An article on Molecular Sieve filters. A guide to understanding filter-drier functions and typesMay 2, 2000
ReprintsOne CommentFilter-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 and will therefore adsorb polarized molecules, such as water, first and hold them tightly. The water molecules are physically separated from the lubricant, minimizing the potential for POE hydrolysis.Activated alumina is formed from aluminum oxide (Al2O3) and is not a highly crystalline material. Both alumina and silica gel show a wide range of pore sizes and neither exhibit any selectivity based on molecular size. 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.Alumina 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.Silica gel is a non-crystalline material with a molecular structure formed by bundles of polymerized silica (SiO2). Gel-type desiccants are indicative of the weaker bond formed between water and the desiccant. Silica gel is the old type of desiccant and is not widely used in today’s filter-driers.
Selecting a desiccant
There are many factors involved when selecting which desiccant material is best for an application. Water capacity, refrigerant and lubricant compatibility, acid capacity, and physical strength are important characteristics of desiccants and should be considered.The first of these, water capacity, is the amount of water the desiccant can hold while maintaining low moisture levels within the refrigeration system.A molecular sieve retains the highest amount of water, while keeping the concentration of water in the refrigerant low. This is due to the strong bond between the molecular sieve and the water.By keeping the water in the system at low levels, freeze-ups, corrosion, and acid formation is minimized. Activated alumina retains a fair amount of water, but the retention isn’t as great as the molecular sieve. This is indicative of co-adsorption of other material. Based on this information, Parker recommends the use of 100% molecular sieve in liquid line filter-driers for maximum water removal. Refrigerant and lubricant compatibility is also essential when selecting a desiccant. Inorganic acids (HCl and HF) form from the decomposition of the refrigerant reacting with an incompatible desiccant and water at elevated temperatures. 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. 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).These organic acids are a sludge-like material that can deposit and plug the system’s expansion device. Parker has tested each of the desiccants used based on their application, to ensure that the formation of these acids is minimized.Acid capacity for activated alumina and molecular sieve is shown in Table 1 (page 92). The varying pore sizes in the activated alumina allow it to be more effective than molecular sieve in removing the larger, organic acid molecules.Alumina is more effective in removing the various acids when it is used in the suction line of the system. When used in the liquid line of a system, there is a potential for the hydrolysis reaction between the POE lubricant and water to occur, actually forming organic acids. This reaction did not occur when the alumina was tested in the suction line. Therefore, for acid cleanup in a system, some manufacturers recommend the use of a suction line filter-drier containing an activated alumina core.Physical strength of the desiccant is another factor to be considered. Desiccants should be strong enough mechanically to resist breaking up when subjected to system vibrations and surges (attrition). Attrition occurs when the desiccant beads rub against one another when it is shaken or vibrated, yielding fine particles. Therefore, the method of retaining the desiccant in the filter-drier (based on drier size and location) plays a major role on the integrity of the desiccant.
Providing filtration
Filtration is the other main function of a filter-drier and is accomplished by different methods. Some driers use only one method; others may use a combination of methods.There are two primary means of mechanical filtration: surface and depth.The simplest form of surface filtration is the screen. The screen is usually a woven wire mesh that catches particles that are larger than the holes in the screen. Until the screen has captured enough particles to provide a layer across the entire surface, particles that are smaller than the holes will pass through the screen. In addition, a particle longer than a hole can pass through if its cross-section is smaller than the hole.As layers of contaminant cover the screen, it will become a depth filter as the layer of contaminant will act as a filter to remove smaller particles that would ordinarily pass through the screen. This layering of contaminant will continue until the pressure drop across the screen reaches the point at which the refrigerant flashes into vapor.Depth filtration takes different forms. The most common depth filters are:
- Bonded desiccant cores
- Rigid fiber glass filters bonded with phenolic resin; and
- Fiberglass pad filters.
Depth filters force the fluid and contaminant to take an indirect route through the filter. Contaminants are trapped in the maze of openings that are spread throughout the filter. Depending on the type of filter, the openings will vary significantly.Bonded desiccant cores have smaller rigid openings than do fiber glass pads. As the flow passes through the media, particles are trapped in the channels, depending upon their size. As the channels fill with particles, the pressure drop will increase until vaporizing occurs as described above.Fiber glass pad filters are not compressed as tightly as bonded, rigid fiber glass filters. The liquid refrigerant with the entrained contaminant flows through the pads. The contaminant will impact the glass fibers and lose some velocity.As the contaminant passes through the media, the velocity will eventually drop to zero, at which point the contaminant will deposit in an opening in the fiberglass. The larger particles will tend to drop out first as their higher mass will tend to cause them to impact on a fiber even though the flow stream will bend around a fiber.As the fiber glass fills with more and more particles, the filtration becomes finer as the filter becomes closer in function to the rigid filter.The core drier picks up particles and the pressure drop increases quickly as the core plugs with contaminant. For the same pressure drop and flow rate, the fiber pad drier can hold up to five times the amount of contaminant as the core drier with equivalent or greater filtration capacity.The core can be used effectively in the suction line drier. In this case, the higher velocity in the suction line will cause the loose fiber glass structure to disintegrate. The rigid cores can be tailored to remove the solid particles that result from compressor breakdown, sludge, and resins.The desiccant bonded in the core will remove water and neutralize acids caused by breakdown of the lubricant. The bonding of the desiccant will preclude the attrition that can occur with loose desiccant beads.
