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19 / November 2020 powderbulk.com Then, that number is corrected for the actual pressure and tem- perature conditions in the filtration equipment, and the resultant num- ber is the final design airflow rate. Taking the surge factor into consid- eration will ensure that the filtration equipment's capacity is adequate for the equipment's purpose. As an example, the very common pressure differential truck-to-silo unloading system is very prone factor concern is mitigated as the primary receiver, such as a cyclone or receiving bin, will dampen the airflow so that there's less of an impact on the airflow to the remote dust collection point. Typical surge factors to apply to the system's design airflow rate are as follows: • Dilute-phase mode: 1.0 • Two-phase mode: 1.5 to 2.0 • Dense-phase mode: 2.5 to 3.0 FIGURE 1 Dilute-phase pneumatic conveying system that uses a multiplier of 1.0 (left) versus a dense-phase pneumatic conveying system that uses a multiplier of 3.0 (right). Note the slug flow in the dense-phase system. Gas Flow performance need to be considered while designing filtration equip- ment for either system. The primary difference to keep in mind is that in a pressure sys- tem, the end of the conveying line — where the venting and filtering take place — will be at near atmo- spheric conditions because the pressure blower is upstream of the filtration equipment, and the back pressure created on the filter hous- ing is from the air moving through the filter elements. Back pressure is the resistance against the airflow through the filtration equipment. Whereas in a vacuum system, because the vacuum blower is downstream of the filtration equip- ment, the filter housing must be designed to withstand the convey- ing system's full operating pressure. Conveying mode. The second consideration is the conveying mode in which the material will be transported. This has a significant influence on the filtration equip- ment design because — depending on the conveying mode chosen — a surge factor over the pneumatic conveying system's design airflow rate needs to be considered. The surge factor is the multiplier used on the pneumatic conveying sys- tem's design airflow rate. The main reason for using the surge factor is that as the solids-to-air ratio increases and the airflow becomes denser, the more likely the slug (amassed material and air moving in a wave formation, as shown in Figure 1) in the pipeline will grow and result in intermittent dis- charge of the dense slugs with less dense periods of airflow. When this happens, there's an instantaneous amount of air and material that discharge into the filtration equip- ment, which could overload the filter elements, creating a greater- than-anticipated back pressure. If the filtration method is a remote dust collector, this surge FIGURE 2 Can velocity (red arrows) is the velocity of the air and solids right before they encounter the filter media, while interstitial velocity (green arrows) is the upward velocity of the air and solids in the open areas of the filter section between the filter elements. Can velocity Interstitial velocity Clean gas outlet Dust Inlet Solids outlet