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4 Moisture Moisture causes several operating problems in RAC systems and understanding the basis of these problems is important. Basically, moisture can be classified as visible and invisible. “Visible” moisture is high concentration of water and can be seen with the eye, and is in liquid form. Occasionally, liquid water is found in systems but this is rather unusual. “Invisible” moisture is water in low concentrations and cannot be seen with the eye, since it is essentially dissolved in the refrigerant. It is important to remember that moisture can easily get into a system but it is difficult to get out. The main situations and activities that can result in moisture entering the system are: • when there is a leak in part of the system that experiences sub- atmospheric pressure (thus, drawing air into the system) • during servicing and repair when the system is opened and exposed to the air and evacuation is not carried out properly • when filters or lubricant are exchanged • during charging with refrigerant and transfer hoses have not been purged properly. The technician should be particularly aware of the possible introduction of moisture when carrying out these activities. There will always be some moisture within any refrigerating system, and complete elimination is almost impossible. However, at very low concentrations the moisture is unlikely to cause any significant problem. On the other hand, if the moisture is present in higher concentrations, then a series of problems can arise. The “tolerable” concentration of moisture differs between refrigerants, oil types, operating temperatures and compressor designs. Nevertheless, in almost all systems – except for ammonia systems – the maximum concentration of moisture should not exceed around 10 ppm. Above these concentrations, the moisture can have negative effects on the system, such as reactions with the oil, causing the unit to malfunction and accelerating burn-out of hermetic compressors. Recognising the presence of moisture The presence of moisture within the system can be recognised by certain observations: • The system will stop due to low suction pressure, and proceed to warm up. Since it is the formation of solid ice within the expansion device that has caused the blockage, the warming will result in the disappearance of the ice and thus the unit will work properly again. However, the process will occur again as the ice once again forms at the expansion device. • Decreasing pressure, where the suction pressure steadily decreases over several hours – even to a vacuum. Then pressure suddenly becomes normal again. This abnormal cycle will keep repeating. • If, during system shutdown, one warms the refrigerant control with a safe resistance heater (hot pad) or radiant heat bulb, the ice will melt. Should the system then begin to work properly, there is definitely moisture in the refrigerant. These symptoms are explained by the following: • The solubility of water in refrigerant reduces with temperature, so as the refrigerant passes through the expansion device, the dissolved moisture may become saturated water. 87 Servicing PracticesPDF Image | Manual for Refrigeration Servicing Technicians
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