Read Part 1 here.
Cleaning for more than one cycle after system “group stop”
Interlocking the dust collector to operate in concert with the system will streamline and improve efficiency and reliability. As previously noted, when using a condition-based cleaning approach, begin with the needs of the system in mind and adjust the dust collection system accordingly. Good start-up and shutdown procedures will contribute to the life cycle of the filter media. When a baghouse is not interlocked with the system and operates independently, overloading, over cleaning and premature filter failure may occur. The baghouse should operate on the principle of resistance pressures, and if the baghouse is cleaning while the system is not in operation, there would be no resistance pressure.
The filter is only the beginning
Filter media are available in many different fabrics, weights and surface finishes to accommodate a systems environment. However, filter media selection is secondary to proper design. In all cases, the filter media is a part of the overall system but the system has to be designed to accommodate all of its components. For example, many ball mill grinding operations have discharge temperatures that vary from 180 – 240 °F. On occasion, temperatures can spike in excess of 280 °F. Many ball mill systems have water sprays or other means to control or react to temperature spikes. In the case of the filter fabric, temperature experienced by the fabric can cause stress if the fabric is rated for a temperature less than present in this environment. The proper interlock here could be to divert hot gases away from the dust collector based on temperature, presenting dilution air into the system, or water sprays (note: these may cause the presence of excess moisture in the gas stream). The fabric selected should be capable of withstanding all of the system’s variables.
Multi-branch vs point-source
Adding too many pick-up points will inhibit the ability of the dust collector to function properly. Consideration should be given to the ROI for dedusting multiple points, adding static pressure to the system, or over drafting on a case-by-case basis. Interlocking the system in multi-point ventilation environments can be challenging. In this respect, the interlock would need to be placed on the vent duct rather than the system itself. Doing so will create an environment where sealing, static pressure and programming would be needed. This arrangement would be less than ideal when balancing the required capture velocity at a point as mentioned above. Instead, a combination of designed containment, hood and duct needs may necessitate the split of multiple point-source collectors in a system instead of one dust collector with multiple ducts and ventilation points. For example, in a plant where the finish mill dust collector had the capacity to pick-up additional ventilation points, additional duct work was installed to capture particulate for several material transfer points. Because of structural steel and other interferences in the environment, a less than ideal ducting arrangement was created to connect the new vent point to the dust collector. In this case, the duct collector was considered to be the system instead of the transport mechanism. As a result, capture velocities were very high, capturing more than just the dust created by impact. This resulted in excessive mechanical wear in the added duct work and increased grain loading to the dust collector, causing it to clean more often and resulting in too much wear on filters. Additionally, the increased static pressure added to the system caused the fan to work against more resistance in the system, resulting in additional electrical consumption.
An example showing a dust collector as a part of a typical mill sweep system.
There are certainly cases where nuisance dust collection can be coupled with process dust, but in most cases, collection efficiencies are negatively impacted.
When able, consolidating return equipment in a dedust recirculation mode will likely save time, money and resources. In nuisance dust systems, situating a dust collector immediately over the conveying component downstream of the collector is optimal. In process dust collection systems, controlling the dust load and direction of flow also contributes to the successful operation of the system.
A typical baghouse unit showing the clean air exit and the collected particulate transport.
When considering the dust collection needs of a facility, it is important to consider these basic dust collection principles to optimise the overall system performance. Considering that a dust collector is a component of a larger system will streamline the production process and improve overall system efficiency, saving time, money and other resources. Interlocking these components to operate in agreement will further ensure that dust collection operational expectations are met. Close attention paid up front to the detail of system design and the relationship of parts to the whole helps to mitigate high costs and frustration later.
Written by Anthony Johnson, CLARCOR Industrial Air, USA. This is an abridged version of the full article, which appeared in the September 2014 issue of World Cement. Subscribers can view the full article by logging in.
Read the article online at: https://www.worldcement.com/the-americas/05092014/the-sum-of-its-parts-part-2-425/