HOW A REFRIGERATOR WORKS
How do you keep your food cool? The answer used to be simple: stick it in a box with a big lump of ice. The first refrigerators were simple devices where you put your food in a box with ice that was regularly delivered. The ice cooled the box, keeping the food cool.
This approach changed early in the 20th century, with the new Frigidare brand launching its first electrically driven models for the home. By 1926, the company had sold more than 200,000 refrigerators and was building new factories to keep up with demand.
Welcome to Appliance Science, a new column all about the science behind your home appliances. You may not realize it, but your appliances are hotbeds of scientific progress, made possible by moon-shot class engineering and technology that would make your ancestors cry with joy. In this column, we'll take you through the science and technology behind the appliances in your home.
The physics of the refrigerator
The core of a modern refrigerator is like a pump pushing water uphill. The water naturally wants to flow downhill, but the pump pushes it to go the other way. Likewise, a refrigerator pulls heat out of the refrigerator and pushes it outside, keeping the inside of the refrigerator cooler than the rest of the room and your milk nice and cold.
The most common way to extract that heat is with a loop of tubes filled with a chemical that boils easily when you reduce the air pressure around it. At normal pressures, the chemical is a liquid. Reduce the pressure, and it boils into a vapor, absorbing energy. That is known as a phase transition. This process transfers energy between the inside and the outside of the fridge, absorbing heat from the inside when the refrigerant chemical boils and turns into a gas, and then dumping this heat outside the box as the gas is pressurized and condenses back down into a liquid.
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The refrigerant moves around this loop, constantly cycling from liquid to gas and back again. A pump on this loop drives this process, pressurizing the refrigerant in the coils on the back of the refrigerator (called the compressor) to around 100 pounds per square inch (psi), about six times the pressure of the atmosphere.
At the end of this compressor is a small valve, called the expansion valve. This opens to let a very small amount of the refrigerant back into the coils on the part of the loop called the evaporator. Inside the evaporator, the low pressure (usually under 6 psi, less than half of atmospheric pressure) makes the liquid boil into a gas, absorbing energy and cooling as it expands. A fan blows air over the evaporator coils and circulates this cool air, producing the familiar cold air blast you feel when you open a running refrigerator.
The compressor then pushes this vapor back into the external part of the loop, where it condenses back into a liquid, releasing the captured heat to the outside air through the condenser coils.
One problem with the early refrigerators using this method is they relied on a variety of noxious, yet phase change-friendly refrigerants, including ammonia and methyl chloride. These fell out of favor after a series of accidents where the refrigerant escaped and injured people. The development of Freon-12, a non-flammable, non-toxic refrigerant, in 1928, helped make refrigerators safer.
Developed by Frigidare (then part of General Motors) and manufactured by DuPont, Freon-12 had all of the benefits of the older refrigerants, but didn't burn or kill people if it was released into the air. The inventor Thomas Midgely famously demonstrated how non-flammable and non-toxic it is by inhaling the gas and blowing it into a candle (don't try that with ammonia or methyl chloride unless you want to die very painfully). Chemists refer to Freon-12 as dichlorodifluoromethane, or its chemical formulae of CCl2F2.
The future of refrigeration
The modern refrigerator hasn't changed that much in a hundred years: although the chemicals inside have changed, the one in your kitchen today works in the same way as the models from the 1930s. The refrigerator of 100 years from now will probably look quite different, though, as engineers are working on a number of new ways of keeping your food cool. These include an interesting quirk of magnets called the magenetocaloric effect, where certain metals change temperature when they move in a magnetic field.
This effect is used in very low temperature refrigerators used in labs, but engineers are now working on ways to make it more practical for home use. We reported recently on how GE engineers are working on magnetocaloric heat pumps to use in their next generation of refrigerators. Other researchers are working on thermoacoustics, where very loud sound waves contained inside a tube (called a standing wave) move heat through the compression and expansion of the gas. Penn State University recently built a prototype that is used by Ben & Jerry's ice cream.
Whatever the technology they may use in the future, refrigerators have fundamentally changed the way that we grow, ship and eat food. The refrigerator means that the food you eat can be grown hundreds of miles away, so you no longer need to live near a farm to get fresh veggies. Simply put, modern cities would not be possible without it, so say thank you the next time you drink a nice cold glass of milk, kept fresh by this miracle of appliance science.