It is an established fact that an HVAC system is made up of so many different kinds of parts and components. Some of these components naturally stand out just because their appearance is striking, and they also have unique purposes. But on the flip side, there are also components that aren’t as striking just because their primary functions are relatively simple — though they are still undeniably important. A perfect example of this kind of HVAC component is the filter drier.
What Is a Filter Drier?
A filter drier is a device used in an HVACR system that is a combination of a filter and a drier. In other words, it has two essential functions: the first is to absorb system contaminants, such as water which can create acids, and the second is to provide physical filtration. Evaluation of each factor is important so as to ensure proper and economical filter drier design.
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Function #1: Absorbing Moisture, Preventing Acids
As indicated by the word “drier,” one of the filter drier’s important functions is the ability to remove moisture from the refrigerant. For this reason, the drier is also oftentimes referred to as dehydrator.
Moisture can come from many sources — a few common examples include trapped air from improper evacuation, system leaks, and motor windings. Another source of unwanted moisture is due to improper handling of polyester (POE) lubricants, which are hygroscopic. This means that they readily absorb moisture. In fact, POEs can pick up more moisture from their surroundings and hold it much tighter than the previously used mineral oils. As a result, this kind of moisture can cause freeze-ups and corrosion of metallic components. Aside from that, water in the system can also cause a reaction with POEs called hydrolysis, thus 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. Essentially speaking, a desiccant is a material that absorbs moisture. In the case of filter driers, there are three most commonly used desiccants, and these are molecular sieve, activated alumina, and silica gel.
Desiccant #1: Molecular Sieve
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 allows for the elimination of the co-adsorption of molecules varying in size. In other words, the uniformity 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 a molecular sieve 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, thus minimizing the potential for POE hydrolysis.
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Desiccant #2: Activated Alumina
Activated alumina is formed from aluminum oxide (Al2O3) and is not a highly crystalline material. Both activated alumina and silica gel show a wide range of pore sizes and neither exhibit any selectivity based on molecular size. Because of the varying pore sizes, they can co-adsorb the much larger refrigerant, lubricant, and organic acid molecules, thus eliminating the surface area available to absorb water.
Furthermore, activated 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.
Desiccant #3: Silica Gel
An old type of desiccant, 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. As a result, silica gel is not widely used in today’s filter driers anymore.
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How to Select a Desiccant for the Filter Drier?
There are many factors involved when selecting which desiccant material is the right fit for a particular filter drier application. Water capacity, refrigerant and lubricant compatibility, acid capacity, and physical strength are all important characteristics of desiccants, and so, they should be thoroughly considered.
The first of these characteristics — water capacity — refers to the amount of water the desiccant can hold while maintaining low moisture levels within the refrigeration system. Out of the three desiccants, the molecular sieve retains the highest amount of water, while keeping the concentration of water in the refrigerant low. This is primarily 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 are minimized. Activated alumina retains a relatively fair amount of water, but the retention isn’t as great as the molecular sieve. This is indicative of the co-adsorption of other materials. For this reason, the molecular sieve is the highly recommended desiccant to be used in liquid-line filter driers for maximum water removal.
Refrigerant and lubricant compatibility is also crucial when selecting a desiccant. Inorganic acids, such as 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.
Meanwhile, 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.
As for acid capacity, the varying pore sizes in the activated alumina allow it to be more effective than molecular sieve in removing the larger, organic acid molecules. In particular, activated 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. As a result, for acid cleanup in a system, some manufacturers recommend the use of a suction line filter drier containing an activated alumina core.
Last but not least, the 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 happens when the desiccant beads rub against one another when it is shaken or vibrated, thus yielding fine particles. Therefore, the method of retaining the desiccant in the filter drier (based on drier size and location) plays a major role in the integrity of the desiccant.
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Function #2: Providing Filtration
Aside from the absorption of moisture and the prevention of acid formation, filtration is the other main function of a filter drier. Simply put, filtration is the process of removing any particle, such as dirt, metal, or chips, from entering the refrigerant flow control. For this reason, a filter is also oftentimes known as a strainer. It is crucial that these particles are filtered out and prevented from going into the metering device of a system. This is because particles can cause blockage to the passage flow of the refrigerant in the expansion valve, which can eventually cause an improper operation to the system.
Filtration in filter driers can be accomplished by different methods. Some driers use only one method while 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. Additionally, 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 various different forms. The most common depth filters are bonded desiccant cores, rigid fiberglass 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 then trapped in the maze of openings that are spread throughout the filter. Depending on the type of filter, the openings will vary greatly.
The openings of bonded desiccant cores are smaller and more rigid than the openings of fiberglass 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 previously.
Meanwhile, fiberglass pad filters are not compressed as tightly as bonded, rigid fiberglass 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 fiberglass 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 a 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 then be used effectively in the suction line drier. In this case, the higher velocity in the suction line will cause the loose fiberglass structure to disintegrate. The rigid cores can be tailored to remove the solid particles that result from compressor breakdown, sludge, and resins. Additionally, the desiccant bonded in the core will remove water and neutralize acids caused by the breakdown of the lubricant. The bonding of the desiccant will preclude the attrition that can occur with loose desiccant beads.
What Are the Various Types of Filter Driers?
The liquid-line type of filter driers is placed after the condenser coil and before the expansion valve. The liquid refrigerant that flows from the condenser to the expansion valve is filtered from the particles and moisture before entering it. A good filter will filter out particles that are 20 microns or above from the system.
Some higher-end devices have sight glasses that enable the technician to look into them to see the level of refrigerant. Some have a chemical in them that indicates the amount of moisture the system has based on the color of the chemical.
Steel Liquid-Line Filter Driers
Steel liquid-line filter driers are intended for use in all sizes and types of systems. The range of physical size, desiccant type, and amount allows them to be applied to virtually any refrigeration and air conditioning system.
This kind of filter drier is physically sized to minimize pressure and provide adequate volume for filtration and drying. The molecular sieve desiccant has the highest weight percent water capacity so that water levels are kept to a minimum. The fiberglass filtration media allows the filter drier to remove and retain large amounts of solid contaminants.
Steel Bi-Flow Filter Driers
In a heat pump or reverse-cycle application, a liquid-line bi-flow filter drier is the best choice. Specifically designed for such applications, the steel shell filter drier incorporates check valves within the shell so that external check valves are not required. These check valves allow flow through the drier in either direction without allowing the escape of contaminants already filtered out.
The desiccant core design allows greater durability as it is being reversed. It also incorporates a molecular sieve for greater water capacity and activated alumina for acid-removal capabilities.
Spun Copper Filter Driers
Spun copper appliance filter driers are designed specifically for fractional-horsepower, low-vibration refrigeration systems. They are manufactured of refrigeration-grade copper tubing, molecular sieve, and typically have a choice of screen material and mesh.
Appliance filter driers are usually installed in the liquid line, as close to the metering device as possible. If the metering device is a capillary tube, the outlet of the filter drier is usually sized to allow brazing the capillary tube into the filter drier. The screen in the filter drier is placed far enough away so that the capillary tube can be inserted into the drier without blocking the refrigerant flow into the capillary tube.
The position of the filter drier should also be as vertical as possible with the flow in the downward direction. This position will allow the drier to act as a liquid seal for the capillary tube, to ensure pure liquid refrigerant flow through the capillary tube. If the filter drier must be installed horizontally, it is recommended that the outlet of the filter drier be angled downward.
The copper liquid-line filter drier must be properly designed when used in larger systems. At higher flow rates, the desiccant bed must be spring-loaded to prevent attrition. For service or field replacement units, a steel filter drier is recommended due to the possibility of higher water and solid particulate levels in the system.
The suction-line type of filter driers is placed after the evaporator and before the compressor. This device is used to protect the compressor, especially after any failure that contaminates the system. In particular, motor failure in the compressor usually causes acid and other contaminants to be introduced into the refrigerant system. Because of this, it is incredibly important for HVAC systems to have the suction-line filter drier installed.
Steel Suction-Line Filter Driers
Solid-core suction-line filter driers are designed for cleanup and are installed in a system suction line. This kind of suction-line filter drier has a design that incorporates a large outside diameter shell, which results in lower pressure drop, shorter lay-in length, and a larger core, thus providing greater filtration area for maximum operating efficiency.
The activated alumina core material has controlled porosity, which effectively removes and holds a maximum number of contaminants with minimum pressure drop. A special binding process protects the core from acid decomposition and allows it to collect and hold inorganic acids and other harmful contaminants present after a motor burnout. Access valves on both the inlet and outlet sides make it easy to measure pressure accurately.
In very dirty systems, enough contaminants will collect in the filter core, causing an increase in pressure drop. The access valves on the steel suction-line filter driers make it easy to measure the pressure drop to determine when the filter drier should be replaced.
The features of the steel liquid-line filter drier, when combined with the steel suction-line filter drier, make the pair an excellent solution for system cleanup. The two driers will quickly and effectively remove the water, sludge, acids, and solid contaminants generated when a system fails. By installing both, the expansion device and the compressor are protected from all forms of contamination.
HVAC systems are complex machines, and so, they are comprised of so many parts and components. Out of these parts and components, one that is incredibly important is the filter drier.
As the name suggests, the filter drier acts both as a filter and a drier. This means that it removes any particle from entering the refrigerant flow while, at the same time, it removes moisture from the refrigerant. With this particular purpose in mind, it is not a surprise that filter driers are a necessity for a lot of HVAC systems.
For that reason, it is incredibly important for HVAC professionals like you to know everything that there is to know about filter driers. You need to know how a filter drier works, what other necessary accessories it needs in order to function properly, and its various types. You need to know all this information so that you will understand filter driers better, thus enriching your HVAC knowledge and service to your customers. Long story, educating yourself on filter driers will help you go a long way in the HVAC industry.
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