流体力学与传热（伍钦）08chapter3-3.pptVIP

The initial pressure drop across the depth filter is generally small than that across a surface filter of comparable efficiency but the build-up of pressure drop as particles are collected is more gradual for a depth filter. Open gravity filters usually operate to a head loss limit of 2.5m water gauge, but pressure filters may use up to 2 or 3times this value. Backwashing is normally employed at a rate sufficient to fluidize the media and to ensure sufficient separation for multiple layer filters. Washing rate of up to 0.6m/min can be achieve this. Normally, backwash water would represent about 1% of filtrate production; 3% would be high and quantities over 5% would be excessive. 3.4 filtration fundamentals 3.4.1 Introduction Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles. A filtration system can be shown schematically suspension medium filtrate In order to obtain fluid flow through the filter medium, a pressure drop Δp has to be applied across the medium. There are four types of driving force: Gravity, Vacuum, Pressure, Centrifugal. There are basically two types of filtration used in practice: Surface filtration: The surface filters are used for cake filtration in which the solids are deposited in the form of a cake on the up-stream side of a relatively thin filter medium. Depth filtration: The depth filter are used for deep bed filtration in which particle deposition takes place inside the medium and cake deposition on the surface is undesirable. In a surface filter, particles of the same size as or larger than the openings wedge into the openings and create smaller passages which remove even smaller particles from the fluid. A filter cake is thus formed, which in turn functions as a medium for the filtration of subsequent input suspension. Surface filters are usually used for suspensions with higher concentrations of solids, say above 1