Guidelines for Using Dielectric Heat Transfer Fluids in Two-Phase Cold Plate-Based Liquid-Cooled Racks, Revision 1.1 (March 2026)

Overview

The guideline outlines best practice for selecting, handling and maintaining dielectric heat transfer fluids in two-phase cold-plate liquid-cooled racks. The fluid works in the secondary closed loop (ASHRAE's TCS) between a heat exchanger such as a CDU and the cold plates in the rack. The goal is multi-vendor solutions in which cooling infrastructure is reused across several IT hardware refresh cycles. The primary audience is design and facility engineers responsible for design, procurement, setup and operation of the TCS loop; equipment suppliers are the secondary audience.

Minimum fluid criteria are low toxicity, non-flammable, ODP near zero, GWP below 700, ASHRAE Std. 34 safety class A1, boiling point between -26.3 C and 60 C at 101.3 kPa and freezing point below -100 C. The typical-property table lists vapour thermal conductivity below 20 mW/mK and liquid below 80 mW/mK at 25 C. Before using the fluid, the safety data sheet (SDS) must be reviewed and users should wear goggles, nitrile or impervious gloves and protective clothing.

Compatibility is the most detailed section. The data-centre operator must certify compatibility of the fluid with all wetted components, including pump seals, heat exchangers and quick-connect seals, at maximum operating temperature; any addition to the wetted-materials list requires re-evaluation of the fluid. Seal weight, stiffness and volume must change by no more than 5% over a period simulating system life. In the example results aluminium, copper, brass, stainless and carbon steel were unchanged with fluoride below 0.5 ppm; the elastomers NBR and Thiokol were not acceptable, while NR, CR and FKM were.

Test methods: metals are exposed at 100 C for 2 weeks and plastics and elastomers at room temperature for 2 weeks. The Soxhlet extraction test measures the fluid's extraction of non-volatile materials such as oils and the sample's absorption of the fluid separately, so it is more informative than simple soak tests; the 72-hour test gives worst-case results at the boiling point, and a positive result is verified with 2 months of monitored functional testing in a working system. The upper operating temperature is 45 C, and above that the wetted-materials list must be re-reviewed.

Mechanical and operating points: because the fluid is non-conductive, a leak does not require immediate server shutdown, and the leak is located with a standard refrigerant sniffer. Total pressure loss between evaporator and condenser should stay below 0.2 bar, liquid and vapour lines differ in size, pressure and flow, fill and drain ports must follow the wetted-materials list, and about 100 um minimal filtration suffices. Ambient should stay within 5-35 C. Routine monitoring covers vapour and liquid pressure and temperature and buffer level; a measured vapour pressure more than 25% from theory calls for a purge. The fluid has no expiry and can be reused or returned to the manufacturer.

Key points

1. Fluids must be low toxicity, non-flammable, ODP near zero, GWP below 700 and ASHRAE 34 class A1.
2. Boiling point is between -26.3 C and 60 C at 101.3 kPa and freezing point below -100 C; vapour thermal conductivity below 20 mW/mK and liquid below 80 mW/mK at 25 C.
3. Seal weight, stiffness and volume must change by no more than 5% over a period simulating system life.
4. In the example, aluminium, copper, brass, stainless and carbon steel were unchanged (fluoride below 0.5 ppm); NBR and Thiokol failed while NR, CR and FKM passed.
5. Metal compatibility is tested at 100 C for 2 weeks and plastics/elastomers at room temperature for 2 weeks, plus a 72 h Soxhlet test and a 2-month system test.
6. The upper operating temperature is 45 C; above it the wetted-materials list must be reviewed again.
7. Because the fluid is non-conductive, a leak does not require immediate server shutdown; leaks are found with a standard refrigerant sniffer.
8. Total pressure loss between evaporator and condenser should be below 0.2 bar; liquid and vapour lines differ in size, pressure and flow.
9. Minimal filtration is about 100 um; ambient during operation should stay within 5-35 C.
10. A vapour pressure more than 25% from the theoretical value requires a purge; the fluid has no expiry and can be reused.

Using it in design

• Apply refrigerant-specific rules to two-phase DLC designs in place of water-loop rules: GWP below 700, A1 safety class, and vapour/liquid line sizing with under 0.2 bar evaporator-to-condenser loss.
• Specify refrigerant leak detection (sniffer or gas sensors) instead of conductive leak rope.
• Require documented fluid compatibility for each wetted material, with the 5% limit and a 72 h Soxhlet test for seals.
• Include about 100 um filtration and fill/drain ports in the design, with extra filtration planned for fill and cleaning.
• Monitor vapour pressure and temperature, liquid pressure and temperature and buffer level, with a purge alarm at 25% deviation from theoretical pressure.
• Trigger room ventilation and refrigerant-concentration checks (for example EN 378 / ASHRAE 15) from the charge in the room.

Who should read it

Mechanical and facility engineers who design and operate two-phase direct-to-chip liquid-cooling infrastructure, and equipment suppliers.

Document contents

1. General Overview
2. Objectives
3. Description of work
4. Compliance with Open Compute Project Tenets
5. Heat Transfer Fluid Typical Properties
6. Approved Dielectric Liquid-Based Heat Transfer Fluid
7. Typical Properties
8. Safety
9. System Design Consideration
10. Compatibility
11. Operating Temperature
12. System Leak Checks
13. Mechanical Design Considerations
14. Fluid Installation and System Operation
15. Start-Up
16. Liquid Filling
17. Operation
18. Monitoring and Maintenance
19. Fluid Lifetime and Disposal
20. Submittals
21. Conclusion

Related standards

• ASHRAE Standard 34
• ASHRAE Standard 15
• EN 378
• OCP Two-Phase Dielectric Fluid Base Specification for cold plates

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Publication details

Authors

Open Compute Project Foundation

Publisher

Open Compute Project Foundation

Year

2026

Document number

Rev 1.1

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