Moisture (water) is an important and universal contaminant in refrigeration systems. The amount of moisture in a refrigerant system must be kept below an allowable maximum for satisfactory operation, efficiency, and longevity. Moisture must be removed from components during manufacture, assembly, and service to minimize the amount of moisture in the completed system. Any moisture that enters during installation or servicing should be removed promptly. Refrigerant filter-driers are the principal devices used to remove contaminants from refrigeration systems. The filter-drier is not a substitute for good workmanship or design, but a maintenance tool necessary for continued and proper system performance. Contaminants removed by filter-driers include moisture, acids, hydrocarbons with a high molecular weight, oil decomposition products, and insoluble material, such as metallic particles and copper oxide. Moisture Other Contaminants System Cleanup Procedure After Hermetic Motor Burnout Contaminant Control During Retrofit Chiller Decontamination System Sampling