Cooling Liquid Products RH-TMM-3

Cooling Liquid Products RH-TMM-3 is supplied as a concentrated ethylene glycol-based heat transfer fluid for closed-loop industrial and electronics cooling circuits. The model designation RH-TMM-3 identifies a non-silicate, non-phosphate corrosion-inhibited formulation. The concentrate is diluted with deionized water to a working concentration of 20 vol% to 50 vol%. The table below lists the main specification and test method matrix for the concentrate.

Dilution water quality exerts the primary control on long-term conductivity. Deionized water with initial conductivity below 10 µS/cm and total hardness below 50 ppm CaCO3 is specified. Hard-water dilution precipitates calcium carboxylates and reduces reserve alkalinity; the resulting deposit appears as white fouling in plate heat exchangers and narrow cold plates. Because the product is phosphate-free, calcium phosphate scale is not formed, but calcium organic acid salts remain a boundary condition. For freeze protection, the freezing point of a 25 vol% aqueous dilution is −12 °C to −15 °C by ASTM D1177. A 50 vol% dilution lowers the freezing point to approximately −37 °C but increases viscosity and reduces heat transfer coefficient further. Concentration should be measured by refractometer or gas chromatography per ASTM D6130, not by pH alone.

Use of RH-TMM-3 is restricted to closed-loop systems. It is not intended for open recirculating cooling towers, potable water systems, or direct discharge to the environment. Release to surface water is regulated because ethylene glycol imposes a biochemical oxygen demand; disposal must follow the applicable industrial discharge permit.

What Are the Electrical Conductivity and Leakage Current Constraints for Direct-to-Chip Cooling?

In liquid-cooled server and power-electronics assemblies where the coolant contacts energized surfaces or wetted connectors, the electrical conductivity of the working fluid must be maintained below 25 µS/cm at 25 °C, measured by ASTM D1125. Conductivity is not static: glycol oxidation, corrosion product dissolution, and plasticizer migration from non-metallic hoses can raise conductivity during service. A side-stream mixed-bed ion exchange filter with a nominal flow rate of 5 L/min per 200 L coolant inventory is a typical control method. The replacement threshold is set at 50 µS/cm for most 48 Vdc to 400 Vdc equipment. Above this value, leakage current across cold plates can exceed 1 mA and may disrupt sensitive differential current monitoring. Published data for RH-TMM-3 in board-level immersion with exposed ball-grid-array packages is limited; therefore, qualification at the specific bus voltage is required before deployment.

The inhibitor package functions in circuits constructed from copper cold plates, nickel-plated fittings, and 316L stainless steel piping by forming a passivating film on copper and aluminum surfaces while maintaining reserve alkalinity to neutralize glycolic acid produced by oxidative degradation. The formulation does not contain silicate, phosphate, or amine corrosion inhibitors. Mixing with amine-based inhibitors is not recommended because copper complexation can destabilize the protective film and accelerate dissolution at temperatures above 80 °C. The product is incompatible with zinc-galvanized pipe, unalloyed magnesium, and natural rubber seals. EPDM, hydrogenated nitrile, and 316L stainless steel are preferred wetted materials.

Galvanic Coupling Boundaries in Mixed-Metal Circuits

Corrosion performance is evaluated with the mixed-metal test of ASTM D1384 and the hot-surface aluminum test of ASTM D4340. In a six-week loop test with 25 vol% dilution at 88 °C, copper weight loss remains below 0.05 mg/week and cast aluminum weight loss below 0.15 mg/week. These values are within the pass/fail limits applied to inhibited glycol coolants; however, the standard test heat flux is not representative of advanced cold plates with local heat flux above 100 W/cm². For such designs, galvanic corrosion at copper-aluminum junctions must be limited by design: maintain anode-to-cathode area ratios above 1:2 and avoid direct contact between copper and zinc-galvanized pipe.

Reserve alkalinity is consumed as glycol degradation acids form. A reserve alkalinity of 11.0–14.0 mL of 0.100 N HCl per 100 mL sample is not a constant field property; measurements below 3.0 mL indicate exhausted buffering and require fluid replacement. In a production-scale 450 kW chiller loop, pH drift from 8.4 to 7.1 over 14 months correlated with a drop in reserve alkalinity from 12.5 mL to 2.7 mL and was traced to continuous make-up water containing 80 µS/cm of dissolved solids.

When an Existing Chiller Loop Is Converted From Uninhibited Water to RH-TMM-3

The substitution raises fluid viscosity and slightly reduces heat transfer performance. A 25 vol% aqueous dilution at 25 °C has a kinematic viscosity of approximately 2.0–2.8 mm²/s, compared with 0.89 mm²/s for water. Fixed-speed centrifugal pumps may still move the required flow if total system pressure drop remains within the pump curve; positive displacement pump bypass flow should be recalculated. The specific heat decreases from 4.18 kJ/(kg·K) for water to approximately 3.3 kJ/(kg·K) for the 25 vol% dilution at 20 °C, and thermal conductivity decreases to approximately 0.42 W/(m·K). A heat exchanger originally sized for water must have 8–12% additional coolant-side surface area to maintain equivalent heat rejection at constant flow rate.

Before filling, residual silicate-based coolant must be removed with a two-pass deionized water flush. Silicate residue can react with calcium ions in make-up water and form hard deposits in low-flow regions. Filtration to 25 µm absolute is recommended after flushing and after the first 72 h of operation. The diluted fluid pH should be confirmed at 8.0–9.5 before load is applied.

Foaming tendency above 50 mL in the ASTM D892 test indicates anti-foam additive depletion or air ingress through pump suction. A positive suction head of at least 0.5 bar at 60 °C is recommended to prevent cavitation in centrifugal pumps. Persistent foam can lead to vapor lock in compact microchannel cold plates and should trigger a fluid sample review.

At heater surface temperatures above 180 °C, glycol oxidative degradation accelerates, producing glycolaldehyde, glycolic acid, and formic acid. The pH buffering system is effective only until reserve alkalinity is depleted. Systems with cartridge heaters or high-flux laser diode cold plates should monitor surface temperature by embedded thermocouple and limit coolant film temperature below 150 °C to avoid rapid inhibitor depletion. For forced-convection loops with heat flux above 150 W/cm², published data for RH-TMM-3 is limited; qualification testing is required rather than reliance on standard corrosion test data.

Distinguishing RH-TMM-3 from Mineral Oil and Deionized Water Formulations

Mineral oil immersion coolants provide electrical conductivity below 0.1 µS/cm and do not require ion exchange, but their specific heat is approximately 1.9 kJ/(kg·K) and thermal conductivity is approximately 0.13 W/(m·K). Inhibited deionized water has higher specific heat but lacks reserve alkalinity and can corrode unpassivated copper if the inhibitor is depleted. RH-TMM-3 occupies an intermediate envelope: specific heat is lower than water but higher than mineral oil, while the inhibited glycol matrix provides freeze protection and buffering that water alone cannot. Compared with silicate-containing coolants, RH-TMM-3 does not form silicate gel deposits in high-heat-flux cold plates. Compared with phosphate-containing coolants, it avoids calcium phosphate scale when make-up water contains dissolved calcium. Compared with propylene glycol-based coolants, RH-TMM-3 generally provides higher thermal conductivity and lower viscosity at equivalent glycol concentration, but it is not suitable where accidental potable water contact is possible. The following table summarizes representative performance boundaries.

RH-TMM-3 is a single-phase heat transfer fluid. It is not suitable for two-phase thermosiphon or immersion boiling modules where vapor formation is intentional; its boiling point for a 25 vol% aqueous dilution is above 100 °C, and surface boiling under high heat flux can cause local inhibitor depletion. Two-phase systems require a dielectric refrigerant with a low boiling point and are outside the operational boundary of this product.

Fluid life is not determined by color or clarity alone. In a closed loop with deionized make-up water, the product may remain in service for 24 months provided pH remains above 8.0 and conductivity below 50 µS/cm. If conductivity exceeds 50 µS/cm, replacement or side-stream ion exchange polishing is required rather than simple inhibitor addition, because dissolved corrosion products cannot be removed by inhibitor replenishment alone.

Concentrate is stored in sealed containers away from zinc, galvanized surfaces, and direct sunlight. Batch preparation should add concentrate to deionized water under gentle agitation; high-shear mixing is not required and may entrain air. A 25 vol% mixture is prepared by blending 250 L of concentrate with 750 L of deionized water at 20 °C; the resulting volume change is not strictly linear and the final density should be confirmed by ASTM D4052.

Compliance Is Required for REACH and RoHS Documentation Before Deployment

The concentrate is supported by a safety data sheet compliant with REACH Regulation (EC) No 1907/2006 and is not intentionally formulated with substances above the maximum concentration values in Directive 2011/65/EU for electrical and electronic equipment. Ethylene glycol is classified under GHS as specific target organ toxicity on repeated exposure. Closed-loop operation, nitrile gloves, and local exhaust ventilation are specified where mist may be generated. The fluid must not be discharged to surface water or storm drains. Disposal must follow local hazardous waste regulations and account for the biochemical oxygen demand of ethylene glycol.