The cold air pumped out by an air conditioning (AC) unit can produce a refreshing oasis on hot summer days. Air conditioners are found in homes, cars, buses, subways, skyscrapers, and shopping centers. They can be small portable units or huge industrial systems. This device provides comfort to millions of people when the temperature rises. But how does an AC unit take that hot air and cool it down?
The Parts of an Air Conditioner
Air conditioners are devices that use a refrigeration cycle to reduce the temperature of input air. The specific thermodynamic process used in air conditioners is called the vapor-compression refrigeration cycle, and the mechanism consists of four parts:
- Evaporator
- Compressor
- Condenser
- Expansion Valve
Refrigerant flows through the circuit created by these parts, and acts as the working fluid. A refrigerant is a compound that has a boiling point below the target temperature, has a high heat of vaporization, and does not cause corrosion, among other properties. Prior to the 1980's, refrigerants were generally made of chlorofluorocarbons (CFC) and other compounds that were found to harm the Earth's ozone layer. Since then, newer and safer refrigerants have replaced these toxic materials.
The Air Conditioning Cycle
When the liquid refrigerant enters the evaporator, the lower pressure causes the refrigerant to evaporate into a vapor. The evaporator is exposed to the air in the zone to be cooled (the interior of the home in the case of a wall-mounted home AC unit), and the evaporation process causes the refrigerant to pull in heat from the air during its phase change.
The vaporized refrigerant passes through a compressor, which increases the pressure of the refrigerant. The refrigerant then enters the condenser, where it returns to liquid form. When it returns to liquid, it releases the heat that it absorbed within the evaporator.
The condenser is exposed to a different zone than that being cooled (the exterior of the home in the case of a wall-mounted home AC unit), allowing the interior zone to retain its lowered temperature. The liquid refrigerant then passes through an expansion valve, where the pressure is reduced, and the process repeats itself.
Fans are used to move cold air out of the AC unit into the air conditioned space and to exhaust the heat from the AC unit. The exhaust is released outside of the cooled space. For this reason, a flow of hot air can he felt when walking past the outside of a window-mounted air conditioner.
The air conditioner takes thermodynamic principles and applies them to a material with just the right properties to make millions of people more comfortable during the dog days of summer.
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