A guide to understanding filter-drier functions and types

May 2, 2000
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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 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.

The contractor's truck: types of filter-driers

Spun copper driers:

Spun copper appliance 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 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 drier is typically sized to allow brazing the capillary tube into the drier. The screen in the 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 drier should be as vertical as possible with the flow in the downward direction. This position will also 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 drier must be installed horizontally, it is recommended that the outlet of the drier be angled downward.

Oem’s should consider a copper liquid line drier as an economical replacement to a steel drier if proper evacuation, brazing, and manufacturing techniques are in place. The added filtration and water capacity of the steel drier are not required when these processes are in place, because they minimize the amount of water and solid particulate introduced into the system.

The copper liquid line 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 drier is recommended due to the possibility of higher water and solid particulate levels in the system.

Steel liquid-line driers:

Steel liquid-line driers are intended for use in all sizes and types of systems. The range of physical size, desiccant type, and amount allow them to be applied to virtually any refrigeration and air conditioning system.

These driers are physically sized to minimize pressure drop 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 contaminant.

Steel suction-line driers:

Solid-core suction-line driers (SLDs) are designed for cleanup and are installed in a system suction line. SLD design incorporates a large outside diameter shell, which results in lower pressure drop, shorter lay-in length, and a larger core, providing greater filtration area for maximum operating efficiency.

The activated alumina core material has controlled porosity, which effectively removes and holds a maximum amount 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 contaminant will collect in the filter core causing an increase in pressure drop. The access valves on the SLD make it easy to measure the pressure drop to determine when the SLD should be replaced.

The features of the steel liquidline filter-drier when combined with the steel suction-line 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 as well as the compressor are protected from all forms of contamination.

Steel bi-flow driers:

In a heat pump or reverse-cycle application, a liquid-line bi-flow 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.

Replaceable drier shells and cores

Replacement filter cores and drier shells are designed to provide flexibility over a wide range of applications and can be used in suction and liquid lines. There are a variety of core formulations currently available on the market to meet the needs of various system conditions.

Core formulations range from all activated alumina, blends of molecular sieve and activated alumina, to 100% molecular sieve. These cores provide removal of moisture, acid, wax, and oil/flux paste, and other solid contaminants such as copper oxides, chips, and other metal fines.

In single- or multiple-core applications, cores may be loaded individually for ease of installation in tight spots. In supermarket use, for example, where water ingression into the system requires frequent service, replaceable core shells allow for easy changeout of the filter-drier element.

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