HVAC Refrigeration Cycle: How It Works

It’s a funny thing about the HVAC refrigeration cycle: Despite its name, the cycle is not just about refrigeration systems.

In fact, the same core principles of the cycle are at work in everything from a small window air conditioning unit or home dehumidifier to a restaurant refrigerator or grocery freezer, to the largest industrial chiller system.

That means understanding the HVAC refrigeration cycle and the basics of how it works is fundamental to success as an HVACR technician.

Four main components of the HVAC refrigeration cycle

At its simplest, the HVAC refrigeration cycle is the process of removing heat from where it is not wanted and transferring it somewhere else.

For example, people think air conditioners create cold air. In reality, an air conditioning system uses the refrigeration cycle to remove heat from the indoor air and send the heat outdoors.

All refrigeration systems include four major components:

• Compressor
• Condenser
• Metering device (also called an expansion device)
• Evaporator

The refrigerant (also called coolant) is piped between these four components in a closed loop system. As the refrigerant circulates, the pressure and state of the refrigerant changes to absorb or release heat.

It is through the continuous circulation and change of state of the refrigerant in the system that the HVAC refrigeration cycle does its job, in everything from a home AC to a 30-ton chiller on a factory roof.

The science of the refrigeration cycle

To better understand the HVAC refrigeration cycle, it’s helpful to keep in mind some basic laws of physics.

About heat and cold:

• Heat (energy) is not created or destroyed — it is transferred. What people experience as “cold” is really an absence of heat.
• Heat naturally flows from a warmer mass to a cooler mass.

About temperature and pressure:

• When you increase the temperature of a vapor (gas), the pressure also increases.
• Similarly, when you apply pressure to a vapor, the temperature also increases. If you decrease the pressure, the temperature drops.
• Pressure changes the boiling point of a liquid, with a lower pressure making it easier for the liquid to boil. For example, water boils at 212° F at sea level but at 160° F on the top of Mt. Everest.

About liquids and gases:

• Evaporation — the changing of a liquid to a gas, or vapor — occurs at a high temperature and high pressure. The amount of heat required is generally measured at the liquid’s boiling point.
• Condensation — the changing of a gas to a liquid — occurs at a low temperature and low pressure.

Putting the laws of physics to work in the refrigeration cycle

In an air conditioning or refrigeration system, the refrigerant is designed to boil at a low temperature corresponding to the system’s design. For instance, in an average AC running under normal conditions with a 75°F indoor temperature, the refrigerant will boil at around 40°F.

Throughout the HVAC refrigeration cycle, the pressure of the refrigerant is manipulated up or down to increase or decrease the temperature so that the refrigerant evaporates or condenses. This changes the state of the refrigerant back and forth between a liquid and a vapor.

This back-and-forth change of state throughout the cycle is what transfers heat and provides the desired cooling effect.

What the four components do in the HVAC refrigeration cycle

What role does each major component play in the cycle of altering the pressure and state of the refrigerant in an HVAC system?

• The mechanical heart of the system is the compressor, which exerts pressure to move the refrigerant; the compressor also creates the high and low pressure sides of the system. The refrigerant enters the compressor as a cool vapor and leaves as a high pressure vapor that is superheated to above the condensing or boiling temperature.
• The condenser is where the superheated refrigerant is cooled and condensed as it winds its way through the condenser coils. The rejected heat is expelled to the outdoors, while the combination of high pressure and heat release turns the refrigerant into a liquid that is subcooled to below the condensing or boiling temperature.
• As the subcooled refrigerant then enters the metering device as a (still hot) high pressure liquid, the device rapidly reduces the pressure. This changes the refrigerant to a mixture of a cold liquid and vapor — roughly 75% liquid and 25% vapor. The refrigerant is at its coldest as it leaves the expansion device.
• This mixture of liquid and vapor makes its way through the evaporator, where heat is transferred from the air to the cooler, low pressure refrigerant. As heat is absorbed by the refrigerant, the air is cooled and the refrigerant boils — eventually returning to the compressor as a cool vapor, to begin the cycle all over again.

Understanding the purpose and function of each of these components helps an HVACR service technician analyze a system to determine if it is running correctly and identify the cause of any malfunction or failure.

What an HVACR technician needs to know

By no means is this a complete explanation of the HVAC refrigeration cycle. For example, depending on the particular type of system, there are other parts that play a vital role, such as:

• An accumulator that prevents liquid refrigerant from passing directly into the compressor
• Various refrigerant lines that carry the coolant between the four main components

But hopefully this overview gives you a good idea of the fundamentals of the refrigeration cycle — plus an appreciation for the importance of the cycle in cooling and refrigeration systems.

To learn more about other technical aspects of working in HVACR, check out our training videos.

HVACR Career Connect NY was created to promote the exceptional quality-of-life benefits of a career in HVAC and Refrigeration service, and also to provide a clear path for getting started in the profession. In doing so, we serve as a resource for employers in the New York City metro area to find and hire smart and capable new technicians. We also serve as an educational resource to support business growth and to help service technicians succeed in their chosen profession.