How Refrigerators Work

The fundamental reason for having a refrigerator is to keep food cold. Cold temperatures help food stay fresh longer. The basic idea behind refrigeration is to slow down the activity of bacteria (which all food contains) so that it takes longer for the bacteria to spoil the food.

For example, bacteria will spoil milk in two or three hours if the milk is left out on the kitchen counter at room temperature. However, by reducing the temperature of the milk, it will stay fresh for a week or two -- the cold temperature inside the refrigerator decreases the activity of the bacteria that much. By freezing the milk you can stop the bacteria altogether, and the milk can last for months (until effects like freezer burn begin to spoil the milk in non-bacterial ways).

Refrigeration and freezing are two of the most common forms of food preservation used today. For more information on other ways to preserve food, see How Food Preservation Works.

If you pour a little rubbing alcohol on your skin, it'll feel cold -- really cold. It isn't refrigerated, so how does this happen? Well, alcohol evaporates at room temperature the way water evaporates at a low temperature in an oven. As it evaporates, it absorbs the heat on the surface of your skin, making your skin cooler. A special coolant called a refrigerant functions in a refrigerator the way alcohol works on your skin, except in a refrigerator, the coolant is trapped inside a series of coils. As it makes a circuit through them, it changes back and forth from a liquid to a gas.

To pull off this frosty feat, a refrigerator uses five major components:

To understand what's happening inside a refrigerator, let's learn a little more about how refrigerants work. You will need:

Add the thermometer to the water filled bowl and place both in the oven. Set the oven to 400 degrees Fahrenheit (204.4 degrees Celsius).

As the oven heats up, the temperature of the water will rise until it hits 212 Fahrenheit (100 degrees Celsius) and it starts boiling. The water temperature will stay at 212 degrees Fahrenheit (100 degrees Celsius) even though it's completely surrounded by the 400 degrees Fahrenheit environment inside the oven. If you let all the water boil away, the temperature on the thermometer will shoot up to 400 degrees Fahrenheit (232.2 degrees Celsius).

Let's look at this experiment another way: Imagine the existence of an exotic creature able to live happily in an oven at 400 degrees Fahrenheit. Let's call him Max. If Max is hanging out in a 400 degree Fahrenheit oven next to a bowl of water boiling away at 212 degrees Fahrenheit (100 degrees Celsius), how is he going to feel about that water? He's going to think the boiling water is really cold. After all, the boiling water is 188 degrees colder than the 400 degrees Fahrenheit that he thinks is comfortable. That's a big temperature difference!

This is exactly what happens when humans deal with liquid nitrogen. We feel comfortable at 70 degrees Fahrenheit (21.1 degrees Celsius), but liquid nitrogen boils at -320 degrees Fahrenheit (-195.5 degrees Celsius). If you had a pot of liquid nitrogen sitting on the kitchen table, its temperature would be boiling away at -320 degrees Fahrenheit (-195.5 degrees Celsius) -- to you, of course, it would feel incredibly cold (so cold it would burn you!).

Modern refrigerators use a regenerating cycle to reuse the same refrigerant over and over again. You can get an idea of how this works by remembering Max and his bowl of water. He could easily create a regenerating cycle by taking the following steps:

By repeating these four steps, Max can reuse the same water over and over again to provide refrigeration.

The refrigerator in your kitchen uses a cycle that is similar to the one described in the previous section. But in your refrigerator, the cycle is continuous. In the following example, we will assume that the refrigerant being used is pure ammonia, which boils at -27 degrees F. This is what happens to keep the refrigerator cool:

By the way, if you have ever turned your car off on a hot summer day when you have had the air conditioner running, you may have heard a hissing noise under the hood. That noise is the sound of high-pressure liquid refrigerant flowing through the expansion valve.

Pure ammonia gas is highly toxic to people and would pose a threat if the refrigerator were to leak, so all home refrigerators don't use pure ammonia. You may have heard of refrigerants know as CFCs (chlorofluorocarbons), originally developed by Du Pont in the 1930s as a non-toxic replacement for ammonia. CFC-12 (dichlorodifluoromethane) has about the same boiling point as ammonia. However, CFC-12 is not toxic to humans, so it is safe to use in your kitchen. Many large industrial refrigerators still use ammonia.

In the 1970s, it was discovered that the CFCs then in use are harmful to the ozone layer, so as of the 1990s, all new refrigerators and air conditioners use refrigerants that are less harmful to the ozone layer.

If you own an RV, chances are you have a gas- or propane-powered refrigerator. These refrigerators are interesting because they have no moving parts and use gas or propane as their primary energy source. Also, they use heat to produce the cold inside the refrigerator.

A gas refrigerator uses ammonia as the coolant, and water, ammonia and hydrogen gas to create a continuous cycle for the ammonia. The refrigerator has five main parts:

It works like this:

You won't need a bag of ice to keep your potato salad cold if you have a handy cooler that plugs into your cigarette lighter. It uses a unique process known as the Peltier effect, a thermoelectric effect, to produce cold temperatures. It's pretty neat, and something we haven't discussed yet.

Named after the French 19th century physicist who discovered it, you can create the Peltier effect yourself using a battery, two pieces of copper wire, and a piece of bismuth or iron wire. Attach the copper wires to the two poles of the battery, and then connect the bismuth or iron wire between the two pieces of copper wire. (The bismuth/iron and copper have to be touching -- it's this connection that causes the Peltier effect.)

The junction where current flows from copper to bismuth will start to get hot, and the junction where current flows from bismuth to the copper junction will get cold. The maximum temperature drop is about 40 degrees Fahrenheit (22.2 degrees Celsius) from the ambient temperature at the hot junction.

As you'd expect, in an electric cooler the hot junction is placed outside the unit, and the cold junction is placed inside. To amplify the effect, coolers contain lots and lots of junctions.

Electric coolers aren't the only unique inventions out there designed to chill your lunch. Solar powered refrigerators are another option. If you plan to spend time camping (or want to start your own hot dog stand), you may want chilled beverages but not have the electricity to power a standard refrigerator. By now, you won't be surprised that a number of energy solutions can provide power to refrigeration systems. In a solar powered refrigerator, a simple solar panel does the honors. Using the sun's rays to make something cold? Now that's ingenious.

Speaking of refrigeration and coldness, have you ever used one of those "instant cold packs" that looks like a plastic bag filled with liquid. You hit it, shake it up and it gets extremely cold. What's going on here?

The liquid inside the cold pack is water. In the water is another plastic bag or tube containing ammonium-nitrate fertilizer. When you hit the cold pack, it breaks the tube so that the water mixes with the fertilizer. This mixture creates an endothermic reaction -- it absorbs heat. The temperature of the solution falls to about 35 F for 10 to 15 minutes.

For more information on refrigeration and related topics, check out the links on the next page.