How do Refrigerant Work

  How Refrigerant Work 

How does a refrigerant move around a chiller or air conditioning unit to remove thermal energy from a room or refrigerator?

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).

Refrigerant boil and evaporate

When we say “refrigerant” we mean a fluid that can easily boil from a liquid into a vapor and also be condensed from a vapor 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. The reason water isn’t typically used as a refrigerant in common air conditioning units is that there are specially made refrigerants designed specifically for this task, and these are able to perform much more efficiently.

Refrigerant Types and Boiling Points

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 vapor with very little thermal energy applied which means the refrigerant can extract heat more rapidly.

How a compressor works

Let's 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 vapor 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.

How a condenser works

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 superheated vapor 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

How the evaporator works

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 that 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 vapor. 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 vapor. Much like heating a pan of water, the heat will cause the water to evaporate into steam vapor and the vapor 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 vapor.

The refrigerant leaves the evaporator as a low temperature, low-pressure vapor. The temperature only changes slightly which confuses many people, but the reason it doesn’t increase dramatically is that it is undergoing a phase change from a liquid to a vapor 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.