How an Air Conditioner Works

We have seen the history of air conditioning and the latest trends in what modern air conditioners offer. Yet, some may not be sure how air is heated or cooled. Let's walk you through quickly how an air conditioner works.

The underlying principle of evaporative cooling is the fact that water must have heat, known as the “heat of vaporization”, applied to it to change from a liquid to a vapor. For example, touching rubbing alcohol irritates your finger because it takes heat away from the skin as the alcohol evaporates. This represents the principle of evaporative cooling. Conversely, when a gas liquefies, it releases a lot of heat, which is how evaporative heating takes place.
Air conditioners work using these principles. A liquid called a “refrigerant” circulates through a closed piping system, repeatedly liquefying and evaporating to cool air. Reversing the refrigerant flow (reverse cycle) heats air.

Substances change state, i.e., solid, liquid, gas, usually when they are heated or cooled. That is called “changing states of matter”. Air conditioners leverage the changes in states of refrigerant to cool or heat room air. The refrigeration cycle continuously changes the states of a refrigerant circulating in a closed piping system.

1. Compressing
When the cooling process starts, the refrigerant at low temperature and pressure is compressed by the compressor. The temperature of the gas increases, whereby making the refrigerant gaseous at high temperature and pressure.

2. Condensing
The gaseous refrigerant is transferred to the condenser (heat changer), where heat is exchanged between the refrigerant and outdoor air. This releases the heat of the refrigerant into the outdoor air. Releasing the heat, the refrigerant changes its state to a medium-temperature and high-pressure liquid.

3. Expansion
The liquefied refrigerant moves to the expansion valve that decreases the pressure of the refrigerant. Depressurized refrigerant expands, dropping the temperature and changing its state into a liquid of low temperature and pressure.

4. Vaporization
The refrigerant turned into a low-temperature and low-pressure liquid at the expansion valve is transferred to the evaporator (heat exchanger), where heat is exchanged between the refrigerant and indoor air. The heat of the indoor air moves to the refrigerant and the air is cooled. The refrigerant picks up the heat from the air, changing its state into a low-temperature and low-pressure gas, and goes back to the compressor again.

A heat pump air conditioner uses a four-way valve that reverses refrigerant flow to offer both cooling and heating for air-conditioned comfort all year round. This function is referred to as a “reverse cycle”. The term “reverse cycle air conditioner”, another name for a heat pump air conditioner, comes from the mechanism of the cycle.

An air conditioner compressor is a component in the system that raises the pressure of the vapor refrigerant to cause changes in the refrigerant temperatures. Most home use air conditioners have compressors inside their outdoor units.

Heat flows from hot to cold. A heat pump reverses that flow by extracting heat from one place and transfers it to another. Depending on whether the cooling or heating mode is selected, it pumps heat in and out using the heat cycle of compression, condensation, expansion, and evaporation.

A power inverter, or simply, inverter, is an electronic device or circuitry that changes direct current (DC) to alternating current (AC). Most domestic and commercial buildings are usually powered by AC. Standard voltages and frequencies vary by country. Changing frequency or voltage requires an inverter that converts AC to DC and, after needed changes in frequency and voltage, converts DC back to AC. Air conditioners with inverters run at controllable motor speeds—an efficient way for energy saving.

A water-cooled chiller system has a chiller that uses water as the refrigerant and can be found in many commercial and industrial applications. The water picks up heat inside the building, returns to the chiller, where it is cooled, then goes back to circulating through the building.

A cooling tower, often located on rooftops, is a heat rejection device that helps refrigerated air conditioning work effectively. Heat exchange taking place in the refrigeration unit increases the temperature of the cooling water. The cooling tower retains and exposes the water to air fanned into the tower to allow evaporation, which lowers the water temperature. Then, the water is passed back to the refrigerant unit and reused.

Free cooling is an economical method of using low external air temperatures in winter to assist in chilling water. The chilled water can be stored until summer and used in the cooling tower to release heat. When the ambient air temperature drops to a set temperature, a modulating valve allows the chilled water to by-pass an existing chiller and run through the free cooling system, which requires much less power to cool the water in the system.
In Europe, free cooling may refer to an outside air cooling system that uses external air inflows to cool air.