How Single Phase and Two Phase Immersion Cooling Differ in Operation

A kilogram of Chemours’ Opteon 2P50 absorbs 115 kJ as it boils. A kilogram of Shell’s hydrocarbon immersion fluid absorbs about a fifth of that across a 10 K temperature difference. That gap is the thermal case for two phase immersion cooling, and it is far narrower than the multiplier the fluid’s maker advertises. The differences that decide daily operation sit in the tank’s pressure envelope and in what each fluid needs over its service life.

Two phase fluid carries four to five times more heat per kilogram

Per kilogram of fluid, two phase immersion moves four to five times the heat of a single phase oil held to a 10 K temperature difference.

The Open Compute Project (OCP) Immersion Requirements define the two methods by what the dielectric liquid does. In single phase immersion the liquid stays liquid and circulates, by pump or by natural convection, through a heat exchanger. In two phase immersion the liquid boils on the hot components, the vapor condenses on a cooled surface, and the condensate returns to the pool by buoyancy-driven flow. Background on the air side is in where the air cooling limit per rack comes from.

Source: Open Compute Project, “Immersion Requirements,” Revision 2.10 (August 18, 2023), section 2.1

Single phase fluid carries heat by changing temperature. Two phase fluid carries it by changing state at a fixed temperature, through its latent heat of vaporization (h_fg, the energy a kilogram absorbs as it turns to vapor). The arithmetic assumes 100 kW of IT heat and a 10 K temperature difference across the single phase fluid, a value chosen for this piece rather than taken from any sheet.

Single phase:  Q = ṁ × cp × ΔT
Two phase:     Q = ṁ × h_fg

Shell S5 X (single phase), cp stated at 40 °C, assumed ΔT = 10 K
  q = 2.274 kJ/kg·K × 10 K = 22.7 kJ/kg

3M Novec 649 (two phase), boiling point 49 °C
  q = h_fg = 88 kJ/kg

Chemours Opteon 2P50 (two phase), boiling point 48.9 °C
  q = h_fg = 115 kJ/kg

Ratio, Novec 649 to S5 X:     88 ÷ 22.7 = 3.9
Ratio, Opteon 2P50 to S5 X:  115 ÷ 22.7 = 5.1

Source: Shell, Technical Data Sheet, Shell Immersion Cooling Fluid S5 X, v1.5 (2021); 3M, Novec 649 Engineered Fluid Product Information (2009); Chemours, Opteon 2P50 product sheet (2024)

The 2P50 sheet prints the unit of its heat of vaporization as kJ/kg·K. Latent heat is energy per unit mass with no temperature term, so this piece reads the value as 115 kJ/kg. A unit error on the property that defines the method is reason enough to request the full data package before sizing a condenser from this sheet.

Table 1. Heat carried per unit of fluid, single phase and two phase immersionkJ/kg · kJ/L · L/s at 100 kW

* S5 X ΔT values are assumptions, not sheet values; 10 K is the base case in the text

* Two phase flow is condensate returning by gravity, not pumped flow

* S5 X density 806 kg/m³ at 15 °C, specific heat 2,274 J/kg·K at 40 °C; per-liter values mix the two conditions

* 2P50 heat of vaporization printed as kJ/kg·K on the sheet; read as kJ/kg

Source: Shell, Technical Data Sheet, Shell Immersion Cooling Fluid S5 X, v1.5 (2021); 3M, Novec 649 Engineered Fluid Product Information (2009); Chemours, Opteon 2P50 product sheet (2024). Calculation from published values.

The per-kilogram ratio belongs to the assumed temperature difference as much as to the fluids. Allowing the oil a 20 K difference doubles its figure to 45.5 kJ/kg and shrinks the two phase advantage to 1.9 to 2.5 times.

Per liter, the ratio at 10 K widens to 7.7 for Novec 649 and 9.1 for 2P50, because the fluorinated fluids are close to twice as dense as the oil.

Chemours’ liquid cooling page states that two phase fluids enable “10 to 100 times greater heat transfer capacity” than single phase alternatives. The arithmetic reaches the bottom of that range only when the oil is held to a 5 K difference, where the 2P50 ratio becomes 10.1 per kilogram.

Nothing in Table 1 approaches 100. The page does not define its term, and I read the multiplier as a claim about boiling heat transfer coefficients at a hot surface, not about how much heat a kilogram of fluid carries out of the tank.

Source: Chemours, “Liquid Cooling for Data Centers,” Opteon application page (accessed September 18, 2026)

Boiling fixes the fluid temperature and makes the tank a pressure system

In two phase immersion, the bulk fluid temperature is set by the boiling point at tank pressure, so temperature control becomes pressure control.

Opteon 2P50 boils at 48.9 °C and Novec 649 at 49 °C, per their sheets. A single phase tank has no such anchor. Its fluid temperature follows flow rate and heat input, which is why OCP requires temperature sensing at the bottom and top of an open bath.

OCP’s minimum management list adds two items for two phase systems only: a pressure and vacuum sensor for the gas phase, and a means to release out-of-bounds pressure or vacuum. It also holds two phase and other volatile-liquid systems to the atmospheric leakage limits for the specific fluid.

Source: Open Compute Project, “Immersion Requirements,” Revision 2.10 (2023), SR 5.1-1 and SR 3.3-2

The vapor volume explains the emphasis. The estimate below treats Novec 649 vapor as an ideal gas at its boiling point, with the molecular weight from 3M’s sheet.

Heat input:          100 kW
Vapor generated:     ṁ = 100 kW ÷ 88 kJ/kg = 1.14 kg/s

Vapor density:       ρv = P × M ÷ (R × T)
                     = (101,325 Pa × 0.316 kg/mol) ÷ (8.314 J/mol·K × 322.15 K)
                     = 11.95 kg/m³

Vapor volume flow:   1.14 kg/s ÷ 11.95 kg/m³ = 0.095 m³/s (95 L/s)
Expansion ratio:     1,600 kg/m³ ÷ 11.95 kg/m³ = 134

At full heat input the tank makes 95 L of vapor per second, 134 times the volume of its source liquid, and all of it must reach the condenser rather than the room.

This is an ideal-gas estimate. OCP notes that vapor density has no industry-standard reference and points to the Peng–Robinson equation of state. Real vapor near saturation is denser than the ideal-gas value, so 95 L/s sits at the high end.

3M’s vapor-pressure fit for Novec 649 returns 97 kPa at 49 °C, within 4 percent of atmospheric pressure, so the sheet’s boiling point and its own correlation agree.

Source: 3M, Novec 649 Engineered Fluid Product Information (2009); Open Compute Project, “Immersion Requirements,” Revision 2.10 (2023), section 4.1

OCP’s “Thermal Loss to Environment” test follows the same pressure logic. For two phase systems it substitutes a single phase proxy fluid to prevent pressure build-up and holds it at the marketed fluid’s critical fluid temperature, not the ambient-plus-20 °C point used for single phase tanks. A two phase thermal loss line is a proxy measurement by design, and I would not set it beside a single phase line without reading both test notes.

OCP lists seal failure and improper maintenance procedures as the causes of gas loss. When each liter of boiling fluid becomes 134 liters of vapor, opening the lid is a fluid-loss event, and the service procedure belongs in the same design review as the condenser.

Source: Open Compute Project, “Immersion Requirements,” Revision 2.10 (2023), SR 6.2-3 and the Fluorochemical and 2-phase Quality Management table

Fluid care, not heat transfer, sets the maintenance calendar

The two methods need different maintenance equipment, and OCP writes a separate fluid quality table for each.

For hydrocarbons and esters, OCP pairs each contaminant with a control: continuous filtration in mechanically circulated systems, a lid on open baths to limit moisture, and acidity and color monitoring to catch oxidation sludge. It advises against mineral oils and sets a flash point floor of 150 °C for new and in-service fluid, and Shell lists S5 X at 200 °C.

Source: Open Compute Project, “Immersion Requirements,” Revision 2.10 (2023), sections 4.2 and 4.3; Shell, Technical Data Sheet, Shell Immersion Cooling Fluid S5 X, v1.5 (2021)

The oil is also thicker. S5 X lists a kinematic viscosity of 9.8 mm²/s at 40 °C.

Novec 649 lists 0.40 cSt (equal to mm²/s) at its sheet’s 25 °C default. With the temperatures unmatched, the 24-fold gap is indicative, not a design number.

In the Table 1 base case, the single phase pumps move 5.46 L/s of that oil through filters and a heat exchanger. The two phase condensate, 0.71 L/s, returns without a pump. Single phase immersion trades latent heat for a pump and a filter, and both are hardware a facility team already knows how to service.

For fluorochemical and two phase fluids, OCP requires a conditioning system that removes water, oils, acids and thermal decomposition products, including the carbonyl fluoride (COF2) and hydrogen fluoride (HF) an electrical arc can produce. It assigns activated alumina or a base wash to those and filtration below 1 µm to particulates. 3M adds that Novec 649 belongs in a sealed system, away from liquid water.

Source: Open Compute Project, “Immersion Requirements,” Revision 2.10 (2023), SR 4.3-4; 3M, Novec 649 Engineered Fluid Product Information (2009)

Elastomer approvals do not carry across fluids. Chemours’ 2P50 sheet rates fluorocarbon (FKM) and fluorosilicone elastomers as poor after two weeks of exposure at 80 °C.

In that test FKM swelled 16.3 percent by volume and fluorosilicone 12.6 percent. Every gasket and hose liner in a two phase tank needs qualification against the specific fluid, and a material list approved for another fluid is not that qualification.

Source: Chemours, Opteon 2P50 product sheet (2024)

The fluid’s supply status belongs on the comparison sheet

Both two phase fluids in Table 1 carry a supply qualification that no thermal property reveals.

3M announced on December 20, 2022 that it would exit PFAS manufacturing and discontinue all of its fluorinated fluids by the end of 2025. Novec 649 is one of those fluids, and that deadline has passed.

Source: 3M, “3M to Exit PFAS Manufacturing by the End of 2025” (December 20, 2022)

Chemours’ product sheet labels Opteon 2P50 a developmental fluid. The July 31, 2023 announcement targeted initial commercialization in 2025 pending regulatory approvals, and the application page still carried a pending-approval footnote when accessed for this piece. Its latent heat is a published value for a fluid whose commercial status the buyer must confirm.

Source: Chemours, “Chemours Announces Development of New Specialty Fluid for Two-Phase Immersion Cooling: Opteon 2P50” (July 31, 2023); Chemours, Opteon 2P50 product sheet (2024)

Four things decide whether a single phase sheet and a two phase sheet can sit side by side:

• The temperature difference behind any single phase heat figure; the same oil spans 11.4 to 45.5 kJ/kg in Table 1.
• The unit and temperature of the latent heat value, and whether vapor density is measured or derived.
• The fluid and temperature behind the thermal loss figure under OCP SR 6.2-3.
• Production and regulatory status, and the elastomers the maker has qualified.

This comparison works from fluid sheets, not measured tank performance. Pump energy, condenser sizing, vapor loss rates and facility-level efficiency sit outside it, and vendor claims on them were not tested. Any choice between the methods needs engineering review for the specific tank, fluid and site.

Two phase immersion earns its fourfold to fivefold advantage per kilogram only on a site that can run a sealed, pressure-controlled tank on a fluid with a confirmed supply. Where either condition is uncertain, the single phase tank’s pump and filter are the simpler problem to own.

References

• Immersion Requirements, Revision 2.10 / Open Compute Project / August 18, 2023 / https://www.opencompute.org/documents/ocp-acs-immersion-requirements-rev-2-1-pdf
• Technical Data Sheet, Shell Immersion Cooling Fluid S5 X, v1.5 / Shell / September 27, 2021 / https://www.shell-livedocs.com/data/published/en/615128f8-db18-4709-bb03-10bf668eb6eb.pdf
• 3M Novec 649 Engineered Fluid, Product Information / 3M / 2009 / https://multimedia.3m.com/mws/media/569865O/3m-novec-engineered-fluid-649.pdf
• Opteon 2P50 Developmental Dielectric Heat Transfer Fluid, product sheet / The Chemours Company / 2024 / https://www.opteon.com/en/-/media/files/opteon/opteon-2p50-gic-product-info-sheet-fnl-100523.pdf
• Chemours Announces Development of New Specialty Fluid for Two-Phase Immersion Cooling: Opteon 2P50 / The Chemours Company / July 31, 2023 / https://www.chemours.com/en/news-media-center/all-news/press-releases/2023/chemours-announces-development-of-new-specialty-fluid-for-two-phase-immersion-cooling-opteon-2p50
• Liquid Cooling for Data Centers / The Chemours Company (Opteon) / undated / https://www.opteon.com/en/applications/two-phase-immersion-cooling
• 3M to Exit PFAS Manufacturing by the End of 2025 / 3M / December 20, 2022 / https://news.3m.com/2022-12-20-3M-to-Exit-PFAS-Manufacturing-by-the-End-of-2025