Refrigeration Cycle Diagram: Four Stages, Components, and Refrigerant State Changes

Key takeaways

• The refrigeration cycle is a thermodynamic process used in HVAC and cooling systems to transfer heat from one area to another.
• The four stages of the refrigeration cycle depend on these four essential components working together: compressor, condenser, expansion device, and evaporator.
• The compressor pressurizes refrigerant gas, the condenser releases heat and condenses it into liquid, the expansion device drops pressure to lower its temperature, and the evaporator absorbs heat to cool the environment.
• Refrigerant continuously cycles between gas and liquid states under changing pressure and temperature to enable effective cooling.
• There are four major refrigeration cycle variants: vapor-compression (most common), absorption (heat-driven), liquefaction (for ultra-low temps), and evaporative (natural but less powerful).
• inFlow’s software helps HVAC businesses track condensers, evaporators, compressors, and other parts and components across trucks and warehouses.

The HVAC industry plays a significant role in our daily lives. It keeps us comfortable in our homes, keeps our food fresh longer, and even preserves medicine. HVAC systems do so through a process called the refrigeration cycle. While the exact mechanisms are complex, the theory is simple. 

In today’s article, we want to get back to basics for anyone working in the HVAC industry and talk a bit about the refrigeration cycle. We’ve even included a helpful refrigeration cycle diagram. So whether you’re a greenhorn just getting started in the industry or a seasoned HVAC professional, this article will give you all the information you need on the refrigeration cycle. 

What is the refrigeration cycle?

You’ve likely heard of the refrigeration cycle if you work in HVAC. After all, it’s the cornerstone of air conditioners, refrigerators, and heat pumps. The refrigeration cycle operates on the principles of thermodynamics and is typically used to cool spaces. It does this by transferring heat from one area to another.

Refrigeration cycle diagram: Components, flow direction, and state points

There are four main components that make the refrigeration cycle possible. They are as follows: compressor, condenser, expansion device, and evaporator. Refrigerant is also important, although it doesn’t play an “active” role. As refrigerant circulates through the system, refrigerant state changes occur: vapor condenses into liquid in the condenser, and liquid evaporates into vapor in the evaporator. Sometimes, it will be a gas and other times, it will be a liquid. 

Each component of the refrigeration cycle has a specific role to play, but they all rely on one another to function. No component is more important than another. Each also represents a stage in the refrigeration cycle. You’ll be able to better observe what we mean by looking at the refrigeration cycle diagram below. Let’s take a closer look. 

Now that you’ve seen the refrigeration cycle diagram and the refrigerant flow direction, let’s examine each of its components in more detail to get a better idea of how it works.  

Compressor stage: Low-pressure vapor becomes high-pressure, high-temperature vapor

It helps to think of the compressor as the heart of an AC system. It’s the component that pushes refrigeration from one part to another, like blood through the body. The compressor establishes the system’s high-pressure and low-pressure sides by raising refrigerant-vapor pressure and maintaining a pressure difference between the condenser and evaporator.

In general, gases will move from high-pressure areas to low-pressure areas. This makes it useful for moving refrigerant from one area of the system to another. 

Condenser stage: Refrigerant rejects heat and condenses at high pressure

The condenser receives hot, high-pressure refrigerant vapor, and heat rejection in the condenser cools and condenses it into a high-pressure liquid that may be subcooled before it reaches the expansion device. It achieves this by taking advantage of heat transfer. Similar to the high-to-low-pressure movement we mentioned above, heat will generally move to colder areas. This principle is part of what turns refrigerant from gas to liquid. Typically, in AC units, the condenser is located outdoors so that the heat expelled by this process can dissipate into the air.  

Expansion stage: Refrigerant pressure and saturation temperature drop

The expansion valve pressure drop throttles high-pressure liquid refrigerant to evaporator pressure, lowering its saturation temperature and causing part of the liquid to flash into vapor. This doesn’t directly lower the temperature, but it does have the same effect. While this may seem like overkill, the expansion device is really what enables the refrigerant to cool your home. Heat exchange alone isn’t enough; it needs a low-pressure environment to expedite the process. 

Evaporator stage: Low-pressure refrigerant absorbs heat and vaporizes

The evaporator is located inside and is actually responsible for cooling the environment. It does this by forcing refrigerant through an area. The refrigerant absorbs the heat and moisture from the region, resulting in cold air. From here, it goes back into the compressor. 

Again, the refrigeration cycle relies on each of these components to function. If one part goes belly up, the system fails. Regular checks are an essential part of keeping things in working condition. 

Compressor types used in HVAC refrigeration

It’s worth noting that there are many types of compressors in use. They all have the same goal but use different methods to achieve it. Each of these methods has its own advantages and disadvantages. Some are quieter; others are more scalable. We won’t go into too much depth here, but it’s best to know the most common types. Overall, centrifugal, rotary-type, reciprocating, and dynamic compressors are the most popular. 

Centrifugal and rotary-type compressors rely on rotation to build pressure, while a reciprocating compressor works more like a piston. Dynamic compressors also rely on rotation, but instead use the motion to speed up an air current. This, in turn, builds pressure. 

Vapor-Compression, Absorption, Evaporative, and Cryogenic Cooling Compared

There are also variants of the refrigeration cycle. In general, there are four types of refrigeration cycles, classified according to their cooling method. It’s important to know what each is and where you can use them. Each method follows the same basic principles shown in the above refrigeration cycle diagram: 

Vapor-compression

This is the most common type of refrigeration, and what we’re focusing on in this article. Compression systems use a compressor to lower the pressure of the refrigerant, cooling it and turning it into a liquid. From here, it moves through the rest of the refrigeration cycle.

Absorption refrigeration

This type of refrigeration cycle uses a heat source to power the refrigeration cycle. You can achieve this by breaking coolant into two separate bodies. One performs evaporative cooling, and then the second body absorbs it. A heat source is necessary to restart the process and align the coolant temperatures again. 

Cryogenic refrigeration and gas liquefaction

Another type of refrigeration involves cooling refrigerant down so far that it becomes a liquid. This makes it easy to store and transport. In general, this is not for consumers. Its normal use case is experiments or massive industry-scale solutions. 

Evaporative cooling

This last one is the simplest refrigeration method. Evaporative systems work by evaporating a liquid, such as water, into the air. This effectively cools the air but can increase humidity. Rather than a man-made cycle, evaporation is part of a more natural cycle. That makes it easy to use but, in some instances, much slower. 

Each method of refrigeration has its pros and cons. Some are cheaper, others are more efficient, and some might not fit a customer’s use case. Depending on the location of your operation, one method might be more effective than another. Be sure to research before adding or creating a product in your company’s catalog. 

HVAC inventory: Tracking refrigeration components across trucks and warehouses

We’ve mentioned it a few times already, but as you can see from the above refrigeration cycle diagram, every component is an integral part of the refrigeration cycle. They all work in unison; if just one is missing, the refrigeration cycle won’t happen. And isn’t that what your customers are paying you for?

This is why proper inventory management for HVAC companies is crucial. But we get it. Field services make it hard to track inventory accurately with technicians spread out all over town. Luckily, inFlow is here to help. With inFlow, you can track your components across multiple locations with our cloud-based inventory system. This means you can set up each truck and warehouse as a separate location so you can see exactly which technicians have what. 

Our field management software makes it easy to monitor your inventory levels and helps you set reorder points for your products. This means inFlow will notify you when you’re running low on something, so you’ll never run out of stock.

We also have a built-in barcoding system that makes it fast and easy for technicians to remove items from their trucks and update their inventory in real-time. Once the job is complete, technicians can use inFlow to create the invoice and bill the client right from the job site.