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How Refrigerators Work
by Marshall Brain

In the kitchen of nearly every home in America there is a refrigerator. Every 15 minutes or so you hear the motor turn on, and it magically keeps things cold. Without refrigeration, we'd be throwing out our leftovers instead of saving them for another meal.

The refrigerator is one of those miracles of modern living that totally changes life. Prior to refrigeration, the only way to preserve meat was to salt it, and iced beverages in the summer were a real luxury.

In this edition of HowStuffWorks, you'll find out how your refrigerator performs its magic. We'll also look at cold packs, electronic coolers and the propane refrigerators found in RVs.

The Purpose of Refrigeration
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.

Parts of a Refrigerator
The basic idea behind a refrigerator is very simple: It uses the evaporation of a liquid to absorb heat. You probably know that when you put water on your skin it makes you feel cool. As the water evaporates, it absorbs heat, creating that cool feeling. Rubbing alcohol feels even cooler because it evaporates at a lower temperature. The liquid, or refrigerant, used in a refrigerator evaporates at an extremely low temperature, so it can create freezing temperatures inside the refrigerator. If you place your refrigerator's refrigerant on your skin (not a good idea), it will freeze your skin as it evaporates.

There are five basic parts to any refrigerator (or air-conditioning system):

  • Compressor
  • Heat-exchanging pipes - serpentine or coiled set of pipes outside the unit
  • Expansion valve
  • Heat-exchanging pipes - serpentine or coiled set of pipes inside the unit
  • Refrigerant - liquid that evaporates inside the refrigerator to create the cold temperatures

    Many industrial installations use pure ammonia as the refrigerant. Pure ammonia evaporates at -27 degrees Fahrenheit (-32 degrees Celsius).

The basic mechanism of a refrigerator works like this:

  1. The compressor compresses the refrigerant gas. This raises the refrigerant's pressure and temperature (orange), so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization.

  2. As it cools, the refrigerant condenses into liquid form (dark blue) and flows through the expansion valve.

  3. When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates (light blue). In evaporating, it absorbs heat, making it cold.

  4. The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.

This is a fairly standard -- and somewhat unsatisfying -- explanation of how a refrigerator works. So let's look at refrigeration using several real-world examples to understand what is truly happening.

Understanding Refrigeration
To understand what is happening inside your refrigerator, it is helpful to understand refrigerants a little better. Here are two experiments that help you see what is happening.

These experiments can help you understand the properties of gases and their role in refrigeration.

Experiment 1
You will need:

  • A pot of water
  • A thermometer that can measure up to at least 250 degrees F
  • A stove
Put the pot of water on the stove, stick the thermometer in it and turn on the burner. You will see (if you are at sea level) that the temperature of the water rises until it hits 212 F. At that point, it will start boiling, but will remain at 212 F -- this is the boiling point of water at sea level. If you live in the mountains, where the air pressure is lower than it is at sea level, the boiling point will be lower -- perhaps between 190 and 200 F. This is why many foods have "high-altitude cooking directions" printed on the box. You have to cook foods longer at high altitudes.

Experiment 2
You will need:

  • An oven-safe glass bowl
  • A thermometer that can measure up to at least 450 F
  • An oven
Put the thermometer in your container of water, put the container in the oven and turn it to 400 F.

As the oven heats up, the temperature of the water will again rise until it hits 212 F, and then start boiling. The water's temperature will stay at 212 F even though it is completely surrounded by an environment that is at 400 F. If you let all of the water boil away (and if the thermometer has the range to handle it), as soon as the water is gone the temperature of the thermometer will shoot up to 400 F.

The second experiment is extremely interesting if you think about it in the following way: Imagine some creature that is able to live happily in an oven at 400 degrees Fahrenheit. This creature thinks 400 F is just great -- the perfect temperature (just like humans think that 70 F is just great). If the creature is hanging out in an oven at 400 F, and there is a cup of water in the oven boiling away at 212 F, how is the creature going to feel about that water? It is going to think that the boiling water is REALLY cold. After all, the boiling water is 188 degrees colder than the 400 F that this creature thinks is comfortable. That's a big temperature difference!

(This is exactly what is happening when we humans deal with liquid nitrogen. We feel comfortable at 70 F. Liquid nitrogen boils at -320 F. So if you had a pot of liquid nitrogen sitting on the kitchen table, its temperature would be -320 F, and it would be boiling away -- to you, of course, it would feel incredibly cold.)

Butane Lighters
If you go to the local store and buy a disposable butane lighter with a clear case (so that you can see the liquid butane inside), what you are seeing is liquid butane stored in a high-pressure container. Butane boils at 31 degrees F at normal atmospheric pressure (14.7 PSI). By keeping butane pressurized in a container, it remains liquid at room temperature. If you took a cup of butane and put it on your kitchen counter, it would boil, and the temperature of the boiling liquid would be 31 F.

The boiling point of butane, by the way, also explains why butane lighters don't work very well on cold winter days. If it is 10 degrees Fahrenheit outside, the butane is well below its boiling point, so it cannot vaporize. Keeping the lighter warm in your pocket is what allows it to work in the winter.

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 again imagining our oven creature and his cup of water. He could create a regenerating cycle by taking the following four steps:

  1. The air temperature in the oven is 400 degrees F. The water in the cup boils away, remaining at 212 F but producing a lot of 400 F steam. Let's say the creature collects this steam in a big bag.
  2. Once all the water boils away, he pressurizes the steam into a steel container. In the process of pressurizing it, its temperature rises to 800 F and it remains steam. So now the steel container is "hot" to the creature because it contains 800 F steam.
  3. The steel container dissipates its excess heat to the air in the oven, and it eventually falls back to 400 F. In the process, the high-pressure steam in the container condenses into pressurized water (just like the butane in a lighter -- see sidebar).
  4. At this point, the creature releases the water from the steel pressurized container into a pot, and it immediately begins to boil, its temperature dropping to 212 F.
By repeating these four steps, the creature now has a way of reusing the same water over and over again to provide refrigeration.

Now let's take a look at how these four steps apply to your refrigerator.

The Refrigeration Cycle
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:

  1. The compressor compresses the ammonia gas. The compressed gas heats up as it is pressurized (orange).
  2. The coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. The ammonia gas condenses into ammonia liquid (dark blue) at high pressure.
  3. The high-pressure ammonia liquid flows through the expansion valve.

    You can think of the expansion valve as a small hole. On one side of the hole is high-pressure ammonia liquid. On the other side of the hole is a low-pressure area (because the compressor is sucking gas out of that side).

  4. The liquid ammonia immediately boils and vaporizes (light blue), its temperature dropping to -27 F. This makes the inside of the refrigerator cold.
  5. The cold ammonia gas is sucked up by the compressor, and the cycle repeats.

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.

In the next section, we'll talk about gas and propane refrigerators.

Gas and Propane Refrigerators
If you own an RV or use a refrigerator where electricity is not available, 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 source of energy. Also, they use heat, in the form of burning propane, to produce the cold inside the refrigerator.

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

  • Generator - generates ammonia gas
  • Separator - separates ammonia gas from water
  • Condenser - where hot ammonia gas is cooled and condensed to create liquid ammonia
  • Evaporator - where liquid ammonia evaporates to create cold temperatures inside the refrigerator
  • Absorber - absorbs the ammonia gas in water
The cycle works like this:
  1. Heat is applied to the generator. The heat comes from burning something like gas, propane or kerosene.
  2. In the generator is a solution of ammonia and water. The heat raises the temperature of the solution to the boiling point of the ammonia.
  3. The boiling solution flows to the separator. In the separator, the water separates from the ammonia gas.
  4. The ammonia gas flows upward to the condenser. The condenser is composed of metal coils and fins that allow the ammonia gas to dissipate its heat and condense into a liquid.
  5. The liquid ammonia makes its way to the evaporator, where it mixes with hydrogen gas and evaporates, producing cold temperatures inside the refrigerator.
  6. The ammonia and hydrogen gases flow to the absorber. Here, the water that has collected in the separator is mixed with the ammonia and hydrogen gases.
  7. The ammonia forms a solution with the water and releases the hydrogen gas, which flows back to the evaporator. The ammonia-and-water solution flows toward the generator to repeat the cycle.
This page offers an extremely detailed description of the process.

Next, we'll look at electric coolers.

Electric Coolers
You may have seen the new coolers that don't use ice, plugging into your car's cigarette lighter instead. These coolers rely on a process known as the Peltier effect, or thermoelectric effect, to produce cold temperatures electronically.

You can create the Peltier effect with a battery, two pieces of copper wire and a piece of bismuth or iron wire. Just connect 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 must touch -- it is this junction that causes the Peltier effect.

The junction where current flows from copper to bismuth will get hot, and the junction where current flows from bismuth to copper the junction will get cold. The maximum temperature drop is about 40 F from the ambient temperature where the hot junction is located.

To create a Peltier cooler, the hot junction is placed outside the refrigerator, and the cold junction is placed inside. Normally, you create a module containing many junctions to amplify the effect. See the links at the end of this article for details on the Peltier effect.

Now let's take a look at what's going on inside a cold pack.

Cold Packs
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.

Check out this page to learn about endothermic reactions.

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