What is the primary cause of frost or ice buildup on the evaporator of a vapor-cycle cooling system?

The primary cause of frost or ice buildup on the evaporator of a vapor-cycle cooling system is inadequate airflow. When airflow across the evaporator is restricted, either due to a dirty air filter, blocked ducts, or a malfunctioning blower, the refrigerant in the evaporator coils does not effectively absorb heat from the cabin air. This reduced heat absorption leads to a drop in temperature of the evaporator coils, allowing moisture in the air to condense and subsequently freeze, resulting in frost or ice accumulation.

In contrast, excessive refrigerant could lead to other types of system inefficiencies but does not directly cause ice buildup on the evaporator. A faulty compressor could impact the system's ability to circulate refrigerant but is not the primary factor for frost formation specifically at the evaporator. High outside temperatures may affect the overall system efficiency, but they do not directly correlate with ice formation on the evaporator unless combined with inadequate airflow, which is the crucial factor for frost accumulation.