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2006 ASHRAE Handook Refrigeration Chapter 6 -
Control of Moisture and Other Contaminants in Refrigerant Systems

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. Moisture must be removed from components during manufacture and assembly to minimize the amount of moisture in the completed assembly. Any moisture that enters during installation or servicing should be removed promptly.

Sources of Moisture
Moisture in a refrigerant system results from
. Inadequate equipment drying in factories and service operations
. Introduction during installation or service operation in the field
. Low-side leaks, resulting in entrance of moisture-laden air
. Leakage of water-cooled heat exchangers
. Oxidation of some hydrocarbon lubricants that produce moisture
. Wet lubricant, refrigerant, or desiccant
. Moisture entering a non-hermetic refrigerant system through hoses and seals

Proper installation and service Procedures as given in ASHRAE Standard 147 minimize the second, third, and forth sources. Lubricants will be discussed in the future. If purchased refrigerants and lubricants meet specifications and are properly handled, the moisture generally remains satisfactory.

Effects of Moisture
Excess moisture in a refrigerating system can cause one or all of the following undesirable effects:
. Ice formation in expansion valves, capillary tubes, or evaporators
. Corrosion of metals
. Copper plating
. Chemical damage to motor insulation in hermetic compressors or other system materials
. Hydrolysis of lubricants and other materials
. Sludge information

Ice or solid hydrate separates from refrigerants if the water concentration is high enough and the temperature low enough. Solid hydrate, a complex molecule of refrigerant and water, can form at temperature higher than those required to separate ice. Liquid water forms at temperatures above those required to separate ice and solid hydrate. Ice forms during refrigerant evaporation when the relative saturation of vapor reaches 100% at temperatures of 32 F or below.

The separation of water as ice or liquid also is related to the solubility of water in a refrigerant.
The solubility varies for different refrigerants and with temperature (Table 1). Various investigators
have obtained different results on water solubility in R-134a and R-123. The data presented here are the best available. The greater the solubility of water in a refrigerant, the less the possibility that ice or liquid water will separator in a refrigerating system. The solubility of water in ammonia, carbon dioxide, and sulfur dioxide is so high that ice or liquid water separation does not occur. The concentration of water by mass at equilibrium is greater in the gas phase than in the liquid phase of R-12 (Elsey and Flowers 1949).


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