In what condition would a cooling coil frost during operation?

Frost formation on a cooling coil typically occurs when there is insufficient airflow over the coil, which can be caused by a dirty prefilter. When the filter is clogged with dust and debris, it restricts the airflow, leading to a decrease in the temperature of the air coming into contact with the coil. As a result, the coil can drop below the freezing point of water, causing moisture in the air to condense and freeze on the coil's surface. This frost buildup impairs the efficiency of the cooling system and can lead to further operational issues.

In contrast, high airflow would likely prevent frost formation due to increased heat exchange, while low refrigerant charge may lead to different issues such as insufficient cooling rather than frost. Finally, high ambient temperatures generally do not promote frost formation; instead, they can lead to increased operating temperatures in the system. Thus, a dirty prefilter is a significant factor in causing cooling coils to frost during operation.