An air conditioning system is made up of 3 main components. They are the compressor, condenser coil and evaporator coil.
While most folks know the function of the compressor, few can tell the differences between the condenser coil and the evaporator coil.
So, what is the difference between an evaporator coil and a condenser coil? Can a condenser be used as an evaporator? Which is bigger between an evaporator and a condenser coil?
The evaporator coil is the indoor unit of the AC where the refrigerant cools the air by absorbing its heat. On the other hand, the condenser coil is the outdoor unit where the refrigerant releases heat to the surrounding.
The refrigerant enters the evaporator coil as a cold liquid but vaporizes (turns to gas) after absorbing heat from the indoor air. Inside the condenser coil, the refrigerant loses heat to the surrounding and exits as a liquid.
In conventional air conditioners, a condenser cannot be used as an evaporator. However, a heat pump’s condenser is used as an evaporator and vice versa during winter when the heat pump is used for heating the house.
Both evaporator and condenser coils have fans which helps to accelerate the rate of heat exchange. The evaporator fan pulls warm air from the house and also pushes out cooled air from the coil to the house. Condenser coil fan blows cooler air over the coil so that the refrigerant can lose heat faster to the surrounding.
The difference between a condenser fan and an evaporator fan is that the condenser uses a propeller fan while the evaporator uses an impeller fan. The blades on an propeller fan are connected to a shaft while those of an impeller fan are attached to a central hub.
Condenser coils are usually bigger than evaporator coils. Apart from releasing the heat absorbed by the refrigerant from the indoor air, condensers also need to eject the heat of compression gained by the refrigerant in the compressor and hence their size.
In a nutshell, both evaporator and condenser coils are heat exchangers. Inside the evaporator coil the refrigerant absorbs heat from the warm indoor air while inside the condenser coil the refrigerant releases the heat to the surrounding.
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Condenser Coil vs Evaporator Coil
As I have mentioned, both evaporator and condenser coils are heat exchangers. In thermodynamics (heat exchange), heat is transferred from a point of high concentration to a point of low concentration.
The above is actually the operating principle behind air conditioning and refrigeration. To achieve that, a refrigerant is needed.
Ideally, a good refrigerant should have a low boiling point. That means that it is able to turn from gas to liquid and back to gas again easily.
When a fluid turns from liquid to gas, the process is called evaporation. It means that the liquid has gained enough heat to vaporize. And that is what happens inside the evaporator coil.
When a liquid evaporates, it takes away heat from its surrounding (latent heat of vaporization). This is exactly what happens when we sweat.
When a fluid turns from gas to liquid, the process is called condensation. It means that the gas has lost enough heat and hence it changes to its liquid state. And that is what happens inside the condenser coil.
For a long term, Freon (R-22) was the refrigerant of choice. It was later discovered that Freon contributed to global warming and hence it was phased out completely in 2020. R-410A is currently the refrigerant of choice.
Let us now look at the differences between condenser and evaporator coils in more details including how they work.
The Condenser Coil
The condenser coil and the compressor form what is known as the outside unit or simply the condenser unit. Since the condenser coil takes the shape of a cube, it means that its inside is hollow.
The compressor is located at the bottom while the condenser fan can be seen at the top of the unit. Condenser coils come in different sizes depending on your house size and air conditioning needs.
A condenser coil is basically copper coils wound into a cube over and over. The reason for having that many coils compacted together is to increases the surface area for heat exchange.
Each part of the coil is in contact with the surrounding air. As the refrigerant moves round the coil, it loses heat to the surrounding air with the help of the fan.
Before the refrigerant enters the condenser coil, it first passes through the compressor. All the AC components are connected using copper tubes.
The refrigerant moves from the compressor to the condenser coil then to the evaporator coil before going back to the compressor. The cycle is repeated again and again until the house is properly cooled.
A compressor functions exactly like a pump. As its name implies, it compresses the refrigerant gas before it enters the condenser coil.
The compressor apart from increasing the pressure of the refrigerant has another very important function. You see, the refrigerant entering the compressor contains heat absorbed from the indoor air.
That heat needs to be released to the outside but there is a problem. It’s during summer so the outside temperature is high as well. As I mentioned earlier, for heat transfer to take place, there must be a temperature differential.
And that is where the compressor comes in. When you compress a fluid, its pressure increases but that is not the only thing that happens. Its temperature also increases drastically.
As such, the refrigerant enters the condenser coil as a high-pressure superheated gas. At that point, the temperature of the refrigerant is higher than that of the surrounding air and hence heat transfer can now take place.
The condenser fan blows air over the coil which absorbs heat from refrigerant and by the time it is exiting the condenser it will have lost so much heat that it will have turned into a liquid.
The outer shell of a condenser unit is made up of fins similar to those of a car radiator. These fins are critical in the dissipation of heat to the surrounding.
The Evaporator Coil
As I has mentioned earlier, the evaporator coil is also known as the indoor unit. If you have a central air conditioner, the evaporator coil will be hidden somewhere inside the house.
On the other hand, if you have a ductless mini-splits air conditioner, the evaporator will most likely be mounted on your wall or ceiling. In that case, each room that needs to be cooled will have its own evaporator coil.
Central air conditioners have only one evaporator coil connected to the different rooms using air ducts. Check out more differences between central air conditioners and mini-splits in this post.
The refrigerant does not enter the evaporator coil straight from the condenser coil. It has to first go through an expansion valve.
An expansion valve is simply a small restriction that controls the amount of refrigerant flowing through it and entering the evaporator coil. You see, when too much refrigerant enters the coil, not all of it will vaporize meaning that some refrigerant will enter the compressor in liquid state.
A compressor is only designed to handle the refrigerant in its gas state. A liquid refrigerant will therefore damage the compressor.
Apart from that, the expansion valve also lowers the pressure of the refrigerant. But why is it important to lower the temperature of the refrigerant?
You see, although the refrigerant is in its liquid state, it is still not as cold as it needs to be in order to cool the indoor air. I said that increasing the pressure of a fluid increases its temperature as well, but the opposite is also true.
By forcing the refrigerant through a restriction, its pressure will decrease and its temperature will also fall dramatically.
Inside the evaporator (assuming you have a central air conditioner), the fan will pull warm air from the house using return air ducts. When the air comes into contact with the cold coil, it will lose the heat to the refrigerant.
The cooled air will then be supplied to the house using supply air ducts. By the time the refrigerant is exiting the evaporator, it will have turned to its gas state and ready for another cycle.
Can a Condenser be used as an Evaporator?
In air conditioning, a condenser coil cannot be used as an evaporator coil. The reason for that is because a condenser coil is designed to handle a high-pressure refrigerant gas while an evaporator coil is designed to handle a low pressure refrigerant liquid.
Having said that, I must also add that it is indeed possible for a condenser to be used as an evaporator with a few tweaks of the HVAC system. That is manifested in how a heat pump works.
When used for cooling, a heat pump works exactly like an air conditioner. The refrigerant moves from the compressor to the condenser coil and then to the evaporator coil.
The process is however different when the heat pump is used to heat the house. You see, during winter, the heat pump system needs to extract heat from the outside, which is opposite of what happens during the summer.
For that to happen though, the direction of flow of the refrigerant has to change. Heat pumps therefore have a reversing valve.
The reversing valve ensures that the refrigerant flows from the compressor to the evaporator and then to the condenser coil.
As such, instead of the high-pressure superheated gas flowing to the condenser coil, it flows to the evaporator coil inside the house. The cold indoor air absorbs heat from the refrigerant and that is how the house is heated.
The refrigerant is then passed through expansion valves where its temperature is reduced way more than the outside temperature. Due to that temperature differential, the refrigerant can extract heat from the surrounding air (at the condenser coil) which is then used to heat the house.
In this case, you can see that the condenser coil is being used as an evaporator coil while the evaporator coil is being used as a condenser coil.
And basically those are the differences between an evaporator coil and and condenser coil. I hope you found this guide useful.