Glycol is a water-miscible coolant that is frequently used in heat transfer and cooling applications. It provides better heat transfer parameters than water, and can be mixed with water to provide a variety of heat transfer characteristics. Glycol comes in two varieties: ethylene glycol and propylene glycol. Though both materials are bad for living things, propylene glycol is most often used near food and ethylene glycol is most often used in industrial applications.
To understand the purpose of glycol, you must first understand how a chiller works. A chiller consists of two key parts: a refrigeration unit which uses electrical energy to produce a cold fluid, and heat transfer coils which move cold fluid from the refrigeration unit to the target area and hot fluid from the target area to the refrigeration unit. Typically, the target area is the inside of a freezer or some other object that you want to cool. The refrigeration unit often consists of a compressor with some compressible heat transfer fluid such as freon.
Every chiller has an operating temperature range. This temperature range is determined by several variables, the most important of which are the boiling point and freezing point of the heat transfer fluid. Glycol is prized as a heat transfer fluid because it can operate at a wide range of temperatures and can be mixed with water. The boiling and freezing points of glycol mixtures are a function of the relative amounts of glycol and water in the mixture.
Pure water freezes at 0 degrees Celsius (32 F) and pure ethylene glycol freezes at -12.9 C (8.9 F). In between, freezing points are non-linear. For instance, a solution of 10% ethylene glycol freezes at -3.4 C (25.9 F), 30% ethylene glycol freezes at -13.7 C (7.3 F) and 60% ethylene glycol freezes at -52.8 C (-63 F). The freezing point of a 60/40 ethylene glycol/water mixture is much lower than that of either pure ethylene glycol or pure water. Mixtures of propylene glycol with water follow a similar pattern, with a 60/40 mixture of propylene glycol with water having a freezing point of -48 C (-55 F).
The low freezing points of glycol mixtures make them ideal for cooling items below the freezing point of water. Thus, glycol/water mixtures are often used to cool freezers and similar environments.
Glycol is useful even when you do not want to cool an item below the freezing temperature of water. The rate of heat transfer is proportional to the difference between the temperature of your coolant and the temperature of the item being cooled. The heat capacity of the coolant, which is a chemical characteristic of the material, is important as well, but we will put it aside for the moment. A 60/40 ethylene glycol/water mixture cooled to -40 C can chill an item at 20 C much more quickly and efficiently than pure water at 10 C. Although ethylene glycol has a lower heat capacity than water (each kilogram of glycol is easier to heat than a kilogram of water), the larger temperature difference allows a glycol mixture to carry heat away much more quickly than pure water.
The low temperatures associated with glycol mixtures make them useful for applications where a chiller must carry a large amount of heat must away quickly. Heat is a byproduct of many chemical reactions; glycol’s ability to carry heat away quickly makes it useful for maintaining the temperatures of chemical reactions. For this reason, propylene glycol/water mixtures are often used to cool fermenters in brewery applications.
Glycol is an important heat transfer fluid in industrial applications. In addition to offering excellent heat transfer parameters, ethylene glycol tends to discourage algae growth in heat transfer equipment. For more information, please contact us. This article is not engineering or food production advice.