TL;DR
- In-row cooling places the air-handling unit between racks — typically every 2-4 racks — so the supply air travels metres rather than across the room.
- Close-coupling reduces fan power, eliminates supply-side mixing, and supports rack densities up to 30-40 kW with air alone (50+ kW with rear-door HX assistance).
- Distinct from perimeter (CRAH) cooling, where large units sit at the room edge and supply air through a raised floor or ducted plenum.
- Standard in modern hyperscale and high-density colocation. The default precursor to liquid cooling for facilities transitioning toward AI workloads.
Overview#
Traditional perimeter cooling — large CRAH or CRAC units around the room edge pushing air through a raised floor — works well at low density but degrades as racks pull more air. By 15-20 kW per rack, the raised floor cannot supply enough air without unreasonably tall floor voids and high static pressure.
In-row cooling answers this by moving the air-handling unit into the row, alongside the racks. Each unit takes warm exhaust from the hot aisle, cools it, and discharges it into the cold aisle a few metres away. The path is short, the supply temperature is tightly controlled per row, and the room itself plays a much smaller role in the thermal model.
Typical Specifications#
| Parameter | Typical range |
|---|---|
| Capacity per unit | 20-100 kW |
| Form factor | Half-rack or full-rack between racks |
| Heat exchange | Chilled water (CW), DX, or hybrid |
| Fan type | EC, variable speed |
| Airflow per unit | 3,000-15,000 m³/h |
| Supply air temperature | 16-25 °C (configurable) |
| Chilled-water inlet | 10-22 °C typical |
| Coverage | 1 unit per 2-4 racks |
When to Use#
- Medium-density deployments (15-30 kW per rack) where perimeter cooling cannot supply enough cold air.
- High-density retrofits where adding more perimeter capacity is impractical — a new in-row unit drops into a vacant rack slot.
- Containment programmes — in-row works naturally with hot-aisle containment because the cold supply is delivered into the contained cold aisle.
- Edge or small-footprint sites where perimeter CRAH would consume too much floor area.
- Pre-DLC transitions — install in-row at 30 kW now, retrofit DLC and convert in-row units to ride-through later.
Trade-offs#
- vs perimeter CRAH: better at high density, more capex per kW, more units to maintain. Standard above ~15 kW per rack.
- vs rear-door heat exchanger: in-row supplies cool air to multiple racks; RDHx captures heat from one rack. Use RDHx when density per rack exceeds in-row capacity.
- vs direct-to-chip: in-row is air-only; DLC reaches into the chassis. Above 40 kW per rack, DLC wins.
- Footprint: each in-row unit consumes a rack position — a 100-rack row may have 25-30 cooling units and 70-75 IT racks.
- Redundancy: in-row is naturally N+1 — losing one unit reduces capacity for adjacent racks; nearby units pick up the slack.
Operational Pitfalls#
- Airflow balancing: each in-row unit's discharge competes with its neighbours. Commissioning must verify per-rack inlet temperatures across the row.
- Chilled-water piping in the row: more units means more pipework, more joints, more potential leak points. Routine inspections matter.
- Maintenance access: pulling a unit for service can affect adjacent racks. Coordinate with workload owners.
- Filter changes: per-unit filters are more frequent maintenance touches than perimeter equivalents.
- Mixed-density rows: a 50 kW rack next to a 10 kW rack uses cooling unevenly. Group densities into similar rows.
References
- ASHRAE TC 9.9 — Thermal Guidelines · ASHRAE
- Uptime Institute — High-Density Cooling Strategies · Uptime Institute
- Open Compute Project — Cooling Sub-project · OCP