How does a refrigerant move thermal energy around a chiller or air conditioning system. It doesn’t matter what type of refrigeration system you use, from the refrigerator in your home, a small split a/c unit all the way up to an industrial chiller. Essentially they all work the same way by passing a refrigerant between the main components of the compressor, condenser, expansion device and the evaporator to remove unwanted heat from one location (e.g. an office) to another (e.g. outside air). Now if you scroll to the bottom of this article you can simply watch a video tutorial on the subject.
For further information on this subject I would recommend visiting the Danfoss website. Danfoss is your go-to source for information and resources that can help you through the cooling industry’s transition to natural and climate-friendly refrigerants. They have a deep understanding of all the new regulations and their effects, and they’re ready to share their knowledge and solutions with you. They’ve also made helpful tools like their refrigerant retrofit guide, Low-GWP Tool, and Coolselector 2 app available for free on their website. You can access them now by visiting Refrigerants.Danfoss.com.
When we say “refrigerant” we mean a fluid that can easily boil from a liquid into a vapour and also be condensed from a vapour back into a liquid. This needs to occur again and again, continuously without fail.
An example of a refrigerant would be water. This is able to evaporate and condense and is easy and safe to use. It’s used in Absorption chillers as a refrigerant, you can find out more about this type of chiller by clicking here. The reason water isn’t typically used as a refrigerant in common air conditioning units is because there are specially made refrigerants designed specifically for this task, and these are able to perform much more efficiently.
Some of the more common refrigerants on the market ate R22, R134A and R410A, although the laws and regulations on refrigerants are tightening and many of these will be phased out in the long run. These common refrigerants all have extremely low boiling points compared to water. This allows it to evaporate into a vapour with very little thermal energy applied which means the refrigerant can extract heat more rapidly.
Lets look at how refrigerant moves around the system. We’ll start with the compressor as this is the heart of the system, it forces the refrigerant around each of the components within the refrigeration system. The refrigerant will enter as a saturated vapour and is a low temperature and low pressure. As the compressor pulls the refrigerant in, it rapidly compresses it, this forces the molecules together so the same amount of molecules fits into a smaller volume. The molecules are all constantly bouncing around and compressing them into a smaller space causes them to collide more often, as they collide they convert their kinetic energy into heat. At the same time, all the energy that is put in by the compressor is converted into internal energy within the refrigerant. This results in the refrigerant increasing in internal energy, enthalpy, temperature and pressure. You’ll know this if you’ve ever used a bike pump, the pump becomes very hot as the pressure increases.
The refrigerant now moves to the condenser. The condenser is where all the unwanted heat is rejected out into the atmosphere. This will include all the heat from the building as well as the heat from the compressor. When the refrigerant enters the condenser, it needs to be at a higher temperature than the ambient air around it, in order for the heat to transfer. The greater the temperature difference, the easier the heat transfer will be. The refrigerant enters as a superheated vapour at high pressure and temperature, it then passes along the tubes of the condenser. During this move, fans will blow air across the condenser (in an air cooled system) to remove the unwanted energy. Much like blowing a hot spoon of soup to cool it down. As the air blows across the tubes, it removes heat from the refrigerant. As the refrigerant gives up its heat it will condense into a liquid so by the time the refrigerant leaves the condenser it will be a completely saturated liquid, still at high pressure, but slightly cooler although it will have decreased in both enthalpy and entropy.
The refrigerant then makes its way to the expansion valve. The expansion valve meters the flow of refrigerant into the evaporator. In this example we’re using a thermal expansion valve which holds back refrigerant, creating a high and low pressure side. The valve will then adjust to allow some refrigerant to flow and this will be part liquid and part vapour. As it passes through it will expand to try and fill the void. As it expands the refrigerant reduces in pressure and temperature, just like if you hold a deodorant spray can and hold the trigger down. The refrigerant leaves the expansion valve at low pressure and temperature then heads straight into the evaporator.
The evaporator receives the refrigerant and another fan blows the warm air of the room across the evaporator coil. The temperature of the room air is higher than the temperature of the cool refrigerant this allows it to absorb more energy and boil the refrigerant completely into a vapour. Much like heating a pan of water, the heat will cause the water to evaporate into steam vapour and the vapour will carry the heat away, if you were to place your hand over the rising steam you will find it’s very hot. Although I wouldn’t recommend this and it can cause injury. Remember we looked earlier at the low boiling point of refrigerants, so room temperature air is enough to boil it into a vapour.
The refrigerant leaves the evaporator as a low temperature, low pressure vapour. The temperature only changes slightly which confuses many people, but the reason it doesn’t increase dramatically is because it is undergoing a phase change from a liquid to a vapour so the thermal energy is being used to break the bonds between the molecules but the enthalpy and entropy will increase and this is where the energy is going. The temperature will only change once the fluid is no longer undergoing a phase change.
And that, is the basics of hot refrigerants work in HVAC refrigeration systems.