hopper design.pdf

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5.0 HOPPER DESIGN

5.1 Bulk solid handling

Measuring the flow properties of bulksolids and how to use this information fothe design of storage vessel.

Definitions:Bin: Any upright container for storing bulksolids.

Silo: A tall bin, where H > 1.5DBunker: A shallow bin, where H < 1.5DHopper: A converging sloping wallsection attached to the bottom of a silo.



5.2 Solid flow pattern

As solid flow from a bin, the boundariesbetween flowing and non-flowing regionsdefine the flow pattern.

Three types of common pattern:i) Funnel flow / core flowii) Mass flowiii) Expanded flow





5.2.1 Flow obstruction

Interruption of solid flow in a bin can becaused by 2 types of obstructions:

i) An arch

Sometimes called as a bridge Formed across a flow channel

ii) Bin opening / rathole

Formed when the flow channel

empties, leaving the surroundingstagnant material in place.

Important in understanding the forcesacting on the wall of the bin and to the

material.



5.3 Types of flow pattern

5.3.1 Funnel flow / core flow:

Occurs in bin with flat bottom or hoppe

having slopes too shallow or too rough toallow solid to slide along the wall duringthe flow.

Funnel flow through an entire bin - Rathole,

formed when stagnant materials gainssufficient strength to remain in place as flochannel empties

Material near to the bin wall becomesstagnant.

First in, last out or do not come out at all.

Rathole / pipe could form.



In severe cases, the material can form abridge or arch over the dischargedopening.

The flow channel may not well defined-particle segregation might occur.-material surrounding the channel may

be unstable-this will cause stop and start flowing,pulsating or “jelly” flow. -could lead to the damage of material

structure.

As bin emptied; solid continually slough ofthe top surface into the channel.



Storage bin having a funnel flow pattern ismost common in industry.

the design do not consider thestagnant materials.

thus, resulting in less dischargedcapacity.

Funnel flow is usually the least costlydesign.

It has several disadvantages whenhandling certain materials:



i) Flow rate from the discharged openingcan be erratic:

Arches tends to form and break. Flow channel becomes unstable.

Upset volumetric feeder installed atthe silo discharged

Powder density at discharged varywidely due to varying stresses in flow.

ii) Fine powders : Flush/aerated uncontrollably

Sudden collapse of rathole/arch



i) Caking/degrading of solid:

Left under consolidating stresses in

the stagnant areas.

ii) A stable rathole/ pipe formed

Stagnant material gain sufficienstrength to remain stagnant.

iii) Level indicator

Would not give correct signal onmaterials level.

Submerged in stagnant area



Despite all of the above, funnel flow is still

adequate for (advantage) :i) Non-caking or non-degradableii) Discharge opening adequately sized to

prevent bridging/ratholing.

However, many mechanical devices couldbe used to promote flow.



5.3.2 Mass flow:

Occurs in bin having steep and smoothhoppers.

Material discharges are fully active.

Flow channel coincides with the bin andhopper walls i.e all materials is in motionand sliding against the wall of bin and

hopper.



Advantages:

i) Erratic flow, channeling and flooding o

powders are avoided.

ii) Stagnant regions in the silo areeliminated.

iii) First in, first out flow occurs. Resulting inminimizing caking, degrading andsegregation during process.

iv) Little particle segregation or eliminated

v) Uniform flow at the hopper outlet

flow is easily controlled

pressures are well predictable.

Di d t



Disadvantages:

i) Friction between moving solids and thesilo.

resulting in erosion of the wall could give rise to contamination of the

solids by the material of the hopperwall.

Serious erosion of the wall material.

ii) For conical hoppers, the slope anglerequired to ensure mass flow dependson the powder-powder friction and thepowder-wall friction.

There is no such thing as mass flowhopper- a hopper that gives mass flowwith one powder may give core flow with

another.



Expanded flow:

Term used to describe flow in a vessel thatcombines a core flow converging hopperwith a mass flow attached below it.

The mass flow hopper section ensures auniform, controlled flow from the outlet. Itsupper diameter is sized such that nostable pipe can form in the core flow

hopper portion above it.



Expanded flow is used where a uniformdischarged in desired, but where space o

cost restrictions rule out a fully mass flowbin.

This arrangement can be used to modifyexisting funnel flow bins to correct flowproblems.

Multiple mass flow hoppers are sometimesmounted under a large core flow silo.





5.4 The Design Philosophy

Blockage or obstruction to flow = arching.

Arch of powder with

sufficient strength toprevent flow

Powder



powders develop strength under the

action of compacting stresses the greater the compacting stress, thegreater the strength developed

(Free-flowing solids such as coarse sand willnever develop compacting stress)



5.5 Flow- no flow criterion

Flow to occur:

strength developed by the solids under theaction of consolidating pressure to supportobstruction to flow is less than gravity floof the solids.

An arch occurs:

when the strength developed by the solidgreater than the stresses acting within thesurface of the arch.

The hopper flow factor (ff )

the ff relates the stress developed in aparticulate solid within the compacting

stress acting in a particular hopper.



powder theindeveloped stress

hopper thein stresscompacting ff

D

C

high value of ff means low flowability.

High σC means greater compaction.

Low σD means more chance of an archforming.

The hopper depends on:

The nature of the solid The nature of the wall material

The slope of the hopper wall



5.5.2 Powder flow function

the unconfined yield stress, σY of the solid

varies with compacting stress, σC ;i.e:

C y fn

the relationship is determinedexperimentally

the relationship is called powder flowfunction





Yi ld l d f il f i ld l i



Yield locus and family of yield loci

D t i ti f d



Determination of y and c

y

c

Critical condition for flow



Critical condition for flow

he limiting condition for flow:

y

c

ff

- to reveal conditions under which theflow will occur.

(a) – powder has a yield stress greater than

σc/ff no flow occurs.

(b) – if actual stress developed < σcrit :

no flow.If actual stress developed > σcrit :

flow occurs.(c) – the powder has a yield stress less

than σc/ff flow occurs.

- For flow, σD> σy or y

C

ff

D t i ti f d d f



Determination of d and fw

Critical outlet dimension



Critical outlet dimension

For a given hopper geometry, the stressdeveloped in the arch is related to the sizeof the hopper outlet, B, and the bulk solid,ρs, of the material.

Minimum outlet dimension, B

g

H B

b

crit

)(

where H(θ) – factor determined by the slopeof the wall.

g – acceleration due to gravity.

For conical hoppers:

60

0.2)(

H

C iti l O tl t Di i



Critical Outlet Dimension



Hopper flow factor values for conical channels, d=50o



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