How Air Conditioning Units Work

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The initial concept behind air conditioning first came about in America, in 1902, from an engineer named Willis Haviland Carrier. He was trying to solve a humidity problem in the office where he worked, by blowing the inside air across chilled pipes. Carrier discovered that as well as reducing the humidity, this process lowered the inside air temperature.

The general concept of how air conditioning works today remains the same as that initial discovery, but the technology behind the process has come on in leaps and bounds. Air conditioning was once considered a luxury for the wealthy, but these days most offices have it, and in America, nearly 90% of all homes have some form of air conditioning to cool their dwellings in the sweltering summer. In very hot and humid climates, especially in working environments, it is considered a necessity rather than a luxury.

How does air conditioning work exactly? In many ways, it is not too dissimilar to the functioning and operation of the common refrigerator. Whilst a refrigerator cools down the inside of a small space for storing food, air conditioning works on a bigger scale, keeping a room or entire building cool using the walls. An air conditioning unit uses a chemical called a refrigerant to remove warm air from a room to the outside, and changing it to cooler air released back into the room. The chemicals are converted from a gas to a liquid and back again on a loop, in a process called phase conversion.

The mechanics and parts of an air conditioner that combine to make it work are quite complex, and the process depends on a number of conditions for optimum performance.

Most air conditioning units operate on a split system, which means that they have a hot side (the condensing unit) and a cold side. The hot side is located outside the home and the cold side is inside the home. An air conditioner consists of a compressor, condenser and an evaporator. The compressor and condenser (the condensing unit) are normally found on the part of the air conditioning unit that is outside, and the evaporator is on the inside. Essentially, the evaporator is the part that receives the liquid refrigerant, the condenser facilities the heat transfer and the compressor is a pump that pressurises the refrigerant. An air conditioning unit also has an expansion device that regulates the flow of the refrigerant chemical into the evaporator.

When warm inside air moves across the evaporator, the refrigerant inside the coil of the evaporator picks up the temperature of the air. It absorbs the heat from the air and turns from a liquid into a gas. The gas refrigerant then passes into the compressor – this is usually located on an outside wall of a home or on the roof of a business. The compressor compresses the gas to a state of higher pressure and temperature. This hot, pressurised gas flows over the condenser. The gas is then condensed back into a liquid as heat is taken away. Most outdoor units also have metal fins on them that help to eliminate the heat more effectively. Once the liquid has been cooled down it is then returned back to the interior of the house or building. The expansion device regulates the flow of liquid refrigerant into the evaporator. This process continues over and over, until the room or building reaches the required temperature according to the set thermostat.


As well as reducing the air temperature, an air conditioning unit serves to reduce the humidity in a room. It does this by absorbing the heat and thus condensing the moisture out of the air as it moves across the evaporator coil. The air is thus drier and cooler.


Air conditioning units also contain filters. The filters help to maintain the efficiency of the air conditioning system, by removing particles that might obstruct the airflow. These particles could include dust or pet fur, for example. When the filters get clogged up, normal airflow is obstructed and dirty particles are then carried into the evaporator coil. Over time, this can affect the function of the coil’s heat-absorbing capacity, and it will need replacing. It is, however, cheaper and easier to change the filters regularly to avoid affecting the functioning of the evaporator coil and the efficiency of the unit.

by Louise Burke