Paying closer attention to the air-filtration systems of pneumatic conveying operations can avoid losses in efficiency in compressed-air usage during filter cleaning
The inefficient use of compressed air in solids-handling processes, such as pneumatic conveying, can waste a significant amount of money. One of the areas that can be improved is in the air-filter cleaning system. Careful attention to the design and operation of the air-filtration system can lead to more efficient use of compressed air.
Air-filtration systems are often underappreciated and overlooked, and are sometimes treated as “fit and forget” equipment in process plants. In cases where process conditions change, such as modifications to a pneumatic conveying system or shifts in the characteristics of the solid materials being handled, inefficiencies can result, unless corresponding changes to the air-filtration system are also considered. Changes in use or deterioration of filter efficiency — which can sometimes be present at installation for a poorly designed filtration system, and sometims develop over time as process conditions change — usually translates into higher air-consumption demands. And compressed air that is ineffectively used translates directly into wasted dollars.
To meet budget constraints or project deadlines, process engineers may unintentionally convert plant operating dollars into “thin air” through the inefficient or excessive use of compressed air. This article provides information that solids handlers should consider for maintaining the efficiency of compressed air use in air-filter cleaning systems.
Pneumatic conveying pressure drop
Pneumatic conveying systems can be troublesome to optimize without prior knowledge of the conveying characteristics of the bulk solids being handled. The importance of using such design information is slowly growing in its uptake among engineers responsible for these solids-handling systems. One of the critically important aspects is that of the pressure drop requirement for a given system that is operating at a given transfer rate with a known bulk solid material. For systems that have been designed purposely for a particular duty, an element of over-design is usually incorporated to allow for foreseeable changes in the bulk solid material. The “same” material from different sources is often actually anything but the same, and can create challenges in terms of processing and handling.
In many cases, pneumatic conveying systems used in plants have not been optimally configured, but rather, they have been required to cope with changing plant requirements over a number of years (or more commonly, decades). Taking into account wear on the equipment, adjustments to pipe routes and changes in the bulk solid material, these conveying systems are often operating on a “knife edge,” where small increases in pressure drop throughout the system (for a variety of reasons) can often lead to unexpected downtime and loss of product.
One aspect of pneumatic conveying systems that can contribute to these problems is the air filter on the receiving silo or bin. When a pneumatic conveying system becomes prone to blocking, one of the first remedial actions plant engineers might take is to increase the airflow. This response brings with it a raft of other unintentional negative consequences, not least of which is excessive air consumption.
Air filters in pneumatic conveying
How can the air filter contribute to excessive air consumption? In understanding the answer to this question, it is important to realize that a “value engineering” approach may have been taken with these conveying operations. Value engineering refers to the idea of reducing costs for equipment that is not considered to warrant an adequate budget. Unsurprisingly, the “out of sight, out of mind” approach to air-filter procurement often results in installed units that may exhibit marginal performance from the moment they are commissioned. The concept of including an element of overdesign for the pneumatic conveying system usually does not apply to the filters.
The types of filters employed in reception vessels can be broken down into two basic styles: mechanically activated and reverse-jet pulse. The former should be applied to batch transfer processes where a significant downtime occurs between loading cycles. In these cases, there is an adequate period within which stimulation of the filters (bags or socks) can be undertaken. The filter media used in such systems relies predominantly on the surface capture of particles, rather than in-depth capture. Therefore, they can be dislodged with relatively high efficiency. The sequencing and operating duration are usually wholly dictated by the operational timing for the process. Such filters can be robust and offer good service. However, these systems are not suitable for continuous processes (where no downtime occurs for cleaning functions).
Continuous processes typically use reverse-jet cleaning systems. In these cases, the cleaning cycle is often completed within 20 ms. For such systems, multiple filter elements are secured into a top plate of the filter housing and a pulse of compressed gas is directed down into multiple or single filter elements. The cleaning function is achieved through the pressure pulse accelerating the filter media, which will decelerate abruptly once it has reached its limit of expansion. This results in the onward loosening and detachment of captured particles ahead of the gas volume flow, which serves to transport particles out of and away from the filter face. As the pulse travels down and dissipates progressively through the filter media, energy is lost while, correspondingly, a degree of gas volume increase occurs. Thus, the cleaning mechanism down through a filter changes. Gas reservoir pressures used in such systems can range from 3 to 5 bars and the pressure applied should be matched to the duty and nature of the filter media. In fact, this style of cleaning can also be applied to batch processes in which sufficient time may exist for a gas back flush to remove particles captured in-depth.
Although many equipment suppliers employ commonly sourced filters, major differences can be found in the methods for applying the pressure pulse. And since compressed gas is equivalent to money in processing operations, judicious usage should be the order of the day.
The most basic arrangement employs a single external air reservoir and multiple air distribution tubes (each serviced by a dedicated solenoid). Such arrangements would typically see the distribution tube passing across the inlets of multiple filters, with the bleed hole in the pipe centered above the inlet of the filter. The effectiveness of this arrangement can vary considerably depending upon whether the filter elements have been correctly sized for a given peak air flowrate and, of course, the nature of the particles being captured. As mentioned previously, the optimal use of the pressure pulse capability of a system is key to the capture-and-release characteristics for a given reverse-jet arrangement. Performance can be enhanced through the use of a nozzle at the bleed hole in the distribution pipe. Such arrangements not only focus and preserve pressure, but act as eductors, drawing additional air into the filter. The combination of minimal pressure loss and enhanced gas volume within the filter assist with particle disengagement and mobilization from the filter.
Key to the longevity of a filter is ability for the compressed-air reservoir to have been sized to cope with air requirements that will usually increase over time, as the filter progresses through its lifecycle. When newly installed, the filter will exhibit a low initial pressure drop, which will progressively increase over a series of “cleaned-to-loaded” cycles of operation. This initial increase in pressure drop will usually stabilize to give a relatively steady “clean” condition as the filter becomes conditioned with embedded particles (Figure 1).
Over time, the mass of irretrievably embedded particles will start to increase to a point where the “dirty” pressure drop contribution of this material begins to dominate that of the mobile particles. During this end-of-life phase of the filter, the difference between the “clean” and “dirty” conditions will decrease. Either through strategic planning or neglect, it is not unusual for filters to be driven to this deteriorating condition.
If the filter cleaning cycle is controlled on a timed basis, then the filter may begin to approach a blinded condition, which gives rise to high pressure drop (sometimes to the extent that the filter housing can be subject to overpressure). This can, in turn, strain sealing gaskets. If the system is controlled on a pressure-drop measurement, then the increased frequency of pulses may match or exceed the re-pressurization capability for a marginally sized air reservoir. This leads to diminishing pulse pressure, and consequently, to reduced cleaning efficiency. This latter effect tends to lead to a runaway deterioration of the serviceability of the filter.
Process variables with strong influence on the ability of a filter to capture (and equally importantly) release particles include particle concentration in the air, and face velocity that develops during operation. Various factors can bring about changes in both these variables. It is fairly common for process plants to change the products that are being manufactured, but sometimes the changes in product have unexpected side effects. An example of this could be a plant whose process-control philosophy is based on measurements of weight, but the plant has changed to a substantially lower-density product. In such a situation, inventory levels in pneumatic receiving bins can increase considerably until the requested weight is achieved. This can bring material levels to a point where there is direct interaction with the incoming charge, creating excessive high dust loadings in the immediate vicinity of the filters (Figure 2).
The net effect is invariably a shortened time period between cleaning cycles (when operating on pressure-drop triggering) and hence much higher compressed-air consumption. On the other hand, an excessive face velocity (embedding particles more strongly) can be generated if factors such as blowdown at the end of a dense-phase batch transfer occurs. Under such circumstances, the filter area may be found to be inadequate if this effect of increased air volume has not been fully taken into account when the filter system was designed and specified. Shifts in particle-size distribution can also be an outcome of the use of excessive air velocities in a pipeline (Figure 3).
Edited by Scottt Jenkins
Richard Farnish is a senior consulting engineer at The Wolfson Center for Bulk Solids Handling Technology at the University of Greenwich (Chatham, Kent ME4 4TB, U.K.; Phone: +44 0208 331 8646; Email: R.J.Farnish@greenwich.ac.uk). The majority of his time at The Wolfson Center is spent undertaking consultancy activities for a wide range of industrial sectors, although he is also involved in the delivery of undergraduate lectures and short courses to industry. A large proportion of his work is linked to troubleshooting bulk solids processes that are underperforming as a result of equipment design issues or product quality problems (segregation, agglomeration, attrition and so on). His research interests relate to optimizing dry-filtration systems. Farnish has worked at the Wolfson Center since 1996. He is a chartered mechanical engineer and a member of the Institution of Mechanical Engineers in (CEng MIMechE) in the U.K.
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