Quick answer
In single-phase immersion the dielectric fluid stays liquid and is pumped through a heat exchanger. In two-phase immersion the fluid boils on the hot chips, the vapour condenses on a coil and drips back. Two-phase can move more heat per litre but needs sealed tanks and engineered fluids now under PFAS scrutiny; single-phase uses simpler hydrocarbon or ester fluids.
Key takeaways
- Single-phase: pumped liquid, simpler fluids, easier servicing.
- Two-phase: boiling and condensing, very high heat flux, tighter vapour control.
- 3M’s exit from PFAS manufacturing reshaped the two-phase fluid market.
General information. Confirm the exact fluid, equipment and qualification with your hardware or CDU supplier before specifying.
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Side by side
| Single-phase | Two-phase | |
|---|---|---|
| How heat moves | Fluid absorbs heat and is pumped to a heat exchanger | Fluid boils at the chip; vapour condenses on a cooled coil |
| Typical fluids | Mineral oils, synthetic hydrocarbons, esters | Engineered low-boiling fluids (historically fluorochemicals) |
| Tank | Open-bath style with lid | Sealed, with vapour management |
| Servicing | Lift servers out; fluid drips off | Must manage vapour losses when opening |
| Fluid cost and losses | Lower cost per litre, low evaporation | Higher cost per litre; vapour losses matter |
| Regulatory exposure | Generally low | PFAS rules affect many fluids |
The PFAS factor
3M announced in December 2022 that it would exit PFAS manufacturing by the end of 2025. Its Novec fluids had been widely used in two-phase immersion, so operators have had to qualify alternatives. Chemours launched Opteon 2P50, a hydrofluoroolefin fluid for two-phase immersion. See Novec alternatives for immersion cooling.
How to choose
- Start from the hardware: which immersion approach and fluids are qualified by the server and tank makers?
- Check your heat density per rack and whether single-phase can handle it.
- Assess operations: servicing frequency, staff training, fluid handling.
- Check regulatory exposure for the fluid in your markets.
- Compare total fluid cost: initial fill, top-ups and end-of-life handling.
Related: dielectric fluids · direct-to-chip vs immersion · immersion fluid suppliers.
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Is two-phase immersion cooling better than single-phase?
It can remove more heat per unit of fluid, but it needs sealed tanks and engineered fluids under regulatory pressure. Single-phase is simpler to operate.
What replaced 3M Novec in two-phase cooling?
Operators are qualifying alternatives such as Chemours Opteon 2P50; check hardware qualification for any fluid.
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