MOISTURE AND AIR CONTAMINATION
In the past the main problem with moisture was ice formation in critical areas such as expansion valves, but it also caused corrosion and damage to motor windings. Lubricants play an important role in determining the effect of contaminants on a refrigeration system. The oil should remain as clean as it is when it entered the compressor (unlike that of the automobile engine which is quickly contaminated by fuel, water, carbon and atmospheric dust). The condition of the compressor oil is therefore a direct indication of the physical and chemical cleanliness of the system. Lubricating oil should be kept in tightly sealed containers to exclude atmospheric moisture. Oil drained from ammonia systems is not used again unless it can be properly filtered and kept dry. Overheating or an electrical fault in the winding of a hermetic or semi-hermetic compressor motor will produce contaminants, including the halogen acids. Eye goggles and rubber gloves should be worn when handling such suspect oil. If shown to be acid, the oil must be removed and carefully disposed of and the system thoroughly cleaned out.
Moisture reacts with POE lubricants and this gives rise to organic acids. These are much weaker than the halogen acids and this needs to be taken into account when the acidity level is measured using an acid test kit. Nevertheless any breakdown of the POE is undesirable and it is recommended that the moisture level be kept below 50 ppm in POE systems. Because POEs have a high affinity for moisture it is essential to keep exposure of the oil to atmospheric air to an absolute minimum. The moisture cannot be fully removed by vacuum procedures and hence appropriate filter-driers are always recommended. Figure 5.4 illustrates a typical expected moisture content variance during the process of installing and commssioning. A compressor which is pre-charged with POE oil is connected first to a factory assembled unit, such as a compressor pack, installed on site observing correct evacuation and sealing procedures, and finally with subsequent running the moisture content is reduced towards 50 ppm by the drier.
Compressor Packaged unit Site Site
Manufacturer manufacturer start final
Figure 5.4 Typical moisture ingress and subsequent removal by the action of the drier in a POE system
Proper evacuation will remove the air content to an acceptable minimum. Oxidation and other high-temperature chemical reactions are more likely to cause damage in the presence of moisture.
When systems are changed over from CFCs or HCFCs to HFC refrigerants, the oil also generally needs to be changed to POE. The original equipment manufacturers recommendation should be sought prior to attempting such a conversion.
Posted in Refrigeration and Air Conditioning