Molten salts, which are also known as salt melts, play an important role as a heat transfer medium in industry. Although water and oil are more common heat transfer fluids, there are many reasons molten salts would be chosen instead.
In this article we are discussing reasons to use molten salts and how they are used in industrial heaters.
Molten salts are used as a heat transfer or heat carrier fluid in industrial applications. They are salts that have been melted until they are liquid.
The typical operating temperature range for molten salts is 285-565 ℃. The most common type of mineral used for molten salts are nitrate salts. It is best to operate at least 20-30 ℃ above a molten salt’s melting point.
One of the biggest reasons molten salts are used for heat transfer is their ability to be used at high operating temperatures. Water boils at 100 ℃ and oil can be used up to approximately 350 ℃.
That leaves molten salts, which can operate at much higher temperatures. Additionally, molten salts are not very corrosive. This means your equipment will remain in good condition with proper maintenance.
If you have decided to use molten salts for your heat transfer fluid, it’s important to understand the types of industrial heaters that use them. Here we are reviewing the types of heating systems that typically use molten salts.
With a circulating system, molten salts are distributed as a heating medium to heat exchangers and other industrial heat users. A typical circulating industrial heating system for molten salts has the following equipment.
The equipment listed above is used as follows. The molten salts are stored in the salt tank.
Initially they are mixed with water, since salts are water soluble. This water/salt mixture is pumped through a closed-loop system with a heater until the water evaporates and the molten salts reach their operating temperature.
The closed-loop system includes pumping through the equipment that uses molten salts as a heating medium before returning to the salt tank. Once at the correct operating temperature the molten salts will start heating as needed.
It is important to consider that the circulation pumps used with molten salts are built specifically for that application. This means the pump chosen must be capable of handling very hot temperatures.
Industrial molten salt heaters that use a circulating system often have a combustion air pre-heater. This equipment is used to heat combustion air that is used at the heater with outgoing hot flue gasses.
Another common piece of equipment included with circulating industrial heater systems for molten salts is a condenser. The condenser uses cold water to condense the water vapor coming from the salt tank during initial heating.
When using salt bath heaters as an industrial heater for molten salts, there is no circulation pump for the system. With a salt bath heater, a vessel is filled with salts and heated to very high temperatures in order to melt the salts.
Inside the vessel is a coil in which the process fluid that needs to be heated is flowing through. This process uses natural convection for heating. It might be chosen because a circulation pump is not needed. A disadvantage to using a salt bath heater is that if the design is poor the heater can be easily damaged.
What industries use molten salts? Molten salts are used across a wide variety of applications including:
The two most common uses for molten salts in industry are thermal fluid heating and thermal storage.
Since molten salts have excellent thermal stability and heat transfer properties, it makes them an attractive candidate for energy storage. They have become more and more popular for use in concentrated solar plants.
Some of the reasons you would opt to use molten salts as your heat transfer fluid include its low melting point, high operating temperature, low corrosivity, low toxicity, and low vapor pressure.
As previously discussed, the #1 reason to use molten salts is their high operating temperature.
As with most things, there are also negatives to using molten salts. The biggest issue with them is their high freezing point ranging from 120-220 ℃. If your heating equipment malfunctions your molten salts will turn solid. This also highlights the importance of having good heat tracing in places where you have molten salts.
Finally, if your application does not require very high operating temperatures, then you would want to reconsider choosing molten salts for your heating medium. In these cases water or oil would probably be the better option.
The first step with using molten salts for industrial heating is deciding if molten salts are the right application for your process.
When it comes to picking an industrial heater system for molten salts you have two choices: circulating systems or salt bath heaters. Your needs, budget, and maintenance capabilities will determine which molten salt heater system is right for your application.
Looking for the right industrial heater to support your molten salt applications? Get in touch with us and a Wattco representative will get you started right away.