Guidelines for Using Water-Based Transfer Fluids in Single-Phase Cold Plate-Based Liquid-Cooled Racks

Overview

The paper is written as one section of a comprehensive liquid-cooled rack specification; wetted materials, quick connects, in-rack manifold, flex hoses, piping, instrumentation and controls, operating conditions and safety must be mutually consistent. The fluid is for the closed secondary loop between the CDU and the cold plates (ASHRAE's TCS). The facility water system side of the CDU is out of scope and assumed to be properly maintained. The initial fill is premade product or on-site mixing with clean water treated to specification, and operating temperature is not expected to exceed 66 C (150 F). It is a companion to OCP's propylene glycol guideline and has been coordinated with ASHRAE and the EE HPC WG.

The fluid properties table gives minimum water quality targets: total suspended solids below 5 ppm (below 1 ppm at initial fill), total dissolved solids below 1,000 ppm after treatment, conductivity below 1,500 uS/cm at 25 C, copper ions below 0.2 ppm and iron ions below 0.1 ppm, pH 8.0-10.5, total hardness below 30 ppm (below 2 ppm for fresh fill) and turbidity below 5 NTU. Microbiological control is below 1 CFU/ml at fill and below 100 CFU/ml in operation. A corrosion inhibitor of azole type (for example tolyltriazole) is above 100 ppm or per treatment plan; nitrites are not used because they can be a nutrient. Chloride is below 50 ppm for 304 stainless steel and below 500 ppm for 316.

The system design chapter ties the wetted materials list closely to the fluid: any addition requires re-evaluation of the fluid, and the contractor, IT supplier and service provider certify compatibility of all wetted components including pump seals, heat exchangers and quick connect seals. Metals include copper, brass with under 15% zinc (annual ICP zinc analysis recommended), 304L and 316L stainless steel (preferred), nickel alloys, brazing fillers (B-Ni-6, the BCuP series, TF-H600F) and elastomers and plastics such as EPDM, Viton, FEP and PTFE; quick connects should be stainless steel and nickel- or chromium-plated ones avoided because plating wears. Typical operating temperature is below 49 C; above 66 C the list must be re-reviewed.

Mechanical design calls for a fully drainable CDU without dead legs, ports to drain and fill, a feeder for injecting chemicals while running, and filtration. Side-stream filtration below 5 micron treating 10% of loop flow protects cold plate microchannels; coarser inline CDU filters (for example 50 micron) do not replace it. For leak checks the system is pressurised to a factor of operating or maximum pressure, with the standard (IEC 62368-1 or ASME B31.n series) agreed between owner and supplier. At first fill the fluid is circulated for 30 minutes at room temperature, and existing systems get two deionised-water flushes. At start-up the loop is leak-checked for 15 minutes and a representative sample is taken from each TCS loop.

The monitoring regime calls for on-site tests weekly for the first month, monthly for the next quarter, then quarterly, until results stabilise; independent laboratory QA testing, including ICP, quarterly for the first year, then every six months if stable. A non-oxidising biocide such as isothiazolone is added only if needed, and frequent biocide additions can build up organics and eventually require a flush. Adjustments, inhibitor additions and analysis reports are recorded. Used water-based fluid can in most cases go to municipal sewers, subject to local regulations and the wastewater facility's rules.

Key points

1. Fluid temperature is not expected to exceed 66 C (150 F); typical TCS loops run below 49 C, and above 66 C the wetted materials list must be re-reviewed.
2. Total suspended solids below 5 ppm (below 1 ppm at initial fill); total dissolved solids below 1,000 ppm after treatment.
3. Conductivity below 1,500 uS/cm at 25 C after treatment; dissolved copper below 0.2 ppm and iron below 0.1 ppm.
4. pH 8.0-10.5; total hardness below 30 ppm (below 2 ppm for fresh fill); turbidity below 5 NTU.
5. Bacteria below 1 CFU/ml at fill and below 100 CFU/ml in operation; azole corrosion inhibitor above 100 ppm; nitrites should not be used.
6. Chloride is below 50 ppm where 304 stainless steel is present and below 500 ppm for 316 stainless steel.
7. Side-stream filtration below 5 micron treating 10% of loop flow is recommended to protect cold plate microchannels.
8. CDU inline filters (for example 50 micron) protect heat exchangers and quick connects but do not replace side-stream filtration.
9. Brass with under 15% zinc is acceptable with annual ICP zinc analysis recommended; nickel- and chromium-plated quick connects should be avoided.
10. On-site tests are weekly for the first month, monthly for a quarter, then quarterly; independent laboratory testing is quarterly in year one, then every six months if stable.

Using it in design

• Write the Table 1 targets (TSS, TDS, conductivity, copper and iron, pH, hardness, turbidity, bacteria, azole inhibitor, chloride) into the fluid specification as acceptance criteria and request a certificate of analysis from the supplier.
• If mixing on site, plan for fresh-fill hardness below 2 ppm, TSS below 1 ppm and bacteria below 1 CFU/ml, and hold the 50 ppm chloride limit wherever 304 stainless steel is used.
• Confirm the wetted materials list (copper, brass, stainless steel, elastomers, brazing alloys) in writing with suppliers, require unplated stainless quick connects and make every list addition a trigger for re-evaluation.
• Add to the CDU a fully drainable, dead-leg-free design, a feeder for chemical injection while running, and a side-stream filter below 5 micron treating 10% of loop flow.
• Contract for the 30-minute first flush, 15-minute leak check, retained commissioning sample, weekly then monthly then quarterly on-site tests, and quarterly independent laboratory testing in the first year.

Who should read it

Facility engineers who design, procure, commission and operate TCS loops for liquid-cooled racks, and water-treatment and equipment suppliers.

Document contents

1. Executive Summary
2. General Overview
3. Objectives
4. Description of Work
5. Heat Transfer Fluid Typical Properties
6. Approved Water-Based Heat Transfer Fluid
7. Typical Properties of Water-Based Heat Transfer Fluid
8. Safety
9. System Design Consideration
10. Compatibility
11. Operating Temperature
12. Leak Checks
13. Mechanical Design Considerations
14. Fluid Installation and System Operations
15. Overview
16. Start-up
17. Cleaning and Filling
18. Operation
19. Monitoring and Maintenance
20. Routine Monitoring
21. Adjustment
22. Fluid Lifetime and Disposal
23. Useful Life
24. Regulation Consideration
25. Fluid Disposal
26. Submittals
27. Conclusion
28. References
29. License

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Publication details

Authors

Dale Sartor; F. Philip Yu; Craig Myers; Sean Barlett; Sean T. Sivapalan

Publisher

Open Compute Project Foundation

Year

2023

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