How Does A Refrigerator Work
All foods have one thing in common: they spoil, or “go bad,” quickly if they are not kept cold. To keep food tasty and safe to eat, it must be chilled. A refrigerator keeps these foods fresh by keeping them cold.
A refrigerator is much more than a box of cold air. It is actually a heat pump. A refrigerator stays chilly by soaking up heat from the food inside it and releasing the heat into the room around it.
Normally, heat moves from a warm place to a cooler place. For example, in the winter heat from a radiator spreads out to warm up the whole room that the radiator is located in. But a refrigerator forces heat to move in the opposite direction. It forces heat to move from a cool place to a warmer place.
Tubes Inside and Out
For a refrigerator to work it use a substance called refrigerant that carries the heat away. Refrigerant flows through the tubes in the refrigerator. Some tubes are located inside the refrigerator’s freezer compartment or inside its walls. Some tubes are under or behind the refrigerator, on its outside.
The refrigerant has two jobs. First it soaks up heat as it flows through the tubes inside the refrigerator. Then it lets go of this heat as it flows through the tubes on the outside.
The refrigerant cools a refrigerator by changing from a liquid to a gas and back again as it travels through its tubes. It soaks up heat when it turns into a gas. Then it loses heat when it turns back into a liquid once more.
You can compare it with your own body that uses evaporation to cool you off on a hot day. On a hot day, sweat trickles out of your skin. Then it turns into a gas called water vapor. This change uses heat energy. The heat comes from your skin. The vapor carries away heat as it drifts into the air. This makes you feel a little bit cooler.
Just the Opposite
The opposite happens when a gas turns into a liquid. Changing from a gas to a liquid is called condensation. You can see condensation on the side of a cold glass on a hot day. Water vapor in the air turns into a liquid when it touches the cold glass. It condenses into water droplets. This change gives off heat. The heat moves into the glass. It makes the cool drink in the glass get warmer, too.
We all know that water start to boil when the water reaches a temperature of 212ºF (100ºC). But other liquids boil at much lower temperatures. In fact, a refrigerant boils at a temperature far below the freezing point of water, which is 32ºF (0ºC). That’s why a liquid refrigerant can boil, or turn into a gas, even inside a chilly refrigerator.
When you heat a liquid it changes into a gas. Cooling a gas can change it back into a liquid. But a refrigerator does not cook refrigerant to turn it into a gas. Neither does it chill the refrigerant to turn it into a liquid. Instead, a refrigerator uses pressure to change the properties of the refrigerant.
Air pressure affects liquids and gases, too. Lowering pressure can make a liquid evaporate. Increasing pressure can make a gas turn into a liquid. A refrigerator turns refrigerant into a gas and then back into a liquid by changing the pressure. It makes the refrigerant switch between soaking up heat and letting it go. The pressure changes happen in different parts of the refrigerator.
First a refrigerator needs an evaporator. The evaporator is a wide tube that loops through the walls of a refrigerator’s freezer section. Its name is a clue to its job: refrigerant evaporates inside it. It needs energy, in the form of heat, to evaporate.
The refrigerant gets this heat from food and air in the freezer. It absorbs the heat, which makes the freezer get colder. The evaporator’s tubes get colder, too. As they get colder, even more heat leaves the freezer, because heat travels from warmer places to colder places.
From a Liquid to a Gas
The refrigerant doesn’t store all this heat. It uses most of the heat to change from a liquid to a gas as it travels through the evaporator. Next, the refrigerant travels down a tube in the wall of the refrigerator’s lower section. It is still a cool gas under low pressure. This tube takes it to the compressor.
In the evaporator, the refrigerant evaporated. In the compressor, it’s compressed! The compressor is a pump that is run by an electric motor. “Compress” means to squash or squeeze. A compressor squeezes refrigerant by forcing it into a smaller space.
The compressor pulls refrigerant out of the evaporator when it runs. The refrigerant is still a cool, low pressure gas at this point. The pressure is low because the gas had lots of room in the evaporator.
But now the compressor squeezes it into a smaller space. This puts the refrigerant under high pressure. High pressure doesn’t just squeeze the refrigerant. It also raises its temperature. The refrigerant is still a gas, but now it is a hot gas instead of a cool one.
The condenser is a thin tube that loops back and forth. It is attached to the outside of the refrigerator, either on the back or underneath. Just like the evaporator and the compressor, the condenser has a name that is a clue to its job. It is where refrigerant condenses. The refrigerant goes from being a gas to being a liquid again.
When the refrigerant left the compressor, it was a hot gas under high pressure. It is still a high pressure, hot gas when it flows into the condenser. The hot refrigerant heats up the condenser as it flows through it.
But now the refrigerant begins to lose its heat, because heat travels from warmer places to cooler places. The refrigerant is hotter than the air in the room, so heat leaves the condenser. You can feel this heat coming out of the back or bottom of a refrigerator.
As it loses its heat, the refrigerant condenses. It turns back into a liquid. So the refrigerant is a liquid again by the time it reaches the end of the condenser and is ready to go back into the refrigerator.
Now the refrigerant is a cool liquid when it comes to the end of the condenser. But it is still under high pressure. The refrigerant needs to evaporate so that it can keep cooling the refrigerator.
The cool, liquid refrigerant flows back into the evaporator through a very skinny tube called an expansion valve. This small tube is thinner than the condenser tube. It is much thinner than the evaporator tube. It is just large enough for the refrigerant to squeeze through.
The refrigerant moves through this tiny tube quickly because it is under high pressure. It shoots out the other side, into the evaporator. It sprays out as tiny drops of liquid, like mist from a can of air freshener.
Cooling the Freezer Again
When the liquid refrigerant bursts out of the skinny tube and into the wide pipe of the evaporator, it is suddenly in a low-pressure area again. Without a lot of pressure squeezing it, the refrigerant quickly starts turning into a gas.
Meanwhile, the refrigerant starts using heat energy as it evaporates. It starts pulling heat out of the freezer. And so the refrigeration cycle continues.
A refrigerator has to keep track of its temperature so that its freezer and chilling section are always cold. This job is done by a part that is called a thermostat. It measures the temperature inside the refrigerator, just as a thermostat in a room measures the room’s temperature. Most refrigerators have just one thermostat. It measures the temperature in the freezer section.
The coldness of the freezer is used to cool the chilling section, too. So if the freezer is very cold, then the chilling section will be just right. If the refrigerator is cold enough, the thermostat turns off the compressor. If the refrigerator is getting too warm, the thermostat turns on the compressor.
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