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Concept
Antifreeze
Summary
An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allowing higher coolant temperature. However, all common antifreeze additives also have lower heat capacities than water, and do reduce water's ability to act as a coolant when added to it.
Because water has good properties as a coolant, water plus antifreeze is used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters. The purpose of antifreeze is to prevent a rigid enclosure from bursting due to expansion when water freezes. Commercially, both the additive (pure concentrate) and the mixture (diluted solution) are called antifreeze, depending on the context. Careful selection of an antifreeze can enable a wide temperature range in which the mixture remains in the liquid phase, which is critical to efficient heat transfer and the proper functioning of heat exchangers. Most if not all commercial antifreeze formulations intended for use in heat transfer applications include anti-corrosion and anti-cavitation agents (that protect the hydraulic circuit from progressive wear).
Water was the original coolant for internal combustion engines. It is cheap, nontoxic, and has a high heat capacity. It however has only a 100 °C liquid range, and it expands upon freezing. To address these problems, alternative coolants with improved properties were developed.
Freezing and boiling points are colligative properties of a solution, which depend on the concentration of dissolved substances. Salts lower the melting points of aqueous solutions. Salts are frequently used for de-icing, but salt solutions are not used for cooling systems because they induce corrosion of metals. Low molecular weight organic compounds tend to have melting points lower than water, which makes them suitable for use as antifreeze agents.
Official source
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