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The Advantages of Using Molten Salts for Process Heating

Molten salts, also known as salt melts, comprise a variety of products that are used across many applications. Some applications that rely on molten salts include steel heat treating and annealing, high-temperature process heating, and thermal storage for solar thermal power plants. Depending on the needs of a specific application, different salts are available to use such as chlorides, bromides, fluorides, organic salts, and nitrates. Many applications use a eutectic blend of potassium nitrate and sodium nitrate.While there are several heat transfer fluid options available, molten salts are more ideal for certain applications due to the specific features unique to them.

Molten Salt

Why Molten Salts?

There are several advantages of molten salts that make them appealing for use in a wide range of applications. The molten salts that are used most frequently in heat transfer applications are nitrate salts due to their low salt melting point, thermophysical properties, low vapor pressure, high operating temperature, corrosion performance, and low toxicity.

Molten salts help increase the maximum temperature limits in which a liquid heat transfer media can be utilized. Although individual salts are usable in these applications, combining two salts together helps reduce the melting point. The reduced melting point enables lower minimum operating temperatures, which minimizes the risk of freezing. 

For example, the melting point of sodium nitrate is 584 °F or 307 °C, and the melting point of potassium nitrate is 631 °F or 333 °C. Combined, the two salts have a melting point of 431 °F or 222 °C. This melting point significantly expands the salt’s operational flexibility for use in various high-temperature applications.

If an application operates at temperatures exceeding 734 °F or 390 °C and requires a type of liquid heat transfer media, it will need to use a molten salt fluid.

Benefits for Industrial Applications?

A wide range of salt blends can help achieve specific operating temperatures required in certain applications. Compared to hydrocarbon fluids, molten salts experience minimal vapor pressure, regardless of how close operating temperatures are to their limits. As a result, high-pressure equipment and piping are almost entirely unneeded. 

Molten salts are also usable at higher temperatures compared to other fluids such as silicone fluids and synthetic oils. They also have thermal stability and good heat transfer properties. Their high efficiency makes them suitable for use as heat transfer and molten salt energy storage media, as they are eco-friendly and can reduce operating costs. 

Request Your Custom Molten Salt Systems

Depending on your application’s requirements, there are three main types of molten salt heating systems you can use. These include circulated molten salt, salt bath heaters, and direct heating for applications such as metal assembly heat treating. 

At Sigma Thermal, we can provide:

  • Circulated molten salt systems that distribute hot liquid salt as a type of heat medium to process heat consumers such as heat exchangers.
  • Salt bath heater systems that use natural convection processes and eliminate the need for circulation pumps. This equipment is designed to operate at high temperatures to supply molten salt energy.

Sigma Thermal’s experts design and manufacture a variety of systems that use molten salt energy, including hot salt tanks, circulated molten salt systems, electric heaters, and salt bath heaters. We have the resources and experience needed to design fully automated custom systems. We can help you meet all of your process needs with our innovative solutions.

To learn more about our molten salt systems and get started on a custom solution for your specific application, request a quote today.

Glycol Heaters

Certain types of heating systems, such as closed-looped thermal fluid heating systems, can use glycol to maintain heat transfer fluid to the desired temperature. While water is an ideal heat transfer medium for a certain range of temperatures, pure water has drawbacks. It can freeze below 32° F and it can corrode the internals of a heating system. Also referred to as antifreeze, glycol lowers the freezing temperature of the mixtures it is added to and it ensures that harsh weather conditions do not cause heating systems to freeze. In addition, glycol raises the boiling point of water to some degree, allowing for higher operating temperatures with less operating pressure than pure water. Glycol is also frequently purchased with corrosion inhibitors that reduce corrosion inside of the system over time. 

Why Are Glycol Heaters Important?

Without adding glycol to a heating system, expensive and potentially dangerous damage to process equipment can occur. If the heating system is shut off for some time during the winter, the system and its pipes can freeze and burst, causing water damage. In addition to preventing system freezing, glycol mixtures are also a very efficient heat transfer medium compared to oils or water alone. Water and glycol mixtures have a better heat capacity than oils and perform well in many types of heaters. There are temperature limitations relative to thermal oils, but within the right range, a water-glycol solution is an excellent choice.

Additionally, glycol heaters are valuable due to the safe heat transfer that they provide. For example, it can be dangerous to have heating elements near the process or materials, but using a glycol heating system keeps the heat source safely away from the process. Thermal oils are also a fire hazard if there is a system leak, while glycol solutions are not flammable. 

Using Glycol in Industrial Heating System Applications

Ethylene and propylene glycol are commonly used in industrial facilities for process cooling. However, they are also used in process heating systems. They are very effective in heating applications due to several properties, such as:

  • High specific heat capacity
  • High thermal conductivity
  • Low viscosity
  • Water miscibility

Water and glycol solutions present many benefits for industrial processes, on the other hand, these are not the only heat transfer mediums available. Synthetic and organic thermal oils are also sometimes selected as they work well in temperatures up to 750° F and produce very little vapor pressure. Water without additives can also be used, although within more limited conditions due to its low-temperature intolerance and boiling point. It is also possible for water to produce a dangerous amount of vapor pressure at certain temperatures. 

A water and glycol mixture can be a preferable choice in many applications since it has a higher boiling point than water alone, which can be increased further by including a higher concentration of glycol. Different types of glycol can be used for varying operating temperatures. For example, while ethylene and propylene glycol are suitable for systems with temperatures between 250–300° F, Tri-ethylene glycol works effectively up to 350° F.

Glycol Heaters from Sigma Thermal

Sigma Thermal’s glycol heaters, such as HC-2 heaters, are designed to meet all area classifications and can perform optimally in even the harshest of environments, including deserts, arctic, and offshore regions. The HC-2 thermal fluid heater is available in capacities from 1 to 100MM Btu/hr, with custom sizes also available. The heater shell is externally insulated using mineral wool insulation which is covered in aluminum cladding. 

Some other design features of glycol heaters from Sigma Thermal include advanced control system options and a double-helical coil design, which offers three passes of flue gas along a conservatively designed coil surface area. Depending on process inlet temperatures, base efficiencies can exceed 88%, and by including an optional economizer, can exceed 93% (LHV basis).

These systems allow flue gases to be cooled significantly, which eliminates the need for most internal shell insulation. Sigma Thermal carries low emissions burners that meet all emissions requirements in standard or engineered burner configurations that can be used with either traditional or alternative fuel sources. 

Sigma Thermal offers customizable glycol heaters that lower fuel and operation costs while improving efficiency. To learn more about our products, please contact us or request a quote to get started.

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