May 24, 2021 Filter-drier
Filter-driers are key components of any refrigeration or air conditioning system. It is responsible for two main purposes: one, to adsorb system contaminants, and two, to provide physical filtration.
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, etc.
Another source is due to improper handling of polyolester (POE) lubricants, which are hygroscopic. 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. Therefore, 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 aluminium oxide (Al2O3) and is not a highly crystalline material. Both alumina and silica gel shows 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 other main function of a filter-drier is filtration. This can be accomplished by different methods. Some driers use only one method where others may use a combination of methods. 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 vapour.
Depth filtration takes different forms. The most common depth filters are:
- Bonded desiccant cores
- Rigid fibreglass filters bonded with phenolic resin; and
- Fibreglass 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 fibreglass 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.
Fibreglass pad filters are not compressed as tightly as bonded, rigid fibreglass filters. The liquid refrigerant with the entrained contaminant flows through the pads. The contaminant will impact the glass fibres 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 fibreglass. The larger particles will tend to drop out first as their higher mass will tend to cause them to impact on a fibre even though the flow stream will bend around a fibre.
As the fibreglass 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 fibre 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 fibreglass 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 the breakdown of the lubricant. The bonding of the desiccant will preclude the attrition that can occur with loose desiccant beads.
There are four types of filter-driers. The first type of filter-driers is spun copper driers. Spun copper 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. These types of 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.
A copper liquid line drier can be used as an economical replacement to 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. A 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.
The second type of filter-driers is steel liquid-line driers. It is 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.
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 fibreglass filtration media allows the filter-drier to remove and retain large amounts of solid contaminant.
The third type of filter-driers is steel suction-line driers. Solid-sore suction-line driers (SLDs) are designed for clean-up 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 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 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 clean-up. 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.
The fourth type of filter-drier is steel bi-flow driers. A Liquid-line bi-flow drier is the best choice in a heat pump or reverse-cycle application. 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.
Filter-driers are available through Quick Time Engineering Inc.
Centrifugal separator
The water and oil are filtered to make pure water available for utilization. This is performed by various types of filters, of which, centrifugal separators are the most popular ones. These filters are used for removal of suspended solids and other impurities in water. Centrifugal separators are driven by the technique of centrifugation. Centrifugation utilizes centrifugal force to separate particles from solution. This process is mainly employed to separate two immiscible substances existing in a solution.
Centrifugal separator features an inlet, outlet, and separator. The mixture is pumped into a cone-shaped working apparatus in the separator. The separator produces a spinning vortex, which leads to the filtration of solids from liquids. The separated solids are collected at the bottom of the separator, and they are purged from there. High density liquid flow out of the separator, along with contaminant, and low-density component will remain inside. Water is one of the denser liquids, so it flows outside and is removed through a discharged outlet. However, lower density fluids such as oil will remain in the center of the vortex. Segregated oil can be easily recovered from the suction orifice of the separator.
Centrifugal separators are available in different designs and capacities. Depending on their designs, they are utilized in different ways across various industries. Centrifugal separator helps improve the efficiency of filtration as well as minimize liquid loss when it is used for pre-filtration. This pre-filtration helps users save on expensive water treatment solutions. They also help in protecting heat exchangers effectively against fouling. Centrifugal separators can remove scale and suspended grit easily. Centrifugal separators are also used for protecting spray nozzles and small orifices in various industrial applications. These separators help remove solids that clog the nozzles of the spray. As a result, the wear and tear of the nozzle can be reduced as well as avoid its regular replacement. Centrifugal liquid separator ability to remove solids from a liquid becomes advantageous in case of applications where the disposal costs are high, or where recovery of high solids is mandatory. This also helps improve the life of seals.
The efficiency of centrifugal separation will depend on the difference between the specific gravity of the liquid and the solid being filtered. The separation efficiency will increase if the difference is large. The separation efficiency is also affected by the particle size. Centrifugal separators are mainly used for liquid-based applications. They are widely used to separate liquid-solid suspensions, liquid-liquid mixtures and solid/gas-liquid mixtures.
Centrifugal separators are used in a variety of industrial applications, it poses many advantages. It has a few moving parts than other separators and has no filters, bag, screens, as well as cartridges, which makes them an ideal choice for various industrial applications. In addition to their design advantages, it also has other benefits. Centrifugal separators are largely maintenance-free owing to the absence of moving parts or other components. It is fitted with an automatic purge valve designed to flush the debris and contaminants automatically. This is another major advantage of centrifugal separator water filters or centrifugal separators used in the industrial process. As the filtration is performed by the spinning of a vortex, no real filters are involved. Therefore, there are no real filters involved. This means there will no accumulation of debris in filters, and there will no breakdown due to the accumulation. Also, there will no need to change the filters more often, as in the case of other liquid separators. Centrifugal separators also have minimal liquid loss where other separators suffer major liquid loss when cleaning sand media filters or automatic strainers. The efficiency of centrifugal separation is 98% of 40 microns in a single pass. However, for centrifugal separator, this is 44 microns. This stands valid for solids at the gravity of 2.6 and water at 1.0.
Centrifugal separator is available through Quick Time Engineering Inc.
Quick Time Engineering Inc is an international company with offices and distribution networks in the USA, Hong Kong, Europe and Malaysia.
In its 20 years of operation since 1998, Quick Time Engineering Inc had emerged from a local engineering company with a single staff that provided solutions in factory automation to become nowadays a company that serves the Oil & Gas industries, EPC contractors, System Integrators and other industrial automation and process control companies worldwide. Customers from over 50 countries worldwide trust us with their need for process control instruments and industrial automation products.
For more information about Quick Time Engineering Inc, visit www.quicktimeonline.com or email enquiry@quicktimeonline.com
