Articles -
June 2025
Immersion Coolants for Data Centers: Selection Criteria
Carter Prokesch
As data centers rapidly scale to meet the demands of AI, machine learning, and high-performance computing (HPC), traditional air-cooling methods are reaching their physical limits. Liquid immersion cooling—where IT equipment is fully submerged in a thermally conductive but electrically insulating dielectric fluid—has emerged as the premier solution for massive heat dissipation. However, selecting the right immersion coolant is a highly complex engineering decision.
Single-Phase vs. Two-Phase Immersion
The first major criteria is determining your cooling architecture:
- Single-Phase Immersion: The coolant remains in a liquid state throughout the entire heat transfer process. Heat is removed via natural or forced convection, and the warm fluid is pumped to a heat exchanger. These fluids (often hydrocarbons, natural esters, or silicones) are highly stable and easier to contain.
- Two-Phase Immersion: The fluid has a low boiling point and turns into vapor upon contact with hot server components (pool boiling). The vapor rises, hits a condenser coil, turns back into liquid, and falls back into the tank. While incredibly efficient for high-density loads, these engineered fluids (often fluorochemicals) require tightly sealed systems and advanced pressure management.
Figures of Merit (FOM)
To objectively compare the heat transfer capabilities of different fluids, engineers use calculated Figures of Merit (FOM). These are mathematical models that account for density, specific heat, thermal conductivity, and dynamic viscosity under specific flow conditions.
For example, in natural convection, heat transfer is heavily dependent on fluid viscosity. Fluids like PAO, GTL (Gas-to-Liquid), and white mineral oils vary wildly in their FOM scores based on their viscosity indexes. In contrast, forced convection (where pumps actively circulate the fluid through heat sink microchannels) relies less on low viscosity and more on the fluid’s specific heat capacity.
Flash Point and Safety
It is vital to take a holistic approach when evaluating these properties. For single-phase immersion coolants, the flashpoint is a critical safety parameter. Unfortunately, there is often an inverse relationship between thermal performance and flashpoint: oils with higher, safer flashpoints generally possess higher viscosities, which negatively impacts their FOM heat transfer scores.
A notable exception includes emerging bio-based renewable hydrocarbons, which manage to strike a unique balance—offering a remarkably low viscosity alongside a stable flashpoint, resulting in higher FOM scores than traditional PAO or white mineral oils.
Summary
Ultimately, there is no one-size-fits-all solution for data center liquid cooling. The choice of coolant is heavily dependent on equipment material compatibility, specific TDP (Thermal Design Power) requirements, facility safety constraints, and overall budget. By rigorously evaluating the thermodynamic properties and FOMs of each chemistry, operators can select the ideal fluid to future-proof their IT infrastructure.
- Follow Dynalene on LinkedIn for more updates.