Vapour Compression Cycle

The vapour compression cycle is a cycle used by conventional refrigerators and air conditioning systems to move heat from one place to another using a refrigerant. Please note that refrigeration systems are heat pumps, they do not “produce cold air”, they cool by removing heat. Cold is simply the absence of heat.

How Air Conditioners and Refrigerators Work

Vapour compression air conditioners and refrigerators operate by using a refrigerant to absorb heat and then radiate it outside. You can either look at the diagram first, or start here. If you have any suggestions, please tell me.

Step 1: The compressor continually forces more and more of the refrigerant in gas form into the condenser, and as it does that, the particles of the gas are forced to move closer and closer to each other until it condenses into a liquid. During the compression process, heat is radiated from the refrigerant by the condenser coil and the fins attached to it which make it easier for the air to absorb heat from the refrigerant, also helping to condense the refrigerant because cooler gases condense more easily.

The condenser fan accelerates the cooling process by blowing the hot air between the condenser fins away and passing cool air between them, because cooler air is capable of absorbing more heat than hot air. As the particles of the refrigerant gas move closer and closer to each other, that causes the refrigerant to release heat.

Step 2: After being condensed in the condenser, the compressor still forces refrigerant into it continuously and that forces it out of the condenser through what is called the capillary tube (not all designs are like this). The capillary tube is a very narrow tube which leads into the evaporator and restricts the flow of refrigerant. As refrigerant enters the evaporator, which is under low pressure (refrigerant evaporates faster in low pressure areas) due to the fact that the suction side of the compressor is attached to the end of it, the refrigerant evaporates in it, and that evaporation process causes the refrigerant to absorb heat.

The cold refrigerant gas then absorbs heat from the evaporator coils (because the gas is cold) and it enters the compressor afterwards, still cold, and absorbs heat from the compressor, cooling it and extending its life. The evaporator or blower fan draws air from the room through the cold evaporator fins, cooling it, and then blows it back into the room. Then step 1 is repeated.

If it helps, think of the evaporator as the heat absorber (and cooler) and the condenser as the heat radiator.

A diagram of the vapour compression refrigeration cycle.

Source

Reverse Cycle Air Conditioning

The cycle mentioned above can be reversed so that heat is transferred from the air outside into the evaporator inside the house. This takes advantage of an air conditioner’s ability to be a very efficient heater.

When an air conditioner is operated in reverse, the evaporator inside acts as the condenser, and the condenser outside acts as the evaporator, so the condenser (now evaporator) outside absorbs heat and transfers it to the evaporator (now condenser) inside the building which then radiates that heat into the rooms with the assistance of fans.

So in normal mode the evaporator absorbs heat and transfers it to the condenser using refrigerant, and in reverse cycle mode, the exact opposite happens. The evaporator and condenser are conceptually the same technology, but they can do each others’ job.

Refrigerant

Refrigeration systems use a variety of refrigerants depending on their application. The two most common refrigerants are R-22 and R-410A for residential air conditioners and R-134a for automobile systems. Please study thoroughly before trying to recharge a system or replace any parts. There are some common mistakes which can cause malfunction such as a failure to vacuum air out of the refrigerant loop.

Components

Vapour compression refrigeration systems usually consist of at least the following parts:

Blower Fan/Evaporator Fan: This fan forces air through the evaporator.

Condenser Fan: This fan passes air between the condenser fins.

Evaporator: An evaporator is usually a copper coil which has aluminium fins attached to it. This is where the refrigerant evaporates and cools the evaporator.

Air Filter: This traps unwanted particles before they enter the evaporator and are blown back into the room.

Condenser: This is usually a copper coil which has aluminium fins attached to it. This is where the refrigerant is cooled and condensed into a liquid.

Compressor: This is usually a black cylindrical device (in the case of building air conditioners) which both forces refrigerant into the condenser and draws it out of the evaporator.

Copper or aluminium tubing: This is used to join the condenser, evaporator, and compressor.

Recharging/Filling

In some areas, you have to have a permit to service air conditioning systems. Please google recharging for more information.

Vacuuming/Refrigerant Recovery

You should recover refrigerant before you try to replace the compressor, drier, condenser, valves, or evaporator not only to ensure that you are in compliance with the law but also because common refrigerants cause ozone layer damage. If you are concerned about the cost of the device required to recover refrigerant in such a way that it does not escape into the atmosphere, then please remember that you need that same device to remove air from the coils before recharging because air conditioners cannot work properly unless you do that.

The device is called a vacuum pump. Please do some research before purchasing one. You can easily damage your air conditioning system if you don’t know what you are doing. Recharging is not as simple as topping it up with gas.

Additional Information and Sources

GSU: Heat Pumps