Design and selection of bag dust collectors

Thread Content

The first step in designing and selecting a pulse bag filter is to understand the conditions of the dust-containing gas that needs to be treated by the filter (including flow rate, temperature, humidity, dust concentration, properties of the dust, etc.). After that, the filter itself, as well as the piping system and auxiliary equipment, are designed and selected. Just like with other dust removal systems, several factors need to be taken into consideration when designing and selecting a bag filter. 1. In the case of pressure loss, in the absence of more appropriate design experience, 75×9.8 Pascals can be used as a typical value at low filtration speeds. However, if a higher filtration speed is used, or if there is a layer of dust that is sticky or has a low porosity, a slightly higher value should be adopted. 2. The filtration rate is the gas-to-cloth ratio. It is the ratio of the flow rate of the filtered gas (m3/min) to the area of the dust removal bag (m2), with units of m3/min·m2 = m/min. This is a unit of velocity; it represents the average speed at which gas passes through the dust removal filter bags, without taking into account the large area occupied by the fibers that make up the fabric. For this reason, it is often also referred to as the “apparent surface velocity”. Filtering speed is a very important factor determining the performance of bag filters. Too high a filtration speed can result in excessive pressure loss, reducing dust removal efficiency and causing the dust collection filters to become clogged and damaged prematurely. However, increasing the filtration speed can reduce the filtration area required by bag filters, allowing smaller dust removal equipment to handle the same volume of gas. There is an optimal filtration velocity for a certain set of design parameters (i.e., dust collector type, cleaning mechanism, fiber material and fabric structure of the dust collection bags, properties of the dust, etc.). Generally, fine dust results in a lower filtration speed compared to coarse dust, and smaller dust collectors often employ a higher filtration speed than larger ones. Pulse and reverse jet dust collectors can achieve a higher filtration speed than vibration and reverse airflow types. For vibrating and counter-current dust collectors used for ordinary dust, the filtration speed can be set at 0.9 m/min. From the perspective of balancing both investment and maintenance, it is recommended that the maximum filtration speeds for feed and grain dust, as well as flour, be 0.98 and 0.75 m/min respectively. 3. Filtering area: The actual fabric area required (in m2) is obtained by dividing the gas flow rate entering the dust collector (in m3/min) by the selected filtering velocity (in m/min). Due to temperature changes and additional increases or decreases in air volume, the volume of dust-containing gas entering the dust collector is not necessarily the same as the volume discharged during the production process. If the dust collector is used in applications where the flow rate varies, it is necessary to decide whether to calculate the filtration area based on the peak flow rate or the average flow rate. Additionally, the inventory of dust collection bag fabric in the dust collector should also be considered to facilitate dust cleaning, inspection, and maintenance. 4. Shell structure: It is relatively easy to determine how many compartments are required for a bag filter. After determining the area of the dust removal bags for each compartment, it is necessary to decide on the most suitable length, diameter, and spacing between the dust removal bags. For dust collection filter bags of normal length, a spacing of not less than 50 millimeters is advisable; for particularly long ones (such as those over 3 meters in length), the spacing should be even greater. Dust removal filter bags should be grouped, with a passage left between the two groups. If there is a channel on only one side, and the dust collection bag has a diameter of 150 millimeters, there should not be more than three or four rows of filter bags per group; for bags with a diameter of 300 millimeters, there should not be more than two rows per group. If there are channels on both sides, there should not be more than eight rows for those with a diameter of 150 millimeters, and no more than four rows for those with a diameter of 300 millimeters. The channel width is 450–600 millimeters. The total number of dust removal filter bags per group depends on the vibration mechanism used. Typically, a set of filter bags is operated by a set of vibration mechanisms. A partitioned chamber can contain several sets of filter bags.