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Industrial dedusting with bag filters

The evolution of the jet pulse bag filter technology

Dipl.-Ing. Theo Schrooten, Astrid Kögel, Dr.-Ing. Gunnar-Marcel Klein,
Intensiv-Filter GmbH & Co. KG, Velbert-Langenberg

The oldest cleaning method for bag filters (still used in mobile filters these days) consists of motorised or manual jolting equipment. With automatic regeneration, a motor is started, either at certain intervals or when reaching a maximum filter resistance. This oscillates the filter element. During the resulting movement, the deposited dust cake detaches from the filter surface and drops down into a dust collector, which is usually cleaned via dust dischargers. Mechanical cleaning is performed after interruption of the filtration. The filter bags are stressed mechanically through shaking during the cleaning operation and therefore have relatively short service lives. The periodic reversal of the flow direction (backwashing filter) is a much gentler cleaning method. Here, the filter system features several separate chambers which are cleaned individually. A combination of both cleaning methods was also often realised. For the last decades, however, the jet pulse method has become the standard cleaning method. The filter media are regenerated through cyclic, intense blasts of compressed air. This briefly causes overpressure in the filter bag during cleaning. The filter bags are briefly inflated, the flow direction is reversed and the filter cake detached (fig. 1). A supporting cage gives the bag the required stability during the filtration phase. Among other things, the cleaning cycles depend on the filter load (volume flow per filter surface and unit of time), the gas density, the raw gas charge and the particle attributes. The regeneration can be controlled in time intervals or via defined differential filter pressures.

Fig. 1: Filtering and cleaning position of a filter bag cleaned with compressed air applied from the outside

Constructive features of modern jet pulse bag filters

Ideally, the raw gas is guided in cross flow to the filter bags to prevent a flow contrary to the particle sedimentation direction. The raw gas is directed across a distribution plate; the pre-separation takes place here and the raw gas flow is homogenised in the filter housing. The particles are separated on the surface of the filter medium resp. on the surface of the filter cake forming there. The corresponding flow resistances result from the pressure losses of the filter cake DpFK and the filter medium directly after jet pulse cleaning (residual pressure loss Dp0). The cleaned gas exits the bag on top. Especially with long bags (e.g. bag length 8 m at a bag diameter of 160 mm) and high filter surface loads, the pressure loss via the bottom of the bag, meaning upon emergence from the bag via the inlet nozzle to the clean gas section, is also of importance. This and all further flow resistances of the filter housing (raw gas inflow up to the filter cake surface, clean gas flow from the bag escape up to the clean gas channel escape) are combined in the pressure loss of the housing DpG. After cleaning of the filter bags, the dust particles sediment into the dust collection chamber and are removed from there mainly via worm conveyors and rotary dischargers. In online operation, the particles in the raw gas chamber are filtered continuously. Directly after jet pulse cleaning, the particle concentration in proximity of the filter bag is very high. Especially with finely disperse dusts showing low agglomeration properties, removed particles may settle on the filter again at this state. This "internal" dust circulation may cause a significant share in the filter cake volume and thus contributes to pressure loss. Therefore, the flow in the filter modules is cut off during cleaning using valves on the raw and/or clean gas side to increase energy efficiency. This so-called offline-mode (with exclusive chamber deactivation semi-offline on the clean gas side) prevents the immediate re-attachment of the dust on neighbouring filter bags. Cleaning with a compressed air pulse with a much lower intensity than that of conventional jet pulse filters is another advantage. Current offline bag filter systems have a modular design and cover a volume flow range from 50,000 m3/h n.c. up to 2 million m3/h n.c. Modern offline jet pulse filters feature flow-optimised components, e.g. flush raw gas valves and a flow-optimised design of the gas flow control in the filter via CFD.

Evolution of the injector technologies

The compressed air injection system for the periodical regeneration of the filter bags perfused from the outside in is of decisive importance for energy-efficient operation. Cleaning has to be performed in such a way that the filter cake is completely detached across the entire bag length. Parallel to that, the rebound of the medium to the support basket must be minimised through the corresponding modulation of the pressure flow. Many injector systems consist of a blast pipe with simple borings through which the compressed air emerges. Secondary air is sucked in through the downstream venturi tube (fig. 2.1), which results in an increase of the static pressure in the filter bag. The feed nozzle (fig. 2.2) with its reduced flow losses is another optimisation. Flaring the nozzles to produce an "ideal nozzle" (fig. 2.3) results in a further increase of efficiency when converting the compressed air energy to a cleaning impulse. The so-called Coanda injector is a very efficient cleaning technology. This cleaning system uses the so-called Coanda effect in which the compressed air emerges from a ring gap and is directed across a curved surface. Here the primary air follows the boundary layer that does not come off due to the geometry of the Coanda injector. This results in an extremely high vacuum in the first injector stage, which sucks up further secondary air and forms a propulsion jet with a much higher air quantity compared to the previously described variants (fig. 3.4). This propulsion jet enters the intake nozzle as second injector stage in which additional secondary air is sucked in. A benchmark of the described systems based on the local effective internal bag pressure measured with piezo-resistive pressure sensors prove that the maximum jet pulse is always achieved with the Coanda injector at tank pressures from 0.2 to 0.5 MPa compared to all other injectors.

Fig. 2: Display of the different jet pulse cleaning systems

1 Blast pipe with boring and venturi intake nozzle

2 Blast pipe with boring and intake nozzle

3 Blast pipe with ideal nozzle and intake nozzle

4 Blast pipe with Coanda injector and intake nozzle

Optimisation of the cleaning control system

These days, cleaning control is performed using the microprocessor technology and field bus systems. Besides controlling the membrane valves, the pneumatically or electrically actuated raw and clean gas valves are addressed and signals emitted by field sensors, e.g. "broken bag monitors" processed. Jet pulses are either clocked with a fixed time control or via differential pressure control with variable cycle times. The continuous control of the compressed air tank pressure is another control parameter for demand-oriented cleaning. The compressed air demand is adapted to the respective local operating conditions by continuous adjustment of the cleaning pressure. The differential filter pressure is a controlled variable of so-called admission pressure-controlled cleaning. The operational parameters of the dedusting facility are maintained at the desired operating point with minimum compressed air demand. The dust accumulation is homogenised and the capacity of the dust extraction components improved. Parallel to this, the service intervals of the filter system can also be prolonged due to the lesser mechanical strain on the filter bags.

Improved bag filter media with reduced pressure loss

The pressure loss of the filter medium and the deposited filter cake makes up the largest share in the costs for the energy required for the operation of the filter system by far. Current developments aim at reducing the residual pressure loss Dp0 and reducing the rise in pressure during the first filtration phase (phase with non-linear pressure rise prior to the onset of actual cake filtration). These developments resulted in new filter media that feature microfibres with a titer of ≤ 1.5 dtex between the supporting fabric and the raw gas side. Their alignment results in a minimal residual pressure loss as well as an almost linear progression of the pressure loss curve in the first filtration phase after jet pulse cleaning. Tests in line with VDI 3926 Page 1 confirm the advantages of these new filter media compared to the presently used needle fleeces and needle fleeces with an ePTFE membrane laminated on top. Although the much more expensive filter media with ePTFE membranes also show a low pressure gradient in the progression of the pressure loss, they start out with a very high residual pressure loss, which can be attributed to the very small pore width of the membrane and irreversible dust deposits in the membrane. In comparison, conventional needle fleeces and microfibre needle fleeces show low residual pressure losses, but at the same time a high increase of the pressure loss curve directly after cleaning. As a result, they just about achieve the pressure level of the ePTFE membrane at the end of the cycle. Validation tests on a semi-technical scale (filter system with 10 bags, D = 160 mm, L = 4000 mm) prove that the differential pressure level can be halved with microfibre-based filter media compared to conventional needle fleeces and needle fleeces containing microfibres (fig. 3).

Fig. 3: Differential pressures between raw and clean gas side, different filter media, test facility with 10 bags

By optimising the cycle times, it is possible to further reduce the pressure loss between the raw and the clean gas side. As filter media based on microfibres show very high degrees of respirable dust, the cycle time can be reduced directly after jet pulse cleaning without increased dust passage. The reduction of the cycle time from the presently standard 300 s to 100 s results in a reduction of the average differential pressure by the factor 4 compared to the present state of technology.

Reducing the LCC for process filters: The new ProJet mega® series

With the design of process filters in the cement and basic materials industries, the aspect of the operating costs and consequently, above all, the energy costs are of particular significance. The period of time drawn upon by operators for the cost comparison of different filter concepts in new investment or upgrade decisions is 10 years in many cases. For a typical filter size of 1.2 million m3/h a.c. in the exhaus gas flow, the operating costs far exceed the investment costs for the filtering installation in this balance sheet period. Thus in the performance specifications for the development of the new process filter series from Intensiv-Filter, the lowering of the operating costs had top priority. Another focal point of the development work was in the control of the extremely high variance in process filtering installations, to allow for various procedural and structural conditions. The aim of the "standardization" project carried out between 2007 and 2009 at Intensiv-Filter was therefore the reduction in part variety (inner design variance) with the simultaneous retention of a maximum number of possible designs (variability). The result is a completely newly developed and modular filter series designed in 3D CAD for the assembly, the ProJet mega® shown in figure 4. From a relatively easy-to-grasp number of constructional basic elements, over 600,000 different versions can be displayed just for row filters with 1-12 filter chambers (each with eight injector tubes x 8 - 17 injectors per row) alone. The versions extend from single row filters with 20,000 m3/h a.c., cover row filters for the mean volume flow range and go up to double row filters with a maximum 12 m bag length, 64 filter chambers (x 136 bags) and a volume flow of 3,000,000 m3/h a.c. ProJet mega® filters are available in black steel, VA or in a mixed design. Depending on the requirements and process conditions, ProJet mega® filters are designed in the following basic jet pulse cleaning designs: Online, online with lockable clean gas and raw gas flaps for online maintenance, semi-offline (one chamber is continuously separated for cleaning by means of flow interruption on the clean gas side) and offline (like semi-offline, but also has automatic raw-gas-side shut down of the chambers to be cleaned). The standardized housing elements and wall modules were optimized using FEM to achieve a maximum bending stiffness and are available in wall thicknesses of 3, 4 or 5 mm. The welded assemblies are pre-fabricated on automatic production facilities with laser robots with a high level of productivity and accuracy. The ProJet mega® series is both turn-key in finished unit supply available, as well as for delivery as a drawing component and as parts (e.g. filter head modules in hardware, housing, hopper and complete periphery, i.e. tubes, dust discharge and steel construction as a drawing component). The reference list for the series first supplied in 2009 already includes numerous process filters in Europe and Asia, both as an electrostatic precipitator retrofit and as a new installation, incl. turn-key projects (Fig. 4).

Fig 4: ProJet mega® bag filter with offline cleaning

Fig 4: ProJet mega® bag filter with offline cleaning

For a significant reduction in the operating costs and thus the LCC, the ProJet mega® series has the following characteristics:

  • Distribution of raw gas via a CFD optimized flow guidance system to achieve a crossflow and top-down flow in the raw gas room and between the filter bags.
  • New design of the clean gas and raw gas flaps with a robust, pneumatic rotary drive and with minimized flow resistance (CFD optimized) and raw gas flaps that are arranged flush with the wall.
  • Intensiv-Filter design of valve block with integrated diaphragm with a particularly large lift and a straight injector tube inserted for the shortest flow paths.
  • The latest injector technology, either with ideal nozzle or with the Intensiv-Filter Coanda injector.
  • All online and offline operating modes are implemented in the standard version, e.g. semi-offline with low pressure cleaning, minimum compressed air consumption and maximum bag service life.
  • Cleaning control system using Intensiv-Filter JetBus Controller® and supply pressure controlled control system; here, the filter differential pressure as a control variable is maintained constantly as a correcting variable using the continuous control of the tank pressure and so that the required compressed air consumption is reduced to a minimum. At the same, the load on the filter bags is minimized and their service life increased.
  • Filter bags "made by Intensiv-Filter" in different designs: e.g. ePTFE/glass fibre membrane media or microfibre needlefelts of the ProTex generation with additional advantages in regard of energy efficiency, e.g. ProTex PI and ProTex m-Aramid for high-temperature applications.

By optimizing the housing flow, the cleaning system and the filter media, the use of filter bags up to 12 m in length is possible in the ProJet mega® series. The cost reductions in industrial dedusting processes that can be achieved in the described technical innovations for the ProJet mega® series are summarized in Fig. 5 in an exemplary case for comparison with conventional types (pressure-surge cleaned bag filters online, conventional needle felt medium, bag lengths 6 m). In this connection - when the process conditions are the same - the investment costs are reduced by 20 % (above all, because of the increase in bag lengths from 6 m to 10 m), the maintenance costs by 20 % (increased service interval for the filter bags because of lower loads) and the energy costs by 45 % (combined effect of the reduction in the filter resistance and the compressed air consumption in semi-offline mode and the lower differential pressure when using the ProTex PI filter medium. In total, the LCC can be reduced by 40 % with the newly developed filter technology over a balance sheet period of 10 years.

Fig. 5: Comparison of investment costs, energy costs, operating costs and LCC for ProJet mega® process filters in contract to a jet pulse filter with conventional design

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