Keeping electrical equipment cool isn’t just about temperature—it’s about reliability, efficiency, and service life. In transformers and other high-voltage assemblies, heat is generated continuously from electrical losses, and that heat has to be moved away from windings and core regions before it creates damaging hot spots.
That’s where a dielectric heat transfer fluid becomes essential. Unlike a standard industrial thermal fluid that only needs to transport heat, a transformer fluid must also insulate electrically while operating over a wide temperature range.
This article explains heat transfer in practical terms, what makes a heat transfer fluid perform well in the real world, and how RawSil HT‑50, a polydimethylsiloxane (PDMS) silicone fluid, fits applications where both cooling and electrical insulation matter.
Heat transfer fundamentals: how heat actually moves
All thermal management systems rely on the same three mechanisms:
1) Conduction
Heat moves through solid materials (like transformer windings, metals, and insulation solids). Conduction is why the heat created inside a winding can reach the fluid boundary.
2) Convection
Heat moving with a fluid as it circulates—this is the main “workhorse” in liquid-filled electrical equipment. The fluid absorbs heat near hot surfaces, flows away, and releases that heat to cooler surfaces (radiators, tank walls, heat exchangers).
3) Radiation
Heat is emitted as electromagnetic energy. Radiation plays a role at higher temperatures, but in most transformer fluid cooling scenarios, convection and conduction dominate.
In practice, the fluid’s job is to support strong convection—either naturally (thermosiphon flow) or forced (pumps)—so heat is continuously transported away from hot spots.
For applications requiring advanced thermal and dielectric performance beyond transformers, see our article on advanced silicone heat transfer solutions.
What makes a heat transfer fluid “high performance” in real equipment?
A fluid can look good on paper but still fail to deliver in a working system if the wrong properties are prioritized. Here are the most important characteristics to understand.
Viscosity: circulation and heat transfer start here
Viscosity controls how easily the fluid flows—especially at startup or at cooler ambient temperatures. If viscosity is too high, flow slows down, convection weakens, and hot spots can form or persist. In forced-flow systems, high viscosity also increases pumping energy.
Specific heat: how much energy the fluid can carry
Specific heat tells you how much energy the fluid can store per unit mass per degree of temperature rise. A higher specific heat helps the system resist temperature spikes and improves the amount of heat carried per unit of flow.
Thermal conductivity: how well heat moves through the fluid film
Thermal conductivity influences how efficiently heat transfers through the thin “boundary layer” of fluid near hot surfaces. Better conductivity generally supports better heat transfer, especially near critical surfaces.
Thermal stability and oxidation resistance: performance over time
Thermal fluids don’t just “run forever.” Heat and oxygen exposure can gradually change fluid chemistry, which may:
- increase viscosity
- form deposits on heat transfer surfaces
- reduce heat transfer efficiency
- shorten service life
Choosing a fluid with strong thermal stability and oxidation resistance helps maintain performance and reduce maintenance burden.
Moisture control: especially important for dielectric systems
For dielectric cooling applications, moisture isn’t just a “contaminant”—it can affect electrical insulation performance. Keeping water content low and maintaining good handling practices matters for long-term reliability.
Fire safety: flash point and fire point are not the same
Two safety-related temperatures are often confused:
- Flash point: when vapors can ignite under test conditions
- Fire point: when the fluid can sustain burning
For electrical equipment where fault events are part of the safety analysis, fire behavior can be a major selection factor.
Why silicone (PDMS) dielectric heat transfer fluids are used in electrical equipment
Silicone-based PDMS fluids are used in transformer and electrical cooling applications because they can combine:
- strong dielectric performance
- stable operation over a wide temperature range
- thermal and chemical stability
- safety-related characteristics (including high fire point behavior, depending on fluid type)
The key is selecting a fluid engineered for electrical equipment—not just a general-purpose industrial thermal fluid. RawSource offers a full range of silicone materials for demanding environments.
Introducing RawSil HT‑50: PDMS heat transfer + dielectric performance in one fluid
RawSil HT‑50 is a polydimethylsiloxane (PDMS) heat transfer fluid designed to provide an optimum combination of heat transfer properties and electrical specifications. It is positioned for use as a cooling and heat transfer fluid for transformers and other electrical equipment.
Key product characteristics
RawSil HT‑50 is described as:
- Meeting IEC and ASTM electrical specification requirements
- Essentially non-toxic and environmentally safe
- Non-halogenated and containing no additives
- Classified as non-hazardous
- Compatible with a wide range of solid electrical insulating materials
- Offering high thermal stability and oxidation resistance
- Having a higher fire point and lower heat release rate than other types of Class K insulating liquids
- Delivering good electrical properties and operating capability over a wide temperature range
RawSil HT‑50 typical properties (for design and comparison)
Below are typical values commonly used when evaluating suitability for transformer and electrical equipment cooling:
Property | Typical Value |
|---|---|
Appearance | Crystal clear liquid |
Density @ 25°C | 0.96 g/cm³ |
Viscosity @ 25°C | 50 cps |
Water content | < 50 ppm |
Specific heat | 1.51 kJ/kg·K |
Thermal conductivity | 0.151 W/(m·K) |
Refractive index @ 25°C | 1.404 |
Breakdown voltage | 50 kV |
Permittivity @ 25°C | 2.5 |
Dissipation factor @ 25°C | 0.0001 |
Volume resistivity @ 25°C | 1.0 × 10¹⁴ Ω·cm |
Flash point (open cup) | 1.0 × 10¹⁴ Ω·cm |
Fire point (open cup) | > 285°C |
Note: “Typical values” are guidance values and should not be treated as specifications. Always evaluate performance in your specific equipment design and operating conditions.
What these numbers mean for real applications
Viscosity (50 cps @ 25°C): flow, cooling, and efficiency
In transformer cooling, fluid movement is central to heat removal. A viscosity profile that supports circulation helps stabilize temperatures and supports better convective heat transport.
Low water content (< 50 ppm): supports insulation performance
In transformer cooling, fluid movement is central to heat removal. A viscosity profile that supports circulation helps stabilize temperatures and supports better convective heat transport.
Electrical properties: designed for dielectric cooling
Breakdown voltage, dissipation factor, permittivity, and resistivity are not “nice-to-have” metrics in electrical equipment—they are core to insulating system behavior. A fluid intended for transformer and electrical equipment use must maintain these properties across operating conditions.
Flash point and fire point: safety-relevant behavior
A high fire point can be a meaningful differentiator in installations where fire safety and risk mitigation are part of the specification. RawSil HT‑50’s listed fire point (open cup) is 370°C.
Typical use cases for RawSil HT‑50
RawSil HT‑50 is positioned for
- liquid-filled transformers
- electrical equipment where both insulation and heat transfer are required
- applications where thermal stability, oxidation resistance, and a wide operating range are important
- environments where fire safety performance is a key selection factor
Storage, handling, shelf life, and packaging
For procurement and maintenance planning, these details matter just as much as performance properties:
- Shelf life: 36 months from date of manufacture in the original container when stored above 0°C and below 32°C (do not store above 38°C).
- Packaging: 200 kg epoxy-coated drums and 950 kg HDPE totes; 50 kg HDPE carboys available on request.
- Storage & handling: use normal safety precautions (gloves and safety goggles); store in original containers in a cool place and protect from direct sunlight.
- Limitations: not represented as suitable for medical or pharmaceutical usage; not recommended for human ingestion or food use.
Closing thought
In transformer and electrical equipment cooling, the best fluid isn’t simply the one with “good heat transfer”—it’s the one that maintains thermal performance, dielectric integrity, and safety behavior consistently over time. RawSil HT‑50 is designed specifically for that overlap: PDMS-based heat transfer capability paired with electrical insulation performance for demanding electrical applications.
Quick FAQ
What is a heat transfer fluid?
A heat transfer fluid is a liquid used to move thermal energy from one part of a system to another—either to heat, cool, or maintain a stable process temperature.
What is a dielectric heat transfer fluid?
A dielectric heat transfer fluid is a fluid that performs both as a coolant and an electrical insulator, commonly used in transformers and other high-voltage equipment.
In transformers and other high-voltage assemblies, heat is generated continuously from electrical losses, and that heat has to be moved away from windings and core regions before it creates damaging hot spots.
Why not use a standard thermal oil in a transformer?
Transformers require the fluid to support electrical insulation performance in addition to heat removal. Many general industrial thermal oils are not designed or qualified for that electrical role.
What should I check first when selecting a transformer cooling fluid?
Start with the operating temperature range, required electrical properties, viscosity behavior, moisture control strategy, thermal stability, compatibility with solid insulation materials, and safety requirements.
