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20 / November 2020 powderbulk.com Air-to-cloth ratio. The fourth consideration is to calculate the filtration area required for the filtration equipment using the air-to-cloth ratio. Divide the final design airflow rate (the pneumatic conveying system's airflow rate multiplied by the appropriate surge factor and corrected for actual fil- tration equipment conditions) by the air-to-cloth ratio to get the total filtration area in square feet. Once this number is determined, the appropriate filtration unit size can be selected. The air-to-cloth ratio to be used highly depends on the material's properties and the filtration media type, a couple of which are shown in Figure 3. Typical air-to-cloth ratios to be used are as follows: • High-pressure, reverse pulse bag filter type: Use 5-to-1 to 10-to-1 ever, if the interstitial velocity is too high, when the filter elements are pulsed during the cleaning cycle, the dust won't fall into the hopper or bin below but will simply be re-entrained on the filter elements when normal airflow through the elements is returned. This situation can lead to increased differential pressure across the filter elements, which will likely lead to an increase in the cleaning cycle duration, fre- quency, or both, which can result in a shorter filter life. The increased back pressure in the filtration equipment can lead to conveying system performance issues as well as particulate emis- sions to atmosphere if a vacuum or pressure relief valve is being used with a low release pressure setting. Undersized filtration equipment can choke when the pneumatic conveying system discharges into the material receiver. This situation normally causes a back pressure to the system that wasn't considered in the design. This back pres- sure could reduce the pneumatic conveying system capacity if the system was designed at the limit or could cause the pressure relief valves on the material receiver to open, emitting fugitive dust around the material receiver environment. In another case, the back pressure could also influence the bin weight if load cells are installed on the bin, which would influence the overall production accuracy. Pressure drop. Pressure drop is the pressure difference between the two filter sides (before the filter and after the filter) in an airflow system. The filtration equipment at the end of the system has its own specific pressure drop in regard to which filtration system type is chosen. No matter what system is chosen, the filtration pressure drop becomes part of the total system pressure drop. to this condition of high instan- taneous air volume because the pressurized truck volume is rapidly released as the truck empties. That puff of dust from the bin vent on top of the truck-unloading silo is a sign of pressurizing the filtration equipment when that instantaneous air bubble enters the filter with too low of a filtration area. This is why giving consideration to a pneumatic conveying system's surge factor is critical to proper operation. Velocity profile. The third con- sideration is the velocity of the air and solids leaving the conveying line and the subsequent can veloc- ity in the filter unit. Can velocity is the velocity of the air and solids right before they encounter the filter media, as shown with red arrows in Figure 2. Can velocity needs to be carefully selected based on the application and the mate- rial properties. While there aren't any real consequences of a too-low velocity in a filtration system, a too-high velocity could wear out the final filter-receiver and could also lead to premature wear of the filtration media. Interstitial velocity is another velocity that needs to be considered when designing and selecting an appropriate filtration unit. Intersti- tial velocity is the upward velocity of the air and solids in the open areas of the filter section between the filter elements, as shown with green arrows in Figure 2. This velocity takes the filter elements into consideration. Interstitial veloc- ity is calculated by taking the final design airflow rate as previously mentioned but using the cross- sectional area of the filter section minus the cross-sectional area of the filter elements themselves to get the true interstitial velocity between the filter elements. Similar to can velocity, there aren't any consequences for having a too-low interstitial velocity. How- FIGURE 3 Not all filters have the same air-to- cloth ratio. A bag filter (right) has an air-to-cloth ratio of 5-to-1 to 10-to-1, while a pleated cartridge filter (left) has an air-to-cloth ratio of 2-to-1.