Reverse-pulse cleaning

Fabric-filter designs fall into three types, depending on the method of cleaning used (1) shaker-cleaned, (2) reverse-flow-cleaned, and (3) reverse-pulse-cleaned. 

In the reverse-pulse-cleaned filter, the bag forms a sleeve drawn over a cylindrical wire cage, which supports the fabric on the clean-gas side (i.e., inside) of the bag. The dust collects on the outside of the bag. 

The off-gas from each furnace is cooled in an evaporative gas cooler and cleaned in a reverse pulse baghouse before being either vented to atmosphere or used in manufacturing sulfuric acid. The baghouse dust from both the smelting and reduction furnaces is combined and recycled through the smelting furnace. 

As a result, there is a growing trend to off-line clean reverse-pulse filters by using bags with multiple compartments. These sections allow the outlet-gas plenum serving a particular section to be closed off from the clean-gas exhaust, thereby stopping the flow of inlet gas. On the dirty-side of the tube sheet, the isolated section is separated by partitions from the neighboring sections where filtration continues. Sections of the filter are cleaned in rotation as with shaker and reverse-flow filters. 

As discussed in the preceding section, filter bags must be periodically cleaned to prevent excessive build-up of dust and to maintain an acceptable pressure drop across the filters. Two of the three designs discussed, reverse-flow and reverse-pulse, depend on an adequate supply of clean air or gas to provide this periodic cleaning. Two factors are critical in these systems the clean-gas supply and the proper cleaning frequency. 

In reverse-pulse applications, most plants rely on plant-air systems as the source for the high-velocity pulses required for cleaning. In many cases, however, the plant-air system is not sufficient for this purpose. Although the pulses required are short (i.e., 100 milliseconds or less), the number and frequency can deplete the supply. Therefore, care must be taken to ensure that both sufficient volume and pressure are available to achieve proper cleaning. 

Barrier filters are cleaned by periodically passing pulsing clean gas through the filter in the reverse direction of normal gas flow. To reduce the overall particulate load, these filters are typically placed downstream from cyclone filters. Barrier filters are effective for removing dry particulates but are less suitable for wet or sticky contaminants such as tars. 

Two principal types of fabric are adaptable to filter use woven fabrics, which are used in shaker and reverse-flow filters and felts, which are used in reverse-pulse filters. The felts made from synthetic fibers are needle felts (i.e., felted on a needle loom) and are normally reinforced with a woven insert. The physical properties and air permeabilities of some typical woven and felt filter fabrics are presented in Tables 17-6 and 17-7. The air permeability of a filter fabric is defined as the flow rate of air in cubic feet per minute (at 70°F, 1 atm) that will pass through 1 ft2 of clean fabric under an applied differential pressure of Vt in water. The resistance coefficient KF of the clean fabric is defined by the equation in Table 17-6, which may be used to calculate the value of KF from the air permeability. If Ap, is taken as 0.5 in water, t as 0.0181 cP (the viscosity of air at 70°F and 1 atm), and Vj as the air permeability, then //, = 27.8/air permeability. 

Bag filter batch load cycle and clean intermittent shaking, reverse pulse, reverse blow ring or sonic cleaifing. Load filter until the gas pressure drop across the filter is >1.5 kPa, then clean. Choice of fabric is critical static charge on fabric, operating temperature, potential for fumes to absorb with moisture to deteriorate bag, and need to select dust removal option to keep the Ap across the bag of 0.5 to 1.5 kPa. Felted material gives higher gas flow rate per unit area than woven, costs 3 to 4 times more, and cannot be cleaned by... 

In the spray calciner, liquid waste is pumped to a nozzle at the top of the calciner where it is atomized by pressurized air, producing droplets with diameters less than 70 irni that are dried and calcined in-fli t in the 700 C-wall-temperature spray chamber. Sintered stainless steel dust filters collect a portion of the powder with a mean diameter of 10 fjm. They are periodically cleaned by a reverse pulse of air. Calcine from the spray chambers and filters drops directly into the melting canister. Frit is fed to the cone of the calciner. 

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