In-Row Cooler

Overview

An in-row cooler (IRC) is a compact water-cooled unit installed in the aisle between server racks, extracting 20–100 kW of heat directly from the hot air plenum. Unlike perimeter CRAC units, IRCs eliminate long ductwork runs and overhead piping, allowing IT teams to cool individual sections of the data center without over-provisioning the entire facility. They are ideal for high-density racks (10–15 kW per rack) and retrofit deployments where adding floor space or perimeter capacity is prohibitive.

IRCs tap into a facility-wide chilled-water loop (supplied by a central chiller, CDU, or economizer). Cold water at 12–18 °C enters a brazed plate heat exchanger, rejecting 40–100 kW of sensible heat to the air stream; warm water returns to the chiller. Dual EC fan motors draw hot server exhaust horizontally through the exchanger at variable speed, modulating airflow via a proportional controller that monitors outlet air temperature.

Water-side flow is often modulated via a 3-way bypass valve to prevent excessive coil pressure drop; a pressure relief valve protects the circuit at 30 psi. Condensate from the cooler body drains via PVC drain line to a floor sump or sewer.

How it works

Water circuit: Chilled water enters at 12 °C via a supply port, flows through 20–30 brazed aluminum plates arranged in a counter-flow pattern against the air stream, and exits at 18–22 °C. Each plate pair increases the surface area for heat transfer; optimal water velocity is 0.5–1.5 m/s (2–3.5 m³/h for a typical unit). A water temperature sensor monitors inlet and outlet to confirm delta-T and detect fouling (if delta-T drops, scaling deposits block plates).

Air circuit: Server exhaust at 35–45 °C is drawn into the unit by two EC fan motors operating in parallel. The fans have integral power electronics commutating at variable voltage (0–10 V input), yielding 500–3000 RPM proportional control. Cold air exits at 16–20 °C, recirculating under the raised floor or returning to the room air handler.

Fan modulation: A micro-controller samples the outlet air temperature every few seconds and issues a proportional-integral (PI) correction to the 0–10 V control module. If outlet temp rises above setpoint (e.g., 18 °C), the PLC ramps voltage upward, increasing fan speed and airflow. If temp drops, voltage decreases to save fan power (up to 70% energy savings vs. fixed-speed fans). The Modbus RTU interface allows the building BMS to read capacity, power draw, and alarms in real time.

Water bypass: A modulating solenoid actuator on the 3-way valve proportions flow between the plate exchanger and a bypass return. If water pressure exceeds 30 psi, the relief valve cracks, bypassing flow to protect the coil from ice-up (freezing at very cold inlet temps) or cavitation.

Condensation: Because IRCs handle sensible cooling only (not latent/dehumidification), minimal condensate forms on the air-side coil. The condensate pan collects stray drops and drains via gravity; an overflow float switch alerts if the drain clogs.

Architecture & Integration

IRCs integrate into a chilled-water loop serving the entire data center. A central chiller or refrigeration rack produces chilled water at 10–15 °C. Distribution piping supplies multiple IRCs in parallel; each unit modulates via bypass valve to accept whatever flow the loop provides. A circulating pump maintains 2–3 bar inlet pressure.

Alternatively, an air-economizer (free cooling from outside air when ambient is below 15 °C) can displace chiller load; the BMS staggers IRC speed and chiller compressor capacity to minimize energy.

In hot-aisle containment, IRCs are placed at the end of each hot aisle with a return-air plenum on the cold side, preventing recirculation and achieving 50–70% efficiency gain vs. open-aisle layouts.

Maintenance & Reliability

Water-side fouling: Hard water or algae deposits reduce plate efficiency over 6–12 months. Annual descaling (citric acid flush) or water treatment prevents buildup.

Fan bearing wear: EC fan motors with sealed bearings typically run 40,000–60,000 hours. Replacement modules (motor + fan + shroud) enable fast turnaround if a bearing seizes.

Drain blockage: Hair, dust, and mineral deposits can clog the 3/4" drain line. Quarterly flushing with warm water or mild vinegar prevents overflow.

Coil leaks: Microcracks in brazed aluminum can develop under thermal stress. A slow water leak (<1 drop/min) is monitored; faster leaks require coil replacement.

Standards & Efficiency

• AHRI 320: Chilled-water cooler performance rating.
• ISO 5226: Vibration limits for in-duct equipment.
• ASHRAE 90.1: Energy code for modulation and controls.
• PUE (Power Usage Effectiveness): IRCs typically cut cooling energy by 25–40% vs. CRAC-only architecture by eliminating ductwork and enabling localized load response.

Data centers with 40+ racks per aisle often achieve PUE = 1.2–1.3 using IRC + hot-aisle containment + water-side economizer.