It's the mixture that makes the difference
Cooling brine, also known as heat transfer fluid, is an aqueous salt solution whose solidification temperatures are far below the freezing point of water. Cooling brines are generally used as refrigerants. The start of solidification of the cooling brine is the decisive application criterion. The start of solidification depends on the concentration and type of dissolved salts.
Depending on the requirements, the concentration required is set or determined in advance. This is done by increasing or reducing the corresponding water content.
Inhibitors in the cooling brines prevent corrosion
Chloride brines (usable up to approx. -45 °C) and carbonate brines are used. If a brine reaches the minimum possible temperature, it is called eutectic brine. Similar to pure substances, these brines solidify and are therefore predestined for cold storage.
Commercial brines contain additives of buffer substances and inhibitors. They prevent the corrosive effect of the brines and improve the pH value. The freezing point can be determined individually or reduced to different extents with a different application concentration.
The composition determines the effect
A sodium chloride solution, for example, has its lowest freezing point at -21.3°C. This is equivalent to the eutectic point. The freezing point for a mixture with ethylene glycol (mixing ratio 50:50), however, is -38 °C. The heat transfer decreases and the specific heat capacity is reduced to 3.5 kJ. By way of comparison, the specific heat capacity of pure water is 4.2 kJ. Another consequence is that the viscosity of the ethylene mixture increases, i.e. the mixture is more viscous. The pumps in the systems have to work more to keep the brines flowing.
Note: chloride-based cooling brines are not suitable for certain materials such as chromium-nickel steels, aluminium or aluminium alloys.
Special mixtures of alcohol and pure water
In addition to the mixtures already mentioned, specific mixtures of water and alcohol (e.g. propylene glycol, ethylene glycol) are also used as cooling brines.
Glycols are mainly used in metal systems because aqueous salt solutions have a high corrosive effect.
Propylene glycol is mainly used in cooling systems for food production and processing. This is because the use of ethylene glycol is prohibited by law.
Areas of application for cooling brines
- refrigeration plants
- Air conditioning and room cooling systems
- cold storage facilities
- heat pumps and heat recovery systems respectively
- Energy transport (cold and warm)
- recooling systems
The refrigerant is the product that withdraws heat energy from the environment through the phase transition from liquid to gas and thereby cools down. It thus generates cold. Natural refrigerants are e.g. ammonia, carbon dioxide, water and hydrocarbons, i.e. these substances occur in nature. In contrast, fluorocarbons, for example, are artificially produced refrigerants.
Cooling brine is another somewhat outdated term for refrigerants. Traditional brine is an aqueous sodium chloride solution. Thus cooling brines are aqueous salt solutions in the classical sense. Due to their ability to be cooled far below the freezing point of the water, they are used for cooling systems.
From a purely physical point of view, refrigerants and heat carriers are heat energy carriers and are the same. Depending on their application, whether the focus is on cold or warm temperatures, they are referred to as either a refrigerant or a heat transfer medium. They serve to transport cold or heat and not to generate it.