Molten Salt as Heat Transfer Fluid

Electric Circulation HeaterThermal fluid systems use all sorts of transfer fluids to provide heat. The heat transfer fluid is heated by a circulation heater and flows to the end target, heating it in turn. Because the end target does not directly contact the heater it is capable of precise heating and prevents damage from contact.

The type of heat transfer fluid varies depending on factors including the end target and the required temperatures.

 

Molten Salts

Although water and oils are the most common heat transfer fluids, molten salts (or salt melts) fill an important niche. These are composed of salts that have been melted to a point where they become a fluid. 

Unlike sodium chloride (table salt), the minerals used in molten salts are not aggressively corrosive. With regular maintenance and proper use, they are safe to use in your heat transfer applications.

Nitrate salts are the most common mineral for molten salt heat transfer fluid. They have a low melting point making them easier to achieve, and a high operating temperature making them effective in high-temperature applications. 

Other benefits of nitrate salts include:

  • Low Corrosivity
  • Low Toxicity
  • Thermophysical Properties
  • Low Vapor Pressure

 

To gain these benefits and to further reduce melting points, sodium nitrate is commonly blended with potassium nitrate to create these transfer fluids. This reduces freeze-up risks and increases the fluid’s versatility.

 

Molten Salts Versus Water and Oil-Based Transfer Fluids

It is important to pick the right thermal fluid and heating equipment for each project. One of the key considerations in choosing between water, oil, and molten salt heat transfer fluids is the operating temperature.

Water heat transfer fluids are a great option where applicable. In most places, water is cheap and readily available. The downside is that water offers the lowest operating temperature. Because of its low boiling point, water-based transfer fluids are a poor choice in processes operating over 100℃. As well, they are prone to freezing anywhere below 0℃ unless mixed with antifreeze.

Oil-based heat transfer fluids are capable of operating at higher temperatures, often used for 100℃ to 350℃. At the upper ranges of this, natural oils tend to degrade making synthetics a necessity.

More expensive synthetics like diphenyl oxide can help these heat transfer fluids operate up to 390℃. However, they remain prone to degradation. This can cause a loss of efficiency, fouling, and produce toxic byproducts.

For temperatures over 390℃, or to avoid degradation risks over 350℃, molten salts are a preferable heat transfer fluid.

 

Molten Salt Operating Temperature

Molten salt heat transfer fluids are ideal for high-temperature operations. While even synthetic oil-based fluids tap out at 390℃, the majority of molten salt applications range from 285℃ to 565℃. 

Switching to molten salt at temperatures as low as 285℃ prevents having to risk degradation, while also benefiting from this heat transfer medium’s low vapor pressure.

Although they are a great option for high-temperature applications, molten salts fluids do have an upper limit. Above 565℃, this transfer fluid begins to degrade. Chloride and fluoride salts are more stable than nitrate salts, so they can be used where the heat transfer fluid needs to be over 600℃. However, these salts are more corrosive than nitrates and have higher melting points, so there is a greater risk of freezing. These issues are more pronounced at higher temperatures.

 

High-Performance Heaters for Heat Transfer Fluids

Whether your operation is best suited to molten salt, water, or oil-based transfer fluids, Wattco is your best option for heating systems. Wattco custom manufactures industrial process heaters for all industries. Our team of engineers helps you select the right heater type, design, materials, and configurations for your project and budget.

Contact Wattco today for electric circulation heater quotes and information.

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