Life changed immensely in the 20th century as air conditioning and refrigeration systems became more efficient, controllable, and even mobile. No longer dependent on the weather for work or play, humans truly made the environment adapt to their needs. Climate control became so reliable and affordable it grew from an invisible luxury to a common necessity. By the end of the century, nearly 70 percent of U.S. households had air conditioning. Now people can live and work in glassed-in or windowless buildings, in porchless houses, or in the warmest and most humid places. In the United States alone, air conditioning reversed a century-long pattern of migration out of the southern cities.
Refrigeration makes transporting fresh food and other perishables possible, and makes home storage for days or weeks practical. By the end of the 20th century, 99.5 percent of U.S. households had a least one refrigerator. Many had separate freezers. People were able to simplify shopping and save money while enjoying a greater diversity and higher quality of food because of this excellent preservation technology.
In an air conditioner, air is cooled and conditioned by units that are similar to domestic refrigerators. Cold liquid refrigerant at low pressure flows through coils on one side. A centrifugal fan draws warm air from the room over the coils. The cooled and conditioned air is returned to the room. The warmed refrigerant evaporates, and then passes into a compressor where it is pressurized. The hot, pressurized gas enters a second set of coils on the exterior side. A second fan draws cool external air over the hot coils to dissipate their heat. In the process, the refrigerant is cooled to below its boiling point and condenses into a liquid. The refrigerant then passes through an expansion valve, where its pressure is suddenly reduced. As this happens its temperature drops, and the cooling cycle begins again.
This may sound simple, but it took the pioneering genius of Willis Carrier to work out the basic principles of cooling and humidity control, and it took innovations by thousands of other engineers before the air conditioner became a real benefit to the average person. Carrier's invention made many technologies possible, especially in fields that require highly controllable environments, such as medical or scientific research, product testing, computer manufacturing, and space travel.
Carrier claimed that while he was standing in a Pittsburgh train station one night in 1902, he realized that air could be dried by saturating it with chilled water to induce condensation. He built the first air conditioner that year, which had the cooling power of 108,000 pounds of ice a day. It was for a Brooklyn printer who couldn't print a decent color image because changes in heat and humidity kept altering the paper's dimensions and misaligning the colored inks.
A refrigerator operates in much the same way as an air conditioner. It moves heat energy from one place to another. Constant cooling is achieved by the circulation of a refrigerant in a closed system, in which it evaporates to a gas and then condenses back again to a liquid in a continuous cycle. If no leakage occurs, the refrigerant lasts indefinitely throughout the entire life of the system.
The use of natural or manufactured ice for refrigeration was widespread until shortly before World War I, when mechanical refrigerators became available. In 1927, General Electric introduced a refrigerator with a "monitor top" containing a hermetically sealed compressor. The 14-cubic-foot refrigerator sold for $525, affordable to just a few, and made GE the industry leader by 1930.
Single-phase electric motors were perfected and reliable by 1920. Frigidaire manufactured the first individual room cooler in 1929, using technology from the household refrigerator. The invention of halocarbon refrigerants by Thomas Midgley in 1928 provided a safe alternative to the toxic and flammable refrigerating fluids previously used. The Frigidaire division of General Motors adopted Freon 12 (dichlorodifluoromethane) refrigerant gas, invented by Midgley and Charles Kettering. Most other refrigerator makers followed suit, replacing ammonia and other more dangerous gases. In the late 1980s, chlorofluorocarbons had demonstrated signs of destroying the Earth's ozone layer, and the production of these chemicals began to be phased out, while the search for a replacement began.
The first air-conditioned automobile, a Packard, was engineered in 1938. The first successful window air conditioner was marketed in 1938 by Philco-York. Mass production of window air conditioners after World War II lowered costs to the point where they were accessible to mass consumers, as were refrigerators. From 1920 to 1930, the cost of a household refrigerator dropped from $600 to $300, and to nearly $150 by 1939.
Refrigeration technology led to the creation of the frozen food industry. In 1914, Clarence Birdseye was fishing in Canada when he noticed that fish caught through the ice froze stiff the instant they were exposed to the air, and they tasted almost fresh when defrosted and cooked weeks later. For several years, he pursued the commercial exploitation of his food-freezing discoveries, learning to freeze cabbages in barrels of seawater. By 1925, Birdseye and Charles Seabrook developed a deep-freezing process for cooked foods. In 1930, Birds Eye Frosted Foods were sold for the first time in Springfield, Mass.
