PG25 Coolant Properties: the Four Ratios a Liquid-Cooling Design Actually Pays

What PG25 Is, and What It Is For

PG25 is a 25 % propylene glycol / 75 % water mixture, the working fluid of the secondary (IT-side) loop in many direct-liquid-cooling designs, including the published K&K reference design. The glycol fraction buys freeze and burst protection for coolant that passes through outdoor-exposed piping and dry coolers, and the inhibitor package manages corrosion across the mixed-metal loop. The loop is closed: the fluid circulates for years and is not consumed in normal duty — the design’s one-time first fill is about 30 tonnes of premixed, supplier-delivered fluid.

PG25 Density, Specific Heat and Viscosity: the Four Ratios That Matter

Substituting PG25 for treated water changes density, specific heat, viscosity and freeze behaviour together, so the honest comparison is a set of ratios, not one number. At 40 °C on generic property correlations, holding heat, temperature difference and pipe diameter constant [MODELED]:

• Flow ×1.045 — volumetric heat capacity falls from about 4,146 to 3,968 kJ/(m³·K), so the same heat at the same ΔT needs 4.5 % more flow.
• Pipe pressure drop ×1.264 — kinematic viscosity about 1.73× water cuts the Reynolds number to 0.603 of the water case and raises the friction factor.
• Pump power ×1.321 — pressure drop times flow.
• In-tube heat-transfer coefficient ×0.69 — the convection penalty, about a 31 % reduction.

The often-quoted “4.5 % more flow” is real and is the smallest of the four. The full derivation is in PG25 is not water plus 4.5 % volume.

The Capacity Derate: a Water-Rated 1.5 MW CDU Is 1.20–1.28 MW on PG25

Where a cooling distribution unit’s nameplate was calibrated on water, charging the loop with PG25 derates the real capability by a factor of about 0.80–0.85 [MODELED] — wide enough to change the unit count of a hall. The reference design carries the bottom of the band, 1.2 MW on a 1.5 MW water-basis nameplate, as the working derate, and registers the calibration fluid of the actual product as an item to close before any request for quotation. The reasoning, and the four conditions a capacity rating needs (Q = U·A·LMTD), are in the derate analysis.

Design Rules the Properties Impose

Three practical consequences carry through the published design. Pump and pipe sizing start from PG25 properties, never from water figures with a flow margin — the penalty lands in pressure drop and pump power, not mainly in flow. Capacity statements are only comparable with their fluid named; the purchased fluid’s supplier property table replaces the generic correlations used for comparison here (CONFIRM item in the design). And for leak response the design treats the mixture as electrically conductive on IT equipment, which makes leak handling an isolation design problem — see 96 isolation valves and the auto-close trade-off.

Download the Drawing Set

Every document is a direct PDF download — no registration, no email gate — with a published SHA-256 checksum on the downloads page:

• Electrical Drawing Book — Public Edition P1 (14 MB, 88 pages)
• Structural Document Set — Public Edition P1 (84 MB, 170 pages)
• GPU Hall Network Reference Design — Public Edition P2 (1.6 MB, 75 pages)
• Liquid Cooling Reference Design — Public Edition P1 (3.1 MB, 38 pages)
• BIM Machine-Audit Sample Pack (5 KB, 4 artifacts + README)
• Storage-Capacity Model — Companion Code (34 KB, code + dataset)
• GPU Hall Design Readiness Checklist (7 KB, 2 pages)
• Machine-Audit Template Pack (3 KB, 3 templates + README)
• Electrical + Structural + Network — Combined ZIP (97 MB, 3 documents)

Published at Design Development — Public Edition P1 status: reference designs for study and evaluation, not construction issues, not sealed by an Engineer of Record. Reuse follows the attribution terms stated inside each document.

Common Questions

What is PG25 coolant?

A 25% propylene glycol / 75% water mixture with a corrosion-inhibitor package, used as the secondary-loop coolant in direct liquid cooling. The glycol fraction provides freeze and burst protection for outdoor-exposed piping; the loop is closed and the fluid is not consumed in normal operation.

How do PG25 properties compare with water?

At 40 °C on generic correlations: about 4.5% more flow for the same heat and ΔT (volumetric heat capacity 4,146 → 3,968 kJ/m³K), kinematic viscosity about 1.73× water, pipe pressure drop about 1.26×, pump power about 1.32×, and in-tube heat transfer coefficient about 0.69× — all MODELED comparison values; the purchased fluid's supplier table governs the design.

Is PG25 electrically conductive?

An inhibited glycol-water mixture is treated as electrically conductive for design purposes on IT equipment: leak response in the published design is built on isolation — valve sectioning and bounded response — rather than on any assumption that the fluid is benign on electronics. Actual conductivity depends on the inhibitor package and fluid condition, which is supplier data.

Why 25% glycol and not more or less?

The fraction is a freeze-protection and property trade: more glycol deepens freeze protection and worsens every thermal-hydraulic property; less does the reverse. 25% is this design's stated basis for its climate exposure; the right fraction for another site is set from that site's minimum temperatures with the supplier's concentration table.

Does a PG25 loop consume coolant?

Not in normal duty: the loop is closed and the published design's water story reflects that — a one-time first fill of about 30 tonnes of premixed fluid, no evaporative consumption by design, and makeup only against service events.