How does an expansion valve function in an A/C unit?

The functionality of an expansion valve in an A/C unit is pivotal as it plays a key role in the refrigeration cycle. Its primary purpose is to reduce the pressure of the refrigerant before it enters the evaporator coil. When the refrigerant passes through the expansion valve, it experiences a drop in pressure, which causes it to undergo a phase change and evaporate. This evaporation process absorbs heat from the air passing over the evaporator coil, thereby cooling that air before it circulates into the space being conditioned.

This cooling effect occurs because the refrigerant, after losing pressure and temperature, becomes a low-pressure, low-temperature gas that is inefficient at holding heat. Upon entering the evaporator coil, it effectively absorbs heat from the environment, making it crucial for the air conditioning process to lower temperatures in the conditioned area.

The other options do not accurately describe the role of the expansion valve. Compressing the refrigerant and condensing it, as suggested in other answers, are functions performed by different components in the A/C system, specifically the compressor and condenser, respectively. Similarly, filtering the air is not a function associated with the expansion valve; that task is handled by air filters in the system.