Liquid Cooling Technologies: How Modern Data Centres Remove Heat Efficiently
- MTS DNC ENERGY CONSULTANTS LIMITED
- 3 days ago
- 6 min read
As rack power densities continue to increase, data centre designers are moving away from relying solely on air cooling. Instead, they are adopting liquid cooling solutions that remove heat much closer to the source, significantly improving efficiency and reducing overall energy consumption.
Today, three liquid cooling technologies dominate the market:
Direct-to-Chip Cooling
Immersion Cooling
Rear-Door Heat Exchangers
Each technology offers distinct advantages depending on the facility type, IT equipment, cooling strategy, and operational requirements.
1. Direct-to-Chip Cooling
Direct-to-Chip (D2C), sometimes referred to as Cold Plate Cooling or Direct Liquid Cooling (DLC), is currently the fastest-growing liquid cooling technology.
Instead of cooling the surrounding air, cold plates are mounted directly onto the components that generate the greatest amount of heat, typically:
CPUs
GPUs
AI accelerators
High-performance memory modules
A coolant circulates through small channels inside the cold plate, absorbing heat before transferring it to a Coolant Distribution Unit (CDU).
How Direct-to-Chip Cooling Works
The process is relatively straightforward:
Chilled coolant leaves the CDU.
The coolant enters cold plates attached to processors.
Heat transfers directly from the chip into the coolant.
Warm coolant returns to the CDU.
The CDU rejects the heat to the facility water system.
The cooled liquid recirculates through the system.
Because heat is removed directly from the processor, thermal resistance is dramatically reduced compared with conventional air cooling.
Advantages of Direct-to-Chip Cooling
✔ Excellent cooling performance for AI workloads
✔ Lower server fan speeds
✔ Reduced energy consumption
✔ Easier maintenance than immersion systems
✔ Compatible with many existing server platforms
✔ Straightforward retrofit into existing facilities
Limitations
Direct-to-chip cooling typically removes 60–80% of the total server heat.
Remaining components still require some air cooling, including:
Memory modules
Power supplies
Hard drives
Network switches
For this reason, Direct-to-Chip systems often operate alongside traditional air cooling.
2. Immersion Cooling
Immersion cooling represents the most advanced form of liquid cooling currently available.
Rather than cooling individual components, the entire server is submerged inside a specially engineered dielectric liquid that does not conduct electricity.
Because every electronic component is surrounded by coolant, heat is removed uniformly across the entire server.
No internal server fans are required.
Single-Phase Immersion Cooling
In a single-phase system:
Servers remain submerged in liquid.
The coolant never boils.
Pumps circulate the warm liquid through external heat exchangers.
Cooled liquid returns to the immersion tank.
The cooling process is simple, reliable, and highly efficient.
Two-Phase Immersion Cooling
Two-phase immersion introduces an additional thermodynamic advantage.
The dielectric fluid has a very low boiling point.
When hot electronic components transfer heat into the fluid:
The liquid boils.
Vapour rises naturally.
Vapour condenses on cooling coils.
Condensed liquid falls back into the tank.
This process exploits the latent heat of vaporisation, allowing enormous quantities of heat to be removed with very little temperature increase.
The result is exceptional thermal performance.
Advantages of Immersion Cooling
✔ Extremely high cooling capacity
✔ Uniform temperatures across all components
✔ No server fans
✔ Minimal HVAC requirements
✔ Excellent support for future AI hardware
✔ Very low operating noise
✔ Outstanding energy efficiency
Challenges
Despite its impressive performance, immersion cooling presents several challenges.
These include:
Higher capital cost
Specialist maintenance procedures
Dedicated server hardware
Fluid management
Training requirements for maintenance staff
Reduced familiarity among operators
Although immersion cooling is expected to become increasingly common, it remains primarily used in hyperscale AI facilities and supercomputing applications.
3. Rear-Door Heat Exchangers
Rear-Door Heat Exchangers (RDHx) provide an excellent compromise between traditional air cooling and full liquid cooling.
Instead of cooling processors directly, a liquid-cooled heat exchanger replaces the rear door of the server cabinet.
Server fans continue moving air through the equipment.
However, before the warm air enters the room, it passes through the rear-door coil where heat is absorbed by circulating water.
Only cooled air leaves the rack.
Advantages
✔ Simple retrofit solution
✔ Minimal server modifications
✔ Reduced room cooling demand
✔ Lower fan energy
✔ Compatible with existing facilities
Limitations
Rear-door heat exchangers still depend upon server airflow.
They cannot remove heat as efficiently as Direct-to-Chip or immersion systems and therefore are generally suited to medium-density racks rather than extreme AI deployments.
Hybrid Cooling Systems
Rather than replacing every air-cooled rack, many modern data centres combine multiple cooling technologies.
This hybrid approach is rapidly becoming the preferred engineering solution.
A typical arrangement might include:
Equipment | Cooling Method |
AI GPU Clusters | Direct-to-Chip |
HPC Servers | Direct-to-Chip |
Legacy Servers | Air Cooling |
Storage Systems | Air Cooling |
Network Equipment | Rear-Door Heat Exchangers |
This strategy allows operators to modernise their facilities without completely rebuilding them.
It also reduces capital expenditure while improving overall energy efficiency.
Understanding Coolant Distribution Units (CDUs)
The Coolant Distribution Unit (CDU) is the heart of every liquid cooling installation.
It performs the same function that an Air Handling Unit performs within an HVAC system.
Its primary responsibilities include:
Pumping coolant
Controlling flowrate
Maintaining pressure
Monitoring temperatures
Removing heat through heat exchangers
Protecting IT equipment
Without a CDU, liquid cooling cannot operate safely.
Primary and Secondary Cooling Loops
Most liquid-cooled data centres use two independent water circuits.
Primary Loop (Facility Water System)
The primary loop rejects heat from the building.
Typical heat rejection equipment includes:
Dry coolers
Cooling towers
Adiabatic coolers
Chiller systems
The water quality within this circuit is similar to conventional building services installations.
Secondary Loop (Technology Cooling System)
The secondary loop circulates coolant directly to the IT equipment.
This loop normally contains:
Treated water
Water-glycol mixtures
Corrosion inhibitors
Filtration systems
The coolant quality is tightly controlled because it comes into close proximity with expensive electronics.
The CDU hydraulically separates these two systems using an internal plate heat exchanger.
This prevents contamination while allowing heat transfer between the loops.
Why Water Quality Matters
Unlike traditional chilled water systems, liquid cooling requires carefully controlled water chemistry.
Poor water quality can cause:
Corrosion
Scale formation
Biological growth
Reduced heat transfer
Equipment damage
For this reason, ASHRAE defines several water quality classifications.
ASHRAE Environmental Classes Explained
ASHRAE TC 9.9 has developed internationally recognised guidance for liquid-cooled data centres.
The three principal classifications are:
Water Quality Classes
These define coolant purity.
Class | Typical Application |
W1 | Ultra-pure water for Direct-to-Chip systems |
W2 | Controlled water chemistry |
W3 | Building water loops |
W4 | Facility cooling systems |
Higher purity generally means greater protection for sensitive IT equipment.
Water Temperature Classes
ASHRAE also specifies maximum supply temperatures.
Examples include:
Class | Maximum Supply Temperature |
W17 | 17°C |
W27 | 27°C |
W32 | 32°C |
W40 | 40°C |
W45 | 45°C |
Higher water temperatures provide several advantages:
Reduced chiller operation
Increased free cooling
Lower energy consumption
Improved waste heat recovery
Better overall PUE
Modern liquid-cooled facilities increasingly operate at warmer temperatures than traditional chilled-water systems.
Surface Temperature Classes
Surface temperature classifications help prevent condensation.
When water temperatures fall below the surrounding dew point, condensation may form on pipework or IT equipment.
ASHRAE therefore defines acceptable operating limits to ensure reliable operation under different environmental conditions.
Engineering Perspective
From a Building Services Engineering standpoint, liquid cooling fundamentally changes how data centres are designed.
Instead of designing around massive airflow volumes, engineers increasingly focus on:
Hydronic pipework networks
Heat exchanger selection
CDU sizing
Pump energy optimisation
Water treatment
Heat recovery opportunities
Building Management System (BMS) integration
Mechanical engineers are becoming as important to modern data centres as electrical engineers, particularly as AI drives rack densities beyond 100 kW.
In the final part of this guide, we will examine the benefits of liquid cooling, compare Power Usage Effectiveness (PUE) and Water Usage Effectiveness (WUE), discuss retrofit strategies, evaluate lifecycle costs, and explore how waste heat recovery is shaping the next generation of sustainable data centres.
How Our Consultants Can Help
At MTS DNC Energy Consultants, we provide:
Heat pump assessments
Part L compliance reports
Building energy modelling
Through NEXUS M&E Design, we also deliver:
Heat loss calculations
Heat pump sizing
Radiator and underfloor heating design
Mechanical ventilation design
We ensure every system is designed for maximum efficiency, compliance, and long-term performance.
Heat pumps work differently. Rather than generating heat through combustion, they transfer heat from the outside environment into a building. By exploiting the vapour compression refrigeration cycle, heat pumps can deliver three to five times more heat energy than the electrical energy they consume, making them one of the most energy-efficient heating technologies available.
Disclaimer
The content shared in these posts is intended for informational purposes only and should not be interpreted as design advice, specifications, or a calculation template. For professional guidance or design services, please contact us through our contact form.
