heat exchanger loop Dielectric fluid level Servers mounted vertically HX + pump to dry cooler
Figure 4. Single-phase immersion tank.
Section 6

Choosing a cooling technology

Rack densityWorkloadApproachWatch out for
< 10 kWEnterprise IT, storageCRAH/CRAC + containmentBypass air, overcooling
10–25 kWCloud, colocationFan walls or in-rowAirflow balancing
25–40 kWAir-cooled GPUActive RDHx, tight containmentHotspots
40–80 kWMixed AI / HPCDirect-to-chip + air residualWater quality, leaks
> 80 kWRack-scale AIDirect-to-chip mandatory, or immersionFloor loading, busway
Section 7

ASHRAE TC 9.9 thermal envelopes

The Thermal Guidelines for Data Processing Environments (5th ed., 2021) recommend 18–27 °C at the server inlet for all air-cooled classes; allowable ranges are wider and define warranty limits. ASHRAE 90.4 separately sets energy limits.

ClassRecommendedAllowableApplication
A118–27 °C15–32 °CEnterprise, mission critical
A218–27 °C10–35 °CMost volume servers
A318–27 °C5–40 °CFree-cooling friendly
A418–27 °C5–45 °CHighly economised sites
H118–22 °C15–25 °CHigh-density air-cooled
Liquid classMax. facility waterHeat rejection
W1717 °CChillers
W2727 °CChillers + economiser
W3232 °CTowers / dry coolers, chiller trim
W4040 °CDry coolers most of the year
W4545 °CDry coolers year-round
W+> 45 °CHeat recovery

Class names follow the 2021 edition; older documents use W1–W5.

Section 8

Water quality and design checks

ParameterTypical OEM guidance (TCS)
FluidTreated DI water or PG25 with approved inhibitors
pH≈7.5–9.0
Conductivity≤25 µS/cm (typical)
Filtration≈50 µm or finer
MaterialsCopper, stainless, approved polymers; no aluminium in copper loops
  • Flush and clean pipework before connecting IT; commission with temporary strainers.
  • Leak detection under CDUs and manifolds, linked to BMS with automatic isolation.
  • Keep TCS supply above room dew point.
  • N+1 CDU pumps and dual power feeds.
  • Per-rack flow and temperature monitoring.
Section 9

The world's largest data centres

Rankings depend on what you measure: operational IT power, planned build-out, or a multi-operator cluster. Basis is stated for each row (as of September 2026).

#FacilityLocationOperatorIT MWBasis
1Inner Mongolia Information ParkHohhot, ChinaChina Telecom, China Mobile, Alibaba, ByteDance…≈3,000+Cluster, combined
2Colossus 2Memphis area, USAxAI≈946Operational est.
3New CarlisleIndiana, USAAmazon / Anthropic≈910Operational est.
4The Citadel CampusNevada, USASwitch≈650Full build-out
5Fairwater AtlantaGeorgia, USAMicrosoft≈636Operational est.
6PrometheusOhio, USAMeta≈562Operational est.
7New AlbanyOhio, USAGoogle≈453Operational est.
8Stargate AbileneTexas, USAOpenAI, Oracle≈400–500Phased, expanding
9Pryor NorthOklahoma, USAGoogle≈368Operational est.
10QTS Atlanta 1Georgia, USAQTS (Blackstone)≈278Operator figure

Sources: Epoch AI (Sep 2026), Blackridge Research, GBC Engineers, Brightlio. Sydney, for scale, is roughly a 950 MW market.

Section 10

Standards and resources

The Engineering ToolBoxFluid properties, psychrometrics, pipe friction, unit conversions
ASHRAETC 9.9 guidelines, Standard 90.4
Uptime InstituteTier I–IV certification, surveys
Open Compute ProjectAdvanced cooling specs, CDU, TCS/FWS
The Green GridPUE, WUE metrics
NABERSAustralian data centre energy rating
CAICT (China Academy of Information and Communications Technology)Chinese data centre and liquid cooling research reports
Epoch AILargest AI data centres tracker

Also: TIA-942, EN 50600, ISO/IEC 30134, AS/NZS 3000, NZ Building Code / NCC.

Section 11

Glossary

TermMeaning
CDUPumps, HX and filters between rack loop and facility water
CRAC / CRAHDX unit / chilled water air handler
Cold plateLiquid-cooled plate on the chip
ImmersionServers submerged in dielectric fluid
Free cooling / economiserCooling without compressors
FWS / TCSFacility water / technology cooling loop
Hot aisle containmentEnclosing the exhaust side of racks
RDHxRear door heat exchanger
Tier IIIConcurrently maintainable
Intelligent computing centreChina's term for AI/GPU data centres