A C Refrigerant Cycle

[Malva Ferster]

Compression is the first step in the refrigeration cycle, and a compressor is the piece of equipment that increases the pressure of the working gas. Refrigerant enters the compressor as low-pressure, low-temperature gas, and leaves the compressor as a high-pressure, high-temperature gas.

Compression can be achieved through a number of different mechanical processes, and because of that, several compressor designs are used in HVAC and refrigeration today. Other examples exist, but some popular choices are:

The condenser, or condenser coil, is one of two types of heat exchangers used in a basic refrigeration loop. This component is supplied with high-temperature high-pressure, vaporized refrigerant coming off the compressor. The condenser removes heat from the hot refrigerant vapor gas vapor until it condenses into a saturated liquid state, a.k.a. condensation.

After doing so, the refrigerant is sent back to the compressor, where the process restarts. And that, in a nutshell, is how a refrigeration loop works. If you have any questions about the refrigeration cycle or its components and how they work, give us a call. We've been helping customers get the most out of their HVAC and refrigeration equipment for nearly 100 years.

As the name suggests, the refrigeration process is a cycle.
We start at the compressor, go through the condenser, then through the restriction, then through the evaporator and finally back to the compressor where the cycle starts all over again.

The Compressor can be thought of as the heart of the process.
It acts like a pump to create the circulation by compressing the refrigerant gas, creating a pressure difference that drives the refrigerant around the circuit in a continuous cycle.

The restriction restricts the liquid refrigerant flow coming from the condenser, and creates a pressure difference between itself and the evaporator. The restriction is more commonly referred to as a METERING DEVICE as it meters the amount of refrigerant entering the evaporator.

The Compressor is the heart of the refrigeration cycle and comes in a vast array of sizes.
In smaller systems it is usually found inside the outdoor unit, next to the condenser. But in large applications of multiple compressors, like in supermarkets, they are usually found separate inside a covered plant room.

Capillary tubes are simply a length of very narrow tube that causes a restriction to the flow of refrigerant.
They are most commonly found on small refrigerators like you have in your home.

The Evaporator & Condenser coils are basically the same type of construction.
A long length of pipe work surrounded by aluminium fins.
They are essentially heat exchangers, similar to the radiator in a car.

The term refrigeration means cooling a space, substance or system to lower and/or maintain its temperature below the ambient one (while the removed heat is rejected at a higher temperature). In other words, refrigeration is artificial (human-made) cooling.

I am trained as an architect and am fairly technically adept; but, honestly, I always get confused about the refrigerant cycle. I have read a bunch of articles like this one over the years, but they never quite stick. This description finally made it clear to me. Thanks! The step by step process you used is very effective.

Thanks for your kind comments Clarke, much appreciated.
This is of course the refrigeration cycle in its simplest form. There are a lot of other components, both electrical & mechanical, that are added to a system in the form of control & safety devices, but no matter how complex a system is, these basic 4 components are the heart of the system and will always be there. If you have that understanding in your head, it makes it easier to comprehend, trace systems out & fault find.

I currently taking a course in R & A, this step by step explanation has really helped me to grasp the basics of refrigeration cycle. I will appreciate if you send me a link to download textbooks with practice questions on this subject. Thanks Torr

There is much to be said of the genius behind this scientific development which really did end up changing the world in a very big way. Necessity was truly the mother of invention in this case, just by the lives it saves in heat related deaths every year alone.

Thanks for your comment Mike, and you are absolutely correct in pointing out that the suction line, in real installations, is larger than the discharge, expansion & liquid lines.
I think people who know that will already be relatively familiar with the refrigeration cycle, and this article was aimed at people with no prior understanding of how the cycle works, and designed to keep things as simple and understandable as possible.

I have been trying for the past 4 months to build my concept on refrigeration cycle. Unfortunately, I could not find any article that give satisfactory answers to my questions. Luckily, today, I accidentally came across your article and read it almost twice to get it all in my mind. Thank you so much for this kind of article.

Great article! It seems like pressure is being used to manipulate the boiling point of the refrigerant as it cycles through the system. Would you be against sharing the boiling point of a typical refrigerant used in residential hvac systems as it passes through the condensor coil on the high pressure side and the evaporator coil on the low pressure side? And how does that compare to outdoor and indoor ambient temperature?

Thank you for taking the time to take us step by step through this cycle. I have linked to your post from a blog that I wrote about geothermal heat pumps, in case people want to better understand how a heat pump can take 55-degree geothermal loops and heat or cool a house with that.

According to the second law of thermodynamics, heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this.[3] An air conditioner requires work to cool a living space, moving heat from the interior being cooled (the heat source) to the outdoors (the heat sink). Similarly, a refrigerator moves heat from inside the cold icebox (the heat source) to the warmer room-temperature air of the kitchen (the heat sink). The operating principle of an ideal heat engine was described mathematically using the Carnot cycle by Sadi Carnot in 1824. An ideal refrigerator or heat pump can be thought of as an ideal heat engine that is operating in a reverse Carnot cycle.[4]

The vapor-compression cycle is used by many refrigeration, air conditioning, and other cooling applications and also within heat pump for heating applications. There are two heat exchangers, one being the condenser, which is hotter and releases heat, and the other being the evaporator, which is colder and accepts heat. For applications which need to operate in both heating and cooling modes, a reversing valve is used to switch the roles of these two heat exchangers.[citation needed]

At the start of the thermodynamic cycle the refrigerant enters the compressor as a low pressure and low temperature vapor. In heat pumps, this refrigerant is typically R32 refrigerant or R290 refrigerant. Then the pressure is increased and the refrigerant leaves as a higher temperature and higher pressure superheated gas. This hot pressurised gas then passes through the condenser where it releases heat to the surroundings as it cools and condenses completely. The cooler high-pressure liquid next passes through the expansion valve (throttle valve) which reduces the pressure abruptly causing the temperature to drop dramatically.[7] The cold low pressure mixture of liquid and vapor next travels through the evaporator where it vaporizes completely as it accepts heat from the surroundings before returning to the compressor as a low pressure low temperature gas to start the cycle again.[8]

Some simpler applications with fixed operating temperatures, such as a domestic refrigerator, may use a fixed speed compressor and fixed aperture expansion valve. Applications that need to operate at a high coefficient of performance in very varied conditions, as is the case with heat pumps where external temperatures and internal heat demand vary considerably through the seasons, typically use a variable speed inverter compressor and an adjustable expansion valve to control the pressures of the cycle more accurately.[citation needed]

The above discussion is based on the ideal vapor-compression refrigeration cycle and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).[4]

In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where heat is more readily available than electricity, such as industrial waste heat, solar thermal energy by solar collectors, or off-the-grid refrigeration in recreational vehicles.

The absorption cycle is similar to the compression cycle, but depends on the partial pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber and a generator. The absorber dissolves the refrigerant in a suitable liquid (dilute solution) and therefore the dilute solution becomes a strong solution. In the generator, on heat addition, the temperature increases, and with it, the partial pressure of the refrigerant vapor is released from the strong solution. However, the generator requires a heat source, which would consume energy unless waste heat is used. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide (absorbent).

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