The refrigeration cycle of an HVAC system is a critical component governed by the principles of thermodynamics. To understand how it works, first we need to grasp four key concepts.

1. Heat of Vaporization

This refers to the amount of heat required to convert a liquid into a gas. You usually measure it at a liquid’s boiling point.

In HVAC, boiling or evaporation means changing a liquid to a gas. Conversely, condensation refers to changing a gas to a liquid. It takes significant amounts of energy to induce changes of state, whether evaporation or condensation.

212 degrees Fahrenheit is the boiling point of water. Water in the pot is at or just below 212-degrees Fahrenheit. The vapor leaving the pot is at least 212-degrees.
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2. Pressure Changes the Boiling Point of a Liquid

Lower pressures make it easier for a liquid to boil, and higher pressures make it more difficult. Let’s use water for the following example.

At sea level water boils at 212 degrees Fahrenheit. However, in the Rocky Mountains where the pressure is lower than at sea level, water boils at roughly 194 degrees Fahrenheit. This pressure difference makes it easier to boil water.

Manipulating a refrigerant’s pressure to change its boiling point is a crucial part of what makes the refrigeration cycle possible.

Diagram of atmospheric pressure in the refrigeration cycle

3. Heat (Energy) Is Neither Created nor Destroyed—Merely Transferred

If we take an 8oz cup of 150 degree coffee and mix it with an 8oz cup of 50 degree iced coffee, we would create a 16oz coffee with a temperature of 100 degrees.

Both amounts of liquid are equal and the mid-point between 150-degrees and 50-degrees is 100-degrees.

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4. There Is No Such Thing as Cold—Only the Absence of Heat

When an air conditioner or refrigerator is cooling a space, it isn’t adding cold air into the space. The purpose of the refrigeration cycle is to remove the heat in a given area and eject it outside. Less heat means a colder room.

You know nothing about the refrigeration cycle John Snow

The Refrigeration Cycle in HVAC

Now we know these principles, we can discuss how the refrigeration cycle works in HVAC. Keep in mind these core principles of the refrigeration cycle always remain the same, even with more complex HVAC systems like chillers.

A refrigeration cycle has four major components: the compressor, condenser, expansion device, and evaporator. Refrigerant remains piped between these four components in the refrigerant loop.

The Compressor

Refrigerant begins as a cool vapor and heads to the first component—the compressor. The compressor is the engine of the refrigeration cycle. It consumes the most power of an HVAC system’s components and forces refrigerant through the system. As the HVAC compresses the cool, gaseous refrigerant, it transforms into a very hot and high-pressure vapor.

Diagram of the refrigeration cycle
Whether in an AC or refrigerator, the principles of the refrigeration cycle remain the same.

After compression, the refrigerant moves to the next component in the refrigeration cycle—the condenser.

The Condenser

The condenser’s job is to cool the refrigerant so it turns from a gas into a liquid, or condenses. This happens when warm outdoor air blows across a condenser coil filled with hot, gaseous refrigerant. This allows heat to transfer from the refrigerant to the cooler outdoor air, where the excess heat diffuses to the atmosphere. The condenser coils wind through the condenser to maximize the surface area of the piping—and heat transfer to the air. The refrigerant turns from a vapor into a hot liquid due to the high pressure and reduction in temperature.

The Expansion Device

The refrigerant then approaches the expansion device as a hot, high-pressure liquid. The expansion device is responsible for quickly driving the pressure of the refrigerant down so it can boil (evaporate) more easily in the evaporator. And that’s it! The expansion device has one sole purpose: to reduce refrigerant pressure. Because the pressure drops so rapidly at the expansion device, the refrigerant turns into a combination of a cold liquid and vapor.

The Evaporator

Once the refrigerant is a cold mix of liquid and gas (vapor), it begins to move through the evaporator. The evaporator is responsible for cooling the air going to a space by boiling (evaporating) the refrigerant flowing through it. This happens when warm air blows across the evaporator as cold refrigerant moves through the evaporator coil. Heat transfers from the air to the refrigerant, which cools the air directly before being vented to a space. Like the condenser coil, the evaporator coil also winds through the evaporator to maximize heat transfer from the refrigerant to the air. The low-pressure liquid refrigerant is easily boiled by the warm air blown across the evaporator and heads back to the compressor as a cool gas/vapor.

Congratulations! You have successfully completed a refrigeration cycle!

Diagram: In a refrigeration cycle, refrigerant is hottest when it leaves the compressor and coldest when it leaves the expansion device. 
The refrigerant is hottest when it leaves the compressor and coldest when it leaves the expansion device.

To summarize—refrigerant in the evaporator absorbs heat (cooling the air), and is expelled from the refrigerant to the outdoor air via the condenser. Simultaneously, the expansion device and compressor help manipulate the pressure of refrigerant to make the cycle possible.

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