데이터 센터의 CDU 장치란 무엇이며 왜 중요한가요?

How a CDU Unit Works: Fluid Circuits, Heat Exchange, and Control Logic

Understanding CDU unit operation requires looking at the two-loop architecture that most modern designs use. The primary loop connects the CDU to the building's chilled water infrastructure or a dry cooler on the roof. The secondary loop — sometimes called the facility-side and IT-side loops respectively — circulates coolant at the temperature and flow rate the servers actually need. A plate-and-frame heat exchanger inside the CDU transfers heat between the two loops without allowing them to mix, which protects IT equipment from the chemical additives and contaminants present in building water systems.

The control logic inside a CDU unit continuously monitors supply and return water temperatures, differential pressure across the heat exchanger, pump speed, flow rate through each rack manifold branch, and ambient conditions. When a GPU cluster suddenly spikes to full compute load, the CDU's PID controllers increase pump speed within seconds and open modulating valves to deliver additional cooling capacity. This dynamic response is one reason liquid-cooled data centers can sustain higher average utilization rates — the cooling system adapts in real time rather than relying on oversized static air volumes.

Modern CDU units also expose their sensor data to the data center's DCIM (Data Center Infrastructure Management) platform via Modbus TCP, BACnet, or SNMP. This telemetry feeds into power usage effectiveness (PUE) calculations and capacity planning dashboards. A facility running CDU units with active DCIM integration can typically achieve a PUE between 1.03 and 1.15 , compared to 1.4–1.6 for equivalent air-cooled facilities (source: Green Grid Technical Forum, Liquid Cooling White Paper WP#49, 2022).

Types of CDU Units Used in Modern Data Centers

Not all CDU units are architecturally identical. The right choice depends on the data center's existing chilled water infrastructure, the target rack density, the cooling approach (direct liquid cooling vs. rear-door heat exchangers vs. immersion), and whether the facility is a new build or a retrofit. Below are the main categories in current deployment.

Row-Level CDU Units

Row-level CDU units are installed at the end of a server row and serve a defined number of racks — typically 6 to 20 racks per unit. They connect to overhead or under-floor chilled water mains and distribute coolant through a manifold to individual rack cold plates or in-row rear-door heat exchangers. Row-level deployment is the most common architecture in enterprise and colocation data centers upgrading from air cooling, because it allows incremental rollout without redesigning the entire facility. Cooling capacity per row-level CDU unit typically ranges from 50 kW to 300 kW , depending on the number of pump circuits and heat exchanger sizing.

Rack-Integrated CDU Units

Rack-integrated CDU units are mounted directly inside or at the top of a single server rack. They handle the cooling loop for that one rack only, making them suitable for ultra-high-density deployments such as AI training nodes where a single rack may draw 60–120 kW. Because the CDU is co-located with the load, supply and return pipe runs are minimal, reducing both pressure drop and installation labor. The trade-off is that each rack requires its own CDU unit, increasing per-unit capital cost and multiplying the number of facility water connections.

Central Campus CDU Units

Large hyperscale facilities sometimes deploy a central CDU unit room that serves an entire data hall or multiple halls simultaneously. Central CDU units are engineered at a larger scale — some units handle 1 MW or more of heat rejection — and interface directly with chillers, cooling towers, or free-cooling economizers. This architecture simplifies facility-level control and maintenance but requires more complex pipe distribution networks and higher upfront civil engineering investment.

Immersion Cooling CDU Units

Single-phase and two-phase immersion cooling systems use a CDU unit to circulate dielectric fluid through tanks in which servers are fully submerged. The CDU in this context is often called a Fluid Distribution Unit (FDU), but the core function is identical — temperature regulation, flow control, and heat rejection to a facility water loop. Immersion-type CDU units must handle fluids with significantly different viscosity, specific heat, and material compatibility requirements compared to water-based systems. Two-phase immersion systems add a condensation recovery circuit to the CDU design, increasing mechanical complexity but enabling near-zero sensible heat loss.

CDU Unit Installation: Pipe Routing, Commissioning, and Common Pitfalls

Even a correctly specified CDU unit will underperform or fail prematurely if the installation is poorly executed. The following points represent lessons learned from actual liquid-cooled data center deployments and are worth including in project specifications and contractor briefing documents.

Pipe Flushing Before Connecting IT Equipment

New copper or stainless-steel pipe systems accumulate flux residue, metal particles, and construction debris during fabrication. If this contamination enters the cold plates on servers or GPU cards, it can block micro-channels with internal diameters as small as 0.5–1.5 mm , reducing cooling performance and potentially voiding the hardware warranty. The CDU unit's secondary loop must be flushed with deionized water at high velocity and filtered through 5-micron absolute filters until turbidity and conductivity readings meet the manufacturer's specification before any IT equipment connection is made.

Air Purging and Degassing

Air trapped in liquid cooling loops causes pump cavitation, reduces effective heat transfer at cold plates, and accelerates corrosion through oxygen exposure. CDU units should be installed with automatic air vents at all high points in the distribution manifold. The initial fill procedure must include a slow fill-and-vent cycle repeated until the circulation loop is fully degassed — a process that can take several hours on a large row-level deployment.

Water Chemistry Management

The CDU unit's secondary loop requires ongoing water quality management. Key parameters to monitor include pH (target range 7.0–8.5 for copper-containing systems), conductivity (typically less than 50 µS/cm for systems with direct cold plate contact), dissolved oxygen (below 20 ppb to minimize corrosion), and biological contamination. Some operators add biocide and corrosion inhibitor packages; others rely on continuous deionization through an ion exchange resin bed installed in a bypass circuit of the CDU unit.

Thermal Expansion Compensation

Liquid cooling pipes expand and contract as temperatures cycle between power-on and shutdown states. For a 20-meter run of copper pipe cycling between 18°C and 45°C, the linear expansion is approximately 9 mm (copper's coefficient of thermal expansion is ~17 µm/m·°C). Expansion loops or flexible braided stainless connectors must be incorporated at regular intervals to prevent stress buildup at pipe joints, which is the most common cause of slow leaks in aging liquid cooling installations.

CDU Unit Maintenance: Schedules, Procedures, and Lifecycle Management

A CDU unit installed in a data center is expected to operate continuously for 10–15 years with minimal downtime. Achieving that service life requires a structured maintenance program covering both the mechanical and electronic subsystems of the unit.

Variable-speed pump drives (VSDs) are among the highest-value components inside a CDU unit and warrant particular attention. Bearing wear in VSD-driven centrifugal pumps typically follows the Weibull distribution, with most failures occurring after 25,000–40,000 operating hours (approximately 3–5 years of continuous operation). Scheduling bearing replacement as a preventive maintenance task at the 30,000-hour mark avoids the much more disruptive scenario of an unplanned pump failure in an active data hall.

미래 방향: CDU 유닛 기술이 향하는 방향

The CDU unit market is evolving rapidly in response to AI infrastructure demands, sustainability mandates, and advances in fluid management technology. 3~7년의 기간을 두고 데이터 센터 프로젝트를 계획하는 사람이라면 몇 가지 추세를 추적할 가치가 있습니다.