Pencil Driers
Pencil driers are found in refrigeration units such as domestic fridges, freezers, beverage coolers as well as air conditioning systems.
Their function is to adsorb system contaminants, such as water, which can create acids, and two, to provide physical filtration by preventing debris from entering the metering device. Debris such as copper shavings and carbonised flakes caused from brazing.
The filter drier should be replaced each time the refrigeration system pipes are opened at any point.
How they work
Drier filters contain molecular sieves which are composed of 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 aluminium 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.
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.