How Air Conditioning Works
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Refrigeration Concepts
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There are two main concepts that make it possible for modern air conditioners to work. The first is that any liquid absorbs a lot of energy when it evaporates or boils, and gives off that same amount of energy when it is condensed from a vapor back to liquid form. To understand this concept a little better, let's look at a pot of water on the stove. When you want to boil water to cook noodles, you put water into a pot on the stove and turn on the burner. The burner heats the water to 212°F (100°C) and no higher. If more heat is added, instead of getting hotter the water boils faster. No matter how much heat you add, the water will not get any hotter until it all boils away.
Effects of pressure
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The second concept is that fluids boil or condense at higher temperatures under pressure than they do at lower pressures.
Let's say you want to cook a pot of beans and you forgot to soak them overnight. You know that it will take all day to cook these beans at 212°F, so you get out the pressure cooker and set it to maintain 15 psi pressure. At 15 psi above atmospheric pressure, the boiling temperature of water is about 250°F. That's why food cooks faster in a pressure cooker than it does in an open pot. That is also why the radiator in your car is pressurized. A car engine is more efficient at 250°F than at 212°F. That's why you should never open your radiator cap when the engine is warm. If you suddenly reduce the pressure while the radiator fluid is above 212°F, it immediately boils and flashes to steam. -
Refrigeration Cycle
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An air conditioner boils its refrigerant at low pressure in order to cool the air in your home, then compresses the refrigerant vapor to a much higher pressure so that it can be condensed by the warmer air outside.
Figure 3 shows a basic refrigeration cycle. Everything to the right of the compressor and the thermal expansion valve is at high pressure and higher temperature. Everything to the left is low pressure and lower temperature. Everything above the evaporator and condenser is a vapor, everything below them is a liquid.
How It Works
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The low-temperature liquid refrigerant is piped through a heat exchanger (identified as the evaporator in Figure 3) in your air conditioning unit, where it absorbs heat energy from the air as it boils to a vapor. This provides the cool air for your home. After the refrigerant is boiled in the evaporator, a compressor pumps the vapor to a higher pressure and sends it outside to a condenser. This is the unit sitting next to your house or on the roof that always seems to be blowing hot air. Air is blown through the condenser to absorb the energy stored in the refrigerant so that it can condense back to a liquid.
The fluid collected in the condenser then goes through a thermal expansion valve or, in smaller systems, an orifice where the pressure is reduced. The low pressure fluid then goes back to the evaporator where the whole cycle starts over.
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