How Refrigerants Work In HVAC Systems
In this article we're going to learn how refrigerants work in HVAC systems
. To become an HVAC engineer it's absolutely essential to understand how a refrigerant works, how it changes phases and how it moves the thermal energy around the system.
It doesn't matter what type of system you look at, whether it's a refrigerator in your home, to simple split units and even large industrial chillers - essentially they all work the same way.
That's because they pass the refrigerant between the main components of the evaporator, the compressor, the condenser and the expansion device.
When we say refrigerant, what we mean is a fluid that can be easily boiled from a liquid to a vapor and also be condensed back from a vapor back to a liquid.
And this needs to happen again and again without failure. So, what is used as a refrigerant? Well, we could use water. Water will work and it is used in absorption chillers.
But the reason we don't typically use water in common refrigeration units is because there are specially made refrigerants designed specifically for the task and they perform much better.
Some of the more common refrigerants that are used currently are R22, R134A and R410A. These refrigerants have extremely low boiling points compared to water, so they need very little heat in order to boil and they evaporate into a vapor and this means they can extract heat more rapidly.
At atmospheric pressure the boiling point of water is 100°C or 212°F, but R134A has a boiling point of just -26.3°C or -15.34°F. So we can use room temperature to boil these and take the heat away from the room and disburse that out to the atmosphere.
Now lets see how the refrigerants actually move around the system. We'll start with the compressor because it's the driving force of the refrigerant, pushing it around the system. There are many different types of compressors but here we will just look at the basic piston type to make it clear and easy to understand how it works.
So the refrigerant enters the compressor as a saturated vapor and it's at a low temperature and low pressure. The compressor pulls the refrigerant in and rapidly compresses it. This forces the molecules together so the same amount of molecules fit into a smaller space.
This increases the collision rate of the molecules because they are all constantly bouncing around. 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 understand this if you've ever used a bike pump the pump gets very hot as the pressure increases.
The refrigerant comes from the compressor around into the condenser. The condenser is where all the unwanted heat is rejected out into the atmosphere. This will include all of 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 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 super heated vapor at high pressure and high temperature and as it passes along the tubes of the condenser, fans will blow air across the condenser to remove the heat, just like you might blow on a hot spoon of soup to cool it down. So as the air blows across the tubes it removes the thermal energy from the refrigerant.
As the refrigerant gives up this 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 the higher pressure but also slightly cooler although it's enthalpy and entropy will have decreased.
Now you'll see this for yourself if you pour boiling water into a glass, then you'll see all the steam start to rise out of it and also the steam, as it comes into contact with the glass, which is at a cooler temperature, will condense and that steam will then form a vapor.
And that will then, as it condenses, will then roll down and make its way to the bottom of the glass again back into a liquid. Next the refrigerant makes it's way from the condenser around and into the expansion device.
In this case we've got a thermal expansion valve. The expansion valve meters the flow of refrigerant into the evaporator. In this example we're using a thermal expansion valve which uses a capillary bolt to control the flow through the valve, depending on the temperature at the outlet of the evaporator.
The valve is a bit like a spray nozzle. It has a high pressure on one side within the bottle and there's a low pressure side just past the nozzle. When the trigger is squeezed it lets the fluid pass and this will be a mixture of liquid and vapor.
So as the refrigerant passes through the expansion valve, it will expand because the pipework after it goes into the evaporator, is at a lower pressure, so the refrigerant will be able to expand to fill this.
As it expands it reduces in pressure and temperature, just like if you hold a deodorant can or a spray paint can and you press the nozzle you'll feel it get colder and the walls of the container will become less pressurized as you do so.
So the refrigerant will leave the expansion valve at a low pressure and temperature and it will head straight into the evaporator. The evaporator receives the refrigerant and the fan blows the warm air across the coils.
The temperature of the air must be higher than the temperature of the cool refrigerant and this allows it to absorb more energy and to boil the refrigerant completely into a vapor. Much like heating a pan of water.
The heat will cause the water to evaporate into steam and the steam will carry away the heat. If you place your hand over the steam of the pan, then you will find it is really hot. So, please don't do that at home.
Remember the refrigerants have very low boiling points, so the room temperature air is enough to boil it into a vapor. The refrigerant leaves as a saturated vapor at a low pressure and temperature. The temperature only changes slightly, which confuses many people, but the reason it does not increase is because it's 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 their 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.
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