Aspects of Rotary Vacuum Filter Design & Performance

The rotary vacuum filter is very basic in design, application and operation. It is also an extremely effective solid/liquid separation device due to its unique methods of handling (discharging) formed cake solids, relatively low operating labor requirements and its ability to filter continuous or batch process flows. The rudiments of a rotary vacuum filter installation include (Figure 1): (1) the filter: comprised of the drum, vat, agitator and solids discharge mechanism; and (2) auxiliary package: consisting of a vacuum source [typically a liquid ring vacuum pump], a means of separating the air and liquid discharge from the filter [usually a vacuum receiver] and a filtrate pump [to transport the liquid from the vacuum receiver to the next step in the process]. The filter is rated by the size [filter area] of the drum and its potential output [typically expressed as pounds per hour of dry solids per square foot of filter area; precoat filters are rated by hydraulics instead of by solids]. The sizing of the auxiliary package components is dependent upon the area of the filter and the type of application. Since rotary vacuum filters can handle such a wide range of materials, expected solids output can range from 5# to 200# per hour per square foot and 2 gallons to 40 gallons per hour per square foot [for precoat discharge]. Discharge Designs The five basic discharge types are: 1. Scraper (Figure 2) 2. Endless Belt (Figure 3) 3. String (Figure 4) 4. Roll, and (Figure 5) 5. Precoat (advancing knife). (Figure 6) Each is designed to be able to discharge specific types of formed cake solids. In essence, these five mechanisms enable the rotary vacuum filter to efficiently handle [i.e. filter a solid-liquid slurry AND discharge the formed solids] a complete spectrum of process slurries. The Scraper Discharge, Endless Belt Discharge, String Discharge, and Roll Discharge have in common that: • the filtered cake solids are formed on a permanent medium, typically a woven synthetic; • the discharge mechanism must separate the solids from the permanent medium (septum); • the solids content of the slurries are relatively high; • the basic designs of the drum, vat and auxiliaries are somewhat “universal”, meaning that one discharge type can be switched out for one of the others. This is not the case for the precoat discharge filter which often requires a totally different filter design.