Coolant

A coolant is a special liquid used to help keep things from getting too hot. Coolants are very important in cars, houses, and many machines. They take away heat and keep systems running safely.

An ideal coolant needs to have a few good qualities. It should be able to hold a lot of heat, flow easily, cost little, and not be harmful. It also should not damage the parts it touches or allow electricity to pass through.

Coolants can stay the same, like staying liquid or gas, or they can change from one form to another. When they change, like from gas to liquid, they can cool things even more, which is how refrigerators work.

Every cooling system has a special container called a coolant reservoir. This holds any extra coolant that might overflow, keeping the system working properly.

Gases

Air is a common type of coolant. It can cool things by moving naturally (called passive cooling) or by using fans to push the air around.

Hydrogen is used as a special coolant because it can take away heat very well and doesn’t weigh much, which helps machines that spin really fast. In big power plants, hydrogen is often used in machines that make electricity.

Some safe gases, like helium and carbon dioxide, are used to cool nuclear reactors. Helium doesn’t easily turn into radioactive materials, making it safe for this use. Sulfur hexafluoride is used to cool and protect high-voltage electrical equipment, like switches and transformers. Steam can also be used as a coolant when a lot of heat needs to be taken away quickly.

Two-phase

Some coolants can be used in both liquid and gas forms in the same system. This helps because when they change from liquid to gas or gas to liquid, they absorb or release a lot of heat, which is very useful for cooling.

Refrigerants are a type of coolant used to reach very low temperatures by changing between liquid and gas. There are many kinds, like halomethanes such as R-12 and R-22, which were used a lot in the past. Today, other types like R-134a and carbon dioxide are used more because of environmental concerns. Water can also be used in this way, for example in boiling water reactors. Liquid gases, like liquid nitrogen, are also used in labs for cooling.

Liquids

Water is the most common coolant. It works well because it can hold a lot of heat and is cheap. It is often used with special additives, like substances that stop rust and antifreeze. Antifreeze is a mix of certain organic chemicals, like ethylene glycol, in water. It is used when the coolant needs to stay working in very cold temperatures or when it needs to handle very high temperatures. Betaine is another coolant made from plant juice. It is not harmful and is easy to dispose of.

Very pure deionized water is used to cool some electrical equipment because it does not conduct electricity well. Heavy water is used in some nuclear reactors as both a coolant and to help control the reaction. Light water reactors, the most common type, use regular water.

Polyalkylene glycol (PAG) is used as a coolant that can handle very high temperatures and does not break down easily. It can also be safe and harmless.

Cutting fluid is a coolant that also helps reduce friction during metal-shaping work. Oils are used when water is not suitable. They can handle very high temperatures without creating high pressure. Mineral oils are used in many machines as both coolants and lubricants. Some vegetable oils, like castor oil, are also used. Mineral oils are used in small electric heaters and in computer systems because they do not conduct electricity.

Silicone oils and fluorocarbon oils are used when a very wide range of temperatures is needed, but they cost more. Transformer oil is used to cool and protect high-power electric transformers. Fuels like kerosene are used as coolants in airplane engines. Waterless coolant is an alternative to water and ethylene glycol. It can handle very high temperatures without boiling and stops rust.

Freons were once used to cool electronics by being poured over them.

Molten metals and salts

Liquid metals like sodium or a mix of sodium and potassium can be used as coolants in special nuclear reactors. For very high temperatures, molten salts like a mix of sodium fluoride and sodium tetrafluoroborate can be used.

Liquid gases

Liquified gases are used as coolants for very cold applications, like in microscopes or for making computer processors work faster. Carbon dioxide is used as a coolant instead of cutting fluids. It keeps tools and materials at normal temperatures and lasts longer. Liquid nitrogen, which is very cold, is the most common coolant. Liquid air is used less because it can cause fires. Even colder temperatures can be reached with liquified neon or liquid helium. Liquid hydrogen is used to cool rocket engines.

Nanofluids

Nanofluids are a new type of coolant made by mixing tiny particles into a liquid like water. These particles improve the coolant’s ability to transfer heat. For example, silver nanorods can increase water’s heat transfer by up to 68%. Alumina nanoparticles can help prevent the formation of steam layers on cooled objects.

Solids

In some cases, solid materials are used as coolants. These materials need a lot of energy to turn into gas, and this energy is taken away by the gases. This method is often used in space travel, for protecting spacecraft when they enter Earth's atmosphere, and for cooling rocket engine parts. It is also used to protect buildings from fire by applying special coatings.

Dry ice and water ice can also be used as coolants when they touch directly with the object being cooled. Sometimes, another fluid helps with the cooling; mixtures of water with ice and dry ice in acetone are popular choices. The turning of water ice into vapor was used to cool the space suits worn during the Apollo space missions.

Biology

Most warm-blooded animals need coolants to keep their body temperature steady. For example, mammals often release salt water, which dries up quickly when it evaporates, helping to cool the body.