Understanding Data Center Cooling Technology: From Legacy Systems to High-Density Solutions

CRAC units are commonly found in small to medium-sized or legacy data centers where simplicity and self-contained cooling are preferred.

They are ideal where infrastructure investment needs to be minimal or where chilled water systems are not yet in place.

Advantages of CRAC

• Simple installation and operation as a self-contained unit.
• No need for extensive chilled water piping or central plant infrastructure.
• Easier to deploy incrementally in existing facilities.

Limitations of CRAC

• Lower cooling capacity (up to ~100 kW per unit), making them less suited for large or high-density data centers.
• Typically less energy efficient compared to chilled water systems.
• Maintenance includes refrigerant handling and condenser upkeep

CRAH units are preferred in larger data centers with existing chilled water infrastructure.

They are suitable when scalable data center cooling and operational efficiency are priorities, especially in facilities that can support central chiller plants and extensive water distribution systems.

Advantages of CRAH

• More energy-efficient than CRAC units due to the use of chilled water.
• Larger capacity units (up to ~250kW per unit) suitable for moderate to large data halls.
• Fewer moving parts compared to refrigerant-based systems, potentially reducing maintenance.

Limitations of CRAH

• Requires robust chilled water piping, pumps, and central plant infrastructure, meaning a higher Capex.
• Often dependent on raised floor designs with perforated tiles for air distribution.
• Return air is typically taken from the top of the unit, which can constrain data hall layouts.

These systems pull hot air from a rear return plenum or corridor and deliver cooled air uniformly across the space.

This integrated setup allows for more precise airflow control and helps prevent hot and cold air from mixing, which can significantly impact cooling performance.

With fans arranged in a stacked, modular array that forms a continuous wall, thermal wall systems offer high airflow capacity while saving valuable floor space.

Their design supports flexible room layouts and makes them especially well-suited for large-scale data centers where CRAH units alone may not provide sufficient cooling or energy efficiency.

Advantages of Thermal Walls

• High cooling capacity (500 kW+ per unit), allowing fewer units to cover large areas.
• Stacked, modular design supports scalable airflow and capacity adjustments.
• Integrated wall placement frees up valuable floor space and removes the need for raised floors and perforated tiles.
• Pulls hot return air from a dedicated mechanical corridor, improving thermal isolation and airflow efficiency.
• Supports flexible room layouts by separating hot and cold air paths, simplifying containment and space planning

Limitations of Thermal Walls

• Requires mechanical corridors or dedicated spaces for hot air return.
• More complex installation compared to traditional CRAH units.
• Initial infrastructure costs can be higher (usually offset by operational savings in large data halls).

RDHx have been around for a while but saw limited adoption in the past.

However, with the rise of ultra-high-density deployments—particularly in AI clusters, hyperscale data centers, and High-Performance Computing (HPC) environments—RDHx is gaining renewed attention as a practical and efficient solution.

By cooling air at the rack level, RDHx reduces or eliminates the need for traditional room-level airflow management, making it an increasingly attractive option in today’s evolving thermal landscape.

Advantages of RDHx

• Cools servers directly at the rack, improving heat transfer efficiency.
• Can be added to a liquid cooling system to help cool the entire rack. While cold plates handle the chips, RDHx cools other components like power supplies and memory. This combination reduces or eliminates the need for traditional air cooling in the data center.
• Enables higher chiller water temperatures, improving chiller efficiency and reducing energy costs.

Limitations of RDHx

• Higher upfront costs due to more complex rack components and cooling plumbing.
• Increased fan power consumption as multiple smaller fans are used instead of fewer large ones.
• May require Coolant Distribution Units (CDUs) to regulate liquid flow and temperature, adding to system complexity.
• Partial room cooling with RDHx requires integration with other cooling methods.

Hot Aisle Containment

This method encloses the hot air exhaust aisle, preventing it from mixing with cold supply air.

It requires a specific ceiling design and is therefore better suited for new builds rather than retrofits.

Cold Aisle Containment

This setup encloses the cold supply aisle to keep cold air focused where it’s needed. However, because the aisle is sealed, accessing equipment inside can be harder during maintenance.

As a result, some operators may avoid fully enclosing the aisle or leave doors open, which reduces the system’s efficiency.