Imagine a summer without ice cream or cold beverages! Societies have spent a great deal of time determining how to keep their food well preserved and edible. For thousands of years, our ancestors tried several methods, such as spicing, salting, smoking, and keeping food in deep pits away from the sun to keep it from rotting. People who were rich enough to afford ice kept their food cold using ice blocks carried from the mountains or shipped from cold places. Ice houses were built to store ice for long-term periods, and it is still common to see people selling ice blocks in some parts of the world.
Starting with the Industrial Revolution, more people started to live in cities in order to work in newly founded industrial plants and factories. Therefore, the rural areas that were responsible for providing the food to cities struggled to provide fresh food. Obviously, the easiest and most economical way was to ship food from the rural areas to the cities. For a long journey like this, the only problem was keeping meat frozen. Several inventors and scientists sought to solve the problem; they installed their “freezers” on ships, such as the Dunedin, paving the way for widespread refrigeration. At the same time in the U.S., refrigerated railroad cars appeared. By the beginning of the 20th century, the household refrigerator became more common. By the early 1900s, families became dependent on this technology, changing the way they lived and ate.
The thermodynamics of refrigeration is based around a simple idea: Have a coolant with a low boiling point, and keep it in a refrigeration cycle. A compressor pumps the coolant into condenser coils, where the coolant gas condenses into a liquid. Warm air is then pumped over the liquid coolant, and the heat exchange provides the “cold” atmosphere in the refrigerator. As the refrigerant evaporates, it is collected and condensed back again. Moreover, the system raises and lowers the pressure throughout the process in order to raise and lower the temperature. Remember PV = nRT? Lowering the pressure lowers the temperature, and vice versa.
At first, compounds like sulfurdioxide (bp: –10 oC), chloromethane (bp: –24.2 oC), and ammonia (bp: –33.34 °C) were used. Soon after these chemicals were introduced, it was learned that chloromethane and sulfurdioxide were highly toxic, and the use of these compounds was abandoned. The number of accidents caused by these refrigerants was also among the reasons to stop using them as coolants. The discovery of chlorofluorocarbons (CFCs) led to the extensive use of CFCs in refrigeration, but they were widely abandoned once it became evident that CFCs were causing ozone depletion.
One particular CFC, Freon-12 (dichlorodifluoromethane-CCl2F2), stays in the atmosphere for an extended period of time in a cycle, destroying ozone. The reaction between Freon-12 and ozone is diagrammed below. Note that in the second reaction, a chlorine radical reacts with an ozone molecule, and in the fourth reaction an oxygen atom that would normally produce an O3 molecule, reacts instead with ClO. Essentially, the chlorine atom is not consumed and repeats the cycle continuously, destroying ozone molecules.
Today, we mostly use carbon dioxide, HFC (hydrofluorocarbon), and HP (high-pressure) refrigerants that have short atmospheric lifetimes, lower ozone depletion, and less global-warming potentials. Nevertheless, the search for the “best” refrigerant is still ongoing in the automotive and refrigerator industries.
The next time you grab an ice-cold drink on a hot day, please remember what a journey refrigerants have been through.
For further reading on the history, development, and future of refrigerants, check out this article.