Chloe Adams
Candle filters are used for water purification, hot gas filtration, and pressure filtration applications. They are simple to assemble and do not require any energy. The filter media can be made from clay, ceramic, porcelain, stainless steel, or synthetic materials. The filters are typically unidirectional and are placed in a filter support vessel. The geometry of the filter elements can vary, but they are usually in the form of a candle or tube, closed at one end, with micro-scale pores. The filters can be cleaned with a soft brush and water every two weeks and should be replaced every 6-12 months.
| Characteristics | Values |
|---|---|
| Filter type | Candle filters, also known as Backwashing Tubular Filters |
| Filter components | Vessel, candles, discharge mechanism |
| Vessel material | Lined carbon steel, stainless steel, alloys |
| Candle material | Cloth, ceramic, metal, clay, porcelain, synthetic, stainless steel, higher alloys, plastic |
| Candle structure | Hollow tubes with a cap on the bottom and a threaded connector on top |
| Outer face material | Punch plate, expanded metal, sintered metal, porous ceramic, stainless steel woven wire cloth, wedge wire, fabric |
| Fabric type | Polypropylene, nylon, teflon, polyester, cotton |
| Filter stages | Filling, Filtration, Discharge |
| Vessel pressure | 6-10 bar |
| Cake discharge | Controlled release of trapped air or gas, manipulating slurry inlet, filtrate outlet, and sludge drain |
| Cake thickness | 5-20 mm |
| Filter candle length | 1-3 m |
| Outer diameter | 60-150 mm |
| Maintenance | Requires frequent cleaning and maintenance, replacement every 6-12 months |
| Use cases | Water filtration, hot gas filtration |
| Advantages | Simple to use and clean, removes pathogens and suspended particles, improves water quality |
| Disadvantages | Prone to breakage, local construction may result in lower quality and efficiency |
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What You'll Learn
Candle filter basics
Candle filters are also known as Backwashing Tubular Filters. They are pressure filters ideal for polishing or thickening. Candle filters do not have any moving parts and can process multiple batches without direct operator supervision.
The filter has three components: the vessel, the candles, and the discharge mechanism. The vessel operates under pressures between 6-10 bar. The candles are named based on their shape and are usually hollow tubes with a cap on the bottom and a threaded connector on top that attaches to a header. The outer face of the tube is usually made of punch plate, expanded metal, sintered metal, or porous ceramic to provide support for the filter media. The filter media can be stainless steel woven wire cloth, wedge wire, or fabric media.
Candle filters can be used for a variety of applications, including water filtration and hot gas filtration. In water filtration, the candle filters are typically made of ceramic or clay and are used to remove turbidity, suspended debris, and pathogens from drinking water. In hot gas filtration, the candle filters are typically made of metal and are closed at one end. The raw gas flows from the outside of the filter candles through the porous wall of the elements to the clean gas part of the filter, and the dust collects on the outer surface of the candles.
Candle filters require frequent maintenance due to particle buildup and reduced flow rates. It is recommended to clean the filters with a soft brush and clean water every 2 weeks. The service life of candle filters varies, but they should be replaced every 6-12 months.
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How to clean candle filters
To clean candle filters, you must first remove the candle from its housing. This may involve undoing screws or disconnecting filters from the system. Next, rinse the candle with water to remove any loose debris and sediment. Gently rub the surface to ensure all trapped particles are removed.
Prepare a cleaning solution by mixing warm water with a mild detergent or vinegar. Soak the candle in the solution for around 30 minutes. The solution will be absorbed into the pores, effectively cleaning the filter. After soaking, gently scrub the surface of the candle with a soft brush or sponge. Focus on areas with visible accumulation or discolouration. Ensure you do not use rough-surfaced materials that could scratch the filter.
After scrubbing, thoroughly rinse the candle under running water to remove any remaining traces of the cleaning solution, detergent, or vinegar. If desired, you can then sterilize the candle by boiling it for a few minutes or using another sterilisation solution to remove bacteria and microorganisms. Once cleaned and dried, the candle is ready for reuse.
It is important to regularly check your candle for any signs of wear, cracks, or damage. Clean your candle at least once a month, as recommended by manufacturers, to maintain optimal performance and extend its lifespan.
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Types of candle filters
Candle filters are used for a variety of applications and can be made from a range of materials. The type of candle filter used depends on the specific requirements and conditions of the filtration process.
One of the most common types of candle filters is the German 'Berkefeld' filter, which is made from diatomaceous earth, a type of porous ceramic. These filters are graded as V—viel (coarse), N—normal, and W—wenig (fine), depending on the rate of water passage at a standard pressure. While V-grade filters are commonly used for clarifying liquids, they are not fine enough to sterilize them by blocking bacteria.
The American 'Mandler' filter candle is another type of diatomaceous earth filter, composed of Kieselguhr, asbestos, and plaster of Paris. Similar to the Berkefeld filters, Mandler filters are graded based on the amount of air pressure they can withstand without producing bubbles when immersed in water.
Porcelain candle filters are also available, with the French 'Chamberland' filter being the most common type. These filters are made from kaolin and sand, and they come in various porosities graded from L1 to L11. L1-grade filters are used for clarifying, while the higher grades are designed for sterilizing.
Asbestos-disc or 'Seitz' filters are considered versatile and suitable for general purposes. They consist of an asbestos disc placed in a metal mount with the rough side facing upwards.
For hot gas filtration, filter candles that are closed at one end are typically used. These filters are vertically installed and hung in a tube sheet, separating the raw and clean sides of the filter vessel. The raw gas flows from the outside of the filter candles, through the porous wall, to the clean gas side, with dust collecting on the outer surface.
Candle filters can also be made from stainless steel wire mesh, which offers high resistance to differential pressure and is suitable for high-temperature operating conditions where plastics, fleece, or paper would not be appropriate.
The choice of filter medium depends on various factors, including the specific application, operating conditions, and the nature of the material being filtered.
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How to assemble candle filters
Candle filters are used to filter liquids with minimal solid content. They are especially useful for filtering flammable, toxic, or corrosive mediums. Candle filters can be made from a variety of materials, including clay, ceramic, stainless steel, and porcelain. They are typically comprised of an upper and lower container, with one or more candles in between, a tap, and a lid.
- Place the candles: The candles should be screwed into the base of the upper container. The number of candles will depend on the desired flow rate, as candles can have very slow flow rates.
- Attach the tap: Connect a tap to the lower container. This will allow for the safe withdrawal of water without risking recontamination.
- Add a lid: Place a lid on top of the upper container to prevent contamination.
- Prepare the filter candle: Before use, clean the water filter candle and its components with clean water to remove any dust or debris.
- Prime the filter candle: Submerge the filter candle in clean water as per the manufacturer's instructions. Priming saturates the material for efficient filtration.
- Install the filter candle: Install the candle according to the manufacturer's guidelines, ensuring a secure fit to prevent leaks.
- Initial flush: Before using the filtered water, run water through the system to remove any loose particles.
- Test the water: After flushing, test the filtered water to ensure its quality.
For industrial candle filters, the process may vary depending on the specific design and requirements. However, the basic components and principles remain consistent.
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How candle filters work
Candle filters are also known as backwashing tubular filters. They are pressure filters ideal for polishing or thickening. They are called candle filters because they resemble upside-down candles.
The filter has three components: the vessel, the candles, and the discharge mechanism. The vessel operates under pressures between 6-10 bar. The candles are cylindrical elements that filter liquids or gases. The liquid or gas to be filtered enters the filter vessel and passes through the candles from outside to inside. The working principle of a candle filter is based on the difference in pressure between the inside and the outside of the candles.
The typical filter candle is made up of a dip pipe, a perforated core, and a sock. The dip pipe allows the flow of filtrate and pressurised gas, and the perforated core has supporting tie rods. The tie rods create an annular space between the filter sock and the perforated core, which helps to maintain a low-pressure drop during operation. The filter sock is installed over the candle and can be made of various synthetic materials. It is the sock that does the actual filtering, trapping particles as small as 0.3 μm.
The filtration process works as follows: increasing pressure forces the filtrate through the filter media on the candles, through the headers, and out the filtrate outlet. Solids are collected on the filter media and form a cake. Spray bars near the headers can clean the cake down the length of the candles. Steam or gas replaces the slurry and is forced through the cake and candles until there is no more filtrate to capture. Air or gas fills the vessel and presses the remaining slurry out of a pipe on the bottom and back into the feed tank. Both the slurry inlet and the filtrate outlet are closed, and the sludge drain on the bottom of the vessel opens. The condensed air or gas trapped in a bubble above the candles and headers gets forced out through the sludge drain. This backwash breaks the cake off the candles and it is discharged. The unit is now ready to begin filling again.
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Frequently asked questions
Candle filters are basic clay filters that are used to remove turbidity, suspended debris, pathogens, and iron from drinking water. Water is poured into the upper container and flows through a candle situated in the bottom. Once the water has passed through the candle, it is collected in the lower container. The filters are simple to put together and do not require any energy.
Candle filters are typically made of clay, but can also be made of porcelain, stainless steel, or synthetic materials. The filters are usually closed at one end and have micro-scale pores that absorb turbidity and suspended debris.
Candle filters require frequent maintenance and must be cleaned with a soft brush and clean water every 2 weeks. The candle surface should be gently scrubbed to remove any accumulated debris. Only clean water should be used for cleaning (no soap, chlorine, or other chemicals). During cleaning, the candle should not be touched with dirty hands or placed on a dirty surface.
