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Single-Phase vs Two-Phase Immersion Cooling

How the two immersion cooling principles work, which fluids they need, and what decides between them.

Updated 9 November 2026 · Published by IsoTec

Short answer

In single-phase immersion cooling, the fluid stays liquid. It absorbs heat from the components and is pumped through a heat exchanger. In two-phase immersion cooling, the fluid boils on hot surfaces; the vapour condenses on a cooled coil above the bath and drips back. Single-phase systems use hydrocarbons or esters, two-phase systems mostly fluorinated fluids.

How single-phase immersion works

Servers stand vertically in a tank or sit in closed chassis filled with fluid. A pump circulates the fluid through a heat exchanger – often in a coolant distribution unit (CDU) – where the heat passes to a water circuit. Some designs rely partly on natural convection. Heat is transferred as sensible heat: the fluid warms up as it flows past the components.

Fluids: synthetic hydrocarbons (PAO, GTL), synthetic esters and natural esters. They have high boiling points and low vapour pressure, so tanks can operate at atmospheric pressure with a lid.

How two-phase immersion works

The fluid has a low boiling point, typically around 50 to 60 °C. It boils directly on the hot surfaces of processors and accelerators. Evaporation absorbs a large amount of heat (latent heat) at an almost constant temperature. The vapour rises, condenses on a water-cooled coil and returns to the bath. Tanks must be well sealed to keep vapour losses low.

Fluids: mostly fluorinated (fluorocarbons, hydrofluoroethers, fluoroketones). Most of them fall under PFAS definitions.

Comparison

	Single-phase	Two-phase
Heat transfer	Sensible heat, pumped or natural convection	Boiling and condensation (latent heat)
Typical fluids	PAO, GTL, synthetic and natural esters	Fluorinated fluids
Tank	Covered open bath or closed chassis, atmospheric pressure	Sealed tank with vapour management
Fluid losses	Low – the fluid does not evaporate noticeably	Vapour losses must be controlled
Fluid cost	Moderate	High
Fire behaviour	Combustible – flash point is decisive	Usually non-flammable
Regulation	Hydrocarbons and esters are not PFAS	Most fluids are PFAS – EU restriction in preparation
Maintenance	Filtration, monitoring of water and acidity	Vapour containment, monitoring of fluid losses and purity

What decides between them?

Heat flux: two-phase systems handle very high local heat flux. Single-phase systems reach high rack densities with good flow design and suitable heat sinks.
Total cost: fluid price, fluid losses and tank complexity favour single-phase systems in most projects.
Regulatory risk: the planned EU PFAS restriction and 3M’s exit from PFAS production affect the supply of two-phase fluids. See PFAS and immersion cooling.
Service: single-phase tanks can be opened for maintenance without vapour handling; servers are lifted out and drip off.

Which fluid for single-phase systems?

For single-phase systems the choice is between hydrocarbons and esters. Esters have higher flash and fire points at comparable viscosity, tolerate more moisture and typically transfer heat better; hydrocarbons have lower permittivity. See immersion cooling fluids compared and the data center immersion cooling guide.

Frequently asked questions

Is single-phase or two-phase immersion cooling more common?

Most immersion deployments use single-phase systems because they are simpler, use lower-cost fluids and do not depend on fluorinated fluids.

What is the boiling point of two-phase immersion fluids?

Typically around 50 to 60 °C, so that the fluid boils on the hot components at atmospheric pressure.

Can esters be used for two-phase immersion?

No. Esters have high boiling points and are used in single-phase systems.

Sources and further reading

Choosing a single-phase fluid?

We help you compare ester fluids for your tank design and test them with your components. IsoTec products are supplied to customers outside Germany.