# Heat-transfer fluid

In fluid thermodynamics, a heat transfer fluid (HTF) is a gas or liquid that takes part in heat transfer by serving as an intermediary in cooling on one side of a process, transporting and storing thermal energy, and heating on another side of a process. Heat transfer fluids are used in countless applications and industrial processes requiring heating or cooling, typically in a closed circuit and in continuous cycles. Cooling water, for instance, cools an engine, while heating water in a hydronic heating system heats the radiator in a room.
Water is the most common heat transfer fluid because of its economy, high heat capacity and favorable transport properties. However, the useful temperature range is restricted by freezing below 0 °C and boiling at elevated temperatures depending on the system pressure. Antifreeze additives can alleviate the freezing problem to some extent. However, many other heat transfer fluids have been developed and used in a huge variety of applications. For higher temperatures, oil or synthetic hydrocarbon- or silicone-based fluids offer lower vapor pressure. Molten salts and molten metals can be used for transferring and storing heat at temperatures above 300 to 400 °C where organic fluids start to decompose. Gases such as water vapor, nitrogen, argon, helium and hydrogen have been used as heat transfer fluids where liquids are not suitable. For gases the pressure typically needs to be elevated to facilitate higher flow rates with low pumping power.
In order to prevent overheating, fluid flows inside a system or a device so as to transfer the heat outside that particular device or system.
They generally have a high boiling point and a high heat capacity. High boiling point prevents the heat transfer liquids from vaporising at high temperatures. High heat capacity enables a small amount of the refrigerant to transfer a large amount of heat very efficiently.
It must be ensured that the heat transfer liquids used should not have a low boiling point. This is because a low boiling point will result in vaporisation of the liquid at low temperatures when they are used to exchange heat with hot substances. This will produce vapors of the liquid in the machine itself where they are used.
Also, the heat transfer fluids should have high heat capacity. The heat capacity denotes the amount of heat the fluid can hold without changing its temperature. In case of liquids, it also shows the amount of heat the liquid can hold before its temperature reaches its boiling point and ultimately vaporises.
If the fluid has low heat capacity, then it will mean that a large amount of the fluid will be required to exchange a relatively small amount of heat. This will increase the cost of using heat transfer fluids and will reduce the efficiency of the process.
In case of liquid heat transfer fluids, usage of their small quantity will result in their vaporisation which can be dangerous for the equipment where they are used. The equipment will be designed for liquids but their vaporisation will include vapors in the flow channel. Also gases occupy larger volume than liquids at the same pressure. The production of vapors will increase the pressure on the walls of the pipe/channel where it will be flowing. This may cause the flow channel to rupture.

## Characteristics of heat transfer fluids

Heat transfer fluids have distinct thermal and chemical properties which determine their suitability for various industrial applications. Key characteristics include:
* Thermal Stability: This refers to a fluid's resistance to irreversible changes in its physical properties at varying temperatures. Fluids with high thermal stability have fewer degradation pathways, leading to longer service lifetimes and less maintenance. The determination of a fluid's thermal stability is often based on tests such as ASTM D6743, which assess degradation products formed under thermal stress.
* Viscosity: The viscosity of a fluid affects its flow characteristics and pumping costs. Lower viscosity fluids are easier to pump and circulate within a system.
* Heat Capacity: A fluid’s heat capacity indicates how much thermal energy it can transport and store, impacting the efficiency of the heat transfer process.
* Thermal Conductivity and Thermal Diffusivity: These properties influence the rate at which heat is transferred through the fluid, affecting how quickly a system can respond to temperature changes.
* Corrosion Potential: The compatibility of a heat transfer fluid with system materials is crucial to minimize corrosion and extend the life of the equipment.
* Freezing and Boiling Points: Fluids should have high boiling and low freezing points to remain in the desired phase during the heat transfer process and to avoid phase change-related issues within the operating temperature range.

## Industrial Applications

Heat transfer fluids are integral to various industrial applications, enabling precise temperature control in manufacturing processes. In the food industry, they are vital for processing meats and snacks. Chemical processes often rely on them for batch reactors and continuous operations. The plastics, rubber, and composites sectors use heat transfer fluids in molding and extrusion processes. They are also critical in petrochemical synthesis and distillation, oil and gas refining, and for converting materials in presses and laminating operations.

## Heat Transfer Fluids in Solar Energy

In solar power plants, heat transfer fluids are used in concentrators like linear Fresnel and parabolic trough systems for efficient energy generation and thermal storage. Molten salts and synthetic heat transfer fluids are utilized based on their ability to function at various temperature ranges, contributing to the generation of electricity and the manufacturing of polysilicon for photovoltaic cells. These fluids assist in the purification and cooling steps of polysilicon production, essential for creating high-purity silicon for solar and electronic applications. Technico-economic analyses are usually performed to select the appropriate heat transfer fluid. Regarding the selection of a low-cost or cost-effective thermal oil, it is important to consider not only the acquisition or purchase cost, but also the operating and replacement costs. An oil that is initially more expensive may prove to be more cost-effective in the long run if it offers higher thermal stability, thereby reducing the frequency of replacement.

### Common Heat Transfer Fluids

The choice of a heat transfer fluid is critical for system efficiency and longevity. Here are some commonly used fluids:
* Water: The most widely used due to its high heat capacity and thermal conductivity.
* Mono-ethylene glycol: Often used in a mixture with water to lower the freezing point for use in colder climates.
* Propylene glycol: Preferred in food production and other industries where toxicity might be a concern.
* Silicone oil: Used for its stability at high temperatures and electrical insulating properties.
* Synthetic and aromatic heat transfer fluids: Employed in high-temperature applications, such as solar power generation and industrial heat processes.
* Molten salts: Utilized in solar energy systems for their capacity for thermal storage and ability to operate at very high temperatures.
* Vegetable oil: Utilized in solar energy systems because they are biodegradable and renewable.