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Wednesday, September 28, 2011

How Does Its Cooling?: A Theoritical View on Air Conditioner & Refrigerator.

1. How Does an Air Conditioner Work?

     Air conditioners and refrigerators work the same way. Instead of cooling just the small, insulated space inside of a refrigerator, an air conditioner cools a room, a whole house, or an entire business.
     Air conditioners use chemicals that easily convert from a gas to a liquid and back again. This chemical is used to transfer heat from the air inside of a home to the outside air.
     The machine has three main parts. They are a compressor, a condenser and an evaporator. The compressor and condenser are usually located on the outside air portion of the air conditioner. The evaporator is located on the inside the house, sometimes as part of a furnace. That's the part that heats your house.
     The working fluid arrives at the compressor as a cool, low-pressure gas. The compressor squeezes the fluid. This packs the molecule of the fluid closer together. The closer the molecules are together, the higher its energy and its temperature.
     The working fluid leaves the compressor as a hot, high pressure gas and flows into the condenser. If you looked at the air conditioner part outside a house, look for the part that has metal fins all around. The fins act just like a radiator in a car and helps the heat go away, or dissipate, more quickly.
     When the working fluid leaves the condenser, its temperature is much cooler and it has changed from a gas to a liquid under high pressure. The liquid goes into the evaporator through a very tiny, narrow hole. On the other side, the liquid's pressure drops. When it does it begins to evaporate into a gas. 
     As the liquid changes to gas and evaporates, it extracts heat from the air around it. The heat in the air is needed to separate the molecules of the fluid from a liquid to a gas. 
     The evaporator also has metal fins to help in exchange the thermal energy with the surrounding air. 
     By the time the working fluid leaves the evaporator, it is a cool, low pressure gas. It then returns to the compressor to begin its trip all over again. 
     Connected to the evaporator is a fan that circulates the air inside the house to blow across the evaporator fins. Hot air is lighter than cold air, so the hot air in the room rises to the top of a room.
     There is a vent there where air is sucked into the air conditioner and goes down ducts. The hot air is used to cool the gas in the evaporator. As the heat is removed from the air, the air is cooled. It is then blown into the house through other ducts usually at the floor level.
     This continues over and over and over until the room reaches the temperature you want the room cooled to. The thermostat senses that the temperature has reached the right setting and turns off the air conditioner. As the room warms up, the thermostat turns the air conditioner back on until the room reaches the temperature.

2. How Does A Refrigerator Work?
     In the summertime, have you ever gotten out of a swimming pool and then felt very cold standing in the sun? That's because the water on your skin is evaporating. The air carries off the water vapor, and with it some of the heat is being taken away from your skin.
      This is similar to what happens inside older refrigerators. Instead of water, though, the refrigerator uses chemicals to do the cooling.
      There are two things that need to be known for refrigeration.
  1. A gas cools on expansion.
  2. When you have two things that are different temperatures that touch or are near each other, the hotter surface cools and the colder surface warms up. This is a law of physics called the Second Law of Thermodynamics.

Old Refrigerators & Today's Refrigerators:-

     If you look at the back or bottom of an older refrigerator, you'll see a long thin tube that loops back and forth. This tube is connected to a pump, which is powered by an electric motor. 
Inside the tube is Freon, a type of gas. Freon is the brand name of the gas. This gas, chemically is called Chloro-Flouro-Carbon or CFC. This gas was found to hurt the environment if it leaks from refrigerators. So now, other chemicals are used in a slightly different process (see next section below).
      CFC starts out as a liquid. The pump pushes the CFC through a lot of coils in the freezer area. There the chemical turns to a vapor. When it does, it soaks up some of the heat that may be in the freezer compartment. As it does this, the coils get colder and the freezer begins to get colder.
      In the regular part of your refrigerator, there are fewer coils and a larger space. So, less heat is soaked up by the coils and the CFC vapor.
     The pump then sucks the CFC as a vapor and forces it through thinner pipes which are on the outside of the refrigerator. By compressing it, the CFC turns back into a liquid and heat is given off and is absorbed by the air around it. That's why it might be a little warmer behind or under your refrigerator.
      Once the CFC passes through the outside coils, the liquid is ready to go back through the freezer and refrigerator over and over.
Modern refrigerators don't use CFC because CFCs are harmful to the atmosphere if released. Instead they use another type of gas called HFC-134a, also called tetrafluoroethane. HFC turns into a liquid when it is cooled to -15.9 degrees Fahrenheit (-26.6 degrees Celsius).
      A motor and compressor squeezes the HFC.  When it is compressed, a gas heats up as it is pressurized. When you pass the compressed gas through the coils on the back or bottom of a modern refrigerator, the warmer gas can lose its heat to the air in the room.
      Remember the law of thermodynamics.
As it cools, the HFC can change into a liquid because it is under a high pressure.
The liquid flows through what's called an expansion valve, a tiny small hole that the liquid has to squeeze through. Between the valve and the compressor, there is a low-pressure area because the compressor is pulling the ammonia gas out of that side.
      When the liquid HFC hits a low pressure area it boils and changes into a gas. This is called vaporizing.
      The coils then go through the freezer and regular part of the refrigerator where the colder liquid in the coil pulls the heat out of the compartments. This makes the inside of the freezer and entire refrigerator cold.
      The compressor sucks up the cold gas, and the gas goes back through the same process over and over.

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