Heat-transfer fluids and heat pipes: How it works

Conduction, convection, boiling and radiation: choosing how heat travels

Heat moves in three ways (conduction through solids, convection carried by moving fluids, and radiation across space), and engineers pick a carrier to suit the temperature: water, thermal oil, molten salt, a boiling refrigerant, a sealed heat pipe or a block of storage material driven on a truck. Before refrigeration, ice cut from ponds was shipped across oceans as stored cold.

Step by step

1. Heat crosses a wall by conductionHeat flows from hot to cold through a solid at a rate set by its thermal conductivity, the area, the temperature difference and the thickness. Copper conducts about 390 W per metre per degree; still air only about 0.023, which is why foam (mostly trapped air) insulates.
2. A pumped fluid carries it away by convectionA liquid is warmed at the hot surface and pumped elsewhere. Synthetic thermal oils such as Therminol VP-1 are rated by their maker from 12 to 400 C, at far lower pressure than water would need at those temperatures; above about 400 C the oil starts to break down.
3. Molten salt for higher temperaturesSolar salt, 60% sodium nitrate and 40% potassium nitrate, is stable up to about 565 C but begins to solidify somewhere around 220 to 240 C, so the cold tank is held near 290 C for a safety margin.
4. Boiling and condensing carries far moreA refrigerant absorbs heat as it evaporates and releases it as it condenses, so a small flow moves a lot of heat. A heat pipe does this in a sealed tube with no pump: a wick returns the liquid by capillary action.
5. Trucked heatMobile stores fill a container with a phase-change material. In the German WärmeEngel project, sodium acetate trihydrate melts at 58 C, each unit holds about 2.5 MWh, and lorries carry landfill-gas waste heat about 16 km to a college.
6. RadiationEvery surface radiates in proportion to the fourth power of its absolute temperature, so radiation dominates at furnace and space temperatures and is blocked by shiny, low-emissivity surfaces.

The science

The key quantity is how much heat a carrier holds per kilogram and how easily it gives it up. Sensible heat (warming a liquid) stores energy in proportion to the temperature rise, so oil and salt are run as hot as their chemistry allows. Latent heat (melting or boiling) stores much more at a fixed temperature, which is why refrigerants, heat pipes, phase-change stores and ice are compact. Published effective conductivities for heat pipes are 10,000 to 100,000 W per metre per degree, tens to hundreds of times copper's, but the real figure falls sharply with length: a 200 mm pipe reaches less than a third of the top value.

The engineering

Oil and salt loops need pumps, heat exchangers and trace heating to stop salt freezing. Solar Two in California melted about 1,300 to 1,400 tonnes of nitrate salt (the final report gives both figures) for its two-tank store. Refrigerant circuits need compressors and leak-tight joints. Heat pipes in laptops and servers are typically copper tubes of 3 to 8 mm with a sintered copper wick and water; Fujikura makes versions 0.6 to 1.5 mm thick. On the Trans-Alaska oil pipeline, 420 miles of pipe sit on 78,000 vertical support members, and in the most sensitive permafrost the supports carry pairs of ammonia-filled tubes that passively pull heat out of the ground.

Key numbers

Sources

1. University Physics Volume 2, 1.6 Mechanisms of Heat Transfer OpenStax, 2016
2. Molten salt suitable heat transfer fluid for concentrating collector fields solarthermalworld.org, 2021
3. Innovation Outlook: Thermal Energy Storage IRENA, 2020
4. Therminol VP-1 heat transfer fluid (company data) Eastman, 2026
5. Design Considerations When Using Heat Pipes (George Meyer) Electronics Cooling, 2016
6. Waste heat is transported to the customer by truck Bayern Innovativ, 2026