Why Use RDHx in an IT Room?

An RDHx (Rear Door Heat Exchanger) is used in IT rooms and data centers to remove heat directly at the server rack level, before hot air mixes into the room. This approach dramatically improves cooling efficiency, supports high rack power densities, and reduces dependence on traditional room-level air conditioning such as CRAC or CRAH units.

1. What Is an RDHx?

An RDHx is a water-cooled heat exchanger mounted on the rear door of a server rack. As hot exhaust air leaves the servers, it passes through the RDHx coil, where heat is transferred to chilled or warm water. The cooled air is then discharged back into the IT room at or near room temperature.

2. Key Reasons to Use RDHx in IT Rooms

2.1 High-Density Rack Cooling

Modern IT racks often exceed 15–30 kW per rack, which is difficult and inefficient to cool using room-based airflow alone.

RDHx systems can:

• Handle 20–80 kW per rack (model and water temperature dependent)

• Prevent hot spots at the rack level

• Enable higher compute density without expanding white space

2.2 Improved Energy Efficiency (Lower PUE)

By capturing heat at the source, RDHx systems:

• Reduce fan energy required for room air circulation

• Lower or eliminate chiller compressor runtime

• Enable warm-water cooling and free cooling

This can reduce cooling energy by 30–50% compared to traditional CRAC-only designs.

2.3 Reduced Airflow Management Complexity

With RDHx:

• Hot/cold aisle containment becomes optional

• Less dependence on raised floors and high static pressure air systems

• More stable inlet temperatures across racks

This simplifies IT room layout and airflow design.

2.4 Compatibility with Free Cooling and Dry Coolers

RDHx systems operate effectively with higher water supply temperatures (e.g. 18–30 °C), making them ideal for:

• Dry coolers

• Cooling towers

• Free-cooling loops

In many climates, this allows compressor-free operation for most of the year.

2.5 Modular and Scalable Deployment

• Installed rack-by-rack as density increases

• No major disruption to existing cooling infrastructure

• Ideal for retrofit of legacy IT rooms

3. How RDHx Works (Simplified)

1. Servers exhaust hot air (typically 35–50 °C)

2. Air passes through RDHx coil mounted on rack rear

3. Heat transfers to circulating water

4. Air exits at ~22–27 °C

5. Heated water is rejected via dry cooler, chiller, or heat recovery system

4. RDHx vs Traditional Room Cooling

5. Additional Operational Benefits

• Lower noise levels in IT room

• Improved temperature uniformity

• Reduced risk of thermal runaway

• Potential for waste heat recovery (office heating, DHW)

6. When RDHx Is the Right Choice

RDHx is especially suitable when:

• Rack densities exceed 10–15 kW

• Space for additional CRAC units is limited

• Energy efficiency targets or carbon reduction goals are strict

• Free cooling or warm-water cooling is planned

• Incremental, modular growth is required

7. Limitations to Consider

• Requires water piping to racks

• Higher upfront cost than basic air cooling

• Proper leak detection and redundancy design needed

These risks are manageable with modern drip-less couplings and monitoring systems.

8. Summary

RDHx is used in IT rooms because it efficiently removes heat at the rack, supports high-density computing, and enables low-energy cooling architectures. For modern data centers and high-performance IT rooms, RDHx bridges the gap between traditional air cooling and full liquid cooling—delivering performance, scalability, and energy savings.