When an industrial process requires heat above approximately 300°C, selecting the right heat transfer fluid affects the entire heating system, including equipment materials, control systems, startup procedures, and maintenance requirements.
Two practical options are thermal oil (organic heat transfer fluid) and molten salt. Each has distinct operating characteristics that influence system design and suitability for different industrial applications.
Operating Temperature and Fluid Properties
The main differences between thermal oil and molten salt involve operating temperature, freezing point, system pressure, and fluid stability.
| Parameter | Thermal Oil | Binary Molten Salt (Solar Salt) | Ternary Molten Salt |
|---|---|---|---|
| Composition | Organic heat transfer oil | 60% NaNO3 + 40% KNO3 | 53% KNO3 + 7% NaNO3 + 40% NaNO2 |
| Maximum operating temperature | 380°C | 575°C | 480°C |
| Freezing point | Typically well below 0°C | 220–240°C | Approximately 142°C |
| System pressure | Below 1.7 MPa, depending on pump head and static pressure | Below 1.0 MPa, depending on pump head | Below 1.0 MPa, depending on pump head |
| Fluid degradation | Oxidation and thermal decomposition | Thermal decomposition at excessive temperatures | Thermal decomposition at excessive temperatures |
| Flammability | Combustible | Nonflammable; strong oxidizer | Nonflammable; strong oxidizer |
These figures represent the operating characteristics described above. Each heat transfer fluid has a specified maximum operating temperature, and exceeding that limit can cause thermal decomposition.
Binary nitrate molten salt remains stable at temperatures up to approximately 575°C, making it suitable for high-temperature applications such as Concentrated Solar Power (CSP).
At the 50 MW Delingha CSP plant in Qinghai, China, where CEEF supplied heating and freeze-protection systems, the molten salt temperature at the receiver outlet reached 566°C.
Freeze Protection and System Design
One of the most important differences between thermal oil and molten salt is their behavior during cooling.
As thermal oil cools, its viscosity increases. Molten salt, however, solidifies when its temperature falls below the freezing point.
Solidified molten salt can block circulation pipelines and create pressure-related risks during remelting. Consequently, molten salt heating systems require specific provisions for drainage, heat tracing, and freeze protection.
Key design measures include:
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Preheating before initial filling: The heater is typically preheated using controlled electric heating or fuel-fired heating before molten salt is introduced into the system.
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Heat tracing and insulation: Pipelines, valves, pumps, and instrument lines require heat tracing and thermal insulation to prevent temperatures from falling below the salt’s freezing point.
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Gravity-drain design: When circulation stops or the system shuts down, molten salt can drain back into the salt storage tank under gravity.
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Automatic temperature monitoring: Monitoring systems can issue warnings and initiate protective actions before temperatures approach the salt’s freezing point.
These measures form part of CEEF’s molten salt system design approach. For this reason, freeze protection is an integral part of the overall system rather than a function of the heater alone.
Material Selection for High-Temperature Operation
Equipment materials must be selected according to the operating temperature.
For systems operating below 350°C, carbon steel is commonly used.
Between 350°C and 480°C, alloy structural steel is typically selected to accommodate higher operating temperatures.
For temperatures exceeding 480°C, stainless steel is generally used for equipment components exposed to the highest temperatures.
Valves, pumps, and sealing components must also be compatible with the operating temperature and resistant to molten salt corrosion.
Molten Salt for Thermal Energy Storage
Another important advantage of molten salt is its ability to store thermal energy.
Molten salt has a relatively high density and can operate across a substantial temperature difference between its hot and cold states, making it suitable for thermal energy storage.
In a two-tank storage arrangement, hot and cold molten salt tanks allow a power plant to store thermal energy and release it when required.
This principle is used in modern solar thermal power plants and can also be applied to industrial processes with significant variations in heating demand.
Choosing Between Thermal Oil and Molten Salt
The appropriate heat transfer fluid depends primarily on the required operating temperature, thermal storage needs, and operating conditions.
| Operating Requirement | Recommended Heat Transfer Fluid |
|---|---|
| Operating temperatures up to 380°C with relatively simple operation | Thermal Oil |
| Process heating or power cycles requiring 350–565°C | Molten Salt |
| Thermal energy storage integrated with a heating system | Molten Salt |
| Simpler startup procedures and intermittent operation | Thermal Oil |
Thermal oil is generally the more practical option when operating temperatures remain within its specified limits and straightforward startup is important.
Molten salt becomes more suitable when higher operating temperatures or integrated thermal energy storage are required, provided that the system incorporates appropriate freeze-protection measures.
CEEF Thermal Oil and Molten Salt Heating Equipment
CEEF manufactures both thermal oil heaters and molten salt heating systems, with equipment capacities covering a wide range of industrial heating requirements.
The equipment referenced here includes:
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46 MW Thermal Oil Heater: Designed for petrochemical industry applications.
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40 MW Ternary Molten Salt Heater: Operating at 450°C for applications in the melamine and alumina industries.
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28 MW Binary Molten Salt Heater: Operating at 565°C for thermal energy storage applications.
CEEF’s heating equipment portfolio also includes smaller-capacity systems starting at 24 kW, supporting different industrial process heating requirements.