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1、12/15/2006,1,Bin and Hopper Design,料箱与料斗的设计 *转载资料，仅供参考*,The Four Big Questions,What is the appropriate flow mode? What is the hopper angle? How large is the outlet for reliable flow? What type of discharger is required and what is the discharge rate?,12/15/2006,2,Hopper Flow Modes,Mass Flow - all th
2、e material in the hopper is in motion, but not necessarily at the same velocity Funnel Flow - centrally moving core, dead or non-moving annular region Expanded Flow - mass flow cone with funnel flow above it,12/15/2006,3,12/15/2006,4,Mass Flow,Typically need 0.75 D to 1D to enforce mass flow,D,Mater
3、ial in motion along the walls,Does not imply plug flow with equal velocity,12/15/2006,5,Funnel Flow,“Dead” or non-flowing region,Active Flow Channel,12/15/2006,6,Expanded Flow,Funnel Flow upper section,Mass Flow bottom section,Problems with Hoppers,Ratholing/Piping,12/15/2006,7,12/15/2006,8,Ratholin
4、g/Piping,Stable Annular Region,Problems with Hoppers,Ratholing/Piping Funnel Flow,12/15/2006,9,12/15/2006,10,Funnel Flow,-Segregation -Inadequate Emptying -Structural Issues,Coarse,Coarse,Fine,Problems with Hoppers,Ratholing/Piping Funnel Flow Arching/Doming,12/15/2006,11,12/15/2006,12,Arching/Domin
5、g,Cohesive Arch preventing material from exiting hopper,Problems with Hoppers,Ratholing/Piping Funnel Flow Arching/Doming Insufficient Flow,12/15/2006,13,12/15/2006,14,Insufficient Flow,- Outlet size too small - Material not sufficiently permeable to permit dilation in conical section - “plop-plop”
6、flow,Material needs to dilate here,Material under compression in the cylinder section,Problems with Hoppers,Ratholing/Piping Funnel Flow Arching/Doming Insufficient Flow Flushing,12/15/2006,15,Flushing,Uncontrolled flow from a hopper due to powder being in an aerated state - occurs only in fine powd
7、ers (rough rule of thumb - Geldart group A and smaller) - causes - improper use of aeration devices, collapse of a rathole,12/15/2006,16,Problems with Hoppers,Ratholing/Piping Funnel Flow Arching/Doming Insufficient Flow Flushing Inadequate Emptying,12/15/2006,17,12/15/2006,18,Inadequate emptying,Us
8、ually occurs in funnel flow silos where the cone angle is insufficient to allow self draining of the bulk solid.,Remaining bulk solid,Problems with Hoppers,Ratholing/Piping Funnel Flow Arching/Doming Insufficient Flow Flushing Inadequate Emptying Mechanical Arching,12/15/2006,19,Mechanical Arching,A
9、kin to a “traffic jam” at the outlet of bin - too many large particle competing for the small outlet 6 x dp,large is the minimum outlet size to prevent mechanical arching, 8-12 x is preferred,12/15/2006,20,Problems with Hoppers,Ratholing/Piping Funnel Flow Arching/Doming Insufficient Flow Flushing I
10、nadequate Emptying Mechanical Arching Time Consolidation - Caking,12/15/2006,21,Time Consolidation - Caking,Many powders will tend to cake as a function of time, humidity, pressure, temperature Particularly a problem for funnel flow silos which are infrequently emptied completely,12/15/2006,22,Segre
11、gation,Mechanisms - Momentum or velocity - Fluidization - Trajectory - Air current - Fines,12/15/2006,23,What the chances for mass flow?,Cone Angle Cumulative % of from horizontal hoppers with mass flow 450 6025 7050 7570 *data from Ter Borg at Bayer,12/15/2006,24,Mass Flow (+/-),+ flow is more cons
12、istent + reduces effects of radial segregation + stress field is more predictable + full bin capacity is utilized + first in/first out - wall wear is higher (esp. for abrasives) - higher stresses on walls - more height is required,12/15/2006,25,Funnel flow (+/-),+ less height required - ratholing -
13、a problem for segregating solids - first in/last out - time consolidation effects can be severe - silo collapse - flooding - reduction of effective storage capacity,12/15/2006,26,How is a hopper designed?,Measure - powder cohesion/interparticle friction - wall friction - compressibility/permeability
14、 Calculate - outlet size - hopper angle for mass flow - discharge rates,12/15/2006,27,12/15/2006,28,What about angle of repose?,Pile of bulk solids,Angle of Repose,Angle of repose is not an adequate indicator of bin design parameters “ In fact, it (the angle of repose) is only useful in the determin
15、ation of the contour of a pile, and its popularity among engineers and investigators is due not to its usefulness but to the ease with which it is measured.” - Andrew W. Jenike Do not use angle of repose to design the angle on a hopper!,12/15/2006,29,Bulk Solids Testing,Wall Friction Testing Powder
16、Shear Testing - measures both powder internal friction and cohesion Compressibility Permeability,12/15/2006,30,Sources of Cohesion (Binding Mechanisms),Solids Bridges -Mineral bridges -Chemical reaction -Partial melting -Binder hardening -Crystallization -Sublimation Interlocking forces,Attraction Forces -van der Waals -Electrostatics -Magnetic Interfacial forces -Liquid
