In-Row vs Perimeter CRAC Cooling for High-Density Racks: A Technical Comparison

Perimeter CRAC units were engineered for a world where average rack densities sat between 3 kW and 6 kW. That world no longer exists in most production data centres. GPU compute nodes, hyperconverged infrastructure, and high-core-count processors routinely push individual racks past 15 kW, with some AI workload deployments reaching 30 kW to 50 kW per rack. At those densities, the physics of perimeter cooling break down before the equipment does.

This post compares perimeter and in-row cooling across four dimensions: airflow mechanics, cooling efficiency, capital and operating cost, and retrofit feasibility. The goal is to give data centre designers, infrastructure managers, and colocation operators a clear basis for deciding which architecture suits their environment.

How Perimeter Cooling Works, and Where It Fails

A perimeter CRAC unit sits at the edge of the data centre floor, typically against a wall or in a corner. It draws warm return air from the room, conditions it across a direct expansion or chilled water coil, and discharges cooled air into a raised floor plenum or directly into the room at low velocity. That air then travels horizontally across the floor, rises through perforated tiles in front of equipment racks, and enters servers from the front face.

The problem is distance. In a room 20 metres wide with CRAC units on opposing walls, air travelling to a rack in the centre of the row has already absorbed heat from adjacent equipment before it arrives. Supply air temperatures at the rack face can be 4°C to 8°C warmer than the CRAC discharge temperature, depending on room layout, cable tray obstruction, and tile perforation percentage. ASHRAE TC 9.9 guidance flags return temperature index (RTI) values above 1.0 as a sign that hot and cold air are mixing excessively. Perimeter-cooled rooms with dense racks almost always produce RTI values well above that threshold.

Beyond mixing losses, perimeter units are sized for room-level heat loads, not rack-level ones. A single 15 kW rack in a room with an average density of 5 kW per rack will create a localised hot spot that the perimeter unit cannot address without overcooling the rest of the room. Operators typically respond by lowering set-points, which increases compressor energy consumption across the entire installation.

How In-Row Cooling Works

In-row cooling places the conditioning unit directly within the rack row, between equipment cabinets. The unit draws hot exhaust air from the hot aisle at the rear of adjacent racks, conditions it, and discharges cooled air directly into the cold aisle at the front. The air path from unit to rack face is typically under two metres.

Because the unit captures heat at the source rather than after it has dispersed into the room, supply air temperature at the rack inlet is far more predictable. A well-configured in-row installation can maintain rack inlet temperatures within 1°C to 2°C of the unit's discharge set-point across the full row. That precision matters when servers are operating at thermal margins set by manufacturers.

In-row units are available in direct expansion (DX), chilled water (CHW), and rear-door heat exchanger configurations. For high-density environments, chilled water in-row units are the most common choice because they can be sized to handle 30 kW to 80 kW per unit without the refrigerant circuit constraints of DX systems. Vertiv's Liebert CW series, Schneider Electric's InRow range, and Stulz's MiniSpace all offer chilled water variants suited to this application.

Efficiency: The Numbers That Matter

The efficiency gap between perimeter and in-row cooling at high densities is measurable and material.

Partial power usage effectiveness (pPUE) for the cooling subsystem in a perimeter-cooled room at 10 kW average rack density typically sits between 1.35 and 1.55, based on field measurements published by the Uptime Institute and ASHRAE research groups. As rack density increases beyond 10 kW, pPUE tends to worsen because perimeter units run harder to compensate for mixing losses and hot spots.

In-row cooling at equivalent or higher rack densities typically achieves pPUE between 1.10 and 1.25 for chilled water configurations connected to a water-cooled plant. The shorter air path reduces fan energy because air does not need to travel far against resistance. Hot and cold air separation is maintained by the physical position of the unit rather than by raised floor pressure management, which eliminates the need for blanking panels, grommet seals, and tile placement discipline to achieve the same result.

Fan energy is a meaningful contributor. Perimeter CRAC units often run large centrifugal fans at high static pressure to force air through a raised floor plenum and across a room. In-row units use smaller fans operating at lower static pressure because the air path is short and direct. EC fan motors in modern in-row units can reduce fan energy by 30% to 50% compared with legacy perimeter unit fan assemblies.

Capital and Operating Cost Comparison

Perimeter CRAC units carry a lower upfront cost per unit than in-row equipment. A floor-standing perimeter unit with a 50 kW cooling capacity might cost between AUD $18,000 and $35,000 depending on refrigerant circuit type and controls specification. An equivalent-capacity chilled water in-row unit from Vertiv, Schneider Electric, or Stulz typically ranges from AUD $25,000 to $55,000, plus the cost of chilled water pipework distribution within the row.

However, the operating cost picture reverses at high densities. Lower pPUE means lower electricity consumption per kilowatt of IT load cooled. At an electricity cost of AUD $0.15 per kWh (a conservative commercial rate for large consumers in eastern Australia), a 500 kW IT load operating at pPUE 1.45 versus 1.15 generates an annual cooling energy cost difference of approximately AUD $197,000. That gap pays back the capital premium of in-row equipment within two to three years in most scenarios.

Maintenance costs are broadly comparable, though in-row chilled water units have fewer refrigerant circuit components to service than DX perimeter units, which reduces the frequency of refrigerant leak checks, compressor inspections, and refrigerant management obligations under Australia's Ozone Protection and Synthetic Greenhouse Gas Management Act.

Retrofit Feasibility

Retrofitting in-row cooling into an existing perimeter-cooled data centre is achievable but requires planning. The key constraints are:

Chilled water availability. In-row CHW units require a chilled water loop within the data centre floor. If the existing building plant only serves perimeter CRAC units, extending pipework to row level involves civil work, pipe sizing calculations, and balancing valve installation. In some facilities, this is straightforward. In others, particularly those in leased premises with limited plant room access, it can be the deciding factor.

Row spacing and cabinet footprint. In-row units occupy one rack unit of floor space per unit within the row. A typical 600 mm wide in-row unit replaces one rack position per 10 to 15 kW of cooling capacity. In dense rows where every rack position is allocated, this requires either row reconfiguration or accepting a reduction in rack count.

Hot aisle containment. In-row cooling performs best with hot aisle containment in place. Without containment, the hot exhaust air the unit is designed to capture disperses into the room before it reaches the unit intake, reducing effectiveness. Retrofitting containment into an operational data centre requires careful sequencing to avoid thermal events during installation.

Controls integration. Modern in-row units from Vertiv, Schneider Electric, and Stulz support BMS integration via Modbus, BACnet, or SNMP. Integrating them into an existing building management system requires controls engineering work, particularly if the existing perimeter units operate on a separate controls platform. Sensor calibration and set-point coordination between row-level and room-level cooling are essential to avoid simultaneous heating and cooling conflicts.

For facilities that cannot accommodate full in-row retrofits, rear-door heat exchangers offer a partial solution. These passive or active units mount on the rear of existing racks and capture exhaust heat before it enters the hot aisle. They do not provide the same degree of control as in-row units, but they can manage localised hot spots without chilled water distribution to row level.

Which Architecture Suits Which Environment

Perimeter CRAC cooling remains appropriate for:

• Data centres with average rack densities below 8 kW
• Smaller server rooms where the distance from perimeter unit to rack is under six metres
• Environments where budget constraints make in-row infrastructure impractical and densities do not justify the investment

In-row cooling is the technically sound choice for:

• Any environment with racks consistently above 10 kW
• GPU compute, AI inference, or HPC deployments where rack densities reach 20 kW to 50 kW
• New data centre builds where chilled water infrastructure can be designed in from the start
• Colocation facilities that need to offer high-density cage products without building separate cooling zones

The transition point is not a fixed number. It depends on room geometry, existing plant capacity, and the distribution of load across the floor. Thermal modelling using computational fluid dynamics (CFD) analysis is the most reliable way to identify where perimeter cooling reaches its limits in a specific room before committing to a retrofit.

Making the Decision

The choice between perimeter and in-row cooling at high densities is ultimately a question of air path length. Perimeter units ask conditioned air to travel metres across a room and through a plenum before reaching the equipment it is meant to cool. In-row units close that distance to under two metres and capture heat at the point of generation. At rack densities above 15 kW, the physics consistently favour the shorter path.

For data centre designers and operators evaluating cooling architecture for high-density environments, CRAC Services Australia provides thermal modelling, airflow analysis, and in-row cooling specification across Brisbane, Sydney, and Melbourne. Visit [https://crac.services](https://crac.services) to discuss your facility's requirements.