Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Single-Phase Cold Plate-Based Liquid Cooled Racks (OCP white paper)

Overview

The paper is designed as one section of a liquid-cooled rack specification, covering the fluid in the closed secondary loop (what ASHRAE calls the Technology Cooling System, TCS) between the CDU and the cold plates in racks and servers; it must be consistent with other sections on wetted materials, quick connects, in-rack manifold, flex hoses, instrumentation and safety. The aim is multi-vendor rack solutions where liquid cooling infrastructure survives several IT refresh cycles. The primary audience is design and facility engineers and managers; the secondary audience is equipment and fluid suppliers. It is an alternative to OCP's water-based fluid paper; the authors disclaim liability and require project-specific verification.

The fluid chapter requires propylene glycol as freeze-point depressant plus an inhibitor package designed for the metals in the loop, with particular emphasis on copper protection. Approved fluids are tested to ASTM D8040-17 on copper and brass coupons, with maximum corrosion rates of 0.22 mpy for copper (5 mg/coupon) and 0.46 mpy for brass (10 mg/coupon). PG25 contains 24.5-29.5% propylene glycol by volume with a freeze point of about -13 to -9 C; PG55 contains 53.0-57.5% with a freeze point of about -38 to -46 C. While glycol stays above 25% by volume the fluid is bio-static and no biocide is needed. In-service limits include pH 8.0-10.5, copper and iron below 2 ppm and chloride and sulfate below 25 ppm.

The system design chapter ties the wetted materials list closely to the fluid: any addition to the list requires re-evaluation of the fluid, and the data centre operator must certify compatibility of all wetted components including pump seals, heat exchangers and quick connect seals. Acceptable metals include 304L and 316L stainless steel and copper, brass with under 15% zinc is allowed but annual zinc analysis is recommended because of dezincification risk, and brazing materials in copper cold plates (B-Ni-6, BCuP-4, BCuP-5) are listed. Quick connects should be stainless steel; nickel- and chromium-plated ones are discouraged because plating can wear away and create a galvanic couple. Operating temperature is not expected to exceed 66 C, with typical operation below 49 C; above 66 C the wetted materials must be re-reviewed.

For filtration, side-stream filtration of 5 micron or finer treating 10% of the loop flow is recommended to protect cold plate microchannels; CDUs may add coarser inline filters, for example 50 micron, to protect heat exchangers, but these do not replace the side-stream filter. The loop should drain completely without dead legs and have ports for adding fluid and injecting chemicals. For loops above 250 gallons or critical systems, continuous monitoring of chemistry, pH, corrosion rate and turbidity is considered. Flush and dilution water should have chloride below 25 ppm, sulfate below 25 ppm and total hardness below 50 ppm (as CaCO3).

For first fill, once the piping is proven leak-free it is filled with the selected glycol fluid, circulated for 30 minutes at room temperature, drained and the inline filters replaced; existing systems get two deionised-water rinses, and chemical cleaners and detergents are avoided unless the fluid supplier advises. At start-up, air is bled, the loop is leak-checked for 15 minutes and a representative sample is taken from each CDU and kept for the life of the fluid as the time-zero reference. Routine on-site testing is quarterly, laboratory analysis annual if deemed necessary, and adjustments and inhibitor additions are recorded. Used fluid must not go to sewers, ground or water bodies.

Key points

1. Fluid operating temperature is not expected to exceed 66 C; typical closed-loop operation is below 49 C, and above 66 C the wetted materials must be re-reviewed.
2. PG25 contains 24.5-29.5% propylene glycol by volume (freeze point about -13 to -9 C); PG55 contains 53.0-57.5% (freeze point about -38 to -46 C).
3. Biocides are not needed while glycol concentration stays above 25% by volume because the fluid is bio-static.
4. Maximum corrosion rates under ASTM D8040-17: copper 0.22 mpy (5 mg/coupon), brass 0.46 mpy (10 mg/coupon).
5. In-service limits: pH 8.0-10.5, copper below 2 ppm, iron below 2 ppm, total hardness below 100 ppm, chloride and sulfate below 25 ppm.
6. Flushing and dilution water should have chloride below 25 ppm, sulfate below 25 ppm and total hardness below 50 ppm (as CaCO3).
7. Side-stream filtration of 5 micron or finer treating 10% of the loop flow protects cold plate microchannels; CDU inline filters (for example 50 micron) do not replace it.
8. Quick connects should be stainless steel; nickel- and chromium-plated ones are discouraged because worn plating creates a galvanic couple.
9. For loops above 250 gallons or critical systems, continuous monitoring of chemistry, pH, corrosion rate and turbidity should be considered.
10. On-site testing is quarterly, annual laboratory analysis if deemed necessary, and the start-up sample is kept for the life of the fluid.

Using it in design

• Write the PG25 or PG55 choice into the fluid specification according to the freeze protection the climate needs, and record that keeping glycol above 25% by volume removes the need for biocide.
• Have the wetted materials list (304L/316L stainless, copper, brass, seals, brazing alloys) confirmed in writing with the fluid supplier, make every addition a trigger for re-evaluating the fluid, and require unplated stainless quick connects.
• Add a 5 micron side-stream filter treating 10% of loop flow, with pressure gauges and dead-leg-free drain and fill ports, and treat the coarse CDU inlet filter as a separate protection.
• Put a 30-minute first flush with the glycol fluid, inline filter replacement, a 15-minute leak check and retention of a time-zero sample from each CDU into the commissioning procedure, and specify flush and dilution water of chloride and sulfate below 25 ppm and hardness below 50 ppm.
• Include quarterly on-site pH and chemistry tests, annual laboratory analysis where needed, record keeping and a no-sewer disposal procedure in the operations plan, and weigh continuous monitoring for large loops.

Who should read it

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

Document contents

1. Executive Summary
2. Introduction
3. General Overview (objectives, description of work)
4. Heat Transfer Fluid Typical Properties
5. Approved Glycol-Based Heat Transfer Fluids
6. Inhibitors and Additives
7. Safety
8. System Design Considerations
9. Wetted Materials
10. Operating Temperature
11. System Leak Checks
12. Mechanical Design Considerations (liquid addition, drainage, filtration, automated monitoring)
13. Fluid Installation and System Operation
14. Cleaning, Flushing and Fill
15. Start-up Procedure
16. Monitoring & Maintenance (routine monitoring, annual analysis)
17. Fluid Lifetime and Disposal
18. Submittals and Records
19. Heat Transfer Fluids

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Publication details

Authors

Keegan Yaroch; John Cuthbert; David Miller; Sean T. Sivapalan; Sean Barlett

Publisher

Open Compute Project Foundation

Year

Year not stated

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