An air conditioning system operates as a heat pump, actively moving heat from the indoor air and transferring that thermal energy outside using a closed-loop refrigeration cycle. This process relies on the physical properties of a specialized chemical compound called a refrigerant, which cycles continuously to absorb, transport, and release the unwanted heat energy.
The Thermodynamic Secret: Latent Heat
The sheer volume of heat absorbed by an air conditioner is possible due to the principle of latent heat, which refers to the energy required to change a substance’s state without altering its temperature. This is distinct from sensible heat, which is the energy that causes a measurable change in temperature. For example, when you boil water, the temperature remains at 100°C while the water turns to steam, because the added energy is hidden as latent heat of vaporization.
The refrigerant is engineered to undergo a phase change from a low-temperature liquid to a gas inside the indoor unit, a process called evaporation. This evaporation requires a significant amount of latent heat energy, which the refrigerant immediately draws from the surrounding warmer indoor air. By absorbing a large quantity of energy to change its state rather than just increasing its temperature, the refrigerant can remove a substantial thermal load from the room.
The Four Core Components
The task of moving heat is managed by four interconnected components that form the closed refrigeration loop:
- The evaporator coil is located indoors, where the refrigerant absorbs heat from the air to transition from liquid to gas.
- The condenser coil is located outdoors and releases the collected heat, causing the gas to turn back into a liquid.
- The compressor acts as the system’s pump, forcing the refrigerant to circulate and raising its pressure and temperature for heat rejection.
- The expansion valve (or metering device) controls the flow of liquid refrigerant and causes a sudden drop in pressure.
Tracing the Refrigerant Cycle
The heat absorption process begins in the indoor unit when the low-pressure, low-temperature liquid refrigerant enters the evaporator coil after passing through the expansion valve. Warm air from the room is blown across the cold coil, causing the heat energy in the air to transfer into the refrigerant. This absorbed heat provides the latent energy needed to boil the refrigerant, causing it to fully evaporate into a low-pressure, low-temperature vapor.
The now heat-laden gas travels to the outdoor unit and enters the compressor. The compressor rapidly squeezes this gas, causing a dramatic increase in both its pressure and its temperature. This action is necessary because heat naturally flows only from a hotter substance to a cooler one; compressing the gas ensures its temperature is significantly higher than the outdoor air, enabling the next step.
The high-pressure, high-temperature gas then flows into the condenser coil, where it is exposed to the cooler outdoor air. As the outdoor fan blows air across the coil, the thermal energy transfers from the superheated refrigerant to the atmosphere. Losing its heat causes the refrigerant vapor to condense, changing its state back into a high-pressure, high-temperature liquid.
To repeat the cycle, the hot liquid must be prepared to absorb heat again by lowering its temperature and pressure. The high-pressure liquid travels to the expansion valve, which restricts the flow and creates a pressure drop. This pressure reduction causes the refrigerant to flash-evaporate partially and cool down significantly, returning it to a low-pressure, low-temperature state. The chilled liquid then returns to the evaporator coil, ready to absorb more heat.