The refrigeration cycle is a critical component of HVAC systems and is governed by the principles of thermodynamics. Before we dive in, let’s get an understanding of four key concepts:

1. Heat of Vaporization

This is the amount of heat required to turn a liquid into a gas. This is often measured at the liquid’s boiling point.

In HVAC, the change of a liquid to a gas is known as boiling or evaporation. Conversely, the change of a gas to a liquid is known as condensation. A significant amount of energy is required to induce changes of state, whether it be evaporation or condensation.

212 degrees Fahrenheit is the boil 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. 

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. Lets use water for the following example:

At sea level water boils at 212 degrees Fahrenheit. However, in the Rockies (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 the refrigerant’s pressure to change its boiling point is a crucial part of what makes the refrigeration cycle possible.

3. Heat (energy) is neither created nor destroyed — it is 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.

4. There is no such thing as cold — only the absence of heat.

When an air conditioner or refrigerator is cooling a space, do not think about it as adding cold air into the space. The purpose of the refrigeration cycle is to remove the heat in a given area and reject it outside. Less heat means a colder room!

Applying it all to HVAC:

Now that we know these principles, we can talk about how the refrigeration cycle works in HVAC. Keep in mind these core principles of the refrigeration cycle will always remain the same, even when we get into more complex HVAC systems like chillers.

The refrigeration cycle contains four major components: the compressor, condenser, expansion device, and evaporator. Refrigerant remains piped between these four components and is contained in the refrigerant loop.

The refrigerant begins as a cool vapor and heads to the first component: the compressor. The compressor is widely considered the engine of the refrigeration cycle; it consumes the most power out of the HVAC system’s components and forces the refrigerant through the system. In the process of being compressed the cool, gaseous refrigerant is turned to a very hot and high-pressure vapor.

Whether it is in an AC or refrigerator, the principles of the cycle remain the same. 

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

The condenser’s job is to cool the refrigerant so that it turns from a gas into a liquid, or condenses. This happens when warm outdoor air is blown across the condenser coil that is filled with hot, gaseous refrigerant. This allows heat to transfer from the refrigerant to the cooler outdoor air, where the excess heat is rejected to the atmosphere. The condenser coils wind through the condenser to maximize the surface area of the piping, and effectively, the 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 refrigerant is now approaching 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.

Now that 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 the space by boiling (evaporating) the refrigerant flowing through it. This happens when warm air is blown 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 it is vented to the 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!

The refrigerant is hottest when it leaves the compressor and coldest when it leaves the expansion device. 

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

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