DX, CDW, and CHW Cooling Architectures: A Technical Comparison for Australian Data Centres

Three cooling architectures dominate data centre mechanical design: direct expansion (DX), condenser water (CDW), and chilled water (CHW). Each moves heat from the IT space to atmosphere through a different refrigerant circuit topology, and each carries a distinct profile of capital cost, operating efficiency, scalability, and climate dependency. Choosing the wrong one at design stage is expensive to correct later.

This comparison covers the engineering fundamentals, the numbers that matter, and the Australian-specific conditions that should influence the decision.

How Each Architecture Works

Direct Expansion (DX)

A DX system refrigerates air directly in the CRAC unit. The compressor, condenser, expansion valve, and evaporator coil are all contained within or immediately adjacent to the precision cooling unit. Refrigerant absorbs heat at the evaporator coil inside the data hall and rejects it at a remote condenser, either air-cooled on the roof or a drycooler circuit.

Brands like Vertiv Liebert PDX, Stulz CyberAir, and Schneider Uniflair InRow all use DX circuits. The system is self-contained, which makes it fast to commission and straightforward to maintain.

Condenser Water (CDW)

A CDW system, sometimes called water-cooled DX, retains the refrigerant compressor in the CRAC unit but rejects heat to a building chilled or condenser water loop rather than to outdoor air. The CRAC unit contains a refrigerant-to-water heat exchanger on the condenser side. That water loop connects to a cooling tower or dry cooler on the roof.

This topology is common in multi-tenant buildings where a shared condenser water plant serves multiple tenants. Vertiv Liebert CW and Stulz MiniSpace water-cooled variants are typical examples.

Chilled Water (CHW)

A CHW system removes the compressor from the CRAC unit entirely. The precision air handler, called a CRAH (computer room air handler), contains only a chilled water coil and fans. Refrigeration happens remotely in a central chiller plant, which may serve dozens of CRAH units across a large data centre. The CRAH is mechanically simple; the complexity sits in the chiller plant, cooling towers, and distribution pipework.

This is the architecture of choice for hyperscale and large enterprise data centres. Schneider Uniflair TDCV, Vertiv Liebert PCW, and Stulz CompTrol CRAH units are designed for CHW systems.

Efficiency Comparison

Cooling efficiency is measured as the coefficient of performance (COP) or, in data centre contexts, the cooling capacity delivered per kilowatt of electrical input.

DX systems typically achieve a COP of 2.5 to 3.5 at design conditions. In Brisbane or western Sydney summer conditions, where ambient temperatures regularly exceed 35°C, an air-cooled DX condenser degrades noticeably. Every 1°C rise in ambient above the design point costs roughly 1 to 1.5% in compressor capacity and increases power draw. A unit rated at COP 3.2 at 35°C ambient may operate at COP 2.6 at 42°C, which is not unusual in Melbourne's west or inland Queensland.

CDW systems decouple the CRAC unit's condenser from outdoor air, so the CRAC unit itself operates at a more stable efficiency. The overall system COP depends on the cooling tower or dry cooler performance, but a well-designed CDW system with a cooling tower can achieve system COP values of 3.5 to 4.5. The cooling tower benefits from wet-bulb temperature rather than dry-bulb, which in most Australian coastal cities is 6 to 10°C lower than dry-bulb on hot days.

CHW systems achieve the highest system-level efficiency when the chiller plant is sized and operated correctly. A modern centrifugal chiller at part load can reach COP values of 5.0 to 7.0. When combined with waterside economisation, where cool overnight or winter conditions allow the chiller to be bypassed entirely, annualised PUE figures below 1.3 are achievable in Melbourne and Sydney. ASHRAE TC 9.9 2021 guidance supports supply air temperatures up to 27°C for A2 class equipment, which allows higher chilled water supply temperatures and further improves chiller COP.

Capital Cost

DX carries the lowest capital cost at small scale. A 50 kW DX CRAC unit with air-cooled condenser can be installed for $30,000 to $60,000 including commissioning. There is no chiller plant, no cooling tower, and no distribution pipework to fund.

CDW costs more than DX because the condenser water plant, pipework, and cooling tower add $80,000 to $200,000 in shared infrastructure. However, that infrastructure is often already present in a co-location building, so the incremental cost for a new tenant may be only the connection fee and the water-cooled CRAC units themselves.

CHW carries the highest capital cost. A chiller plant sized for 500 kW of IT load, including chillers, cooling towers, primary and secondary pumping, and CRAH units, typically costs $400,000 to $900,000 depending on redundancy requirements. The N+1 or 2N redundancy configurations required for Tier III and Tier IV facilities multiply that figure further.

Below roughly 200 kW of IT load, CHW rarely justifies its capital cost. Above 500 kW, the operating cost savings and scalability advantages begin to outweigh the capital premium within five to seven years.

Operating Cost

Operating cost is dominated by compressor energy in DX and CDW systems, and by chiller and pump energy in CHW systems.

For a 500 kW IT load facility operating 8,760 hours per year in Sydney:

• DX at COP 3.0: approximately 1,460 MWh per year in cooling energy
• CDW at COP 4.0: approximately 1,095 MWh per year
• CHW at COP 5.5 with partial economisation: approximately 700 to 800 MWh per year

At a commercial electricity rate of $0.18 per kWh (approximate 2026 large-site rate), that difference between DX and CHW is $120,000 to $135,000 per year for a 500 kW facility. Over a ten-year asset life, the CHW system's operating cost advantage easily exceeds its capital cost premium.

CDW systems also carry water treatment and cooling tower compliance costs under AS/NZS 3666, which requires documented water treatment programmes and Legionella risk management plans. These add $8,000 to $20,000 per year depending on tower size and local council requirements.

Scalability

DX scales poorly above 500 kW. Adding cooling capacity means adding discrete CRAC units, each with its own refrigerant circuit, compressor, and condenser. Refrigerant pipework runs become longer and harder to manage. Balancing airflow across multiple units in a large hall requires careful placement and often results in hot spots.

CDW scales moderately well within a building's condenser water plant capacity. Adding CRAC units is straightforward as long as condenser water flow and cooling tower capacity are available. The constraint is the shared plant.

CHW scales best. Adding CRAH units to an existing chilled water loop is low-cost and low-risk. The chiller plant can be expanded modularly. Large data centres routinely operate CHW systems serving 5 MW to 50 MW of IT load from a central plant with multiple chillers in parallel, each brought online as load grows.

Climate Suitability for Australian Conditions

Australia's climate varies enough across the three main data centre markets to influence architecture selection.

Brisbane and South-East Queensland have high summer dry-bulb temperatures (35 to 40°C) but moderate wet-bulb temperatures (24 to 26°C). DX air-cooled systems work but suffer efficiency losses in summer peaks. CDW and CHW with cooling towers perform well because the wet-bulb advantage is significant. Waterside economisation hours are limited compared to Melbourne.

Sydney offers moderate dry-bulb peaks (38 to 42°C in western suburbs, lower on the coast) and wet-bulb temperatures of 22 to 25°C. All three architectures are viable. CHW with cooling towers and partial economisation delivers good annualised efficiency. Sydney's water supply reliability and water treatment requirements under AS/NZS 3666 are manageable.

Melbourne has the best climate for waterside economisation. Overnight temperatures regularly fall below 15°C, and winter wet-bulb temperatures of 8 to 12°C allow cooling towers to reject heat without mechanical refrigeration for extended periods. A CHW system in Melbourne with a properly designed economiser bypass can run compressor-free for 2,000 to 3,500 hours per year, cutting chiller energy by 25 to 40% annually. This is the strongest argument for CHW in the Victorian market.

When to Choose Each Architecture

Choose DX when:

• IT load is below 150 kW
• The facility is a single-room server room or edge deployment
• Budget for infrastructure is constrained
• Speed of deployment matters more than long-term efficiency
• The site has no existing water infrastructure

Choose CDW when:

• The facility is in a multi-tenant building with an existing condenser water plant
• IT load is 100 kW to 500 kW
• Air-cooled DX efficiency is unacceptable but a full chiller plant is not justified
• The building owner manages the shared plant and its AS/NZS 3666 obligations

Choose CHW when:

• IT load exceeds 300 kW and is expected to grow
• Long-term operating cost and PUE targets are primary design drivers
• The site is in Melbourne or another climate with meaningful economisation potential
• Redundancy requirements are Tier III or above, where centralised plant with N+1 chillers is more cost-effective than duplicating DX units
• The organisation has mechanical engineering staff or a service partner capable of managing a chiller plant

Refrigerant Considerations

DX and CDW systems contain refrigerant in the CRAC unit. Under Australia's obligations under the Kigali Amendment, high-GWP refrigerants including R410A (GWP 2,088) face accelerating phase-down through the 2020s. New DX and CDW equipment is moving to R32 (GWP 675) and R513A (GWP 631). This transition affects spare parts planning and service agreements for any DX or CDW installation.

CHW systems contain refrigerant only in the chiller plant, which is typically located in a plant room rather than the data hall. This separation simplifies refrigerant management and reduces the risk of refrigerant-related downtime in the IT space itself.

Making the Decision

The right architecture depends on load size, budget, climate, and the organisation's appetite for infrastructure complexity. DX is not inferior to CHW; it is appropriate for a different scale and context. A 100 kW edge data centre in Brisbane does not need a chiller plant. A 2 MW facility in Melbourne should not be running air-cooled DX.

For facilities in the 200 kW to 500 kW range, the decision between CDW and CHW often comes down to whether a chiller plant can be shared with other building loads, and whether the site's climate offers enough economisation hours to justify the CHW capital premium.

CRAC Services Australia works across all three architectures, servicing Vertiv Liebert, Schneider Uniflair, and Stulz equipment in Brisbane, Sydney, and Melbourne. For guidance on architecture selection, thermal modelling, or maintenance of existing systems, visit [crac.services](https://crac.services).