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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. In this edition of How Stuff Works we look at how your refrigerator performs its magic, and in the process you will also learn how your home and car air conditioners work. We will also look at propane refrigerators found in RVs, the new electronic coolers and cold packs!
The Purpose of Refrigeration
Let's start by looking at the reason why you have a refrigerator in your kitchen. The fundamental reason for its existence is to keep food cold in order to help the 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 by storing it in the refrigerator, the milk 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 many months (until effects such as 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 the How Stuff Works article entitled How Food Preservation Works.
Parts of a Refrigerator
The basic idea behind a refrigerator is very simple: A refrigerator uses the evaporation of a liquid to absorb heat. You probably know that when you put water on your skin it feels cool. As the water evaporates it absorbs heat and creates the cool feeling. Rubbing alcohol feels 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 were to place your refrigerator's refrigerant on your skin it would freeze your skin as it evaporates.
The diagram at the right shows you the basic mechanical layout of any refrigerator or air conditioning system. There are 5 basic parts:
This is a fairly standard explanation of how a refrigerator works. However, if you have heard this explanation before you know it is not very satisfying - it does not necessarily make a lot of intuitive sense. So let's look at refrigeration using several real-world examples to help understand what is truly happening.
Understanding Refrigeration
There are two physical processes that help to understand what occurs inside a refrigerator:
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Experiment 1
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Experiment 2 As the oven heats up, the temperature of the water will again rise until it hits 212 degrees F and then start boiling. Then the water's temperature will stay at 212 degrees F even though it is completely surrounded by an environment that is at 400 degrees F. If you were to let all of the water boil away (and if the thermometer has the range to handle it), then as soon as the water is gone the temperature of the thermometer will immediately rise to 400 degrees 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 a 400 degree F oven. This creature thinks 400 degrees F is just great - the perfect temperature (just like humans think that 70 degrees F is just great). If the creature is hanging out in a 400 degree F oven and there is a cup of water in the oven boiling away at 212 degrees F, how is the creature going to feel about that water? It is going to think the boiling water is REALLY cold. After all, the boiling water is 188 degrees colder than the 400 degrees that this creature feels is comfortable. That's a big temperature difference!
This is exactly what is happening, by the way, when we humans deal with liquid nitrogen. We feel comfortable at 70 degrees F. Liquid nitrogen boils at -320 degrees F. So if you have a pot of liquid nitrogen sitting on the kitchen table, the liquid nitrogen is boiling away at its boiling temperature of -320 degrees F and we feel that it is very cold.
What if you wanted to keep the liquid nitrogen from boiling away (or if the 400 degree creature wanted to keep his water from boiling away)? You have two options. You can either insulate the liquid nitrogen (generally done by putting it in a thermos (a vacuum flask) called a dewar). That way it cannot absorb heat, so it cannot boil. Or you can put it in a high-pressure container. The pressure will keep it from boiling.
Butane LightersThe 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 F 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. |
So, having heard all of this, you can see that one way to make a refrigerator would be to have a big tank of liquid nitrogen beside your house. You could pump the liquid nitrogen into your refrigerator and let it boil away. Liquid nitrogen is the refrigerant in this refrigerator, and it is not recycled. That would work great if you didn't mind having to pay for liquid nitrogen (it's about 30 cents a gallon right now) or having a truck come by your house once every week or so to deliver it.
The way modern refrigerators work is they 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 400 degree creature and his pot of water. The air temperature in the oven is 400 degrees F. The water boils away, remaining at 212 degrees F but producing a lot of 400 degree F steam. Let's say the creature collected the steam in a big bag. Once all the water had boiled away, let's say he then pressurized the steam into a steel container. In the process of pressurizing it, its temperature would rise to 800 degrees F and it would remain steam. So now the steel container is "hot" to the creature - 800 degrees F - because it contains 800 degree steam. However, the steel container would dissipate its excess heat to the air in the oven, eventually falling back to 400 degrees F. In the process, the high-pressure steam in the container would condense into pressurized water (just like the butane in the lighter). At that point the creature could release the water from the steel pressurized container into a pot and the water would immediately begin boiling. Its temperature would drop to 212 degrees F. The creature now has a way of reusing the same water to provide refrigeration.
The refrigerator in your kitchen uses a similar cycle, but in a refrigerator the cycle is continuous. For example, if the refrigerant is pure ammonia, the ammonia boils at -27 degrees F. The compressor compresses ammonia gas. The compressed gas heats up as it is pressurized (orange). The coils on the back of the refrigerator let the hot ammonia gas dissipate its heat. The ammonia gas then condenses into ammonia liquid (dark blue) at high pressure (just like the butane liquid in a butane lighter is liquid because it is at high pressure in the lighter). Now 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). The liquid ammonia immediately boils and vaporizes (light blue), lowering the temperature to -27 degrees F. This makes the inside of the refrigerator cold. The cold ammonia gas is then 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 will often hear a hissing noise under the hood. That noise is the sound of high-pressure liquid refrigerant hissing through the expansion valve.
Pure ammonia gas is highly toxic to people and would pose a threat if the refrigerator ever leaked, so all home refrigerators use safer refrigerants instead. 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 CFCs are harmful to the ozone layer, so in the 1990s all new refrigerators and air conditioners use refrigerants that do not harm the ozone layer.
Gas and Propane Refrigerators
If you own an RV or use a refrigerator where electricity is not available, chances
are you have a gas-powered or propane-powered refrierator. Gas refrigerators are interesting
because they have no moving parts and use gas or propane as their primary source of energy. They
are also odd because 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 5 main parts:
The liquid ammonia now makes its way to the evaporator. Here it mixes with hydrogen gas and evaporates, producing cold temperatures inside the refrigerator. The ammonia and hydrogen gases flow to the absorber. Here the water collected in the separator is mixed with the ammonia/hydrogen gases. 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.
The following link provides a nice description of both batch and continuous gas refrigerators: Small Ammonia Refrigerators.
Electric Coolers
You may have seen the new coolers that don't use ice but instead plug into your car's cigarette lighter.
These coolers rely on a process known as the "Peltier effect" or the "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 wire. Connect the copper wires to the two poles of the battery and the connect the bismuth wire between the two pieces of copper wire. The bismuth and copper must touch - it is the junction that causes the effect. At the point where current flows from copper to bismuth the junction will get hot, and at the other junction between bismuth and copper the junction will get cold. The maximum temperature drop is about 40 degrees F from the ambient temperature where the hot junction is located. The hot junction is placed outside the refrigerator and the cold junction is placed inside.
For more information, the Frequently asked questions about Peltier Effect page is helpful, as is the Advanced Thermoelectric Products page. See also the Melcor page.
Cold Packs
Speaking of refrigeration and coldness, have you ever used one of those "instant cold packs"? You've probably
seen them - they look like a plastic bag filled with liquid. You hit one, shake it up and it gets very 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 and the water mixes with the fertilizer. It turns out this is an endothermic reaction - it absorbs heat. The temperature of the solution falls to about 35 degrees F for 10 to 15 minutes.
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