Cleanable filters are used to separate solid particles from gases when cleaning exhaust air having high concentrations of dust. The cleaning effect is based on the so-called "surface filtration". In surface filtration, the particles are mostly separated when contacting the particle layer (dust cake) forming on the surface of the filter media. After a specified pressure drop is reached, or at fixed intervals, the filter media are cleaned off so that the filtration process can be repeated periodically. The separated dust can be recovered.
The types of cleanable filters differ in terms of the geometric arrangement of the filter media, the gas flow and the cleaning method.
|Cleaning||shaking, back-flushing or compressed air|
|Typical parameters:||Residual dust concentration||1 - 20 mg/m³ (filter cassette < 0,001 mg/m³)|
|Particle size distribution||< 0,1 - 100 µm|
|Temperature||-40 - +260 °C|
|Filter media||Needle felts, spunbonds, wet-laid papery nonwovens|
|Typical characteristics||Weight per unit area||150 - 700 g/m³|
|Air-to-cloth ratio||30 - 180 m³/m²/h|
|Differential pressure at commissioning||200 - 500 Pa*|
|Differential pressure in operation||500 - 1.500 Pa*|
|Geometry||bag filter, pocket filter, cartridge filter, pleated element filter, cassette filter, rigid filter|
*sintered filter elements are brandspecific, partly significantly higher
- In bag filters, the filter element is usually a cylindrical tube. The filter bags are produced in various diameters and lengths. They cover the complete spectrum of < 0.5 m² to > 100,00 m². Bag filters are used to filter exhaust air at low and medium temperatures of up to approx. 260°C, with the filter bag holding back the dust as the air flows through. Filter bags must be regularly cleaned, usually with compressed-air blasts. The cleaning effect of each blast of compressed air is intensified by the sudden expansion of the filter bag. Bag filters are therefore best suited for sticky/strongly adhesive dust.
- Pocket filters are used to remove dust from smaller quantities of gas. The operating temperatures correspond to bag filters. The filter medium is stretched over flat, plate-like frames open on one side for the clean gas to escape. The gas flows from the outside inward. Pocket filters are cleaned regularly, usually with compressed-air blasts. The cleaning effect of each blast of compressed air is intensified by the sudden expansion of the filter pocket. This effect is only marginally weaker in pocket filters than in bag filters. Pocket filters are therefore also best suited for sticky/strongly adhesive dust.
- Cartridge filtersare an alternative to bag filters that is being used more and more. The filter medium is folded into a star shape and placed over a cylindrical supporting cage. The gas flows from the outside inward, and the filter is cleaned by pressure shocks or low-pressure washing. Cartridge filters are used only for dust that is easy to clean off, since otherwise the folds become clogged with dust. The operating temperature is limited to approximately 200 °C.
- Pleated element filtersare an alternative to pocket filters. Two folded filter media are placed on top of one another, then glued or welded together where they come into contact. The gas flows from the outside inward, and the filter is cleaned by pressure shocks or low-pressure washing. Pleated filters are used only for dust that is easy to clean off, since otherwise the folds become clogged with dust. The operating temperatures are typically around 130 °C.
- In cassette filters ("HEPA filters"), the dust-laden gas usually enters the raw-gas chamber of the filter housing in the lower part, where it is pre-separated, and then flows through the first filtering stage. Entrained fine dust is separated on the outside of the folds of the filter cassette. Filter cassettes are cleaned pneumatically. A nozzle pipe slowly moves back and forth over the entire width and length of the filter cassette. The compressed air blows out the filter cassette downward, removing dust from the filter elements. The clean gas from the first filtering stage can then pass through a second, uncleaned filtering stage if desired (safety filter, police filter). The operating temperature range is limited to a maximum of 180 °C.
- Rigid filters mostly consist of a very porous sintered plastic framework on which often a PTFE membrane is laminated. The dust separation is only on the membrane then. The self-supporting rigid body below the membrane has no filtering function anymore. On the one hand the anti-adhesive PTFE membrane can be used for sticky dusts, on the other hand, the rigid body elements lack blowing up in spite of jet-pulse and with it a kinetic support of the blowoff process.
Ordinarily, the particle-laden gas flows through the filter elements from the outside inward: The air flow deposits the filtrate onto the outside of the filter media. This forms a layer of dust, known as the filter cake.
The filter media must be cleaned periodically. Brief blasts of compressed air, mechanical vibration or back-flushing with air are used to remove the dust cake. The dust layer falls down, e.g., into a collection hopper that is regularly emptied (e.g., by a rotary feeder).
The operating performance depends on a number of factors, which need to be considered when designing a dust extractor. The have a decisive influence on te functionality of the entire system.
Primary determining factors:
|Chemial and physical properties of the carrier gas||corrosive, dry / dew point|
|Chemial and physical properties of particles||particle size distribution, density, particle form, abrasiveness, agglomerating, fibrous, hygroscopic, sticky, corrosive, liquid aerosols existing|
|Physical properties of the separated dust||bulk density, bridging, free-flowing, dumping angel|
|Process parameter||volume flow, raw gas dust concentration, operating pressure, temperature|
|Operating method of the plant||continuous, discontinuous|
|Necessities depending on the installation site||maximum dimensions - length, width, height (incl. free space for filter media change)|
|Technical explosion and flame parameters of carrier gas and particles|
Secondary determining factors:
|Safety requirements||Explosion protection concept, required clean gas value, dust exposition of employees during service works (dust release during change of filter media and container), disposal of the dust|
|Production features||cleanability (CIP), FDA suitability, suitable for food, decontamination|
|Economic features||24h operating, online filter change, service friendliness, cost analyses over the plant lifecycle|
The primary and secondary features are the parameters for designing a filtering installation and set up the basis for the following constructional characteristics of procedural filter plants:
Kind and form of filter elements:
- filter confection and geometry: bag, pocket, cartridge, rigid-body, pleated elements, cassette, etc.
- filter material: polyester, polypropylene, PTFE and much more
Design of the filter plant:
- air-to-cloth ratio
- geometry: rectangular, round
- raw gas channel: horizontal, vertical (axial), tangential, pre-separator, downflow principle, terminal velocity
- cleaning principle: jet-puls, back-flush, shaking
- design of the dust discharge: discharge, hopper inclination
- kind of filter media change, raw gas side, clean gas side, horizontal, vertical, dust-free, safe-change, wetting
Besides the primary - mostly procedural - influencing factors, the safety-related parameters and production-related features become increasingly important.
Filtering separators (stand alone unit) in principle consist of:
- Filter head with regeneration device (today typically compressed air cleaning)
- Intermediate base for holding the filter elements
- Filter elements
- Dust collecting bin
- Dust discharge in various designs
- Additional components, e.g., cleaning controller and discharge elements
|Fabric filter with mechanical cleaning||Cassette filter with back flushing (1)||Filter with jet-pulse cleaning|
|Gas volume flow||500 m³/h||300 - 30,000 m³/h||100 - >2,000,000 m³/h|
|Filter area||> 5m²||10 - 300 m²||0,5 - >20,000 m²|
|Air-to-cloth ratio||0.7 - 1.5 m³/m² min||0.5 - 1.7 m³/m² min||0.5 - 500 g/m³|
|Dust load||0.5 to 500 g/m³||< 1 g/m³||0.5 to 500 g/m³|
|Filter material||fabric||micro-fibreglass or nonwoven with PTFE membrane||needle-felt, nonwoven, sintered plastic|
(1) Infastaub uses back flushing only for cleanable cassette filter type MKR, on which the above mentioned figures rely to. Reverse air cleaning filter with filter pockets or so-called lay-flat-bag elements of diverse producers can cope with volume flows up to 100,000 m³/h with accordingly high filter surfaces.
After technical clarification, very different plant designs are available. These can be commercially compared. But today, a simple cost consideration existing of investment costs and expecting filter life time is not timely anymore.
This shouuld be changed by an economic consideration over the entire plant life time. The following features should be considered:
- investment costs
- energy costs
- energy costs of filter cleaning (pressurized air, respectively nitrogen or similar / power)
- costs filter media as well as general costs for spare parts over the entire plant life time (including costs for assembly)
- production downtime costs during filter media change
- estimated costs for dismantling and possibly decontamination
Every on the market existing cleanable filter design has because of their special features its reason for existence.
The following table shall only highlight the special weaks and strengthens of the construction designs, in order to find the appropriate solution for a filter project. Only single stage plant up to a volume flow of about 100,000 m³/h are considered. The presented ratings are relatively to the particular comparing design, beginning with the most appropriate (+++). Features, that have only slight deviations, are not displayed in the overview.