When a system pipework is opened to atmosphere during a replacement operation, it is possible that air will enter the system. Air contains moisture, and only a small amount of moisture in a system which has a capillary for the refrigerant control can result in that moisture freezing. This leads to a complete loss of refrigeration.
Filter driers are normally capable of dealing with small quantities of moisture. However, it is recommended that a drier be changed when leaks are detected, especially on the low side.
A complete loss of the refrigerant charge owing to leakage can result in a compressor operating on vacuum, drawing in air. In this case the drier could become saturated and moisture will circulate through it. When this occurs, freezing can take place at the expansion valve; this is tantamount to a complete blockage in the liquid line.
When the symptoms of plant operation indicate a complete blockage, and no temperature difference is obvious at the filter drier, it is natural to suspect that the expansion valve or perhaps a solenoid valve is at fault. The simple expedient of applying a cloth dipped in hot water will determine the presence of moisture. Warming up the expansion valve will melt the ice in the valve and the flow of refrigerant will resume, but only until such time as the temperature at the expansion valve is low enough to form ice and restrict the liquid flow once more. A blowtorch should never be used for this purpose, for obvious reasons.
In most cases, fitting a new filter drier will overcome the problem. However, it is stressed that this drier should be removed after a suitable running period and another new drier fitted. Should the condition persist after a drier has been replaced, the system must be discharged of refrigerant, evacuated and then recharged.