RawSil HT-50: PDMS Dielectric Heat Transfer Fluid for Transformers

If you spec or buy cooling fluid for liquid-filled transformers, the fluid has to do two jobs at once: move heat away from the windings and insulate electrically across the whole operating range. A general industrial thermal fluid only has to transport heat. A transformer fluid that fails on dielectric strength or fire behavior is a safety and warranty problem, not just a cooling shortfall.

This piece covers what actually drives selection (viscosity, specific heat, dielectric properties, and fire point) and where RawSil HT-50, a polydimethylsiloxane (PDMS) silicone fluid, fits when both cooling and insulation matter.

How heat actually moves in a liquid-filled transformer

Three mechanisms run at once. Conduction carries heat through solids (windings, core, solid insulation) to the fluid boundary. Convection is the workhorse: fluid absorbs heat at hot surfaces, circulates (by thermosiphon or pump), and dumps it at radiators and tank walls. Radiation matters only at higher surface temperatures and is usually secondary.

The practical takeaway: the fluid’s main job is to sustain strong convection. If circulation weakens, hot spots form and insulation ages faster. For deeper coverage, see our article on advanced silicone heat transfer solutions.

The properties that decide selection

Viscosity controls flow at startup and cold ambient. Too high, and convection weakens and pumping energy climbs. RawSil HT-50 runs 50 cSt at 25°C, low enough to circulate well in both natural and forced-flow designs.

Specific heat sets how much energy the fluid carries per unit mass per degree — higher resists temperature spikes. Thermal conductivity governs transfer across the boundary film at hot surfaces. Thermal stability and oxidation resistance decide service life: heat and oxygen can raise viscosity, lay down deposits, and cut transfer efficiency over time, driving maintenance cost.

Moisture is not just a contaminant in a dielectric system — water degrades insulation performance, so low water content and disciplined handling matter. Fire behavior is its own axis: flash point is when vapors can ignite under test; fire point is when the fluid sustains burning. For equipment where fault arcs are in the safety case, fire point is often a primary selection factor.

Why PDMS silicone fluids are used in electrical equipment

Silicone (PDMS) fluids earn their place in transformer cooling because they combine strong dielectric performance, stable operation across a wide temperature range, thermal and chemical stability, and a high fire point relative to mineral oils. The trade-off is cost: silicone fluids carry a price premium over conventional transformer oil, so they are specified where the wide-range stability and fire performance justify it, not as a drop-in for every unit.

The discipline is to pick a fluid engineered for electrical equipment, not a general-purpose thermal fluid. RawSource offers a full range of silicone materials for demanding service.

RawSil HT-50: PDMS heat transfer plus dielectric performance

RawSil HT-50 is a polydimethylsiloxane heat transfer fluid built to pair heat-transfer capability with electrical specifications for transformers and other liquid-filled equipment. Per its data sheet it is positioned as:

• Meeting IEC and ASTM electrical specification requirements
• Non-halogenated and containing no additives
• Compatible with a wide range of solid electrical insulating materials
• High in thermal stability and oxidation resistance
• Higher in fire point and lower in heat release rate than other Class K insulating liquids
• Stable across a wide operating temperature range

Refer to the current Safety Data Sheet for hazard classification, handling, and disposal; do not rely on summary claims in place of the SDS.

RawSil HT-50 typical properties (for design comparison)

Note: typical values are guidance for design comparison, not a specification. Confirm against the current data sheet and validate in your own equipment and operating conditions.

What the numbers mean in service

Viscosity (50 cSt @ 25°C): low enough to keep convection strong at normal ambient, which stabilizes winding temperatures and limits hot-spot formation.

Water content (< 50 ppm): in a dielectric system, low moisture protects insulation performance and breakdown voltage over time.

Electrical properties: breakdown voltage, dissipation factor, permittivity, and resistivity are core to insulation behavior, not nice-to-haves. The fluid has to hold them across the operating range.

Fire point (> 285°C, open cup): a high fire point is a meaningful differentiator where fire risk is part of the specification, which is the main reason silicone fluids displace mineral oil in some installations.

Typical applications

• Liquid-filled transformers
• Electrical equipment requiring both insulation and heat transfer
• Service demanding thermal stability, oxidation resistance, and a wide operating range
• Installations where fire-safety performance is a key selection factor

Storage, handling, and packaging

For procurement and maintenance planning, the logistics matter as much as the spec sheet:

• Shelf life: 36 months from manufacture in the original sealed container, stored above 0°C and below 32°C (do not exceed 38°C).
• Packaging: 200 kg epoxy-coated drums and 950 kg HDPE totes; 50 kg HDPE carboys on request.
• Handling: follow the SDS — use gloves and eye protection, keep in original containers in a cool place, and protect from direct sunlight.
• Limitations: not represented as suitable for medical, pharmaceutical, food, or ingestion use.

Bottom line

In transformer and electrical-equipment cooling, the right fluid holds thermal performance, dielectric integrity, and fire behavior together over years of service. RawSil HT-50 is built for that overlap — PDMS heat transfer paired with electrical insulation. Send your transformer rating, operating range, and applicable IEC/ASTM spec and RawSource will confirm fit and quote bulk supply.

FAQ: RawSil HT-50 heat transfer fluid

What is RawSil HT-50?

RawSil HT-50 is a polydimethylsiloxane (PDMS) silicone heat transfer fluid engineered for liquid-filled transformers and other electrical equipment. It combines convective cooling with dielectric insulation and is positioned to meet IEC and ASTM electrical specification requirements.

Why use a silicone (PDMS) fluid instead of mineral transformer oil?

PDMS fluids offer stable performance over a wider temperature range, strong thermal and oxidation stability, and a higher fire point than typical mineral oils, which matters where fire risk is part of the safety case. The trade-off is a price premium, so they are specified where that stability and fire performance justify the cost.

What viscosity and fire point does RawSil HT-50 have?

Typical viscosity is 50 cSt at 25°C, low enough to sustain convection at normal ambient temperatures, and fire point (open cup) is greater than 285°C. Treat these as typical design-comparison values and confirm against the current data sheet for your application.

How is RawSil HT-50 supplied and stored?

It ships in 200 kg epoxy-coated drums and 950 kg HDPE totes, with 50 kg HDPE carboys on request. Shelf life is 36 months from manufacture in the original sealed container stored above 0°C and below 32°C. Follow the Safety Data Sheet for handling and disposal.

What applications is RawSil HT-50 used in?

It is used in liquid-filled transformers and electrical equipment that need both insulation and heat transfer, in service requiring thermal stability and a wide operating range, and where fire-safety performance drives fluid selection. It is not represented for medical, pharmaceutical, food, or ingestion use.

RawSource Editorial

Commercial & Sourcing Desk