Can Rear Door Water Cooled Heat Exchanger Be Used with Free Cooling Systems?

Rear Door Water-Cooled Heat Exchangers (RDHx) are particularly well suited for use with free cooling systems, and this is one of their key advantages in modern, energy-efficient data centers.

How RDHx Works with Free Cooling

RDHx transfers heat from server exhaust air directly to a water loop. When this water loop is connected to a free cooling source, mechanical refrigeration can be reduced or completely eliminated during suitable ambient conditions.

Typical free cooling sources include:

• Dry coolers

• Adiabatic dry coolers

• Cooling towers

• Hybrid air-water heat rejection systems

As long as the water temperature supplied to the RDHx is below the server exhaust air temperature, effective heat removal is achieved.

Suitable Water Temperature Range

RDHx systems can operate with:

• Chilled water: 7–18 °C

• Warm water / free cooling water: 18–35 °C (or higher, depending on design)

This wide operating range allows data centers to maximize free cooling hours throughout the year.

Energy Efficiency Benefits

Using RDHx with free cooling systems delivers several measurable benefits:

• Significant reduction in chiller runtime

• Lower overall energy consumption

• Improved Power Usage Effectiveness (PUE)

• Compatibility with high supply air temperature strategies

System Configurations

Common configurations include:

• RDHx + dry cooler for full free cooling in cool climates

• RDHx + adiabatic cooler for extended free cooling in warmer climates

• RDHx + cooling tower with plate heat exchanger separation

A buffer tank and variable-speed pumps are often used to stabilize water temperatures and flow.

Key Design Considerations

When integrating RDHx with free cooling, attention should be given to:

• Water flow rate and pressure drop across the rear door coil

• Ambient design temperature and climate conditions

• Control strategy for bypass or partial mechanical cooling

• Water quality and corrosion protection

Typical Applications

• High-density server racks (30–80 kW per rack)

• AI and HPC data centers

• Retrofit projects aiming to reduce cooling energy

• Facilities targeting low-carbon or carbon-neutral operation

Summary

Rear Door Water-Cooled Heat Exchangers are fully compatible with free cooling systems and are often selected specifically to enable or maximize free cooling operation. When properly engineered, they can deliver high rack-level cooling performance while substantially reducing operating costs and environmental impact.