The principle of the refrigeration cycle is to move heated air from one location (the source) to another (the heat sink). In the case of household appliances this can either be used to heat or cool an area or in the case of heat pumps, can be used for both.
The process of moving warm air is dictated by the second law of thermodynamics which does not allow warm air to spontaneously move into a cooler area. This requires work in the form of energy usually in combination with mechanical pressure.
The workings of the refrigeration cycle concerns the evaporation and condensation of what is known as the “working liquid” or the refrigerant as it cycles through a closed system. In most refrigeration systems the refrigerant used is often a freon; a CFC (chlorofluorocarbon), used for its stability, non flammability and ability evaporate and condense with ease.
The Refrigeration Cycle: Understanding Key Concepts
The refrigeration cycle can be quite a complicated topic and before jumping right in it’s wise to familiarize yourself with some basic scientific concepts behind it just so it’s all easier to understand.
Changes in State
All materials change state when a certain amount of energy is put into it. Usually heat energy is utilized to change the state of matter but not always. Take water for example:
- H2O is ice (solid state) when below 32 degrees Fahrenheit.
- Ice becomes water (liquid state) at above 32 degrees Fahrenheit.
- Water becomes steam (gaseous state) at 212 degrees Fahrenheit.
A gas can have even more energy applied to it to achieve the lesser known state of matter plasma which has many applications such as cutting metal.
The process can also occur in reverse and it’s the point at which a liquid changes into a gas (evaporation) and then turns from a gas back into a liquid (condensation) is what we are concerned with for the refrigeration cycle in HVAC.
The Utilization of Pressure
Another method that can be used to change the state of a material is to put it under pressures that are too extreme for it, forcing it to change state. Pressure can also be used to lower the temperature thresholds which the material requires to change state.
This is because pressure facilitates heat transfer as heat tends to travel form high pressure to low pressure (keep this in mind when we explain the refrigeration cycle).
Under high pressures for example, water is being pushed to solidify and so if you were trying to evaporate water under high pressures, more energy (more heat) would be needed. Whereas water would be much easier to evaporate under lower pressures.
With that being said, you would have an easier time boiling water up on a mountain where air pressures are lower than you would at sea level.
Within a household appliance that uses the refrigeration cycle, pressure is manipulated to alter the temperature threshold of the refrigerant.
Cold is the Absence of Heat
You can not add cold to something, cold is not a form of energy. You can only add or remove heat energy. The removal of heat energy in turn makes something cold.
An appliance that uses the refrigeration cycle, such as an air conditioner or evaporative cooler, is not adding cold air into the room, it is actually removing heat and putting it somewhere else. It may feel like it’s adding coldness into the room as you can feel the fan blowing cold air in, but that cold air you feel is the air from the room with the heat removed.
Conservation of Energy
The conservation of energy is a law in chemistry that dictates how energy behaves. Namely, energy cannot be created or destroyed but can only be transferred between forms. As an example of this, when you light a fire, heat energy is transferred throughout different materials and into other forms of energy such as light.
The transfer of heat is also a vital aspect of the refrigeration process as heat is transferred to and from the refrigerant at different stages in the refrigeration cycle.
The Refrigeration Cycle Explained
Now that you have an understanding of the basic principles involved in the refrigeration cycle you can apply this knowledge to how it is used in by your HVAC system components. Some HVAC systems or appliances are more complex than others but it’s important to remember the refrigeration cycle remains the same.
The refrigeration cycle is composed of six main components. The refrigerant is the CFC chemical used as your working liquid, the refrigeration loop which allows the refrigerant to travel between the other four components which are:
- The compressor - Increases the pressure of the refrigerant
- The condenser - Cools the refrigerant and ejects hot air
- The expansion device - Reduces the pressure of the refrigerant
- The evaporator - Boils the refrigerant and ejects cool air
Step 1: Compression
The refrigerant starts in the system as a cool vapour (mix of liquid and gas) and travels into the condenser. The condenser is considered the heart of a refrigeration system as it pushes the refrigerant through the refrigeration loop and draws the more power than any of the other components.
After the refrigerant enters the compressor it greatly increases the pressure of the refrigerant. With the use of pistons and a crankshaft it decreases the space available to the refrigerant in the cylinder in which it is contained.
The sudden increase in pressure results in the cool vapour that enters the compressor leaving as an extremely hot vapour.
Step 2: Condensation
After leaving the compressor, the hot vaporous refrigerant travels into the condenser; a component which contains many coils to increase its surface area and a fan. The refrigerant travels through the condenser coils whilst the condenser fan blows ambient air from outside across it the coils.
The outside air is cooler and lower in pressure than the refrigerant and so heat transfers from the refrigerant to the air which is then ejected outside. This results in the temperature of the refrigerant dropping and it changing from a hot vapour to a hot liquid.
Step 3: Expansion
The refrigerant is a hot liquid which is still at high pressure as it enters the expansion device but not for long! The sol job of the expansion device is to dramatically reduce the pressure of the refrigerant.
This sudden drop in pressure causes the refrigerant to cool down significantly but it can now be boiled by the evaporator.
Step 4: Evaporation
The evaporator also contains coils and a fan similar to the condenser and for the same reason, to increase surface area and better facilitate heat transfer between mediums. The cooled liquid refrigerant that enters the condenser is now low pressure and travelling through the condenser coils while the condenser fan blows warm air from the room over the coils.
This results in the refrigerant being boiled and evaporating, in effect the heat form the room air is being transferred to the refrigerant, the now cool air is then ejected back into the room. This causes the refrigerant to become a cooled vapour travelling on its way back to the compressor.
And that is the refrigerant cycle.
To Summarize the Refrigeration Cycle
The refrigerant cycle that you have just read depicts a system which takes heat from the warm air of a room and ejects it outside through the evaporation and condensation of a refrigerant with the utilization of pressure.
It is called the vapour compression refrigeration cycle.
In a less complicated look at the system it can be broken down into two stages:
The refrigerant is pressurized by the compressor to allow heat to transfer from the refrigerant to the outside in the condenser. This functions to eject warm air from the HVAC system and condense the refrigerant from vapour to liquid.
The liquid refrigerant is then depressurized and cooled by the expansion device allowing heat to transfer from the room to the refrigerant. This functions to eject cool air from the HVAC system into the room and evaporate the refrigerant from liquid to vapour.
This may be a basic look at the refrigeration cycle but it is the main concept of it. A real household appliance system will have some other components such as metering devices and driers that help the process along.
Types of Refrigeration Cycle
Other than the vapour compression refrigeration cycle that you have already seen and that is most commonly found in industrial and domestic HVAC systems, there are other less often used types.
Vapour Absorption Refrigeration System
The vapour absorption refrigeration system, also known as the ammonia refrigeration system works in a very similar way to the vapor compression system but it differs in the method that the temperature of the refrigerant is raised.
Instead of a compressor, an absorber and a generator is used to dissolve the refrigerant in an absorbent (if ammonia was used as the refrigerant, water was used as the adsorbent); raise its temperature and also its pressure. Sometimes a liquid pump is used to drive the process depending on the size of the system.
The vapour absorption refrigeration system was widely used in the early 20th century but is now outdated as the vapour compression refrigeration cycle has a higher coefficient performance.
Gas Refrigeration Cycle
In this refrigeration system, a gas which does not change state throughout the process is used. As there is no change in the state of matter of the gas used, a compressor and evaporator is not needed and instead heat exchangers take their place.
Gas cycle systems are not commonly used because they have a low efficiency and require the systems to be of an overly large size to give similar outputs to appliances which use the vapour compression refrigeration system.
Stirling Cycle
Stirling engines can be used by both wood stove fans and heat pumps to move heat from one place to another. The principle of a Stirling engine is that mechanical functions are driven by the pressure that results from the differentiation of temperature between two fluids.
About the Author
Dave Miller is a HVAC technician with over 10 years in the industry. Dave created HeatTalk with the ambition for it to become a resource for individuals looking for answers, whether they be a layman, student or a professional.