It is an unfortunate reality that most aspects of bulk solids handling within processes or plants are subject to ‘value engineering’. This close control of funds is often a result of the allocation of an unrealistic CAPEX for a given project.
In terms of what would classically be considered solids handling equipment it can manifest as storage equipment that has poor discharge performance (or no discharge at all!), feeders that offer unstable/unreliable function or pneumatic conveying systems that block or consume excessive energy.
An often-unseen victim of cost cutting are regenerative dust filters (such as those employed to receiving vessels or used for venting of plant). If the solids handling is subject to financial review, then it is almost certain that an early victim in such an exercise will be the dust capture equipment.
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If it is accepted that the importance of correct solids handling equipment design is often poorly understood, then it goes without saying that ‘lower tier’ items such as filters are likely suffer equally, or even more so. In this respect any shortcomings in the performance of a given air filter can represent a considerable vulnerability to overall plant efficiency or reliability.
Depending upon where in the process scheme a filter is employed, the impact on the process can be significant (and seemingly hard to identify in some cases). Examples of flow stoppages are not unheard of where filters are used to handle air leakage associated with rotary valve operation on positive pressure pneumatic conveying systems or where air leakage pathways extend into buffer hoppers.
In such circumstances any shortcoming in the design or operation of the filter can result in over pressure and air leakage into the active flow channel for equipment that is interfaced to the rotary valve or into the main silo that may be connected to a buffer hopper. In such instances the gas volume flow rate relative to the permeability of the bulk solids being handled can lead to a reduction or complete stoppage of flow from above.
Common causes of such a scenario can relate to marginal sizing of the filter area, whereby the air leakage rate of a rotary valve (in its “as supplied” condition) form the basis for determining the face velocity – and hence filter area required.
Initially, the filter area may be sufficient to control face velocities to approximately 35mm/s (this is a typical value that should vary to suit bulk material properties) -on the assumption that the full installed filter area is fully functional throughout the anticipated service life of the filter.
In some instances, a fan will be installed to ensure that the planned face velocity is maintained. However, any deterioration in the functional filter area will obviously correlate into an increase in face velocity over the active face area, and hence stronger/deeper particle embedment, which in turn contributes to a reduction in cleaning efficiency.
More specific to reverse pulse type filters (Fig 1) is the issue of embedment and its impact on cleaning function. The removal of captured particles in reverse pulse systems (whether bag or cartridge type filters) is a function of the interaction of a pulse of compressed air that is delivered into the throat of a filter.
The way that such a pulse interacts with the filter media to obtain a cleaning function varies depending upon the structure of a given filter. In the case of a bag/sleeve type filter, which is supported against a frame, the pulse will provide a high rate of acceleration of the fabric which will end abruptly once the filter reaches its elastic limit.
The energetic deceleration of the fabric provides momentum to project the captured particles from within. It can be readily understood that the level energy introduced into the filter is of great importance – and, of course, at an operational level this takes the form of the pressure setting for the air reservoir.
Equally pertinent to the cleaning is the pulse duration (usually in the 50-100msec range) which is provided through a quick acting solenoid at the air reservoir and interfaced to the blast tube. Such blast tubes would typically extend across the inlets to multiple filter elements or along a single envelope type element.
These tubes are typified by being constant bore and employing ~6mm outlets above each filter centre line. These penetrations may take the form of a simple hole or have a short downward stub (the purpose of which is to centralise the pulse within the filter element and therefore provide a degree of symmetry to the pulse propagation).
A large proportion of filter arrays do not employ venturi above the filter inlet, which would otherwise serve to induct additional air into the pulse ‘cone’ that is generated as pressurised gas exits through the holes in the blast tubes.
The failure to incorporate venturi structures beneath the blast tubes has been shown to increase energy consumption (for a comparable level of cleaning) by up to 30% (Morris, 1984). The reduction in cleaning efficiency when venturi are not employed arises from the ‘cone’ of pressure pulse expansion only intercepting with the interior wall of a filter at a point ~20cm down from the inlet.
This effect generates a gas induction through the filter media over this distance, which in turn dictates that an outward pressure can only develop below this interception point – thus cleaning of the filter cannot occur over the induction zone.
Fig 1 Idealised layout for a reverse jet filter array
The obvious implication of this effect is that although the array filter area is available for particle capture initially, the cleaning function only occurs over a proportion of the total area – with the consequence of higher face velocities over the capture area (Fig 2).
In the case of dealing with air leakage from a rotary valve, it is important to bear in mind that the gas volume to be handled will only increase with time as the clearances at the rotary valve tips become greater. Thus, a marginally sized filter can quickly move into a loading regime that begins to cause problems with cleaning efficiency (and hence further functional area loss over time).
In some cases increasing pulse pressure or frequency (which on the face of it seems like a logical move) serves to increase the rate of filter deterioration by virtue of causing a depletion rate of compressed air from the external reservoir (if it has also been marginally sized).
In this circumstance, pulse pressure lessens which reduces cleaning effectiveness which in turn can result in control settings being modified -and the whole cycle can start again. This type of filter failure is further compounded by the uniformity of hole size along the length of the blast tube – with a deterioration in pressure pulse delivered being by no means unusual for wide, multi filter arrays (i.e. long blast tubes).
This can lead to an uneven distribution of cleaning efficiency across the filter housing as well as down each filter. The proximity of filters to an inlet also serves to generate an inhomogeneous particle capture across the filter array.
In the event that an averaged pressure drop value is being used as the indicator for the overall filter array it is evident that a risk exists that a proportion of the replaces filters may still have a useful service life remining. However, the down time and economics associated with servicing often dictates that a complete array may be changed out.
Fig 2 Post cleaning evidence of uneven particle embedment and, consequently, establishment of preferential channelling (Koch 2008) – image c/o University of Greenwich
In conclusion, this brief article can only capture a few of the many aspects of filter design and operation that critically affect performance and life cycle considerations. It is difficult to pick out any one aspect of bulk solids handling equipment that should be prioritised, as all elements of equipment are equally critical to the efficient operation of process – however the particle capture aspects of a plant should be given an equal weighting of importance. Sadly, this if often not the case.
*Morris, W, 1984. Cleaning mechanisms in pulse-jet fabric filters. Filtration and Separation, Volume 21, pp. 50-54
*Koch., 2008. The permeability distribution (PD) method for filter media characterisation. Aerosol Science and Technology, Volume 42, pp. 423-444.
For further information please contact wolfson-enquiries@gre.ac.uk or call 020 8331 8646
