What is the function of a sight glass in a refrigeration system?

The function of a sight glass in a refrigeration system is to provide a visual indication of the refrigerant's condition, specifically whether it is in a liquid state and free of bubbles. It is typically installed in the liquid line, before the expansion valve. A clear sight glass indicates that the refrigerant is in a subcooled liquid state, which is desirable for efficient system operation. The absence of bubbles indicates that the refrigerant is fully condensed and that there is sufficient liquid refrigerant to supply the evaporator. The presence of bubbles in the sight glass can indicate several problems, such as a low refrigerant charge, a restriction in the liquid line, or insufficient subcooling. Bubbles indicate that the refrigerant is flashing (vaporizing) before it reaches the expansion valve, which reduces the system's cooling capacity. Some sight glasses also include a moisture indicator, which changes color to indicate the presence of moisture in the refrigerant. Moisture in the system can lead to the formation of acids and sludge, which can damage the compressor and other components. For example, if the sight glass shows bubbles and the moisture indicator is green, it indicates a low refrigerant charge and a dry system. If the sight glass is clear but the moisture indicator is yellow, it indicates a wet system that needs to be evacuated and recharged. The sight glass is a valuable tool for diagnosing problems and assessing the overall health of the refrigeration system.