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Refrigeration: The Anatomy Behind a Breath of Cold Air
You can pop the meals out of your fridge or freezer to enjoy any time. Read on to learn how these ice boxes keep things chill.
Like an inscrutable monolith, it stands in your kitchen. It may emit a whirr or gurgle every so often, but in general a refrigerator’s workings are not easy to intuit. Its moving parts are hidden, and the most important element of all is a shape-shifting substance called a refrigerant. Several different chemicals can do the job, but all share the property of changing state at less extreme temperatures than most substances. (Their boiling points are typically a couple dozen degrees Fahrenheit below zero.) They morph easily between a liquid and a gas, thereby producing blasts of arctic air each time you open the unit’s door.
Refrigerant keeps things cool by acting as a sort of shuttle, picking up heat from inside the chamber and dropping it off outside, over and over again. (That's why the back of your fridge gets warm.) Use your x-ray vision to bore into the area behind the food compartment, and you’ll find the refrigerant—here appearing as a chilled gas—circulating through small tubes and absorbing any heat inside the chamber.
As it leaves, the now-warm gas enters the compressor. As you may remember from science class or the time you got trapped in a storage closet full of thermodynamics textbooks, pressure is proportional to temperature, so compressing the gas also heats it further. Since heat naturally flows from warmer areas to cooler ones, this step helps propel the refrigerant into the series of tubes at the back of the fridge known as the condenser. The maze of thin tubes creates a lot of surface area so the heat can quickly radiate away into the kitchen.
As the gas loses heat, it condenses back into a liquid and flows through an expansion valve. If you’ve ever used a can of compressed air, you’ll have noticed that it becomes frigid as you dispense the gas and relieve the pressure inside. The expansion valve works the same way, controlling the flow of refrigerant between areas of higher and lower pressure. As the valve dispenses refrigerant into the low-pressure area called the evaporator, the refrigerant has room to expand into a (still very cold) gas. And the cycle begins again, at least until the desired temperature is reached and a thermostat shuts off the system.