【文章內(nèi)容簡介】 sing. Because the cleaning air is very small pared with the main process air, there is no need to partmentalize this type of bag filter. The pulse jet cleaning is generally very efficient and, because the operation of all the bags is continuous, the air to cloth ratio is much higher 12 than with the other two types and consequently requires less space within the plant Fertilizer applications are not the easiest due to the sometimes sticky and hygroscopic nature of the dust. Our experience with reverse air bag filters has been poor whereas pulsejet type filters have been found to be the most effective for use in our industry. We will, therefore, concentrate on pulse jet filters for the remainder of this paper. 2. Filter bag design features Filter Bags / Cages In theory filter bags can be pleated or smooth. Bags can be cylindrical or envelope shaped. We remend only smooth, cylindrical bags. These bags are typically about 130 mm in diameter. We also remend that bag length be limited to no more than 12 ft (3657 mm) to ensure efficient cleaning. The material and finish applied to the bag will depend on the properties of the dust being handled. However for DAP, polypropylene with a glazed finish is remended. As mentioned earlier, a metal wire cage is inserted into each bag as a support. Galvanized cages are typically the standard, but we remend stainless cages. We have experienced some problems with bridging between the outermost bags in the unit and the outside casing. To avoid this we have had success with removing all bags around the periphery to increase the space between the casing and the nearest bag. This effectively downrates the standard offering from the vendor but does avoid the bridging problem. Pulse Jet Cleaning System The pressed air injection is normally controlled by two timers。 one for frequency and the other for duration. The duration of the pulse is very short – of the order of seconds. The frequency is set by monitoring the rise in pressure drop and pulse cleaning often enough to prevent a preset pressure drop from being exceeded. Typical pressure drops are in the range of 75 to 150 mm. It is possible to replace the frequency timer with a pressure switch which actuates the 13 pulse cleaning to maintain pressure drop although we have not found this to be necessary. Double diaphragm solenoid valves are remended because they offer extremely quick opening times and improved bag cleaning. Access to the Outlet Air Plenum If the unit is to be placed outdoors, a walkin plenum is specified to gain access to the outlet air plenum. For indoor locations, a walkin plenum is not required. Either way, a series of quick release inspection / maintenance hatches are provided in the top of the unit (see Figure 2 below). Opening of these hatches allows access to the pulse pipes, venturis, bag cages and bags. If the units are installed indoors, headroom must be checked to make sure that the bag cages can be fully removed. In case of headroom limitations, split bag cages can be specified. Figure 2 Venturis are normally attached to the cages and the bags are held in place with quick release spring steel bands (See Figure 3 below). Figure 3 Dirty Air Inlet 14 Typically vendors provide the choice of dirty air inlets on the side of the housing or in the side of the hopper. Hopper entry is remended by vendors when dust loadings are very high or the dust is very heavy. In this way the hopper supposedly functions as a precollector. Our experience with hopper entry has not been good with build up problems being encountered in the hopper. We therefore remend sidehousing entry. Hopper Vendor’s standard offerings typically have multiple hoppers each with its own discharge, (especially for larger units), as shown in Figure 2 above. We remend against that arrangement as we have found that hopper build up occurs where one hopper j