What usually causes partial or complete clogging in capillary tubes?

The correct answer is poor preventive maintenance resulting in excessive head pressure. In refrigeration systems, capillary tubes play a critical role in regulating the flow of refrigerant. When preventive maintenance is neglected, systems can experience excessive head pressure due to factors like refrigerant leaks, overcharging, or insufficient airflow. This excessive pressure can lead to a buildup of contaminants or debris within the capillary tube, resulting in partial or complete clogging.

Effective preventive maintenance practices help to ensure that the system operates within the appropriate parameters, addressing potential issues before they lead to more significant problems, such as clogging in the capillary tubes. Thus, maintaining proper conditions and regularly checking system components can prevent excessive pressure buildup, which is a primary contributor to clogs.