![]() Organic acids can form from the breakdown of the lubricant in the presence of an incompatible desiccant and water (elevated temperatures will increase this reaction). ![]() Inorganic acids formed will attack the crystalline structure of the molecular sieve and break it down as well as attack metal surfaces in the system. Inorganic acids (HCl and HF) form from the decomposition of the refrigerant reacting with an incompatible desiccant and water at elevated temperatures. Refrigerant and lubricant compatibility is also essential when selecting a desiccant. Based on this information, Parker recommends the use of 100% molecular sieve in liquid line filter-driers for maximum water removal. This is indicative of co-adsorption of other material. Activated alumina retains a fair amount of water, but the retention isn’t as great as the molecular sieve. This is due to the strong bond between the molecular sieve and the water.īy keeping the water in the system at low levels, freeze-ups, corrosion, and acid formation is minimized. The first of these, water capacity, is the amount of water the desiccant can hold while maintaining low moisture levels within the refrigeration system.Ī molecular sieve retains the highest amount of water, while keeping the concentration of water in the refrigerant low. Water capacity, refrigerant and lubricant compatibility, acid capacity, and physical strength are important characteristics of desiccants and should be considered. Selecting a desiccant There are many factors involved when selecting which desiccant material is best for an application. Silica gel is the old type of desiccant and is not widely used in today’s filter-driers. Gel-type desiccants are indicative of the weaker bond formed between water and the desiccant. Silica gel is a non-crystalline material with a molecular structure formed by bundles of polymerized silica (SiO 2). Due to the varying pore sizes, they can co-adsorb the much larger refrigerant, lubricant, and organic acid molecules, eliminating the surface area available to adsorb water.Īlumina can also aid in the hydrolysis of the POE lubricants creating organic acids since both water and lubricant are adsorbed into the pore openings of the alumina. Both alumina and silica gel show a wide range of pore sizes and neither exhibit any selectivity based on molecular size. The water molecules are physically separated from the lubricant, minimizing the potential for POE hydrolysis.Īctivated alumina is formed from aluminum oxide (Al 2O 3) and is not a highly crystalline material. The surface of this desiccant is charged positively with cations, which act as a magnet and will therefore adsorb polarized molecules, such as water, first and hold them tightly. This permits molecules, such as water, to be adsorbed, while allowing other larger molecules, such as the refrigerant, lubricant, and organic acids, to pass by. This uniformity eliminates the co-adsorption of molecules varying in size. The crystal structure is honeycombed with regularly spaced cavities or pores.Įach of these cavities or pores are uniform in size. Molecular sieves are crystalline sodium alumina-silicates (synthetic zeolites) having cubic crystals, which selectively adsorb molecules based on molecular size and polarity. The three most commonly used desiccants are molecular sieve, activated alumina, and silica gel. This is accomplished by the use of desiccants within the filter-drier. To prevent the formation of these acids, the water within the system must be minimized.
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