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How an Air Conditioner Works | The Refrigeration Cycle

August 18th, 2017

Why does school go so slow but summer vacation so fast? While we may not be able to answer that mystery, we can certainly help you get in the school state of mind by answering the question: How does an air conditioner work?

How Does an Air Conditioner Work?

There are two laws of physics that we should review before explaining the inner workings of your air conditioning system.

  1. Combined Gas Law

The first is the relationship between pressure and temperature, known as the combined gas law since it combines Boyle’s Law, Charles’s Law, and Gay-Lussac’s Law:

  • Boyle’s Law states that the pressure-volume product is constant.
  • Charles’s Law shows that the volume is proportional to the absolute temperature.
  • Gay-Lussac’s Law says that the pressure is proportional to the absolute temperature.

In simple English, the combined gas law says that whenever you heat up a gas, pressure also increases. And vice versa, whenever you pressurize a gas, heat also increases.

  • If pressure increases, so does its temperature. This is why a tire heats up as you pump it with air.
  • If pressure decreases, so does its temperature. This is why an aerosol can gets colder as you depress the nozzle and release pressure.

The way an air conditioner uses this combined law is by pressurizing and depressurizing the refrigerant to increase or decrease its temperature.

  1. 2nd Law of Thermodynamics

The second law of physics you need to know is the 2nd Law of Thermodynamics:

If you paid attention in school, you might remember that the second law of thermodynamics states that heat flows from hotter to colder bodies naturally. You can only transfer heat from a colder body to a warmer body through some kind of external work.

Air Conditioning 101: The Fundamentals

Air conditioners transfer heat from the indoors to the outdoors.

Although you may think that air conditioners create cold air, they actually extract heat from the indoor air and send it outside.

When heat is removed from the indoor air, the air is cooled down. It’s best to think of the air conditioning process as heat flowing from the indoors to the outdoors.

The Refrigeration Cycle

An air conditioner works using a thermodynamic cycle called the refrigeration cycle. It does this by changing the pressure and state of the refrigerant to absorb or release heat.

The refrigerant (aka coolant) absorbs heat from inside of your home and then pumps it outside.

Most air conditioners are air-source, split systems. What this means is that there is one unit inside and one unit outside, which is why it is called a split system.

The air-source part refers to the place where the thermal energy is dumped, the outside air. There are other potential places where the heat can be transferred, such as water or ground, known as water-source, or ground-source systems.

The inside unit is normally inside the house somewhere, in the attic, basement, closet or crawl space. The outside unit is normally located on the side or back of the building.

Other kinds of air conditioning systems, such as ground-source and water-source, follow the refrigeration cycle, but some of the specifics, such as location and parts may differ.

Here are the basic parts of the refrigeration cycle (the same process that your refrigerator used to keep food cold):

  1. Air flows over the indoor coils, which contain extremely cold refrigerant.

When air flows over the cold coils, heat from the air gets transferred to the refrigerant inside the coils. After the air flows over the coils, it gets cold, normally dropping around 20 degrees.

This process follows the 2nd law of thermodynamics, which says that heat naturally (spontaneously) flows from a warmer body to a cooler body.

After the refrigerant absorbs the heat, its state changes from a liquid to a vapor. This warmer refrigerant gas then gets transferred to the compressor (step 2 in the refrigeration cycle).

  1. Warmer, vaporized refrigerant gets compressed (pressurized) to a hot temperature.

Even though the refrigerant has absorbed heat from the indoor air, it is still fairly cool. The still cool, but warmer vaporized gas enters the compressor (located in the outside unit) to increase its pressure and temperature.

We increase the temperature of the refrigerant because it needs to be warmer than the outdoor air. Remember the 2nd law of thermodynamics again—heat flows from warmer to cooler bodies.

If the refrigerant is 120 degrees and the outdoor air is 90 degrees, the outdoor air is cooler, which means the heat from the refrigerant will flow in the direction we want—outside. If the temperature outside is 120 degrees, the compressor will have to work extra hard to increase the temperature of the refrigerant to a higher temperature.

After the refrigerant’s temperature is increased above that of the outdoor air’s temperature, it then flows into another set of coils, known as the condenser coils (also located outside).

  1. Very hot refrigerant flows into condenser coils where it loses heat to the outdoor air.

Since the refrigerant has been compressed (pressurized), it is now hotter than the outdoor air. A condenser fan blows hot outdoor air over the even hotter outdoor condenser coils.

As outdoor air flows over the outdoor coils, heat is removed from the refrigerant and released into the outdoor air. Again, this is due to the 2nd law of thermodynamics.

After the refrigerant loses thermal energy to the outdoor air, it condenses back into a liquid and gets pumped back inside.

  1. The still warm refrigerant from the outdoor unit needs to get cold.

When the refrigerant leaves your outdoor condenser unit, its temperature is still pretty high. The refrigerant’s temperature will need to drop significantly before it can absorb more heat from the indoor air.

The metering device, usually a thermostatic expansion valve, is a special device that depressurizes the refrigerant, causing a drop in temperature. It does this by expanding the refrigerant into a larger volume.

The refrigerant needs to be colder than the indoor air in order to absorb heat. Once the refrigerant gets cooled down, it flows back into the evaporator coils where it begins the refrigeration cycle again.

Hopefully, this helps you understand the basic workings of an air conditioner. The refrigeration cycle is basically the same for your freezer and refrigerator.

As August, and with it, summer, comes to an end, a new school year is just around the corner. If you ever need any more HVAC lessons, Service Champions is here to help. Even if it means help with your next science test, don’t hesitate to contact us.

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