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Modern Marvel: How Does an Air Conditioner Work?

by Daniel Estevao on August 11, 2017

How Does an Air Conditioner Work? - Heating & Cooling ScienceThere are many things that are a mystery to us. Are there aliens? Why did the chicken cross the road? How does an air conditioner work?

Well, you may not have been pondering the answer to that last one, but now that we’ve got your attention — do you know how an air conditioner works? Most people don’t.

To help get you into a back-to-school mindset, we’re going over Air Conditioning 101.

Sure the basic principle is simple enough — an air conditioner cools a space by taking heat from inside and dumping it outside. How it does this, however, is a bit more complicated.

What is an Air Conditioner?

Air conditioners are heat exchangers. They take warm air from the room, blow it over cold coils to remove the heat, and then return the cooled air to the room. The heat that has been collected by the cold coils (known as evaporator coils) is then pumped outdoors via refrigerant.

Technically, an air conditioner is a form of heat pump; however, the term “heat pump” is typically used for a system that pumps heat in both directions in order to provide cooling and heating.

The refrigerant flows through your indoor and outdoor air conditioning units in a closed loop. It moves from a gas state to a liquid state and back again as it flows through your air conditioning system.

How the Refrigeration Cycle Works

In order to know how air conditioning works, there are a few scientific principles we should go over first:

Hot to Cold: The 2nd Law of Thermodynamics

The 2nd law of thermodynamics states that heat flows naturally from hot to cold. That is why your home needs air conditioning in the first place. The outdoor heat flows naturally into your cool home.

The only way that you can get heat to flow in the other direction is through mechanical means. You air conditioner does this by making the refrigerant so cold that the heat from inside your home gets transferred to the refrigerant, and then raises the temperature of the refrigerant to higher than the outdoor air so that it flows outward.

Heat will not flow in the opposite direction, meaning from a lower temperature to a higher temperature, on its own. That’s why you need air conditioning.

Changing Refrigerant Temperature: The Combined Gas Law

The combined gas law, which combines Boyle’s Law, Charles’ Law, and Gay-Lussac’s Law together, demonstrates:

  • Higher volume equals lower pressure.
  • Higher temperature equals higher pressure (and vice versa)
  • Higher temperature equals higher volume.

The formula for the combined gas law is PV/T=k.

P = pressure

V = volume

T = temperature

k = constant

When you pressurize the refrigerant, its temperature increases. When you depressurize the refrigerant, the temperature decreases.

You can experience this law in action when your pump up a bicycle tire. As you add pressure to the tire, it starts to get warmer. Try touching your bicycle tire the next time you pump it full of air and you will notice the heat increasing.

Similarly, when you release pressure from an aerosol can, you will notice that the can gets colder.

Using the same basic principle, your air conditioner pressurizes and depressurizes the refrigerant to raise or lower its temperature.

The Refrigeration Cycle

Warm room air is drawn into the air conditioner via your room’s return ducts. The warm indoor air is blown over very cold indoor coils (called evaporator coils). As the warm indoor air flows over the cold coils, it causes the refrigerant to evaporate. In the process, heat is absorbed by the refrigerant in the coils as it turns into gas.

As the warm air flows over the cold coils, it loses heat and becomes the cold air you feel from your supply vents. So, the warm air enters your indoor unit warm and after flowing over cold coils, returns back into the rooms of your home colder than when it entered.

After the refrigerant has absorbed heat and turned into a gas, it then travels to the compressor where it gets pressurized, increasing its temperature.

To return to a liquid state, it must give up its temperature. The refrigerant loses its temperature by having outdoor air blow over the condenser coils with help from the condenser fan.

The condenser coils contain the hot, compressed refrigerant gas. As the outdoor air flows over the even hotter refrigerant, the heat flows naturally from the hotter refrigerant to the cooler outdoor air.

As the refrigerant in the condenser coils loses heat and becomes cooler, it is turned back into a liquid and returned to the cooling coils to start the process over again. However, it is still too warm to absorb heat from your indoor air.

In order to get the refrigerant really cold again, it has to be depressurized. An expansion valve decreases the refrigerant’s temperature by expanding its volume and depressurizing the liquid refrigerant.

Refrigeration Cycle Summary:

  • A low-pressure gas enters the compressor, where it is pressurized and turned into a hot, high-pressure gas. Remember the combined gas law—when you increase pressure, you increase temperature at the same time.
  • The hot, pressurized gas loses heat when the condenser fan blows outdoor air over the coils. The refrigerant loses heat to the outdoor environment.
  • As the hot, pressurized gas loses heat, it turns into a liquid.
  • The liquid refrigerant gets depressurized, usually by a thermostatic expansion valve, causing it to become cold.

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 science or HVAC lessons, Pacific Heating & Cooling is here to help.

We’ve been proudly serving our communities in Federal Way, Graham, Spanaway, University Place, Steilacoom, Sumner, Lakewood, Puyallup, Tacoma and Gig Harbor since 1984.

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