- Email: [email protected]
Filter media: New filter barrier permits filtration and backwashing
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Feature Filtration+Separation November/December 2013
Filter media:
New filter barrier permits filtration and backwashing A
new filter uses flexible elements to enlarge apertures during backwashing. Backwashing needs less energy and is effective. A vertical fixed drum filter can be used to obtain filtration with simultaneous filtering and backwashing. Dr Ian Doig explains.
Introduction Many filters open their apertures to enhance backwashing. These filter types include: 1. Cylindrical cartridges comprising a coiled helical flat spring, where apertures are the spaces between neighbouring coils of the helix. During filtration, external pressure compresses the coil to its narrowest slot apertures. While backwashing, internal pressure extends the coil lengthwise, which opens the apertures. The flat spring has coils each wide in its radial direction and thin in its depth. Protrusions hold the gap (the filter aperture) between coils constant under external (filtering) pressure. Internal (backwashing) pressure opens the apertures to enhance backwashing. 2. Cylindrical cartridges comprising a pleated cloth. While filtering, external pressure keeps the pleats in a cylindrical array. Filter cake forms on the pleats. While backwashing, internal pressure extends the pleats outwards, increasing the circumference of the cartridge to a cylindrical shape and dislodging the filter cake1. 3. Pulse jet filters used with tubular dust filters. Dust is filtered from air outside the tubular bag to the inside. A pulse of compressed air periodically enters the bag through a Venturi nozzle at the top of the bag to deliver a shock wave of air within the tube, which dislodges dust (filter cake) collected around the tube outside.
Figure 1: Section of a comb showing elements (teeth) with filter apertures between the teeth.
Figure 2: Side view of assembled teeth (elements) and combs showing teeth pressed flat against underlying combs when filtering (fluid enters from above – apertures between teeth at their minimum width). Teeth are arranged in groups of three with third tooth being most flexible.
Filtration is temporarily halted while pulsed backwashing occurs2. 4. Filaments arranged as brushes, which lie randomly flat and overlap other filaments to provide a filter mat while
filtering. Filaments are swept upwards by a backwashing fluid to open the apertures between filaments3. Filters of types 1 and 2 operate batch-wise. Filtration continues until the filtration flux
Feature Filtration+Separation November/December 2013
Figure 3: Side view of assembled teeth (elements) and combs showing teeth opened by fluid from below when backwashing. First teeth yield least; third teeth yield most. Apertures are progressively larger from the first to the third teeth.
Figure 4: Side view of several assembled teeth (elements) and combs showing longer teeth overlaying other teeth and combs to form a deeper filtration barrier.
Figure 6: Example of an alternative screen or cloth where warp elements are attached at both ends to stiff weft elements. During filtration, the elements are pressed against underlying rods to maintain the filtration apertures. While backwashing, fluid from beneath the elements lifts the elements to open the apertures.
Figure 5: Plan view of vertical cylindrical filter enclosed within a vertical shell. Filtration is from outside the cylindrical filter to the inside. A series of rotating arms provide local intense sprays backwashing small areas of the filter while filtration proceeds elsewhere.
Note: Figures 6 and 7 show warp elements grouped with four elements in each group. The first warp element remains stiff, while the second warp element yields more and all fourth warp elements yield most to lift away from the support bars. This provides enlarged apertures during backwashing. During filtration all warp elements rest upon stiff support bars and all apertures are approximately the same size.
diminishes to a set value when filtration ceases and backwashing begins. The cycle, filtrationbackwashing- filtration- backwashing, is continuously repeated. Filters of type 3 operate with filtration occurring simultaneously on several tubular bags, while (pulse-jet) backwashing occurs
Figure 7: A section of the screen with elements raised by backwashing fluid to open the apertures (Apertures shown are larger than normal).
within one tubular bag to remove filter cake progressively in clumps. These clumps fall to the floor of the housing containing the tubular bags. Accumulated clumps are periodically removed. Filters of type 4 can operate batch-wise, or filtration and backwashing can occur
simultaneously. The filter barrier comprises groups of fibres, or a cloth or a screen arranged and supported as a vertical cylinder. Continuous filtration proceeds from outside of the vertical cylindrically formed cloth or mat while continuously rotating jets each provide highly localised backwashing, which blast small areas of the filter cloth from inside the
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Feature Filtration+Separation November/December 2013
Figure 8: Example of an alternative screen or cloth where warp elements are grouped into pairs, and are attached at both ends to stiff weft elements. The screen filters - warp elements are pressed against underlying support rods to maintain the filtration apertures.
cylindrical filter barrier to remove filter cake progressively in clumps. These fall to the floor of the tank containing the filter barrier and are periodically removed – see Figure 5 4. A disadvantage of the type 3 filter mat3 is that the apertures formed by filaments are randomly swept back by the backwash to form a filtration barrier. This produces apertures of various sizes. The larger apertures allow undesirably large particles to enter the filtrate, accompanied by a disproportionately high local flux rate. A recent innovation provides uniformly sized apertures while retaining other advantages of the type 4 filter4.
New developments The new filters have two forms. In the first form, the apertures are the gaps between the teeth of a series of combs in a parallel array, where the teeth of one comb overlap, and rest upon, the backs of one or more other combs in the array. During filtration, fluid carrying dispersed particles passes through the gaps (or series of gaps where the teeth of more than one comb overlap other teeth), and particles larger than the gaps remain trapped between the teeth. One or more underlying combs support the teeth against the flow of fluid during filtration and prevent unwanted opening of the apertures. During backwashing, teeth are lifted away from the underlying combs and the gaps between teeth increase, thereby enlarging the gaps to enhance backwashing efficiently using less energy. In an improvement, teeth form groups comprising first and second teeth, or first, second and third teeth etc. where the first tooth yields less during backwash than the second, and the second tooth yields less during backwash than the third, and so on. Hence, during backwashing, first teeth in each group yield the least (or do not yield), second teeth yield more and all subsequent teeth yield
Figure 9: The screen or cloth of Figure 8 is now being backwashed from below. The more flexible warp element of each pair is lifted by the backwash to provide enlarged apertures for backwashing.
more than the earlier ones. The gap during backwashing becomes progressively greater within each group after the first tooth. This provides progressively larger apertures within each group during backwashing and directs the removed filter cake downwards. In a second form, teeth of the first type become elements that are not straight but are uniformly spaced to provide filtration apertures that are the gaps between elements. The elements are held in an array by lateral elements joined to elements where they intersect to form a screen (or a specially woven filter cloth where the elements become warp elements and the lateral elements become weft elements). The gap between adjacent lateral (weft) elements is at least five times the gap between (warp) elements. Because there are elements that are not straight, they are able to deflect resiliently and slightly twist when a fluid passes between them, thereby opening the gaps between elements. When fluid passes between elements during filtration, deflection and twisting is prevented by one or more underlying stiff transverse bars located between adjacent lateral elements. When fluid passes between elements during backwashing, deflection is not prevented and the gaps open. Note that the elements are sufficiently resilient to sustain their non-straight orientation while deflecting – see Figures 6-9. Similarly to the first form, the improvement comprises elements forming groups of first and second elements, or first, second and third elements etc. where the first elements yield less during backwash than the second, the second elements yield less during backwash than the third and so on. Hence, during backwashing, first elements yield the least, second elements yield more and all subsequent elements yield more than the previous ones. The gap during backwashing becomes progressively greater in each group away from first elements – see Figures 6-9. This provides progressively larger apertures during
backwashing and directs the removed filter cake downwards.
Advantages and disadvantages The advantages of the new filters include: • The filtration aperture remains approximately constant. • The backwashing force need only be a small one to open apertures. • A minimum amount of filtered fluid is required by the jets of Figure 5 to provide effective backwashing. • Filtration and backwashing occurs simultaneously – filtration is continuous, simpler and more efficient than rotary vacuum drum filtration – without the complexities of vacuum, and its limited filtration pressure. • Simultaneous filtration and backwashing equipment can be smaller, cheaper and more efficient than rotary vacuum drum filtration. • Compressed air may be used in place of filtered fluid to improve filtration productivity. • Overall efficiency per unit volume of filtrate is higher. The only disadvantage is that where separated granules are the product, separate equipment is required for granule washing and drying operations.
•
References 1. Dr. M filter: www.drm.ch 2. wikipedia.org/wiki/Dust.collector#pulse jet 3. US 6,103,132 Seyfried et al 15 Aug 2000 4. WO2009/000401 pub. 15 Oct. 2009 Doig, I.D.
Contact: Dr. Ian D. Doig E-mail: [email protected]
Feature Filtration+Separation November/December 2013
Filter media:
New filter barrier permits filtration and backwashing A
new filter uses flexible elements to enlarge apertures during backwashing. Backwashing needs less energy and is effective. A vertical fixed drum filter can be used to obtain filtration with simultaneous filtering and backwashing. Dr Ian Doig explains.
Introduction Many filters open their apertures to enhance backwashing. These filter types include: 1. Cylindrical cartridges comprising a coiled helical flat spring, where apertures are the spaces between neighbouring coils of the helix. During filtration, external pressure compresses the coil to its narrowest slot apertures. While backwashing, internal pressure extends the coil lengthwise, which opens the apertures. The flat spring has coils each wide in its radial direction and thin in its depth. Protrusions hold the gap (the filter aperture) between coils constant under external (filtering) pressure. Internal (backwashing) pressure opens the apertures to enhance backwashing. 2. Cylindrical cartridges comprising a pleated cloth. While filtering, external pressure keeps the pleats in a cylindrical array. Filter cake forms on the pleats. While backwashing, internal pressure extends the pleats outwards, increasing the circumference of the cartridge to a cylindrical shape and dislodging the filter cake1. 3. Pulse jet filters used with tubular dust filters. Dust is filtered from air outside the tubular bag to the inside. A pulse of compressed air periodically enters the bag through a Venturi nozzle at the top of the bag to deliver a shock wave of air within the tube, which dislodges dust (filter cake) collected around the tube outside.
Figure 1: Section of a comb showing elements (teeth) with filter apertures between the teeth.
Figure 2: Side view of assembled teeth (elements) and combs showing teeth pressed flat against underlying combs when filtering (fluid enters from above – apertures between teeth at their minimum width). Teeth are arranged in groups of three with third tooth being most flexible.
Filtration is temporarily halted while pulsed backwashing occurs2. 4. Filaments arranged as brushes, which lie randomly flat and overlap other filaments to provide a filter mat while
filtering. Filaments are swept upwards by a backwashing fluid to open the apertures between filaments3. Filters of types 1 and 2 operate batch-wise. Filtration continues until the filtration flux
Feature Filtration+Separation November/December 2013
Figure 3: Side view of assembled teeth (elements) and combs showing teeth opened by fluid from below when backwashing. First teeth yield least; third teeth yield most. Apertures are progressively larger from the first to the third teeth.
Figure 4: Side view of several assembled teeth (elements) and combs showing longer teeth overlaying other teeth and combs to form a deeper filtration barrier.
Figure 6: Example of an alternative screen or cloth where warp elements are attached at both ends to stiff weft elements. During filtration, the elements are pressed against underlying rods to maintain the filtration apertures. While backwashing, fluid from beneath the elements lifts the elements to open the apertures.
Figure 5: Plan view of vertical cylindrical filter enclosed within a vertical shell. Filtration is from outside the cylindrical filter to the inside. A series of rotating arms provide local intense sprays backwashing small areas of the filter while filtration proceeds elsewhere.
Note: Figures 6 and 7 show warp elements grouped with four elements in each group. The first warp element remains stiff, while the second warp element yields more and all fourth warp elements yield most to lift away from the support bars. This provides enlarged apertures during backwashing. During filtration all warp elements rest upon stiff support bars and all apertures are approximately the same size.
diminishes to a set value when filtration ceases and backwashing begins. The cycle, filtrationbackwashing- filtration- backwashing, is continuously repeated. Filters of type 3 operate with filtration occurring simultaneously on several tubular bags, while (pulse-jet) backwashing occurs
Figure 7: A section of the screen with elements raised by backwashing fluid to open the apertures (Apertures shown are larger than normal).
within one tubular bag to remove filter cake progressively in clumps. These clumps fall to the floor of the housing containing the tubular bags. Accumulated clumps are periodically removed. Filters of type 4 can operate batch-wise, or filtration and backwashing can occur
simultaneously. The filter barrier comprises groups of fibres, or a cloth or a screen arranged and supported as a vertical cylinder. Continuous filtration proceeds from outside of the vertical cylindrically formed cloth or mat while continuously rotating jets each provide highly localised backwashing, which blast small areas of the filter cloth from inside the
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42
Feature Filtration+Separation November/December 2013
Figure 8: Example of an alternative screen or cloth where warp elements are grouped into pairs, and are attached at both ends to stiff weft elements. The screen filters - warp elements are pressed against underlying support rods to maintain the filtration apertures.
cylindrical filter barrier to remove filter cake progressively in clumps. These fall to the floor of the tank containing the filter barrier and are periodically removed – see Figure 5 4. A disadvantage of the type 3 filter mat3 is that the apertures formed by filaments are randomly swept back by the backwash to form a filtration barrier. This produces apertures of various sizes. The larger apertures allow undesirably large particles to enter the filtrate, accompanied by a disproportionately high local flux rate. A recent innovation provides uniformly sized apertures while retaining other advantages of the type 4 filter4.
New developments The new filters have two forms. In the first form, the apertures are the gaps between the teeth of a series of combs in a parallel array, where the teeth of one comb overlap, and rest upon, the backs of one or more other combs in the array. During filtration, fluid carrying dispersed particles passes through the gaps (or series of gaps where the teeth of more than one comb overlap other teeth), and particles larger than the gaps remain trapped between the teeth. One or more underlying combs support the teeth against the flow of fluid during filtration and prevent unwanted opening of the apertures. During backwashing, teeth are lifted away from the underlying combs and the gaps between teeth increase, thereby enlarging the gaps to enhance backwashing efficiently using less energy. In an improvement, teeth form groups comprising first and second teeth, or first, second and third teeth etc. where the first tooth yields less during backwash than the second, and the second tooth yields less during backwash than the third, and so on. Hence, during backwashing, first teeth in each group yield the least (or do not yield), second teeth yield more and all subsequent teeth yield
Figure 9: The screen or cloth of Figure 8 is now being backwashed from below. The more flexible warp element of each pair is lifted by the backwash to provide enlarged apertures for backwashing.
more than the earlier ones. The gap during backwashing becomes progressively greater within each group after the first tooth. This provides progressively larger apertures within each group during backwashing and directs the removed filter cake downwards. In a second form, teeth of the first type become elements that are not straight but are uniformly spaced to provide filtration apertures that are the gaps between elements. The elements are held in an array by lateral elements joined to elements where they intersect to form a screen (or a specially woven filter cloth where the elements become warp elements and the lateral elements become weft elements). The gap between adjacent lateral (weft) elements is at least five times the gap between (warp) elements. Because there are elements that are not straight, they are able to deflect resiliently and slightly twist when a fluid passes between them, thereby opening the gaps between elements. When fluid passes between elements during filtration, deflection and twisting is prevented by one or more underlying stiff transverse bars located between adjacent lateral elements. When fluid passes between elements during backwashing, deflection is not prevented and the gaps open. Note that the elements are sufficiently resilient to sustain their non-straight orientation while deflecting – see Figures 6-9. Similarly to the first form, the improvement comprises elements forming groups of first and second elements, or first, second and third elements etc. where the first elements yield less during backwash than the second, the second elements yield less during backwash than the third and so on. Hence, during backwashing, first elements yield the least, second elements yield more and all subsequent elements yield more than the previous ones. The gap during backwashing becomes progressively greater in each group away from first elements – see Figures 6-9. This provides progressively larger apertures during
backwashing and directs the removed filter cake downwards.
Advantages and disadvantages The advantages of the new filters include: • The filtration aperture remains approximately constant. • The backwashing force need only be a small one to open apertures. • A minimum amount of filtered fluid is required by the jets of Figure 5 to provide effective backwashing. • Filtration and backwashing occurs simultaneously – filtration is continuous, simpler and more efficient than rotary vacuum drum filtration – without the complexities of vacuum, and its limited filtration pressure. • Simultaneous filtration and backwashing equipment can be smaller, cheaper and more efficient than rotary vacuum drum filtration. • Compressed air may be used in place of filtered fluid to improve filtration productivity. • Overall efficiency per unit volume of filtrate is higher. The only disadvantage is that where separated granules are the product, separate equipment is required for granule washing and drying operations.
•
References 1. Dr. M filter: www.drm.ch 2. wikipedia.org/wiki/Dust.collector#pulse jet 3. US 6,103,132 Seyfried et al 15 Aug 2000 4. WO2009/000401 pub. 15 Oct. 2009 Doig, I.D.
Contact: Dr. Ian D. Doig E-mail: [email protected]