- 1 1. Importance Of Textile Filters Used In Various Fields
- 2 2. Air conditioner filters
- 3 3. Bag house
- 4 4. Automotive filters
- 5 5. Water Purification (Reverse Osmosis)
- 6 6. Conclusion
Textile filters play a major role in many industries even though it appears to be very small in size. It can be used for filtration of both air and liquid. The usage of filter fabric varies right from material type to structure according to the end use. Since it has wide range of applications, here we focus only on filters which are used in air conditioner, bag house, automotives and water purification (Reverse Osmosis) briefly.
1. Importance Of Textile Filters Used In Various Fields
Textiles which are specifically designed and engineered to be capable of using in products, processes or services of mostly non-commercial are termed as industrial textile and it can also be called as high tech textiles. Since industrial textile is a wide field, here we focus only on filter.
Filtration is a mechanical or physical operation, which is used for separation of solids from fluid or air by interposing a medium through which only the fluid or air can pass. Oversize solids in are retained by the filter. Filtration is used to separate particles and fluid in a suspension, where the fluid can be a liquid, a gas. Depending on the application, either one or both the components may be isolated. Filtration applications using textiles have wide scope as they are used in various areas like vacuum cleaners, air conditioner, bag house, automotives, chemical filtration and water purification (Reverse Osmosis), power stations, petrochemical plants, sewage disposal, etc., Textile materials basically woven or non-woven are suitable for filtration.
Objective of filter medium is to maximize the possibility of collision and the subsequent retention of the suspended particles with fibrous structure. Porosity has major effect on filtration, when porosity increases pressure drop tends to decrease but if flow rate increases pressure drop increases. Some major principles involved in filtration process are interception, inertial disposition, random diffusion, electrostatic disposition and gravity.
2. Air conditioner filters
Air conditioning is an integral part of many modern homes and air filters are important to the smooth and proper functioning of the ac. Air conditioner filters are needed because they keep dust out of the home and provide clean, cool air.
The air conditioning coil is designed with closely spaced fins that are pressed on the actual refrigerant tubes. These fins collect a lot of dust and dirt. An air conditioner’s filter is what prevents dust and grime from building up on these coils, besides improving indoor air quality as well. Filters come in variety of sizes and can be either permanent or washable.
2.1 Places of usage
Removal of all air borne contaminants is simply not practical in most facilities, so once problematic pollutants are identified, it’s time to look at filter efficiency. Filtration efficiency defines how well the filter cleans indoor air by removing air borne particles. Low-efficiency filters (25% e.fficiency on 3-10 micron particles) are typically used to keep lint and dust from clogging the heating and cooling coils of an HVAC (Heating Ventilation Air Conditioning) system. Medium and high efficiency filters (95% efficiency on 3-10 micron particles) are typically used to remove micro organisms and prevent them from entering into the building.
2.2 Types of filters
2.2.1 Fibreglass filters
It is a throw away type air filter. Here layered fibreglass fibres are laid over each other to form the filter media and typically are reinforced with a metal grating that supports the fibreglass to prevent failure and collapse.
2.2.2 Pleated synthetic filters
This is the best HVAC filter available, as pleated filter which has large filter media area provides lower pressure drop and great contaminant holding capacity. Pleated synthetic filters are similar to fibreglass filters but typically have a superior dust stopping ability. Lower pressure drop reduces fan energy requirements.
Pleated air filters used in HVAC systems are made with a wide range of filter media, including fibreglass, polyester, paper and synthetic non woven material. Recent technologies in non-woven have allowed for improvement in both performance and value of synthetic filter media over standard cotton/poly blends used for years in HVAC filters. Unlike traditional cotton/poly media, the synthetic filter media in more modern filters can be made of thermally bonded a continuous hydrophobic, polyolefin fibre that doesn’t absorb moisture. This is important in resisting bacterial growth and it keeps shed fibres from getting into the HVAC coils or into the air space of the building. Moreover, synthetic filter media can be manufactured without the use of chemical binders, meaning that humidity will not affect the structure of the filter. Unlike poly/cotton filter media, synthetic filter media can be made with a gradient density structure that provides a solid mechanical foundation to maintain high efficiency over the useful life of the filter.
2.3 Cause of a dirty filter
Old and dirty air filters in an air conditioner will have a negative impact of performance of the system, causing it to cool inefficiently and affect the quality of air inside the room or house. This will put people with asthma and allergies at greater risk, as well as waste energy and money.
Since the filter is an important component in an air conditioner, it can’t be neglected, so it has to be either replaced or cleaned periodically for better performance.
3. Bag house
A bag house or fabric filter is an air pollution control device that removes particulates out of air or gas released from commercial processes. Power plants, steel mills, pharmaceutical producers and other industrial companies often use bag houses to control emission of air pollutants.
3.1 Working of bag house
Bag house have long, cylindrical bags made of woven or felted fabrics as a filter medium. Dust laden gas or in other words dirty air enters the bag house through hopper and is directed into bag house compartment. The gas is made to pass through the bags, a layer of dust accumulates on the filter media surface until air can no longer move through it. When a sufficient pressure drop occurs, the cleaning process begins. Cleaning can take place either when bag house is on or off, but once the filter media is cleaned the filtering starts.
The cleaning can be done in three different ways namely mechanical shake method, reverse air method and pulse jet method.
3.2 Types of filter
Fabric filter bags are mostly oval or round tubes, typically 15-30 feet long 5-12 inch diameter either made of woven or non-woven material. Depending on chemical or moisture content of gas stream, its temperature and other conditions bags may be constructed out of cotton, nylon, polyester, fibreglass or any other material
3.2.1 Non woven filters
Non woven filters are made by needle punching method. After they are made with suitable fibres depending on the conditions, they are attached to a woven backing. Felted filters contain randomly placed fibres supported by this scrim. In a membrane filter, a thin, porous membrane is bound to the scrim. This type of filters are mostly preferred if pulse jet method of cleaning is used as it can give increased efficiency and minimal residual dust build up due to aggressive cleaning action.
3.2.2 Woven filters
Woven filters have a definite repeated pattern. So it is used in lower energy cleaning methods such as shaking or reverse air method. Various weave patterns such as plain weave or twill weave increase or decrease the amount of space between the individual fibres. The size of the space affects the strength and permeability of the fabric. A tighter weave with finer counts, higher ends per inch, higher picks per inch and higher cover factor corresponds with lower permeability and therefore efficient capture of fine particles.
Due to continuous contact of dirt with filters and continuous cleaning of it have an impact on both filters life and performance. A thin layer of Teflon membrane may be adhered to the filtering side of the fabric, keeping dust particles from becoming embedded in filter media fibres this method can improve the life of the filter media without altering the performance.
4. Automotive filters
While driving, there is a chance for irritating and potentially dangerous airborne particles to enter in, which can either harm occupants of the vehicle and also the engine. As far as liquids go, they have impurities in the oil that can cause harm to the engine’s system and organic materials in fuel can also be harmful. Let us discuss few filters which are responsible for keeping everything running clean.
4.1 Engine air filter
The engine air filter is there to filter out any impurities in the air that is taken in by the engine for combustion. The impurities may include any type of abrasive particle that is present in air including dust and dirt. So this helps the overall combustion process work properly, which ensures the engine is running right. In order for the vehicle to meet the optimal fuel efficiency, the vehicle needs to be combusting properly so, without the proper combustion fuel mileage and power will be lost.
Cabin air filters which are very similar to engine air filters catches any pollen that are floating in air, it prevents those harmful and allergic particles from entering the cabin thereby making the air inside the car much cleaner.
The filtration material maybe pleated paper or foam or cotton gauze. They have to be replaced once a year for effective filtration performance.
4.2 Oil filter
In fluids section of the vehicle, the oil filter is more comprehensively designed since oil filter is far important to the life of the engine than the engine air filter and also have greater influence on performance of the vehicle. Inside the engine, there are a lot of parts working together. As a result, the normal wear and tear involves little metal shavings that can build up and block the flow of components and end up in the oil pan. When the oil is cycled back through, the metal shavings will come with it and potentially harm the engine. Since engine oil is there to keep the engine cool and working effortlessly, it’s important that these shavings are removed this is the oil filter’s job. It not only filters them but also the contaminants present in the oil.
For filtration earlier bulk cotton or various woven fabrics like linen were used. When disposable filter became popular, cellulose and papers became popular to minimize production cost, low cost disposable spin on oil filters used these media. Finally synthetic media oil filters were introduced where polyester is used where better quality is acquired. Fibreglass and metal fabrics can also be used for oil filtration for obtaining extremely fine mesh. They are all made by needle punching technique. Oil filters has to be replaced on regular basis whenever the oil is replaced for smooth performance.
5. Water Purification (Reverse Osmosis)
Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids and gases from contaminated water. Reverse osmosis is one of the membrane filtration process which is usually followed for more effective purification of water in houses and even in tertiary treatment of effluent water.
5.1 Understanding reverse osmosis
Reverse Osmosis is a technology that is used to remove a large majority of contaminants from water by pushing the water under pressure through a semi-permeable membrane.
Osmosis is a naturally occurring phenomenon and one of the important process in nature. It is a process where a weaker saline solution will tend to migrate to a strong saline solution. A semi-permeable membrane is a membrane that will allow some atoms or molecules to pass but not others. It will allow air molecules to pass through but not pests or anything larger the holes in screen door. Reverse osmosis is the process of osmosis in reverse. Whereas osmosis occurs naturally without energy required, to reverse the process of osmosis we need to apply energy to the more saline solution. A reverse osmosis membrane as mentioned earlier allows water molecules passage but not majority of dissolved salts, organics, bacteria and pyrogens. However we need to push the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralise or deionise) water in the process allowing pure water through while holding back a majority of contaminants. When pressure is applied to the concentrated solution, the water molecules are forced through the semi permeable membrane and since the contaminants can pass through they are held back.
5.2 Reverse osmosis working
Reverse osmosis works by using a high pressure pump to increase the pressure on the salt side of the reverse osmosis and force the water across the semi permeable membrane, leaving almost 95% – 99% of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure. The desalinated water that is demineralised or deionised is called permeate water. The water stream that carries the concentrated contaminants that did not pass through the semi permeable membrane are called reject stream. As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure) the water molecules pass through the semi-permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the reject stream (also known as the concentrate or brine stream), which goes to drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it. It is important to understand that an RO system employs cross filtration rather than standard filtration where the contaminants are collected within the filter media. With cross filtration, the solution passes through the filter, or crosses the filter, with two outlets: the filtered water goes one way and the contaminated water goes another way. To avoid build up of contaminants, cross flow filtration allows water to sweep away contaminant build up and also allow enough turbulence to keep the membrane surface clean.
5.3 Semi permeable membrane
A semi permeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by diffusion. The first reverse osmosis membrane was made from cellulose acetate as an integrally skinned asymmetric semi-permeable membrane in 1960. But nowadays in the process of reverse osmosis TFC (Thin Film Composite) membrane is used. These are semi permeable membranes manufactured especially for use in water purification or desalination systems. This membrane can be considered as a molecular sieve constructed in the form of a film from two or more layered materials. TFC membranes are typically made out of a thin polyamide layer (<200 nm) deposited on top of a polyethersulfone or polysulfone porous layer (about 50 microns) on top of a non-woven polyester fabric support sheet. The membrane is prepared in successive steps of (i) solution casting of polysulfone on the fabric, (ii) interfacial in situ polymerization of the polyamide barrier layer on the polysulfone, and (iii) introduction of the protective layer. The three layer configuration gives the desired properties of high rejection of undesired materials (like salts), high filtration rate, and good mechanical strength. The polyamide top layer is responsible for the high rejection and is chosen primarily for its permeability to water and relative impermeable to various dissolved impurities including salt ions and other small molecules.
Most of the contaminants that are caught by a reverse osmosis membrane go with the flow and are washed away by the high pressure water. However, contaminants can accumulate on the membrane itself. This can lead to problems as the membrane loses its effectiveness in sifting impurities out. The accumulated dirt also ends up slowing down the RO process, resulting in less clean water for your use. Hence the basic rule is that RO membrane has to be replaced for every two or three years.
Filtration is one of the major part in industrial textiles which has a very wide range of usage in many areas. The major areas where air and liquid filters are used and their importance were discussed. Even in other areas the textile material of the filter may vary whereas the filtration mechanism remains the same as discussed in any of the above industrial filter idea.