What is the primary purpose of an expansion device in a refrigeration system?

The primary purpose of an expansion device in a refrigeration system is to reduce refrigerant pressure. As refrigerant flows from the high-pressure side of the system to the low-pressure side, the expansion device plays a critical role in adjusting the refrigerant to a lower pressure before it enters the evaporator. This reduction in pressure allows the refrigerant to expand and absorb heat more effectively from the environment it is cooling.

When the refrigerant experiences this drop in pressure, it also cools significantly, which is essential for the heat exchange process that occurs in the evaporator. The low-pressure, low-temperature refrigerant then absorbs heat from the surrounding area, allowing the refrigeration cycle to continue efficiently. This process not only helps in cooling but also facilitates phase change, as the refrigerant transitions from a liquid to a vapor state in the evaporator.

In the context of the choices given, increasing refrigerant pressure or maintaining oil levels does not represent the function of an expansion device, nor does recycling refrigerant. Instead, the reduction of refrigerant pressure is fundamental for the effective operation of the refrigeration cycle.