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Topology · 10 min read

DX vs CDW vs CHW Data Centre Cooling Compared

CRAC Services Engineering

Direct expansion, condenser water, and chilled water are the three precision-cooling topologies you'll encounter in Australian data centres. Here's what differentiates them, where each wins, and how to size them.

The three topologies

Every data hall and server room in Australia is cooled using one of three precision-cooling topologies — or sometimes a hybrid of two. The choice determines both the CapEx envelope and the next 15 years of OpEx.

DX (Direct Expansion): Refrigerant compressor inside the indoor cooling unit. Outdoor condenser per indoor unit. Self-contained per unit.

CDW (Condenser Water): Refrigerant compressor inside the indoor cooling unit. Heat rejected via a building condenser-water loop to a centralised cooling tower or dry cooler.

CHW (Chilled Water): No refrigerant in the indoor cooling unit. Cooling provided by chilled water from a centralised chiller plant.

From highest equipment count and individual flexibility (DX) to lowest equipment count and highest centralised efficiency (CHW).

Equipment count and physical footprint

DX (15-200 kW per unit): every indoor unit has a paired outdoor condenser. A 500 kW data hall needs 5-7 indoor units (plus N+1) and 5-7 outdoor condensers. Total roof or pad area requirement is significant. Refrigerant pipework runs between every indoor and its condenser.

CDW (15-300 kW per unit): every indoor unit connects to a single building condenser-water loop. A 500 kW data hall still needs 5-7 indoor units, but the outdoor footprint is one cooling tower or dry cooler array, not 5-7 condensers. Refrigerant stays inside indoor units.

CHW (25-500 kW per unit): every indoor unit connects to a chiller plant via chilled-water pipework. Indoor unit count is similar but the indoor units are more compact (no compressor). Outdoor footprint is the chiller plant — typically rooftop or basement.

Efficiency at part load

This is the dominant lifecycle-cost differentiator:

  • DX compressors are most efficient at 80-100% load. At 30% load (typical operating point for most data centres), efficiency drops by 25-40%.
  • CDW compressors behave similarly to DX at part load — slightly better because the heat rejection (cooling tower) operates more efficiently at part load.
  • CHW chillers are dramatically better at part load. Magnetic-bearing chillers (Daikin McQuay, Smardt) deliver part-load efficiency 40-60% better than full-load. Centrifugal chillers with VFDs are similar.

For a 1 MW data centre running at 40% IT load average, the difference in annual cooling power consumption between DX and CHW can be 30-50% — meaning $100,000-$200,000/year on a typical commercial electricity tariff.

Resilience and concurrent maintainability

DX: N+1 redundancy comes from extra indoor units. Loss of one indoor unit means one outdoor condenser idle. The redundancy is granular — each unit is a complete cooling chain.

CDW: N+1 redundancy comes from extra indoor units (sharing the cooling tower) plus extra cooling tower capacity. Single point of failure on the cooling tower water loop, mitigated by N+1 tower modules.

CHW: N+1 redundancy comes from extra indoor units (sharing the chiller plant) plus extra chiller plant. The chiller plant becomes the resilience focus — typically 2N or N+1 chillers, redundant pumps, redundant cooling towers (water-cooled chillers).

For Tier III (concurrent maintainability) and Tier IV (fault tolerance), CHW with redundant chiller plant is the standard architecture. DX is harder to make concurrently maintainable at large scale.

CapEx ranges (indicative, 2026)

For a 500 kW IT-load data centre:

  • DX: $400-$800 per kW IT (cooling equipment + outdoor condensers + refrigerant pipework). Total $200,000-$400,000.
  • CDW: $500-$900 per kW IT (cooling equipment + cooling tower + pipework). Total $250,000-$450,000.
  • CHW: $700-$1,400 per kW IT (cooling equipment + chiller plant + pipework + pumps). Total $350,000-$700,000.

CHW wins on lifecycle cost above ~500 kW IT due to part-load efficiency. Below 200 kW, DX wins clearly. The 200-500 kW band is where CDW often wins — centralised heat rejection without the chiller plant CapEx.

Refrigerant exposure

  • DX: refrigerant in every indoor unit + every outdoor condenser. Multiple charge points. Substantial leak inspection scope.
  • CDW: refrigerant in every indoor unit only. Outdoor side is water (with biocide / inhibitor). Smaller refrigerant footprint.
  • CHW: refrigerant only in centralised chiller plant. Indoor side is water-only. Smallest refrigerant compliance footprint per kW.

For sites with refrigerant management compliance burden (Defence, pharma, hospitals), CHW substantially simplifies the regulated refrigerant scope.

When to choose which (decision matrix)

Choose DX when:

  • IT load < 200 kW
  • No existing chiller plant
  • Independent indoor unit redundancy preferred
  • Site has rooftop space for outdoor condensers
  • Mining / remote sites without centralised plant

Choose CDW when:

  • IT load 200-800 kW
  • Constrained outdoor space (urban data centres)
  • Existing cooling tower in the building (commercial high-rise data halls)
  • Centralised heat rejection preferred but chiller plant CapEx hard to justify

Choose CHW when:

  • IT load > 500 kW
  • Existing chilled-water plant (hospitals, large commercial)
  • High-density / liquid-cooling future plans
  • Tier III / Tier IV resilience required
  • Hyperscale or wholesale data centre build

High-density and AI/GPU considerations

For AI training racks at 30-100 kW per rack, all three air-cooling topologies struggle. The current best practice mixes CHW perimeter cooling with rear-door heat exchangers (RDHx) on high-density racks, transitioning to direct-to-chip liquid cooling at the very high density end.

The chiller plant infrastructure carries through. CHW data halls retrofit naturally to liquid cooling — DX data halls require a chiller plant addition before retrofit is economic.

When to call us

We design, install, and service all three topologies + the high-density / liquid-cooling adaptations. If you're scoping a new build or a major refit, we can model the right topology for your load profile, growth plan, and capital budget.

[Request a Quote](/contact#quick-quote).

References

  • ASHRAE TC 9.9 — Thermal Guidelines for Data Processing Environments
  • AS/NZS 1668.2 — Mechanical ventilation
  • AS/NZS 3666 — Cooling tower / water systems
  • AS/NZS 5149 — Refrigerating systems