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Without a valvebody, the drum can only be used for precoat discharge applications. Universal and valveless drum designs have applied vacuum only to the surface of the drum by means of the filtrate pipes. All liquid and air are contained within the filtrate pipes. The interior of the drum is dry and at atmospheric pressure (Figures 7, 8). The unique drum design (particular to Italian filters) in Figure 10 changes the design of the auxiliary equipment package. Compare the schematic in Figure 11 with the “typical” schematic for RVF’s in Figure 1.Special DesignsCake Wash Appropriate wash liquor can be applied to the formed cake solids if the process requires a high degree of mother liquor recovery. As a rule of thumb, 90% of the mother liquor can be recovered with the application of wash liquor equal to 2 displacements of the mother liquor content in the filtered cake solids. It is essential that the wash liquor be applied to the cake solids with uniform distribution. Spray nozzles are preferred, but are a high maintenance item. Drip pipes do not give good distribution unless combined with a wash blanket (another high maintenance item). Weak liquor separation is possible only if the drum is of universal design. An additional vacuum receiver and filtrate pump would be required. Cake wash can be accomplished on all discharge and drum designs (Figure 12). Knock-Out Receiver Precoat discharge
Introduction The most common type of continuous vacuum filter. The rotary drum filter in which liquor is sucked through a moving septum to deposit a cake of solids. Cake is moved out of the filtering zone, washed, sucked dry and dislodged from the septum.