Hitek Filter Driers A guide to understanding filter-drier functions and types Filter-driers are a key component in any refrigeration or air conditioning system. This article offers acr technicians a description of the basic function of these devices and differences between the various types currently available. A filter-drier in a refrigeration or air conditioning system has two essential functions: one, to adsorb system contaminants, such as water, which can create acids, and two, to provide physical filtration. Evaluation of each factor is necessary to ensure proper and economical drier design. Absorbing moisture, preventing acids The ability to remove water from a refrigeration system is the most important function of a drier. Water can come from many sources, such as trapped air from improper evacuation, system leaks, and motor windings, to name a few. Another source is due to improper handling of polyolester (POE) lubricants, which are hygroscopic; that is, they readily absorb moisture. POEs can pick up more moisture from their surroundings and hold it much tighter than the previously used mineral oils. This water can cause freeze-ups and corrosion of metallic components. Water in the system can also cause a reaction with POEs called hydrolysis, forming organic acids. To prevent the formation of these acids, the water within the system must be minimized. This is accomplished by the use of desiccants within the filter-drier. The three most commonly used desiccants are molecular sieve, activated alumina, and silica gel. Molecular sieves are crystalline sodium alumina-silicates (synthetic zeolites) having cubic crystals, which selectively adsorb molecules based on molecular size and polarity. The crystal structure is honeycombed with regularly spaced cavities or pores. Each of these cavities or pores are uniform in size. This uniformity eliminates the co-adsorption of molecules varying in size. 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. The surface of this desiccant is charged positively with cations, which act as a magnet
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