Material Compatibility in Immersion Cooling, Revision 1.0 (Nov 28, 2022)

Overview

The document begins from the premise that in immersion cooling the fluid contacts individual components directly, so fluid-material compatibility becomes a design constraint. It classifies fluids as single-phase (mineral oils, GTL and PAO synthetic hydrocarbons, natural and synthetic esters, biobased renewable hydrocarbons) and two-phase (PFCs, PFPE, fluoroketones and HFOs). Mineral oils are not recommended for immersion cooling because impurities such as sulphur, nitrogen and aromatics cause compatibility problems.

Required fluid properties are gathered in a datasheet table taken from Immersion Requirements rev 2, with a named method and format for each property: dielectric strength ASTM D1816/IEC 60156, resistivity ASTM D1169, specific heat ASTM E1269, thermal conductivity ASTM D7896 and viscosity ASTM D7042. Flash, fire and auto-ignition points, pour point, sulphur, acidity, biodegradability and GWP/ODP must also be declared. Minimum requirements: dielectric strength at least 6 kV/mm, resistivity 2.00 GOhm.m (single-phase) and 0.20 GOhm.m (two-phase lifetime value), closed-cup flash point at least 150 C, auto-ignition at least 300 C and single-phase sulphur under 10 ppm.

Single-phase failure mechanisms are grouped under six headings: dissolution of PVC cables and plastics, swelling of rubbers and seals by absorption, metal corrosion, environmental stress cracking of amorphous polymers, oxidation forming acids and moisture causing short circuits. For two-phase systems primary (fluid-material) and secondary (material-material, with the fluid as a vector) compatibility are distinguished: non-volatile hydrocarbons distil onto boiling surfaces and lower heat transfer, and this is reversible with adequate filtration.

The standard fluid-ageing test runs at 80 C (lower if the fluid boils below 80 C) for 336 hours in 0.8 litres of fluid, measuring fluid colour, breakdown voltage, DDF and acid value and sample dimensions, mass, colour and Shore hardness before and after. Property changes under 10% are acceptable, 10-20% case by case and over 20% unacceptable. For two-phase fluorochemicals Soxhlet extraction is recommended, separating mass lost by extraction from mass gained by swelling. The appendix holds compatibility charts for six fluid families and warns that they are guidance developed with specific fluids and plastic formulations only.

Key points

1. Mineral oils are not recommended for immersion cooling because of sulphur, nitrogen and aromatic impurities.
2. Datasheet methods: dielectric strength ASTM D1816/IEC 60156, resistivity ASTM D1169, specific heat ASTM E1269, thermal conductivity ASTM D7896, viscosity ASTM D7042 at 0-60 C.
3. Flash point (ASTM D93), fire point (ASTM D92), auto-ignition (ASTM E659), pour point, sulphur, acidity (IEC 62021), biodegradability (OECD 301) and GWP/ODP must also be declared.
4. Minimum requirements: dielectric strength at least 6 kV/mm; resistivity at least 2.00 GOhm.m (single-phase) / 0.20 GOhm.m (two-phase, lifetime value); closed-cup flash point at least 150 C; auto-ignition at least 300 C; single-phase sulphur under 10 ppm.
5. Single-phase failure mechanisms: dissolution of PVC cables and plastics, swelling of rubbers and seals, metal corrosion, environmental stress cracking of amorphous polymers, oxidation forming acids, moisture short circuits.
6. Two-phase systems distinguish primary (fluid-material) and secondary (material-material) incompatibility; extracted hydrocarbons deposit on boiling surfaces and lower heat transfer, which is reversible with adequate filtration.
7. Standard fluid-ageing test: 80 C (lower if the fluid boils below 80 C), 336 hours, 0.8 L of fluid; fluid colour, breakdown voltage, DDF, acid value and sample dimensions, mass, colour and Shore hardness measured before and after.
8. Acceptance criteria: property change under 10% acceptable, 10-20% case by case, over 20% unacceptable.
9. For two-phase fluorochemical systems Soxhlet extraction is recommended: under 2% mass loss indicates very good compatibility, over 6% mass gain indicates poor compatibility.
10. Appendix charts cover six fluid families; for example the GTL chart lists nitrile, fluoroelastomer and PTFE as acceptable and EPDM and silicone rubber as unacceptable; charts are guidance only.

Using it in design

• Store the Table 1 properties (flash, fire and auto-ignition point, dielectric strength, resistivity, viscosity curve, GWP) in immersion fluid catalogue entries.
• Flag fluids that miss the Table 2 minimums, for example flash point under 150 C, in the fire-protection review.
• Give tank, pump, seal and cable material selections a compatibility status from the 80 C / 336 h ageing test, with a warning when a material lacks test evidence or exceeds a 20% change.
• Exclude elastomers that come out unacceptable in the fluid, such as EPDM and silicone rubber in the GTL chart, from seal and hose selection; consult the chosen fluid's own chart and run your own test.
• In two-phase designs ask for Soxhlet results and provide filtration (for example activated carbon) against extractable load, which otherwise builds up on boiling surfaces.
• Ask board and cable suppliers for compatibility test evidence with the specific fluid; the same component can behave differently in different fluids.

Who should read it

Mechanical and hardware engineers designing immersion-cooled systems, fluid and tank suppliers and reliability test teams.

Document contents

1. Glossary
2. Introduction
3. Purpose
4. Scope
5. Immersion Cooling Fluids Classification
6. Single Phase Fluids
7. Two Phase Fluids
8. Required Fluid Specifications
9. Minimum Dielectric Requirements for Single and Two Phase Fluids
10. Defining Material Compatibility
11. Material Compatibility for Single Phase Fluids
12. Material Compatibility for Two Phase Fluids
13. Recommended Test Methods
14. Material Compatibility Criteria Selection
15. Standardizing Test Methods for Two Phase Fluids
16. Soxhlet Extraction Material Compatibility Test
17. Experiment Method
18. Meaning of the test results, extractables
19. Appendix: References
20. Appendix: Material Compatibility Charts for Various Single and Two Phase Fluids

Related standards

• OCP ACS Immersion Requirements Rev 2
• ASTM D1816
• IEC 60156
• ASTM D93
• IEC 62021
• OECD 301

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Publication details

Authors

Open Compute Project Foundation

Publisher

Open Compute Project Foundation

Year

2022

Document number

Rev 1.0

Document language

English

Source type

Industry body

Access

Open licence — downloadable

Licence

CC BY-SA 4.0 — share with attribution under the same licence

Licence evidence

www.opencompute.org

Citation