The main difference between single-phase cooling fluids and two-phase cooling fluids is how they absorb and transfer heat. Single-phase fluids remain liquid throughout the cooling process. Two-phase fluids absorb heat by boiling into vapour, then turn back into liquid when cooled. Both methods can be used in immersion cooling, where electronic equipment sits directly in a suitable electrically insulating fluid.
This difference affects fluid selection, equipment design, maintenance and heat removal. Understanding how each method works helps explain why a fluid suitable for one system may not suit another.
What Does “Phase” Mean in Cooling Fluids?
Here, “phase” means the fluid’s physical state: liquid or vapour.
- Single-phase cooling: The fluid heats up and cools down while staying liquid.
- Two-phase cooling: The cooling cycle includes both liquid and vapour.
These terms also apply to certain cold-plate cooling systems. This article focuses on immersion cooling fluids, which directly contact electronic components. The Open Compute Project distinguishes immersion cooling from cold-plate systems, where coolant flows through plates attached to components.
What Are Single-Phase Cooling Fluids?
Single-phase cooling fluids absorb heat from electronic components without boiling during normal operation. As the fluid absorbs heat, its temperature rises. The system then removes that heat and returns cooled fluid to the equipment.
How Does Single-Phase Immersion Cooling Work?
A typical cooling cycle follows these steps:
- Electronic equipment is submerged in a suitable dielectric fluid.
- Heat passes from the components into the surrounding liquid.
- The warmed fluid moves through the system using pumps or natural convection.
- A heat exchanger transfers the heat to a separate cooling circuit.
- The cooled fluid continues circulating.
Natural convection means warmer, less dense fluid rises while cooler fluid moves down. Therefore, a pump is not required for fluid circulation in every single-phase design.
Single-phase immersion fluids include suitable hydrocarbons, natural and synthetic esters, and some fluorochemical fluids. Their suitability depends on the specific formulation and operating conditions.
Example: Shell identifies its Immersion Cooling Fluid S3 X as a synthetic single-phase fluid for applications including high-performance computing and edge computing. It illustrates how a purpose-designed fluid can cool electronic equipment while remaining liquid.
What Are Two-Phase Cooling Fluids?
Two-phase cooling fluids are selected to boil within the system’s intended operating conditions. Heat from electronic components causes some of the liquid to become vapour.
This process uses latent heat of vaporisation: the heat absorbed when a liquid changes into vapour.
How Does Two-Phase Immersion Cooling Work?
In a typical tank-based system:
- Electronic components are submerged in dielectric fluid.
- Heat causes the fluid to boil at hot component surfaces.
- The resulting vapour rises towards a condenser.
- The condenser removes heat, turning vapour back into liquid.
- The liquid returns to the bath, repeating the cycle.
The system must contain and recover vapour to limit fluid loss. Boiling and condensation can move the immersion fluid without an internal circulation pump, although the wider cooling installation may still use pumps.
Example: Chemours’ product information describes Opteon 2P50 as a developmental dielectric fluid intended for two-phase immersion and electronic cooling. This illustrates a fluid designed specifically for phase-change heat transfer.
How Do Single-Phase and Two-Phase Cooling Fluids Compare?
| Comparison point | Single-phase cooling fluids | Two-phase cooling fluids |
|---|---|---|
| State during cooling | Remain liquid | Change between liquid and vapour |
| Main heat absorption method | Liquid temperature increases | Liquid boils and absorbs heat during the phase change |
| Intended boiling behaviour | Boiling is avoided during normal operation | Boiling is part of normal operation |
| Typical fluid movement | Pumped circulation or natural convection | Vapour rises; condensed liquid returns |
| Heat removal equipment | Heat exchanger | Condenser |
| Fluid containment | Prevents leakage and contamination | Must also control vapour escape |
| System considerations | Liquid flow and heat exchanger performance | Boiling performance, condensation and vapour management |
These differences describe the operating methods. Actual equipment arrangements vary between system designs.
Which Method Provides Better Cooling?
Two-phase cooling can offer strong heat removal at hot surfaces because it uses boiling. However, this does not mean every two-phase installation outperforms every single-phase installation.
Both methods need effective thermal design. Single-phase systems depend on delivering sufficient liquid flow to hot components. Two-phase systems depend on effective boiling surfaces and adequate condensation. The Open Compute Project highlights the need to optimise heat transfer around high-power components in both approaches.
For a practical comparison, ask suppliers for tested performance under the same equipment load and operating conditions. A fluid category alone does not establish a system’s cooling capacity.
What Should You Check Before Selecting a Cooling Fluid?
Does the Fluid Match the Operating Temperature?
A single-phase fluid must remain liquid within the intended operating range. A two-phase fluid needs boiling characteristics that suit the system’s cooling conditions.
For single-phase cooling, viscosity, or resistance to flow, also matters. A lower-viscosity fluid generally flows more easily through equipment, but selection must consider the complete cooling design.
Is It Compatible With the Equipment?
Electrical insulation is only one requirement. The fluid must also be compatible with materials it touches.
Checks can include:
- Circuit boards and electronic components.
- Cable insulation.
- Plastics and seals.
- Thermal interface materials.
The Open Compute Project discusses material testing because immersion fluids can extract substances from certain materials or change their properties. Two-phase boiling and condensation introduce additional compatibility considerations.
Is the Fluid Currently Available and Supported?
Current supply information matters, especially when older articles mention legacy fluids. 3M states that it completed its exit from PFAS manufacturing at the end of 2025. It also notes that some sales and customer transitions involving previously manufactured products continued beyond that date.
For projects referencing older 3M fluids, confirm stock availability, technical support and the qualification requirements for any replacement. The manufacturing exit does not mean all two-phase cooling fluids are unavailable.
Frequently Asked Questions
Q1. Does “two-phase” mean two different fluids are mixed?
Ans: No. It refers to liquid and vapour being involved in the cooling cycle. It does not describe the number of ingredients in the fluid.
Q2. Is boiling a sign of overheating in two-phase cooling?
Ans: Boiling is an intended heat removal process in a correctly designed two-phase system. The equipment must still operate within its specified temperature and cooling limits.
Q3. Can a single-phase fluid be replaced directly with a two-phase fluid?
Ans: Do not assume they are interchangeable. Their intended operating behaviour differs, and a two-phase installation needs suitable condensation and vapour management. Any change requires assessment by the system and fluid suppliers.
Q4. Is single-phase immersion cooling suitable for high-performance computing?
Ans: Yes. Purpose-designed single-phase fluids are offered for high-performance computing. Suitability for a particular installation depends on the fluid, hardware and cooling system working together within their supported conditions.
