What are the criteria for selecting the heat transfer medium in an external coil reactor?

The selection of heat transfer media for external coil reactors requires comprehensive consideration of key parameters such as temperature range, thermal stability, safety, and economy. For high-temperature conditions (above 300℃), molten salts (potassium nitrate-sodium nitrite mixture) or heat transfer oils (biphenyl-diphenyl ether) are typically chosen, characterized by high boiling points, large heat capacities, and low vapor pressures. In the medium-temperature range (150-300℃), saturated steam or mineral oil is often used. Steam has the advantage of high latent heat of phase change, while mineral oil is suitable for precise temperature control. For low-temperature heat transfer (-50℃ to 150℃), ethylene glycol aqueous solutions or liquid ammonia are commonly used, balancing antifreeze performance and heat transfer efficiency. The thermophysical properties of the medium directly affect the heat transfer effect; media with high thermal conductivity can improve the heat exchange efficiency of the coil, while media with low viscosity are beneficial for circulation.

Chemical compatibility is another important consideration. Organic heat transfer oils should be avoided in strongly oxidizing environments, and heat carriers containing metal ions should be excluded from acidic media. Safety requirements include flash point, auto-ignition point, and toxicity level. In explosion-proof applications, non-flammable media such as high-temperature water should be selected. Economic evaluation needs to consider both initial cost and operating consumption. Molten salt systems have high initial investment but long service life, while heat transfer oils require regular replacement but are easy to maintain. The environmental friendliness of the medium is also increasingly important, with biodegradable synthetic ester oils gradually replacing traditional mineral oils. System design must also consider the compatibility of the medium's thermal expansion coefficient with the coil material to avoid structural damage caused by thermal stress.