As AI, machine learning and high-performance computing push rack densities far beyond the limits of traditional air cooling, Immersion Liquid Cooling has emerged as one of the most efficient and scalable approaches to modern Data Centre Cooling. This is where Castrol ON DC 15 comes in.

A purpose-engineered, single-phase dielectric coolant designed specifically for immersion cooling of electronic and electrical systems. Combining exceptional thermal performance, outstanding dielectric protection and broad material compatibility, helping data centres unlock higher compute densities with lower energy consumption.

This guide brings together Castrol’s official product data and material compatibility guidance to explain why Castrol ON DC 15 is fast becoming a preferred data centre cooling fluid for immersion cooling deployments worldwide.

Why Immersion Liquid Cooling Is Transforming Data Centre Cooling

Traditional air cooling struggles to cope with the thermal demands of today’s processors. With rising Thermal Design Power (TDP) in CPUs and GPUs, airflow alone can no longer remove heat efficiently.

Immersion Liquid Cooling solves this by submerging IT hardware directly into a non-conductive dielectric fluid. Heat is transferred straight from electronic components into the fluid, which is then circulated through a heat exchanger.

This approach delivers:

• Significantly higher cooling efficiency

• Support for ultra-high rack densities

• Reduced power consumption

• Lower fan energy and noise

• Improved hardware reliability

However, immersion cooling only performs as well as the fluid it uses, which is where Castrol immersion fluids excel.

Introducing Castrol ON DC 15 Immersion Cooling Fluid

Castrol ON DC 15 is a synthetic hydrocarbon-based dielectric coolant developed specifically for single-phase immersion cooling of electronic and electrical systems.

It is engineered to deliver:

• Excellent flow characteristics

• High thermal performance

• Outstanding dielectric protection

• Superior oxidative and hydrolytic stability

• Excellent low-temperature performance

This makes DC 15 an ideal data centre cooling fluid for hyperscale, edge, AI and HPC environments.

Key Advantages of Castrol ON DC 15

Optimised for Immersion Liquid Cooling

DC 15 is formulated to circulate easily through immersion cooling tanks and heat exchangers thanks to its low viscosity and excellent flow behaviour. This ensures rapid heat removal from submerged IT equipment.

Outstanding Thermal Performance

With strong thermal conductivity and high specific heat capacity, Castrol ON DC 15 efficiently absorbs and transports heat away from high-power processors.

Superior Dielectric Protection

As a fully dielectric fluid, Castrol ON DC 15 provides:

• Very low electrical conductivity (<35 pS/m)

• High dielectric strength (>14 kV/mm)

• High breakdown voltage (>35 kV)

This allows electronic components to operate safely while fully submerged.

Long Fluid Life and Stability

Castrol ON DC 15 delivers:

• Excellent oxidative stability

• Strong hydrolytic stability

• High flash and fire points

• Low pour point for cold-start environments

This ensures long service life even in demanding operating conditions.

Typical Performance Characteristics

According to Castrol’s Product Data Sheet, Castrol ON DC 15 delivers the following typical properties:

• Viscosity @ 40°C: 7.5 mm²/s

• Density @ 15.6°C: 835 kg/m³

• Pour Point: -45°C

• Thermal Conductivity @ 40°C: 0.126 W/mK

• Specific Heat Capacity @ 40°C: 2.0 kJ/kgK

• Electrical Conductivity @ 25°C: <35 pS/m

• Dielectric Constant: 2.09

• Flash Point: 166°C

• Fire Point: 197°C

• Appearance: Clear & bright

These characteristics make Castrol ON DC 15 highly effective for continuous, high-density immersion cooling operation.

Material Compatibility for Immersion Cooling Systems

Material compatibility is critical in Immersion Liquid Cooling systems. Every elastomer, plastic and seal used in submerged environments must be compatible with the cooling fluid.

Castrol ON DC 15 has been tested across a wide range of commonly used materials and shows good compatibility with most commercially available elastomers and plastics.

Materials with Good Compatibility

Castrol ON DC 15 demonstrates good compatibility with:

• Fluorocarbon rubber (FKM)

• Thermoplastic polyurethane (TPU)

• Acrylic rubber (ACM)

• Acetal copolymer (POM)

• Polyurethane rubber (PUR)

• Polycarbonate (PC)

• Hydrogenated nitrile rubber (HNBR)

• ABS

• Nylon (PA6 & PA66)

• EPDM

• PTFE

• Polypropylene (PP)

• High-density polyethylene (PE-HD)

• Silicone rubber (VMQ)

• PEEK

• Ultra-high molecular weight polyethylene (PE-UHMW)

Materials Requiring Careful Evaluation

Some materials show moderate or poor compatibility depending on grade and manufacturer, including:

• Chloroprene (CR)

• Polystyrene (PS)

• Soft PVC

• Certain polyester films

• Low-density polyethylene (PE-LD)

Castrol recommends testing the exact grade of material before deployment in any immersion cooling system.

Designed for Modern Data Centre Cooling Infrastructure

Castrol ON DC 15 is designed for use in:

• Single-phase immersion cooling tanks

• Electrical and electronic cooling systems

• AI and HPC data centres

• Edge computing facilities

• Hyperscale data centres

It enables operators to deploy high-density compute with lower cooling energy, reduced infrastructure footprint, and improved sustainability.

Why Data Centres Choose Castrol ON DC 15

Data centre operators select Castrol ON DC 15 because it delivers:

• Proven immersion cooling performance

• Excellent dielectric safety

• High thermal efficiency

• Long operational life

• Broad material compatibility

• Backing from Castrol’s global technical support network

It is engineered specifically for the demands of modern Immersion Liquid Cooling and next-generation Data Centre Cooling Fluid applications.

Final Thoughts: Powering the Future of Immersion Liquid Cooling

As the data centre industry accelerates toward higher rack densities and lower carbon footprints, immersion cooling is rapidly becoming the new standard.

Castrol ON DC 15 provides the performance, protection and reliability required to support this shift — delivering safe, efficient and scalable cooling for the world’s most demanding digital infrastructure.

For operators planning the next generation of immersion-cooled data centres, Castrol ON DC 15 offers a proven and future-ready cooling foundation.

Need Technical Support?

For compatibility testing, deployment guidance or technical documentation, contact your Kernow Oils representative or visit:

Castrol ON DC 15 PDS