A high-temperature process loop runs hot for years: a jacketed reactor, a hot-oil heater, a bank of heat exchangers. The thermal fluid in that loop has two ways to ruin a plant’s week. It can catch fire if it leaks onto something hot, and it can slowly oxidize and coke until it fouls the system and stops carrying heat. Silicone heat transfer oil is engineered to do neither, which is why it is specified where fire safety and long, clean service life matter more than the lowest fluid price.
The short version: silicone heat transfer oil (RawSil HT-50) is a low-moisture (under 50 ppm water) 50 cSt polydimethylsiloxane (PDMS) thermal fluid. It is not a generic 50 cSt silicone oil; it is a purified, electrical- and heat-transfer-grade fluid. It carries heat across a wide temperature range with high oxidation and thermal stability, a high fire point above 340 C, low fouling, and no additives to deplete. It is used in process heating, heat exchangers, hot-oil and thermal-fluid heater systems, jacketed reactors, and fire-safe transformer cooling.
Low-moisture 50 cSt is not generic 50 cSt silicone oil
This is the distinction that matters at the purchase order. A standard 50 cSt silicone oil and RawSil HT-50 share a viscosity, but they are not the same product. RawSil HT-50 is a low-moisture grade, with water content held below 50 ppm, purified and qualified for heat-transfer and electrical service.
That low moisture is not cosmetic. Water depresses dielectric breakdown voltage, which is why the same low-moisture grade doubles as a transformer fluid, and at high temperature trace water drives hydrolysis and degradation. Buying a generic 50 cSt silicone oil for a thermal loop, on the assumption that viscosity is the only spec, is how a fluid underperforms or fails an electrical test. The grade, not just the viscosity, is what you are specifying.
The data
| Property | Typical value |
|---|---|
| Chemistry | Polydimethylsiloxane (PDMS), 50 cSt |
| Water content | below 50 ppm |
| Density at 25 C | 0.96 g/cm3 |
| Viscosity at 25 C | 50 cSt |
| Specific heat | 1.51 kJ/(kg.K) |
| Thermal conductivity | 0.151 W/(m.K) |
| Flash point (open cup) | 300 C |
| Fire point (open cup) | above 340 C |
| Pour point | below minus 50 C |
| Standards | IEC 60836, ASTM D4652 |
PDMS (CAS 63148-62-9, PubChem; electrical grade per ASTM D4652). Values are typical reference figures; the Certificate of Analysis governs the lot you buy.
Why silicone for heat transfer
Silicone heat transfer oil earns its premium on four properties:
- Fire safety. Its fire point above 340 C and high flash point make it a less-flammable fluid, far safer than mineral hot oil if it leaks onto hot surfaces.
- Thermal and oxidation stability. It resists thermal breakdown and oxidation and carries no additives to deplete, so it does not coke and foul the way mineral oils do, giving very long, low-maintenance service life.
- Wide temperature range. With a pour point below minus 50 C and high-temperature stability, it carries heat across a broad operating window without thickening in the cold or breaking down hot.
- Clean and non-toxic. It is non-toxic and non-halogenated, avoiding the toxicity and disposal burdens of some aromatic synthetic fluids.
Where it is used
| System | Role |
|---|---|
| Hot-oil and thermal-fluid heater loops | Fire-safe heat transfer medium |
| Heat exchangers | Stable, non-fouling heat carrier |
| Jacketed reactors and process heating | Even, controllable indirect heating |
| High-temperature industrial processes | Long-life thermal fluid |
| Transformers and electrical equipment | Fire-safe dielectric cooling, covered in silicone transformer fluid |
How it compares to other heat transfer fluids
Mineral hot oils are cheaper but flammable and prone to coking and a shorter life. Aromatic synthetic fluids reach higher peak temperatures than silicone but bring flammability and toxicity and handling concerns. Water and glycol systems are limited to low temperatures. Silicone heat transfer oil sits where fire safety, stability, cold flow, and non-toxicity matter, and its honest limit is that its top continuous-use temperature is lower than some aromatic synthetics. Match the fluid to whether fire safety and longevity or peak temperature dominates.
The environmental note
Silicone heat transfer oil is non-toxic and non-halogenated, which are specific, real advantages. It is not, however, readily biodegradable; PDMS is environmentally persistent, so it is managed and disposed of as an industrial fluid rather than marketed as a “green” product.
Buying silicone heat transfer oil
RawSource supplies Silicone Transformer Oil (STO-50) — 50 cSt PDMS Dielectric Fluid (RawSil HT-50, the low-moisture 50 cSt silicone heat transfer oil, CAS 63148-62-9) for industrial heat-transfer and electrical service, in epoxy-coated drums and HDPE totes, with CoA and SDS documentation. It meets IEC 60836 and ASTM D4652 and is a drop-in for comparable 50 cSt PDMS fluids, as covered in RawSil HT-50 as a 50 cSt PDMS drop-in replacement. Tell us your operating temperature, system, and standards for a quote.
Frequently asked questions
What is silicone heat transfer oil?
It is a polydimethylsiloxane (PDMS) thermal fluid used to carry heat in industrial systems. The grade supplied as RawSil HT-50 is a low-moisture 50 cSt fluid with high thermal and oxidation stability, a fire point above 340 C, and a wide operating temperature range, meeting IEC 60836 and ASTM D4652.
Is silicone heat transfer oil the same as 50 cSt silicone oil?
No. RawSil HT-50 shares the 50 cSt viscosity but is a low-moisture (under 50 ppm water), purified heat-transfer and electrical grade. A generic 50 cSt silicone oil is not qualified for thermal or dielectric service; the grade, not just the viscosity, is what you specify.
Why use silicone instead of mineral hot oil for heat transfer?
Fire safety and life. Silicone heat transfer oil has a fire point above 340 C versus a much lower value for mineral hot oil, and it resists oxidation and coking with no additives to deplete, so it lasts far longer with less fouling and maintenance.
What temperature range does silicone heat transfer oil cover?
It has a pour point below minus 50 C and high-temperature stability, giving a wide operating window. Its honest limit is that its top continuous-use temperature is lower than some aromatic synthetic fluids, which trade that range for flammability and toxicity.
Can the same fluid be used for transformers and heat transfer?
Yes. Because RawSil HT-50 is a low-moisture, electrical-grade PDMS, the same fluid serves as a fire-safe dielectric coolant in transformers and as a heat transfer fluid in thermal systems. The transformer application is detailed separately.
Editorial note. This article is general technical and procurement guidance for industrial and process professionals and is not engineering or regulatory advice. Physical, thermal, and fire-property figures are typical reference values from the product technical data; they are not a guaranteed specification, and the Certificate of Analysis governs the material you buy. Fluid selection, temperature limits, and standards compliance must be confirmed for your system and jurisdiction. The fluid is for industrial and professional use and is not represented as suitable for medical, pharmaceutical, or food use. Always consult the current Safety Data Sheet (SDS) before handling. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
