Air Slide DKE292_Ch18

Fluidized Motion Conveying Systems

INTRODUCTION

A l t h o u g h t h e s e c o n v e y i n g sy sy s t e m s h a v e b e e n i n u s e f o r e l l o v e r o n e h u n d r e d years, they h ave b een rather neglec ted. Th is is desp ite the fact that they are wid ely used for materials such as cement, fly ash and a l u m i n a , a n d a r e v e r y e c o n o m i c a l t o operate. Th e m ain prob lem is that they have, until recently, on ly been able to operate on a downward incline and as a consequence have been referred to as airassisted g r a v i t y c o n v e y o r s "air slides". They have been developed operate ho rizo nta lly and have co nsiderab le potential for further development. dense F l u i d i z e d m o t i o n c o n v e y i n g can be regarded as an extreme form p h a se s e p n e u m a t i c c o n v e y i n g . It is e s s e n t i a l l y extension this method, with b u l k p a r t i c u l a t e a t e r i a l m a d e t o f l o w a l o n g a c h a n n e l . I n t h e a i r -a -a s s is is t e d g r a v i t c o n v e y o r t h e c h a n n e l i s i n c l i n e d a t a s h a l l o w d o w n w a r d a n g l e a n d th th e p r e d o i n a n t factor fact or causing flow is the gravitational force on the m aterial. It this for th is reason p o t e n t i a l l y v e ry operthat air-assisted gravity conveying systems ry e c o n o m i c a l ate. 1.

Conveying Technique

technique achieve conveying essentially aerated state in the b u l k particulate material, from the moment that it is fed into the upper end of the channel, to the point at which it is discharged.

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2004 by Marcel Dekker, Inc. All Rights Reserved.

Chapter 18

Figure 18.1

The principle of air-assisted gravity conve ying.

o t h e r gas, This achieved means of the continuous introduction air, a t a r e l a t i v e l y l o w f l o w r a t e a n d p r e s s u r e , i n t o a p l e n m c h a m b e r , i n order fluidize the m aterial. The prin cip le of operation is illus trated ith the sketch in Figure 1 7 . 1 . T h e a i r passes t h r o u g h f a l s e bottom, m e m b r a n e , made f s u i t a b l e rous material, which runs e n t i r e l e n g t h o f t h e c h a n n e l . i t h p o w d e re re d m a t e r i a l s , a n d those c o n t a i n i n g f i n e p a r t i c le le s , t h e c h a n n e l i s g e n e r a l l y e n c l o s e d , a s s h o w in F i g u r e 1 8 . 2 [1], B y t h i s m e a n s t h e e n t i r e c o n v e y i n g s y s t e m c a n b e t o t a l l y e n closed. The fluidizing air, after passing through the bed of material sliding on the m e m b r a n e , f l o w s over t h e t o p o f t h e b e d o f m a t e r i a l a n d i s v e n t e d t h r o u g h s u i t able filter unit. Since bulk solid material kept live m e a n s o f t h e steady f l o w air, slope, e v e n w h e n m a t e r i a l f l o w s f r e e l y down angle inclination relatively small. quantity of air used k e p t t o t h e a b s o l u t e m i n i m u m necesorder sary reduce both inter-particulate forces, and the frictional forces between particles a n d t h e internal channel surfaces, sufficient allow the mao t i o n c o n v e y i n g s y s te te m s , t e r i a l t o f l o w [1]. T h e a i r r e q u i r e m e n t s f o r f l u i d i z e d therefore, relatively low and so they n e e d t o b e a i n t a i n e d b e t w e e n r ea ea s o n a b l y c lo lo s e l i m i t s i n o r d e r t o o p t i m i z e c o n v e y i n g c o n d i t i o n s

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Fluidized Motion Conveying

Conveying Channel

Porous Membrane

Supply Hopper

Section

Figure 1 8 . 2

1.2

-X

Plenum Chamber

T y p i c a l air-assisted g r a v i t y c o n v e y o r .

System Advantages

F l u i d i z e d m o t i o n c o n v e y i n g h a s a l l t h e a d v an an t a g e s o f p n e u a t i c c o n v e y i n g , b u t th e w i t h f e w o f t h e d i s a d v a n t a g e s . I t p r o v i d e s a t o t a l l y e n c l o s e d e n v i r o n m e n t f o r th layout, and has no vi ng parts. i t h a i rr - a s si s i s te te d material, is very flexible g r a v i t y c o n v e y o r s t h e o n l y d r a w b a c k i s t h e fact t h a t m a t e r i a l c a n o n l y b e c o n v e y e d n d o w n w a r d g r a d i e n t , b u t a s m e n t i o n e d above, system does h a v e d e v e l o p e n t p o t e n t i a l t h a t i s a k i n g h o r i z o n t a l c o n v e y i n g , a t l e a s t,t , a d i s t i n c t p o s s i b i l i t y . A p a r t i c u l a r a d v a n t a g e over p n e u m a t i c c o n v e y i n g i s t h a t t h e c o n v e y i n g v e l o c i t y i s v e r y l o w . I n d i l u t e p h a s e p n e u a t i c c o n v e y i n g , s o l i d s l o a d i n g r a t io i o s th th a t very low and conveying velocities c o n s e q u e n t l y re r e l a t iv iv e l y can be achieved u c h h i g h e r t h a n a lm lm o s t alternative high. s result, pow er requirem ents e m . O p e r a t i n g p r o b l e m s a s s o c ia ia t e d ith abrasive partim e c h a n i c a l c o n v e y i n g s y s t em c l e s , s u c h a s t h e e r o s i v e wear system components, degradation friable particles, can be so severe that pneumatic conveying, s m e a n s transport, considered such materials. often material can be conveyed f, n p n e u m a t i c c o n v e y i n g s y s t e m , dense phase, power r e q u i r e m e n t s w i l l lower operating problems will generally reduced. In fluidized otion conveyo rs, ho ever, so lids load ing ratios are even h i g h e r a n d c o n v e y i n g v e l o c i t ie ie s a r e u c h l o w e r t h a n th th o s e i n d e n s e p h a s e c o n v e y i n g . A s a r e s u l t, t, p o w e r r e q u i r e e n t s a r e o n a p a r w i t h b e l t c o n v e y o r s , a n

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Cha pter 18

operating problems associated with abrasive existent.

.3

f r i a b l e materials

almost non-

Design Tolerance

general principle very simple this fluidized motion conveying method c o n v e y i n g h a s a particular a d v a n t a g e being essentially 'workable'. With pn eum atic c onveying systems systems it is critical that the con vey ing line inlet air velocity correctly specified. Because air is c o m p r e s s i b l e , very much higher pressures used pneu ma tic conveying than fluidized motion conveying, correct i n l e t v e l o c i t y achieved maintained n pneumatic e n s u r i n g that c o n v e y i n g s y s t e m is not s i m p l e m a t te te r . t h i s inlet air velo city is too h igh the material flo w rate rate may b e reduced , the p o w e r r e q u i r e m e n t s w i l l b e e x c e s s i v e , a n d o p e r a t i n g p r o b l e m s w i l l be severe. this velocity is too low, the the ater ial may not conv ey at al l, and the p ip el in e is b l o c k . W i t h fluidized m o t i o n c o n v e y o r s great deal latitude availlikely able in the design installations, p r o v i d e d t h a t a few b a s i c r e q u i r e m e n t s met, they w i l l g e n e r a l l y o p e ra ra te te i t h o u t t r o u b l e .

.4

Historical

i s n o t k n o w n h e n a e r a t i o n o f a b u l k s o l i d m a t e r i a l w a s first used as an aid to c o n v e y i n g , but one of the earliest relevant patents appears have been that Germany, proposed the use of air, entering open 1895 [2]. Dodge, cha nne l through slits in the base, transport coarse grained materials, such g r a i n . R e a l p r o g r e s s in the m e t h o d c o n v e y i n g w a s n o t m a d e until s o m e thirty years later when it was found t h a t gravity conveying aerated powders ideally suited t o t h e c o n v e y i n g cement. G e r m a n c o m p a n y P o l y s i u s w a s a p i o n e e r in the d e v e l o p m e n t airassisted assisted gravity con vey ing. They were fol low ed b y t h e H u r o n P o r t l a n d C e m e n t Company America. Huron's plant A l p e n a , M i c h i g a n , was one of the first make extensive commercial use of this method conveying. They employed t h e y c a m e to be c a l l e d , various stages of the p r o d u c t i o n p r o c e s s , "Airslides", finished cement. third organization that from g r i n d i n g m i l l d i s c h a r g e played a pro ine nt part in deve loping and esta blish ing air-gravity conveyors was F u l l e r C o m p a n y , w h i c h m a n u f a c t u r e d t h e m u n d e r l i c e n s e f ro r o m H u r o n [1 ]. air-assisted gravity conveyor first c a m e p r o m i n e n c e for the Although transport of cement, and is s t i l l idely used for this mate rial, an y other types types of a s e. e. P u l v e r i z e d f u e l ash, b u l k p a n i c u l a t e m a t e r i a l are now h a n d l e d w i t h r e l a t i v e e as a l u m i n u m industry, are io n i n d u s t ry r y , a n d a l u i n a from p o w e r g e n e r a t io from commonly conveyed this means, well diverse substances such coal dust, sand, num erous plasti metal powders. s o m e d e t a i l in the p u b l i s h e d T y p i c a l o f t h e large i n s t a l l a t i o n s described literature are a 55,000 storage plant and an 88,000 ship load ing plant, both h a n d l i n g a l u m i n a [ 3 ] , and a C a n a d i a n a l u m i n u m s m e l t e r c a p a bl bl e handling

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Fluidized Fluidized Motion Conveying y e a r [ 4 J . a r i o u s s iz iz e s convey ing channe used 175,000 ton of a l u m i n a t h e s e i n s t a l l a t i o n s , o n e o f t h e l a rg rg e s t b e i n g a 3 f t w i d e c h a n n e l h i c h c o u l d f r o m surge hopper o m a i n s il i l o t rate 1650 t o n / h . transport a l u m i n a fr

1.5

Conveying Principles

C o n s i d e r i n g t h e a d v a n t a g e s that f l u i d i z e d m o t i o n c o n v e y o r s c a n offer o v e r o t h e r f o r m s o f b l k so s o l i d s t r a n sp s p o r t , p a r t i c u l a r l y in in t e r m s o f l o w p o w e r c o n s u p t i o n , t h e t h e s e c o n v e y o r s is not as w i d e s p r e a d might expected. some extent c o n f i d e n c e o n t h e part of the d e s i g n e n g i n e e r , t h i s m a y b e t h e r e s u l t f lack s i n c e ev e v e n a i r -g - g r a v i ty t y c o n v e y i n g r e m a i n s s o m e t h i n g of an art. enable systems to be o p t i m a l l y d e s i g n e d , r a t h e r t h a n o v e r In order air-float, d e s i g n e d , s o m e u n d e r s t a n d i n g of the p h e n o m e n a i n v o l v e d fluidized motion conveying, necessary. Observation f p a r t i c u l a t e b u l k so so l i d b e i n g c o n this means along d u c t w i l l i m m e d i a t e l y s u g g e s t s i m i l a r i t y o l i q u i d veyed evident that continuo us supply f l o w i n g i n a n i n c l i n e d c h a n n e l . w i l l also that necessary maintain l i q u i d l i k e state of the m a t e r i a l has a c l o s e finity to the gas f l u i d i z a t i o n process. basic principles static f l u i d i z a t i o n , t h e r e f o r e , are first e x t e n d e d d e a l w i t h the flow of fluidized b u l k p a r t i c u l a t e m a t e r i a l s . design, construction operation practical air-assisted gravity conveyors then discussed some i n t e r e s ti ti n g v a r i a t i o n o n t h e l e n g t h . C o n s i d e r a t i o n is finally g i v e n o n u m b e r conventional air-float conveyor in which the transported material flows along a even upward i n c l i n e d channel. horizontal,

THE FLOW OF FLUIDIZED MATERIALS W h e n p a r t i c u l a t e m a t e r i a l s b e c o m e fluidized u n d e r influence f continuous u p w a r d f l o w o f a gas, they tend d i s p l a y m a n y of the characteristics liquids. m a i n t a i n h o r i z o n t a l free surface, O n e o f t h e s e characteristics is the ability a n o t h e r is the a b i l i t y to flow from h i g h e r o lower level. This means that a fluidized m a t e r i a l w i l l flow from a hole in the side of a vessel. If a horizo ntal p ipe fitted to the h o l e m a t e ri ri a l w o u l d c o n t i n u t o f l o w , p r o v i d e d t h a t pipe was not so long that complete d e - f l u i d i z a t i o n occurred along its length. If it were possible keep m a t e r i a l in its fluidized c o n d i t i o n , as it passed along pipe, the flow could be maintained almost indefinitely.

.1

Pipeline Conveying

pne um atic conv eying, materials that have very good retention properties ta n c e , q u i t e n a t u r a l l y . g e n e r a l l y b e c o n v e y e d i n d e n s e p h a s e o v e r a r e a s o n a b l e d i s ta A flow of high pressure air is all that is req uire d to keep the m ate rial on the ove pipeline. through materials with poor r e t e n t i o n p r o p e r t i e s it is necesintroduce into material s o m e m e a n s, s, c o n t i n u o u s l y a l o n g sary

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Chapter 18

50

length of the the pip elin e, in order to achieve den se phase flow , as discusse d in the previous chapter. 2.1.1

The Gattys System

A p a t e n t e d m e t h o d w h i c h c a n g i v e a m a t e r i a l a r t i f i c i a l air retention properties is the Gattys Trace A i r ' system . In th is system air at a rel ativ ely low pressu re is supp l i e d c o n t i n u o u s l y to the m a t e r i a l in the p i p e l i n e t h r o u g h internal perforated pipe which runs whole length of the convey ing line motive force c o m e s ffrom rom pressure drop along conveying line created p u m p i n g air in at the u p s t r e a m end, as in c o n v e n t i o n a l p n e u m a t i c c o n v e y i n g . a l t e r n a t iv i v e , a l t h o u g h u n p r a c t i c a l , m e t h o is to h a v e c o n t i n u o u s p o rt r t io io n of the pi pe line all itself made of a porou s aterial. From an external source of the material could be fluidized through the porous section of pipe, and the high pressure pipe line wo uld provide m o t i v e f o rc rc e . T h i s p r i n c i p l e within forms basis of the more recent developments that allow c h a n n e l to run h o r i z o n t a l l y . If If t h e p i p e l i n e w e r e t o b e i n c l i n e d , g r a v i t y w o u l d p r o v i d e t h e m o t i v e force and the air s u p p l y w i t h i n p i p e li li n e w o u l d n o t b e n e e d e d . .2

Fluidized Flow

This combination gravitational force with fluidization provides basis f p o t e n t i a l l y v e ry r y e c o n o m i c a l m e t h o d o f tr t r a n s p o r t i n g b u l k s o l i d s . F i g u r e 18.3 s h o w s different approach to the same concept c o n t i n u o u s fluidized flow. This illus-a s si s i st s t e d g r a v it it y trates q u i t e s i m p l y t h e f u n d a m e n t a l p r i n c i p l e o n w h i c h t h e a i r -a conveyor operates. 2.2.1

Angle

Repose

M o s t f re r e e f l o w i n g p a r t i c u l a t e m a t e r i a ls ls d i s p l a y n a t u r a l a n g l e repose around 35 to 40 d e g r e e s , i l l u s t r a t e d i n F i g u r e 18.3a. I n o r d e r t o g e t s u c h a m a t e r i a l t o f l o w c o n t i n u o u s l y , u n d e r g r a v it i t y a l o n e , on an i n c l i n e d s u r f a c e , w o u l d necessary for the s l o p e o f t h e s u r f a c e to be greater than t h i s a n g l e repose, shown Figure Materials that exhibit some degree c o h e s i v e n e s s h a v e m u c h l ar ar g e r angles repose. Such materials will often flow, even s t e e p l y i n c l i n e d surfaces, without s o m e form assistance, such v i b r a t i o n of the s u r f a c e . introd u c t i o n of air to a b u l k p a r t i c u l a t e m a t e ri ri a l also provide means promoting flow.

This can be achieved supporting m a t e r i a l n p l a t e m a d e f suita b l e p o r o u s s u b s t a n c e a n d a l l o w i n g a i r t o fl fl o w u p w a r d s t h r o u g h t h e m e m b r a n e this means into the m a t e r i a l . introduced into material significantly r e d u c e t h e n a t u r a l a n g l e o f r e p o s e . T h e m a t e r i a l w i l l t h e n f l o w c o n t i n u o u s l y f ro ro m p l a t e w h e n it is i n c l i n e d t v e r y s h a l l o w a n g l e a n g l e of the plate need only Figfluidized angle greater than repose o f t h e m a t e r i a l , shown 18.3c. m o s t f r ee ee f l o i n g m a t e r i a l s th t h e f l u i d i z e d angle of repose is between a b o u t 2 and d e g r e e s .

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Fluidized Motion Motion Conv eying

Influence Figure 1 8 . 3 material, (b) no aeration

aeration horizontal pile aeration a n g l e of repose, (a) steep i n c l i n e , and (c) with aeration shallow incline.

Channel Flow

2.

T h i s p h e n o m e n o n o f fluidized flow can form the basis of a sim ple and reliab le method that r e q u i r e d s c h a n n e l h a v i n g porous bulk solids transport. b a se se t h r o u g h ich air can flow . Th e air ust be availab le over the entire leng th of the c h a n n e l and so a p l e n u m chamber needs to be p r o v i d e d b e n e a t h , as s h o w n in F i g u r e 1 8 . 2 . 2.3.1

Starling Flow

is an e s s e n t i a l r e q u i r e m e n t t h a t s u f f i c i e n t should pass into through m a t e r i a l i n t h e c h a n n e l to to c a u s e i t t o f l o w . T h e p o r o u s b a s e , t h e r e f o r e , u s t b e o f h i g h e n o u g h resistance e n s u r e t h a t w h e n p a r t of it is c o v e r e d m a t e r i a l , the air does not by-pass this section. Air flow will always have the tendency of taking the path least resistance. resistan ce. This s p a r t i c u l a r p r o b l e m start u p , a s i l l u s t r a t e d F i g u r e 18.4. m a t e r i a l o n t h e c h a n n e l b e c o m e s s ta ta r v e air, will f l o w anychannel. starting flow, therefore, the air velocity into further d o w n s ta t a ti t i o na n a rryy m a t e r i a l m u s t ex e x c ee ee d t h e m i n i m m v a l u e o f f l u i d i z i n g a i r v e l o c i t y f o r

material,

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even when

large part o f t h e p o r o u s m e m b r a n e


uncovered.

Chapter 18

508

Porous Membrane (Distributor)

Plenum C h a m b e r Figure 18.4

2.12

Starting f l o w o f material

inclined channel.

Channel Slope other e s s e n t i a l c o n d i t i o n t o b e m e t i s t h a t

sufficient downward slope p e r m i t steady c o n t i n u o u s f l o w o f t h e f l u i d i z e d a t e r i a l . P r o v i d e t h a t t h e s e c o n satisfied, air-assisted gravity conveyor will normally prove to be a ditions trouble free very economical method transporting wide range powdered g r a n u l a r m a t e r ia ia l s . depend appearance o f t h e f l o w i n g aerated p o w d e r i n t h e c h a n n e l upon number p r o p e r t i e s t h a t m i g h t together termed 'flowability' of the m a t e r i a l . Thus very free flowing material having relatively natural likely f l u i d i z e very w e l l . S u c h angle material would have repose w o u l d channel inclined good f l o w a b i l i t y a n d i n t h i s state w o u l d f l o w s m o o t h l y a l o n g j u s t o n e o r t o d e g r e es e s t o th th e h o r i z o n t a l , a s i l l u s t r a t e d e a r l i e r i n F i g u r e 1 8 . 2 . ia l o u l d s h o w d i s t i n c t l i q u i d l i k e V i s u a l o b s e r v a t i o n o f t h e f l o w i n g m a t e r ia characteristics such s smooth s l i g h t l y r ip ip p l e d s u r f a c e . partial obstruction flow could set up a p l u m e , and a more s ubstan tial obstruction cou ld set up a contrast, material that s t a n d i n g wave. cohesive show arkedly differ e n t b e h a v i o r i n a n a i r -a -a s s i s t e d g r a v i t y c o n v e y o r [ 5 , 2.3.3

Cohesive Materials

V e r y c o h e s i v e m a t e r i a ls ls a r e u n s u i t a b l e f o r c o n v e y i n g i n c h a n n e l s i n t h i s m a n n e r . a t e r i a l s t h a t a re r e o n l y s l i g h t l y c o h e s iv iv e , h o w e v e r , c a n u s u a l l y b e c o n v e y e d p r o -

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vided that slope of the c h a n n e l greater than about 6 t o 1 0 degrees. Observation these materials s u g g e s t s that particles a r e n o t f l u i d i z e d , move virtua l l y a s a s o li li d m a s s o f m a t e r i a l s l i d i n g a l o n g t h e c h a n n e l , a s i l l u s t r a t e d i n F i g u r e 18.5. I r r e g u l a r z i g z a g cracks in the f l o w i n g b e d o f m a t e r i a l , a n d t h e craggy pearance of its free surface, suggests similarities to the c h a n n e l i n g slugging behavior that can occur in stationary stationary f luid ized beds. T h e s e c o h e s i v e m a t e r i a l s expected could well exhibit such characteristics.

2.3. 2.3.44

Flow Mechanis

is not clear as to what is the do ina nt factor factor that that causes the im pro ved f l o w a b i l that results from continuous aeration materials. could result from filtering through solid particles reducing contact forces between them. Alternatively could come from f o r m a t i o n of air layers between the bed particles a n d t h e c h a n n e l s u r f a c e s , w i t h c o n s e q u e n t s h a r p r e d u c t i o n of the b o u n d a r y s h e a r stresses. SYSTEM P A R A M E T E R INFLUENCES

T h e m a i n d i f f i c u l t y w i t h t h e s t u d y o f t h e f l o w o f f l u i d i z e d so lids, and the reason why it will difficult develop e x a c t m a t h e m a t i c a l a n a l y s i s , is the large number variables involved.

Cohesive Material

Layers

ir

Figure 18.5

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Flow

cohesive material

inclined channel.


Chapter 18

51

With parameters such particle shape, e x a m p l e , t h e r e s l a c k f precise c o n v e n i e n t d e f i n i t i o n that can be used modeling. shortage of ext h e s e m o r e c o m p l e x p a r a m e t e r s a d d s to the p e r i m e n t a l data o n t h e i n f l u e n c e already co nsiderable problem s. order s i t u a t i o n , it is as w e l l i n i t i a l l y to set aside all the sim plif variables that contribute what may be termed 'nature' o f t h e p a r t i c u l a t e b u l k made between flow behavs o l i d . T h u s o n l y very general comparison w i l l attempt investigate, depth, effects ior of d i f f e r e n t m a t e r i a l s , w i t h material characteristics such particle density, particle size shape, their distributions.

3.1

Variables Considere

study then becomes restricted to the flow f g i v e n a e r a t e d m a t e r i a l in an m a i n v a r i a b l e s , th t h e r e f o r e , are the mass flow rate of the m a t e inclined c h a n n e l . width slope of the c h a n n e l , depth of the f l o w i n g b e d o f m a t e r i a l , rial, and the sup erficia l veloc ity of the air. No te th at the su per ficia l air veloc ity is giv en b y t h e v o l u m e t r i c f l o w r a t e of the air, divided by the surface area of the p o r o u s c h a n n e l base. M o s t of the experimental work theoretical analyses carried by various researchers have concentrated o n t h e r e l a t i o n s h i p s a m o n g t h es e s e five parameters [7|. 3.1.1

Depth Control

T h e a j o r i t y o f c o m m e r c i a l i n s t a l l a t i o n s i n v o l v i n g a i r -a -a s s i st st e d g r a v i t y c o n v e y i n g rely some form flooded feed to the u p p e r end of the c o n v e y i n g ch ch a n n e l . these the depth of the flow ing bed is of little practical interest, prov ided that it does increase to the extent that c h a n n e l b e co co m e s b l o c k e d . experimental vestigations, however, it is l i k e l y that the bed depth would be the independent usually c o n t r o l l e d w i t h o u t u n d u e diffivariable, as the other parameters culty. c o n s i d e r a t i o n of the i n f l u e n c e variables such as the c h a n n e l s l o p e , u s e f u l insight into perficial air velocity, and solids mass flow rate, gives mechanism of fluidized flow in inclined channels.

.2

Depth

Channel Slope

superficial g e n e r a l , for a given solids mass flow rate velocity, depth would tend increase as the i n c l i n a t i o n of the c o n v e y i n g of the f l o w i n g ste ep slopes this effect is not very sig nifica nt, but c h a n n e l is reduce d. A t rela tively steep lo p e a p p r o a c h e minimum which flow occur, depth as the c h a n n e l s lo of bed increases rap idly . This effect is illustr ated in Figure 18.6. increase in the solids mass flow rate would result n s h i f t of the c u r v e s u p w a r d s . similar result w o u l d caused by a chan ge in the superficial velocevident from s h a p e of the c u r v e s F i g u r e 1 8 . 6 t h a t t h e r e is an apity. proximate m i n i m u m slope w h i c h fluidized material w i l l flow.

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Motion C onveying Fluidized Motion

C o n s t a n t Superficial Velocity

Increasing M a t e r i a l Mass Flow Rate

Channel Slope Influence

Figure 18.6

c h a n n e l slope and material flow rate on

depth.

actual value t h i s i n i m m c h a n n e l s lo l o p e d ep ep e n d s m a i n l y u p o n n a t u r e o f t h e m a t e r i a l i n v o l v e d a n d , to t o a l e ss ss e r e x t e n t , u p o n t h e s o l i d s m a s s f l o w rate and the superficial air velocity. Attempting to convey at slope less than this id t h i c k e n i n g o f t h e m a t e r i a l b e d t o t h e p o i n t a t m i n i m u m v a l u e c a n r e s u l t i n a r a p id which channel becomes blocked. Conveying s l o p e s m u c h greater t h a n does make m i n i m u m n e c e s s a r y does y i e l d s i g n i f i c a n t a d v a n t ag ag e , best use of available headroom. 3.2.1

Mathematical Analysis

i s n o t easy express mathematically r e l a t i o n s h i p between t h e b e d d e p t h channel slope, since m u c h depends u p o n n a t u r e o f t h e c o n v e y e d material. form expression that b e e n proposed

m

Ch

Sina

where

aterial flow rat

=

= = =

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bed depth channel width coefficient index cha nne l slope


(1)

Chapter 18

index, aspect ratio has a value between 1 an an d 3 d e p e n d i n g u p o n defined as the bed depth divided channel width of the flow. Aspect ratio equal c o e f f i c i e n t C is not constant, depends upon h/b. U n f o r t u n a t e l y n a t u r e of the conveyed material, upon flow conditions. An alternative expression from Ref 7 is:

b w h e r e PA g

=

Sina~K

----

(2)

b u l k d e n s i ty ty o f m a t e r i a l g r a v i t a t i o n a l a c c e l e r a t io io n re constants

are constants for a given bulk participate material. It has been form of the r e l a t i o n s h i p f o u n d t h a t E q u a t i o n 2 can represent quite closely tween depth channel slope, indicated Figure 18.6. A t t h e present time, however, insufficient inform ation available o n t h e values constants Whilst it s e e m s probable that for different bulk solids, these constants will d e p e n d p r i m a r i l y o n p a r t i c le l e si si zzee a n d d e n s i t y , uch more e xper imen tal work to be

3.3

Bed Depth

Superficial Air V elocit

superficial s i m i l a r v a r i a ti t i o n of bed d e p t h o c c u r s s r e s u l t varying velocity, shown Figure 18.7. t h i s case the set of c u r v e s s h o w n plotted constant solids m a s s flow rate, with each curve representing different chanslope. Again there appears to be a t e n d e n c y t o w a r d s optimum flow which, la t e d t o t h e s lo lo p e o f t h e c o n v e y i n g c h a n n e l . R e d u c i n g p r e v i o u s l y r e m a r k e d , i s r e la less than optimum cause f l o w i n g m a t e r i a l to bethe air flow rate c o m e d e - f l u i d i z e d . T h i s r e s u l t s n s u d d e n f a l l in the m a t e r i a l f l o w v e l o c i t y and a c o n s e q u e n t t h i c k e n i n g of the bed, often to the p o i n t total cessation flow. the other hand, an increase of the air flow rate much above the optimum produces little advantage, and is merely wasteful of energy.

3. 3.

Mathematical Analysis

N o r e l i a b l e m a t h e m a t i c a l m o d e l h a s y e t b e en en f o u n d t h a t w i l l a l l o w p r e d i c t i o n o f variation o f t h e depth o f t h e flowing suspension with superficial velocity. make problem even more difficult, there appears to be considerable inconsistency between diff eren t ind bulk solids with regard to the q u a n t i t y required relation m i n i m u m f l u i d i z i n g velocity. This, perhaps, is not surp r i s i n g since very s im ila r situation exists with regard t o t h e p n e u m a t i c c o n v e y i n g o f b u l k m a t e r i a ls ls .

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C o n s t a n t Mass F l o w Rate

Optimum Operating Band

ffl

Increasing Channel Slope

Superficial Figure 18.7

Influence

superficial

Velocity

velocity

channel slope on bed depth.

T h u s , w h i l s t m o s t free f l o w i n g m a t e r i a l s c a n b e c o n v e y e d s a t i s f a c t o r i l y a t s u p e r f i c i a l velocities

a r o u n d 2 to 3

f , s o m e b u l k s o l i d s r e q u i r e air flows t h a t

much higher multiples f A g a i n , f u r t h e r e x p e r i m e n t a l w o r k n e e d s to be undertaken investigate possibility predicting superficial influence v e l o c i t y o n f l o w b e h a v i o r f r o m e a s i l y m e a s u r e d c h a r a c t e r i s ti ti c s o f b u l k s o l i d . h a v e to be in t e r m s of air r e t e n t i o n p e r m e a b i l i t y , ra ra t h e r t h a n p a r t i These c l e s iz iz e a n d d e n s i t y

3.4

Bed Depth

Solids Mass Flow Rate

Observation a c t u a l flows of fluidized m a t e r i a l s inclined ch ann els suggest that a l t h o u g h the bed d e p t h w i l l i n c r e a s e if the solids mass flow rate increased, shown F i g u r e 18.6, r e l a t i o n s h i p is not one of d i r e c t p r o p o r t i o n a l i t y . It has, fact, fact, been fo nd that the bed d e p t h t e n d s vary as the s q u a r e r o o t of the s o l i d s m a s s flow rate, indicated Equation T h i s m e a n s t h a t i n m o s t p r a c t ic ic a l s i t u a t i o n s t h e r e l a t i o n s h i p b e t w e e n t h e b e d d e p t h and the m a t e r i a l m a s s flow rate a l m o s t l i n e a r , but as the m a t e r i a l flow rate is reduced t o w a r d s z e r o , t h e b e d d e p t h b e g i n s decrease sharply. This illustrated in Figure 18.8.

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Chapter 18

Increasing Channel Slope

C o n s t a n t Superficial Air Velocity

Material M a s s Flow rate Figure 18.8

3.4.1 3.4.1

I n f l u e n c e of m a t e r i a l f l o w rate

channel slope

depth.

Ch annel Clea ring

It is peculiarity air-assisted gravity conveyors that when solid s fee is reduced, flow becomes unstable, then stops, before the bed depth b e c o m e s zero. This means that base of the channel cannot completely cleared of the conveyed aterial sim ply by shu tting off the so lids feed.

.5

Solids Flow Rate

Ch annel Widt

T h e r e is no s a t i s f a c t o r y e x p e r i m e n t a l i n f o r m a t i o n in the l i t e r a t u r e on the r e l a t i o n ships between channel width, bed depth and the mass flow rate of the conveyed m a t e r i a l . Some researchers have attempted compare data collected from chann e l s o f t w o a n d three different widths, but the range been severely restricted and the r e s u l t s , t h e r e f o r e , h a v e b e e n i n c o n c l u s i v e . Because of the r e l a t i v e l y l a r g e q u a n t i t i e s of m ateria l that can be transported transported b y t h e w i d e r a i r -g -g r a v i ty t y c o n v e y o r s , p r o h i b i t i v e c o st st s h a v e l i m i t e d research rigs to channel widths being m a x i m u m about inches. Industrial installations c o m m o n l y up to a b o u t feet width, sometimes wider. Some caution, small experimentherefore, should e x e r c i se s e d w h e n p r o j e c t i n g d a t a g a t h e re re d tal rigs these greater widths. Naturally, if the c h a n n e l s l o p e , s u p e r f i c i a l v e l o c i t y and bed d e p t h k e p t c o n s t a n t,t , t h e m a t e r i a l m a s s f l o w r a te te s h o u l d b e a p p r o x i m a t e l y p r o p o r t i o n a l t o c h a n n e l w i d t h . It is m o r e r e a l i s t i c , h o w e v e r , recognize that conveying chan-

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u s u a l l y o p e ra r a ttee d t c o n s t a n t as a s p ec e c t r a ti t i o ( b ed ed d e p t h / c h a n n e l i d t h nels therefore conveying capacity n o r m a l l y t a k e n to be p r o p o r t i o n a l to the s q u a r e o f th t h e w i d t h . T h i s i s d e p i c t e d g r a p h i c a l l y i n F i g u r e 1 8 .9 .9 .

.6

Other Influences

A l t h o u g h , for a g i v e n b u l k p a r t i c u l a t e m a t e r i a l , ain parameter influencing those discussed above, there several other syste s ystem m infl ue nc es that flow cause changes o c cu cu r d u r i n g c o n v e y i n g . most significant these moisparticle segregation. ture, electrostatic charging

3.6.1

Moisture

is ell kno wn that changes in the mo istu re content of powde rs can serio usl affect t h e i r h a n d l i n g c h a r a c t e r i s t i c s , this e s p e c i a l l y tr t r u e in the case fluidization moisture may be beneficial fluidized flow. W h i l s t s m a l l i n c r e a s e reducing t e n d e n c y o f t h e material hold electrostatic charge, much moisture c a u se se n o r m a l l y f r e e - f l o w i n g m a t e r i a l s become cohesive that they cannot fluidized. Although q u a n t i t i e s re re q u i r e d relatively small, would fluidizing r e c o m m e n d e d t h a t the air for f l u i d i z i n g s h o u l d d r i e d , p a r t i c u l a r l y if the c o n veyed material hygroscopic. Since conveying system itself almost totally enclosed, m ateria ls are unlikely to absorb moisture from the atmosphere during conveying.

Increasing Density

u.

Constant Aspect Ratio

Channel Width Figure 18.9

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Influence

channel width

density

conveying capacity.


Chapter 18

3.6.2

Electrostatic Charging

s i m i l a r effect Electrostatic charging have both flow and fluidizing as drying, however, will help t h i s case, m e n t i o n e d a bo bo v e w i t h moisture. respect m o i s t u r e c o n t e n t . If the c o n v e y e d m a t e r i a l p o t e n t i a l l y e x p l o s i v e , electrostatic charging could present considerable hazard. t h i s case material c o u l d b e f l u i d i z e d w i t h n i tr tr o g e n

3.6.3

Segregation

fluidized beds. This tenThere s natural tendency segregation occur dency for the coarser particles to d r i f t d o w n t o w a r d s t h e p o r o u s m e m b r a n e c a n a l so s o o c c u in flowing f l u i d i z e d m a t e r i a l s . relatively steeply inclined, there would Where channel short, little o p p o r t u n i t y segregation occur. longer ch anne ls, however, probb e c o m e significant. coarse particles e x t r e m e cases d e p o s i t c o n t i n u o u s l y b u i l d u p o n t h e b o t t o m of the channel u n t i l t h e m a t e r i a l f l o w ceases altogether.

M A T E R I A L INFLUENCES b u l k particulate material having good fluidizing characteristics will, Almost inclined surface. Such materials can, when suitably aerated, flow easily down therefore, transported satisfactorily in an air-assisted gravity conveyor. t h o u g h it is often stated that being easily f l u i d i z a b l e is an e s s e n t i a l r e q u i re re m e n t conveying this man ner, many aterials that slightly cohesive also conveyed. Very cohesive aterials, how ever, generally unsuitable air-assisted virtue ia l s t h a t cohesive sticky g r a v i t y c o n v e y i n g . a t e r ia being d a m p c o m e i n t o t h i s c a t e g o r y , as do p o w d e r s extremely fine particle size that have channel surface, tendency smear over hence blind porous membrane.

4.1

Geldart's Classification

Geldart work c l a s s i f y i n g b u l k s o l i d s a c c o r d in in g their fluidization b e h a v i o r [ 8 ] p r o v i d e s a u s e f u l gu ide to the suitab ility of powders and granu lar air-assisted gravity conveying. Geldart's classification is in t e r m s materials mean particle size, and the d i f f e r e n c e density between p a r t i c l e s and the material identified. fluidizing m e d i u m . F o u r d i f f e r e n t g r o u p s this representation, p arti classification presented F i g u r e 18.10. density rather than density d i f f e r e n c e em ployed. Thi because fluidizing d e n s i t y of the air can be d i s r e g a r d e d as it is n e g l i g i b l e c o m p a r e d w i t h w i t h air, almost any particle.

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'f

100

_C

I 2 \N

20

50

100

500

Mean Particle Si/.e Figure 1 8 . 1 0 ambient air.

4.1.1

Geldart's classification

1000

5000

|am

f l u i d i z a t i o n b e h a v i o r f o r f l u i d i / a t i o n with

Fine Granular Materials

general, materials Group w h i c h i n c l u d e s m o s t fine g r a n u l a r m a t e r i a l s , mean particle size range from a b o u t 50 to 1 0 0 0 m i c r o n , density range 75 to 250 lb/ft are the easiest convey. These will flow very well very shalc h a n n e l s lo l o p e s. s. W h e n supply f f , the bed c o l l a p s e s f l u i d i z i n g air is s h u t o ff r a p i d l y and the flow stops. s result t h e r e u n l i k e l y to be any p r o b l e m s w i t h retention.

4. .2

Large Granular Materials

Group c o v e rs r s m a t e r i a l s larger particle size high density gran ular aterials. T h e s e m a t e r i a l s c a n u s u a l l y be conveyed in the s a m e m a n n e r a s t h e g r o u p B materials. quantity required, however, tends b e c o m e rather fluidizing large. s r e s u l t o t h e r f o r m s transport, such belt conveyors, m igh t prov to be m o r e s u i ta ta b l e .

4.1.3

Air Retentive Materials

Group aterials typically includes powders v e r y fine g r a n u l a r m a t e r i a l s p a r t i c l e d e n s i t y . T h e s e a t e r i a l s s h o u l d c o n v e y w e l l in an air-assisted air-assisted grav ity

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Chapter 18

51

conveyor, they have tendency c o n t i n u e f l o w i n g for a t i m e after fluidizing air has been shut off. This property of air r e t e n t i v e m a t e r i a l s m u s t taken into account when designing conveying systems, since angle repose material, illustrated Figure 18.3a, cannot relied upon stop material flow.

4.1.4

Cohesive Materials

Group inclu des cohesive materials that fluidize satisfactorily difficult h i g h inter-particulate f o rc rc e s r e s u l t i n g f r o m very small particle size, cause e l e ct c t ro r o s t at a t ic i c e f f e c ts ts o r h i g h m o i s t u r e c o n t e n t . T h e d i v i d i n l i n e b e t w e e n g r o u p s C and A is very i n d i s t i n c t and the o n l y w a y o f properly assessing suitability air-assisted gravity conveying is by p r a c t i c a l e x p e r i m e n t doubtful materials small scale test rig. previously m e n t i o n e d , i t m a y b e f o u n d that apparently unsuitable matericombination flowing will, y sliding, move continu ously along slope o f t h e c h a n n e l , h o w e v e r , w i l l need to be greater than inclined c h a nn e l . that g r o u p A and B m a t e r i a l s .

4.2

Material Suitability

At the present time it is s t i l l n e c e ss ss a ry r y t o re re l y h e a v i l y o n e x p e r i e n c e w h e n m a k i n g an assessment whether given material suitable air-assisted gravity conveying. Geldart's chart should help th is respect, respect, ho ever, laboratory tests should provide confirmation d o u b t . M a n y different k i n d s powders case g r a n u l a r m a t e r i a l s h a v e , and are being, transported transported su cce ssfu lly air-assisted gravity conveyor. P ractical ractical info rm ation obtained, unf ortu na tely , tends t o b e j e a l ously guarded b y t h e c o n v e y i n g e q u i p m e n t m a n u f a c t u r e r s . T h i s d o e s h a v e s l o w i n g system d e v e l o p m e n t advancement of the technology. effect

PRA CTICAL AIR-ASSISTED AI R-ASSISTED GRAVITY CONVEYING A s h a s b e e n p r e v i o u s l y e x p l a i n e d , c o n v e y i n g n d o w n w a r d s l o p e has the great advantage gravity assist flow of the aerated bulk solid. This is the traditional, energy application air-float conveying, commercial units a v a i l a b l e u n d e r variety trade names. F i g u r e 1 8 . 1 1 represents basic air-gravity conveyor which c o n v e y e d b u l k s o l i d fl f l o w s c o n t i n u o u s l y u n d e r g r a v i t y fr fr o m i n l e t to the d i s c h a r g e p o i n t . this form device a l so so i d e l y e m p l o y e d discharge aids, flow a s s i s t o r s , m o u n t e d o n t h e f l o o r inside s i l o s , b u n k e r s , r a i l w a y w a g o n s , t r u c k s , e tc tc . s s e ls ls s u c h t h e s e to be m a d e w i t h virtually flat base, T h e y e n a b l e c o n t a i n i n g v e ss and thu s to hav e a sub stan tially greater greater capac ity [9, 10]. In t h e se se a p p l i c a t i o n s t h e channel generally very much shorter than when used for the transport materials.

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Material Feed

Inspection Cover

Vent Filter U n i t

Plenum Chamber

Supply Filtered

Figure 18.11

.1

Arrangement

typical air-gravity conveyor.

Material Discharge

Channel Construction

T h e b a s i c c o n s t r u c t i o n o f a p r a c t i c a l a i r -g -g r a v i t y c o n v e y o r i s v e r y s i m p l e , a n d t h i s is one of its main a d v a n t a g e s o v e r o t h e r m e t h o d s o f b u l k s o l i d s t r a n s p o r t . F o r t r a n s p o r t a p p l i c a t i o n s t h e c o n v e y o r c o n s i s t s e s s e n t i a l l y o f t w o U - s e c ti ti o n c h a n n e l s , w i t h o n e i n v e r t e d , a n d t h e p o r o u s m e m b r a n e c l a m p e d b e t w e e n t h e m . F i g u r e 18.12 s h o w s t y p i c a l c l a m p i n g a r ra r a n g e m e n t for the duct sections membrane.

Conveying Channel

Porous Membrane

Inspection C o v e r (may glazed) .- enum Chamber

Figure 18.12

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Typical conveying duct section.


Chapter 18

52

lower channel serves s p l e n u m c h a m b e r w h i c h air is s u p p l i e d o v e r a l l l e n g t h of the c o n v e y i n g s y s t e m . o n e o r more points, d e p e n d i n g u p o n presence of the covered channel renders c o n v e y o r v i r t u a l l y f r ee ee f r o m problems dust leakage. ou ld, how ever, also also operate oper ate qu ite satisfactorily as an open ch ann el, as it i t often must wh en o perating as a discharge aid. c o n v e y o r s can be m a n u f a c t u r e d n range standard bolt-together components, which include straight and curved sections of various widths. A range o f accessories accessori es are also av aila ble , such as flow diverters, i n l e t a n d d i s c h a r g e ports, inspection win do s, gate gate valv es, access ports scrap traps.

5.2

Material Feeding

W h e r e precise control of the m a t e r i a l flow rate is not r e q u i r e d , flooded feed from s u p p l y h o p p e r to the c o n v e y i n g d u c t s h o u l d satisfactory. system then little effectively self-regulating and, with free f l o w i n g powders, there should s l o p e of the c h a n n e l risk of the c o n v e y o r b e c o m i n g c h o k e d , p r o v i d e d t h a t the flow rate of the fluidizing air are sufficient. baffle Some measure solids flow control m a y b e a c h i e v e d w i t h gate c o n v e y i n g d u c t , p o s i t i o n e d c lo lo s e to the i n l e t f r o m t h e h o p p e r . P l a c i n g a flow in r e g u l a t i n g gate near o u t l e t end o f the c o n v e y o r generally a d v i s a b l e as the w h o l e c h a n n e l c o u l d fill w i t h m a t e r i a l b a c k i n g u p f r o m t h e gate. P r o b l e m s w o u l d t h e n o c c u r w i t h v e n t i n g of the f l u i d i z i n g air and with erratic flushing of the m a t e rial un der the the gate as it open s. Solids flow control at the inlet e n d , a l t h o u g h b a s i c a l l y m o r e r e l i a b l e , d o e s present problem l o n g c h a n n e l s b e c a u s e of the c o n s i d e r a b l e d e l a y b e t w e e n m a k i n g a n a d j u s t m e n t t o t h e c o n t r o l g a te te , a n d s e e i n g t h e e f f e c t o f t h i s a d j u s t m e n t fact, where it is i m p o r t a n t control mateat the l o w e r end of the c h a n n e l . install becomes alm ost essential r i a l flow rate w i t h i n r e l a t i v e l y c l o s l i m i t s , b u f f e r hopper close to the d i s c h a r g e p o i n t . s o m e form r e g u l a t i n g g a t e in the c o n v e y i n g d u c t , material As an alternative o c o n v e n t i o n a l r o t ar ar y v a n e flow control could take place at the h o p p e r o u t l e t . ideal for the p u r p o s e . For a consistent free f l o w feeder screw feeder would material it is possible that a pinch valve or an iris v a l v e w o u l d b e s u i t a b l e

.3

Porous Membrane

variety us m e m b r a n e . S o m e different m a t e r i a l s m a y b e e m p l o y e d as the p o r o us typical examples wo ven cotton, cotton, polyester b eltin g, sintered plastic, ceram ic l a m i n a t e d s t a in i n l e s s st st e el el m e s h . tiles,

5.4

Venting

Wh en the convey or is covered, it is necessary for the top ch ann el to be adequately vented th rou gh su itab le filters. filters. ith short short conveyors it may be sufficient to rely on powder from o u t l e t end of the c h a n n e l , then the air e s c a p i n g w i t h

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t h r o u g h the vent system of the discharg e hoppe r, if one is in use. If the con vey ing system l o n g , or if t h e r e s p o s s i b i l i t y of the c h a n n e l o u t l e t b e c o m i n g ch ch o k e d w i t h m a t e r i a l , it is better i n l e t and the v e n t from two or mo re points betwe en outlet.

5.5

cce ss Port

is likely prove u s e f u l have inspection access ports fitted convenient positions along d u c t , e s p e c i a l l y in the r e g i o n of the inlet o u t l e t , and in o t h e r positions where blockage occur. i n s p e c t io io n p o i n t shown earlier F i g u r e 18.12. T h e s e can be glazed r e q u i r e d .

.6

Material Discharge

end, to telescopic telescopic loa ding spouts. Som care s h o u l d b e t a k e n w i t h t h e v e n t i n g o f from the conveying duct a v o id id e x c e s s iv iv e b l o i n g t h r o u g discharge point, o t h e r w i s e t h e r e s h o u l d b e n o p r o b l e m w i t h t h i s pa p a r of the p l a n t . Controlling the location which material discharged f r o m a n airassisted gravity c o n v e y o r l i k e l y to be more satisfactory than controlling rate of discharge. discharge. Usin g approp riate bends, dive rters and outlet ports it is po ssible to c o n s tr t r u c t q u i t e c o m p l e x sy s y s te te m s . F i g u r e 1 8 . 1 3 i l l u s t r a t e s ingenious simple solution to the problem of auto atically co ntro lling the feed of ma terial to a stockpile.

F i g u r e 18.13

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Stockpile f e e d i n g t h r o u g h

u l t ip i p l e discharge outlets.


52

Chapter 18

T h i s s h o w s a n o v er e r h e a d a i r -g - g r a v i t y co co n v e y o r d i s c h a r g i n g t h e f l u i d i z e d r i s i n g l e v e l of the stockoutlet spouts u n t i l terial down each f s u c c e s s i o n pile causes them lo w o v e r cease d i s c h a r g i n g . M a t e r i a l w i l l a u t o m a t i c a l l y f lo channel membrane, f lo lo w t h r o u g top of any blocked discharge spouts, along next outlet available. Many functions, such t h i s , can be carried automatically, without need of any valves m o v i n g p a r t s in the system, particularly

5.7

Components

air-gravity conveying systems their versatility, One of the advantages these systems sys tems su pp ly the standard components which most manufacturers bolt together suit user's particular requirement. a d d i t i o n to the basic straight channel units intake discharge sections, components normally a v a i l ab ab l e i n c l u d e t h e f o l l o w i n g : Bends: right ha nd left hand. - p ie i e c e s t o d i v i d e t h e f l o w from c h a n n e l i n t o two or three, or to recombine into one. F l o w diverters, often used in co nju nc tion ith side discharg e boxes, to allow operator direct the flow as r e q u i r e d . Flow control gates or baffles: for either manual or automatic operation. Material traps f o r t h e c o l l e c t i o n subsequent removal heavy impurities in the flow construction basically quite straightforward. these c o m p o n e n t s a n e x a m p l e , typica l patter f f l o w diverter, or side discharge box, is illustrated in F i g u r e 18.14.

DESIGN P A R A M E T E R S terms of system design, the main parameter to consider, in order to achieve the desired m ate rial flow flow rate, is i s the cha nn el idt h. The co rrect spe cification of the air requirements cha nnel slope, however essential ensuring that system w i l l operate operate satisfactorily. These three param eters are conside red in deta il, alon with influence of conveying distance.

6.1

Ch annel

idt

a i n p a ra r a m e t e r g o v e r n in in g capacity of an air-gravity conveyor is the c h a n width. In the literature published by manufacturers of these conveyors, and in o t h e r s o u r c e s g i v i n g b a s i c d e s i g n d a ta ta , q u a n t i t i e s d e s c r i b e d 'typical capacities' given. Such capacities g e n e r a l l y g i v e n s f u n c t i o n o n l y of the c h a n n e l width, with little, a n y , i n d i c a t i o n o f h o w s u c h data w o u l d modified differtypes c o n v e y e d m a t e r i a l , and for different slopes and air flow rates.

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Diverter Plate

Material Discharge

Figure 18.14

Duct diverter facility.

T h i s , h o w e v e r , i s n o t a s u n r e a s o n a b l e as it first a p p e a r s v i e w of the fact that, provided s l o p e a n d a i r f l o w rate exceed required minimum optivalues for the particu lar aterial being conveyed, they w i l l have little influe n c e o n t h e m a t e r i a l f l o w rate. These p o i n t s w e r e i l l u s t r a t e d e a r l i e r Figures 18.6 18.8. 6.1.1

Modeling

u s e f u l p r e l i m i n a r y e s ti t i m a t o f t h e c h a n n e l w i d t h r e q u i r e d f o r a g i v e n a p p l i c a ti ti o n may be made regarding constant average velocity and the bulk density velocity de nsity are fact, both f u n c t i o n s o f t h e the flowing suspension. velocity. t a k i n g th th e m b e i n g c o n s ta ta n t , channel slope and the fluidizing width of a convey or re qu ired to handle a given mass flow rate of a material given approximately

(3)

where

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Chapter 18

52

expansion ratio of bed mass flow rat

material

conveying aspect ratio b

p

bulk density

material

velocity o f t h e c o n v e y e d m a t e r i a l

t a k i n g s u i t a b l e average v a l u e s of the q u a n t i t i e s

intro-

i n p l a c e o f t h e b u l k d e n s i t y t, a c o n v e n i e n t 'rule-ofducing the particle density t h u m b ' e q u a t i o n m a y b e proposed

0-65 Capability

m a s s flow rate

conveyed material, m

given

t o n / h and the p a r t i c l e

density, lb/ft t h e c h a n n e l w i d t h , b , w i l l be given in feet. This relationship been used plot chart presented F i g u r e 1 8 . 1 5 , h i c h p r o vi v i de de s q u i c k a p p r o x i m a t e c h a n n e l s i z e for a g i v e n a p p l i c a t i o n . reference for d e t e r m i n i n g s h o u l d b e n ot o t e d t h a t n o r m a l i n d u s t r i a l p r a c t i c e w o u l d b e u n l i k e l y to perwidest channels o p e r a t e w i t h conveying aspect ratio 0-5, high used for the the chart, and so caution sh ou ld be exercised in th is respect w he n sin above equations chart.

.2

Channel Slope

Equation 18.2 been proposed show relationship between channel that the use of slope and the o t h e r system parameters. However, it was p o i n t e d this equation restricted y lack i n f o r m a t i o n on the v a l u e s of the c o n s t a n t s resort laboratory At the present time, therefore, it is still necessary tests if an accurate i n d i c a t i o n of the o p t i m u m c h a n n e l s l o p e which convey given material required. most indu strial app lications air-gravity air-gravity convey ors installed w i t h slope of 2 to 10 degrees. lower level inclination depends very much upon type material being transported. degree i n i t i a l aeration o f t h e conveyed material, and the n a t u r e o f t h e p o r o u s m e m b r a n e , also i n f l u e n c e m i n i m u m slope that can be u s e d . g e n e r a l a b o u t 1° is s u f f i c i e n t v e ry r y f re re e f l o w i n g m a t e r i a l s . l t h o u g h plant layout, w i l l n e c e s s a r i l y be the o p t i m u m suit s u c h a low a n g l e m a x i m u m f l o w , and a s l o p e around 3° may be more appropriate. More

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h e s i v e m a t e r i a l s m a y r e q u i r e a m i n i m u m slope of 7 to 10 deg rees fo r satisfac tory transport, c o n t i n u o u s t ro r o u b l e free o p e r a t i o n . Particle Density lb/ft

6000

4000

10

2000

iooo

0

1

Channel Width - f Figure 18.1

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.3

Conveying Distance

Provided that the continuous dow nw ard slope can be ain taine d, there is generl i m i t to the l e n g t h because ally conveying channel that can be used. This gravity provides m o t i v e f o r c e a n d n o t p r e s s u r e d r o p , as in p n e u m a t i c c o n v e y ing, and horizontal fluidized motion conveyors. Air-assisted gravity conveyors of 500 ft or m o r e length are not u n k n o w n . is necessary, of course, to arrange th e air sup pl y so that a u n i f o r m pressure exists beneath distributor, p o r o u s m e m b r a n e , and in v e r y l o n g c o n v e y o r s usual provide inlets several points along length of the plenum chamb e r . I t m a y also advisable vent conveying channel several points prevent the b u i l d - u p of an excessive velocity over the top o f the aterial being conveyed.

6.4

Air Requirements

In order

s p e c i f y the air r e q u i r e m e n t s of an a i r - a s s i s t e d g r a v i t y c o n v e y o r it is necessary establish v o l u m e t r i c f l o w r a t e of the air t h r o u g h p o r o u s base o f t h e c h a n n e l a n d t h e pressure w i t h i n plenum chamber. 6.4.1

A ir Sup ply Pressure

pressure of the air in the p l e n u m c h a m b e r c l e a r l y f u n c t i o n of the resist a n c e of the p o r o u s base of the c h a n n e l , but it a l s o d e p e n d s u p o n d e p t h of the conveyed material in the channel. For the purpose a n a l y s i s it can be a s s u m e d f u l l y s u p p o r t e d by the a i r . It is t h e n p o s s i b l e that conveyed material estimate pressure o n t h e u p p e r s u r f a c e o f t h e p o r o u s m e m b r a n e s i mp l e fluid e c h a n i c s , fo f o r an a n y req uir ed aspect aspect ratio o f t h e f l o w i n g b e d . K n o w l e d g e o f t h e p e r m e a b i l i t y o f th th e p o r o u s b a s e w o u l d t h e n p e r m i t t h e p r e s s u r e in the to be For the u n i t area, unit pressure drop expressed as the v o l u m e t r i c flow rate i a l , it can be a c r o s s it. If t h i s i n f o r m a t i o n is not a v a i l a b l e for the m e m b r a n e m a t e r ia measured quite easily n permeameter. f l o w r e s i s t a n c e s of the air ffrom rom mover to the p l e n u m c h a m b e r , and of the air t h r o u g h v e n t i n g s y s te te m , m u s t then added give the air s u p p l y p r e s su s u r e n e e d e d for the s p e c i f i c a t i o n . practice, however, it is d i f f i c u l t predict with any confidence optimum value this parameter because o f t h e u n c e r t a i n t y o v e r actual pressure drop flowing bed of material. across m e n t i o n e d p r e v i o u s l y , it is e s s e n t i a l t h a t p o r o u s m e m b r a n e is of s u f f i c i e n t l y high resistance ensure uniform distribution c o n v ey ey e d m a t e r i a l . T y p i c a l l y pressure of the air in the p l e n u m of air into chamber is fou nd to be approximately to 20 inch water gauge.

6.4.2

Volumetric Flow Rate

T h e f l o w r a te t e o f a ir i r t h a t m u s t b e s u p p l i e d t o t h e a i r -g -g r a v i t y c o n v e y o r d e p e n d principally upon width of the c h a n n e l and the nature of the b u l k length

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p a r t i c u l a t e m a t e r i a l t o b e c o n v e y e d . T h e a i r flow rate most conveniently u n i t area o f t h e p o r o u s c h a n n e l expressed t e r m s o f t h e v o l u m e t r i c f lo l o w r at at e base. This, c o u r s e , is the s u p e r f i c i a l v e l o c i t y o f t h e air, from plenum chaminto conveyed material. The value of the superficial velocity that is required, and must be maintained, can be predicted approximately from a knowledge of the fluidization characteristics o f t h e b u l k p a r t ic ic u l a t e m a t e r i a l . B o t c h a n n e l s l o p e a n d t h e material flow rate, h o w e v e r , w i l l a l s o h a v e influence. optimum value superficial air velocity which c o n v e y o r c a n b e operated e c o n o m i c a l l y , w i t h o u t u n d u e risk s t o p p a g e of the m a t e r i a l f l o w , l i k e l y to be b e t w e e n t w o a n d t h r e e t i m e s

t h e m i n i m u m v e l o ci c i ty ty , f, at which the material could be fluidized, as illustrated earlier F i g u r e 18.7. very free f l o w i n g m a t e r i a l s , n relatively steep i n c l i n e , an air v e l o c i t y only slightly in excess of the minimum fluidizing velocity may be sufficient. may be needed. In very fine powders, however, air velocities up to ten times addition being wasteful energy, operation at too h i g h an air v e l o c i t y c a u s e p r o b l e m s s r e s u l t of fine p a r t i c l e s b e i n g e n t r a i n e d in the air stream leavt h e t o p s u r f a c e o f t h e m a t e r i a l b e i n g c o n v ey ey e d a l o n g t h e c h a n n e l . The designer, therefore, requires some knowledge, not only of the m i n i m u m fluidizing velocity of the m a t e r i a l to be c o n v e y e d , also of the air velocity which entrainment can begin. Many methods of predicting for bulk particulate materials are to be found in the published literature. F i g u r e 1 8 . 1 6 is a chart based o n o n e o f these correlations m a t e r i a l s fluidized w i t h air at a c o n d i t i o n close n o r m a l a m b i e n t [1], A l s o s h o w n this chart approximate values s t i l l a i r . The air v e l o c i t y terminal velocity p a r t i c l e s in free f a l l w h i c h p a r t i c l e e n t r a i n e n t c a n b e g i n co c o r r e sp sp o n d s a p p r o x i m a t e l y t o t h i s v e l o c i t y . F o r a particulate material known particle size density, Figure 18.16 allows fairly reliable estimate to be m a d e of the m i n i m u m fluidizing v e l o c i t y . W i t h k n o w l e d g e of the d i a m e t e r of the smallest particles in the material, Figure 1 8 . 1 6 a l s o a l l o w s p r e d i c t i o n of the air v e l o c i t y w h i c h t h e s e fine particles begin t o b e carried upwards f r o m t h e s u r f a c e o f t h e b e d . A p p r o x i m a t e r a n g e s of the t y p e s fluidization behavior, given G e l d a r t ' s c l a s s i f i c a t i o n , a r e a l so so s h o w n o n F i g u r e 1 8 . 1 6 . T h e y a r e s u p e r i m p o s e d o n t h e l i n e s c o r r e s p o n d i n g t o th t h e m i n i m u m f l u i d i z in in g c o n d i t i o n , t h u s h e l p i n g t o provide a useful predictio n of the likely beh avio r of a particulate material in an airassisted gravity conveyor. 7

OPERATING PROBLEMS

been stated that air-assisted gravity conveyors usually trouble free operation, hilst thi true, there a r e o n e o r t w o w a y s which problems may arise.

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1000

io 4J

c.

/3

Group

20

50

100

200

Mean Particle Size - |am Figure 1 8 . 1 6 M i n i m u m f l u i d i z i n g v e l o c i t y a n d t e r m i n a l v e l o c i t y f o r a b e d o f p a r t i c le le s fluidized with air.

One potential source t r o u b l e is the p o r o u s m e m b r a n e t h a t f o r m s base of the conveying channel. There are many examples of installations in which the s a m e e m b r a n e h a s b e e n i n u s e c o n t i n u o u s l y f o r a n u b e r o f y e ar a r s. s. I n o t h e r necessary q u i t e f r e q u e n t intervals. T h e r e cases, h o w e v e r , r e p l a c e m e n t p r o b a b l y little that can be done about b l i n d i n g of the t he po res in the top surface of the

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membrane, precautions c a n b e taken against deterioration o f t h e u n d e r s i d e ens urin g that the m ain air supply is adequately filtered. further precaution c o n c e r n s need for the p o r o u s m e m b r a n e withstand a certain amount of use. It appears appear s to be co on practice for op eratives attempt r e l i e v e s u s p e c te t e d b l o c k a g e s w i t h the aid of an iron bar, similar implement, wielded against outside of the c h a n n e l . i n s p e c t i o n ports provided these often accessed with t h e n o t u n c o m m o n r e s u l t t h a t porous distributor cracked, in the case c e r a m i c tiles, p u n c t u r e d , in the case woven fabrics. Blockage of the c o n v e y i n g c h a n n e l occur unless porous unlikely me mb rane is damaged , or the nature of the conveyed ma terial changes drastically such becoming wet. Both these cases would tend cause local, complete a t i c f l o w in the c o n v e y i n g c h a n n e l d e - f l u i d i z a t i o n of the f l o w i n g m a t e r i a l . E r r at unlikely to be caused by the air-gravity conveying system itself, unless slope s h a l l o w or the bed depth is too great. It is m o r e p r o b a b l e t h a t feed to the c h a n n e l w o u l d be at fault, s result m a t e r i a l a r c h i n g in the h o p p e r s u p p l y i n g conveyor, example.

8

HORIZONTAL AND UPWARD TRANSPOR

e x a m p l e of the air-gravity conveyor, already been established, through that a f l u i d i z e d m a t e r i a l w i l l flow a l o n g a c h a n n e l , i n t h e m a n n e r o f a l i q u i d , provided that there is an i n p u t energy to the m a t e r i a l s u f f i c i e n t maintain flow. v i e w of the many positive features that air-gravity conveying has to offer, attempts devise modificais not surprising that there have been n u m b e r t i o n s to the basic system that would permit material to be transported horizontally or on an u p w a r d s l o p e . such example uses series s te t e p p ed ed a i r - g r a v i t y c h a n n e l s , j o i n e d gether vertical l i f t s . Other methods, some them exhibiting considerable i n g e n u i t y , rely on in clin ed air j e t s , or on the pressure gradient set up in a partitioned channel, provide forward motivation necessary for the m a t e r i a l . n u m b e r of these systems a re ex am ined in deta il below

8.1

The Jet-Stream Jet-Stream and Similar Similar Conveyors

simplest method generating f l o w f fluidized material along h o r i z o n t a l base and/or sides of the c h a n n e l n series c h a n n e l is to introduce through forward facing jets. number interesting variations this approach have been proposed.

8.1.1

Distributor Types

Jet-Stream conveyor, similar types, have all the air e n t e r i n g channel through louvers, or angled slits, in the base. Such a base is illustrated in F i g u r e

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Chapter 18.17. Provided that s i g n i f ic i c a n t c o m p o n e n t o f t h e velocity is in the h o r i z o n t a l direction, m a t e r i a l t o b e c o n v e y e d w i l l 'float' a l o n g o n t h e r e s u l t i n g c u s h i o n

air.

Jet-Stream itself [ 1 1 , 12], air at very pressure f l o w s f r o m p l e n u m chamber through a flat plate punched with a series of louvers, which forms the base of the c o n v e y i n g channel. staglouvers laid out in rows, offset gered e n s u r e u n i f o r m d i s t r i b u t i o n of air jets, w h i l s t m a i n t a i n i n g a d e q u a t e gidity of the p l a t e . s p a c i n g of the p e r f o r a t i o n s , t h e i r s h a p e a n d t h e percentage o f o p e n a r ea ea m a y b e v a r i e d d e p e n d i n g u p o n t h e m a t e r i a l b e i n g c o n v e y e d . W i t h fine granular materials t h e d e p t h o f t h e louvers would very s m a l l compared l e s i nt n t o th th e p l e n u m c h a m b e r . w i t h t h e w i d t h , t o m i n i m i z e b a c k f l o w o f p a r t i c le

8.1.2

Operating

Experience

amount p u b l i s h e d d a ta ta a v a i l a b l e o n t h e v a r i o u s m e t h o d s ho rizontal con veying is somewhat limited. first w o r k o n t h e m u l t i - l o u v e r e d base-plate pears have been that Futer [12] conveyed shelled corn, Mesh (1100 m i c r o n ) sand and 35 Mesh (500 micron) aggregate, rates of u p t ton/h, n c h a n n e l 12 in w i d e and 10 ft l o n g . K o v a c s a r a d i [ 1 3 ] u s i n s i m i l a r basep l a te te s c o n v e y e d g r a n u l a t e d s u g a r n c h a n n e l 6 w i d e and 25 ft l o n g . From p u b l i s h e d l i t e r a tu t u r e appears that conveying channels fitted with louvered slotted base-plates have proved to be v e r y s u c c e s s f u l transporting cartons and packets, but rather less so for bulk p a r t i c u l a t e materials, especially where these are of fine particle size. The main d i f f i c u l t y w h e n c o n v e y i n g s u c h very m a t e r i a l s s e e m s to be in m a i n t a i n i n g u n i f o r m f l o w o f a i r , s i n c e resistance of the d i s t r i b u t o r p l a te t e m e a n s t h a the air flow is s e r i o u s l y a f f e c te t e d by the amount m a t e r i a l in the c h a n n e l .

Figure 18.17

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Perforated plate distributor.


Fluidized Motion Conveying T h e r e l a t i v e l y h i g h v e l o c i t y o f th t h e a i r j e t s f ro ro m t h e d i s t r i b u t o r , h i c h c a n b e up to 6 0 0 0 ft/min [ 1 2 J , cause unacceptable degradation friable materials. been sh ow n [13] that backing slotted plate with porous fabric give s o m e i m p r o v e m e n t in air d i s t r i b u t i o n , but the r e d u c t i o n in air velocity from slots largely destroyed c a p a b i l i t y of the conveyor operate o n t h e h o r i z o n t a l .

8.1.3

Combination Conveyor

interesting variant o f t h e c o n v e y i n g c h a n n e l i t h slotted base illustrated in Figure 1 8 . 1 8 [14]. This example again represents an attempt to o v e r c o m e fluidizing separating from d i s a d v a n t a g e s o f t h e simple slotted base p r o p u l s i o n a i r b y t h e i n g e n i o u s d e v ic ic e m a k i n g a n g l e d s l i t s in the p o r o u s b a s e p l a t e . T h e r e s u l t i n g j e t s o f a i r t h u s d r i v e t h e f l u i d i z e d a t e r i a l u p t h e s l o p i n g s ec ec tions along channel.

.2

The Pneumatic Escalator

have described a device that has s o m e affinity to the T a n a k a [15, Shinohara air jet c o n v e y o r s which they called 'Pneumatic Escalator". construction t h i s d e v i c e i s s i m i l a r t o th t h e c o n v e n t i o n a l a i r - g r a v i t y c o n v e y o r , e x c ep ep t t h a t p l a s t i c plates are fitted across t h e c o n v e y i n g c h a n n e l t o f o r m a s e r i e s o f i n c l i n e d c e l l s . illustrated system Figure 18.19. passing through porous membrane lifts particles up the inclined plates from cell to the next. c o m m o n w i t h o t h e r f o r m s of air jet c o n v e y o r , however, q u a n t i t y of air r e q u i r e d t e n d s to be l a r g e . A l t h o u g h device peared w o r k w e l l o n a n upward slope o f 3 ° , w i t h c o n v e y i n g still p o s s i b l e t slope 26°, m o s t of the channels tested w e r e o n l y inches width. would appear have been tried n commercial scale.

P o r o u s Plates Angled with Slits B e t w e e n

Plenum

Chamber

Air Supply Figure 1 8 . 1 8

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combination

p o r o u s distributor

directional


jets.

Chapter

32

Porous Membrane

Air Supply Figure 1 8 . 1 9

8.3

Plenum Chamber

pneumatic escalator.

The Isler Conveyor

A rather different approach to upward air-float conveying has been used by Isler [ 1 7 ] . e d e v e lo l o p e d a s y s te te m i n h i c h a p r e s s u r e g r a d i e n t w a s s e t u p i n t h e c o n veyed a t e r ia i a l . T h i s w a s a c h ie ie v e d b y d i v i d i n g b o t h t h e p l e n u m c h a m b e r a n d t h e m a t e r i a l f l o w c h a n n e l i n t o s e p ar a r a te te c o m p a r t m e n t s , s u p p l i e d w i t h a i r a t d i f f e r e n t f l o w r a t e s. s . T h i s sy s y s t e m i s i l l u s t r a t e d i n F i g u r e 1 8 . 20 20 . T h e t e s t c h a n n e l w a s 2 0 f long wide.

Air Control Valves

Inlet Figure 1 8 . 2 0

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Isler c o n v e y o r .


Material Discharge

Fluidized Motion Conveying Cement conveyed t rate of 20 t o n / h , a l t h o u g h t h i s c l a i m e d to be well below capacity o f t h e c h a n n e l . maximum upward slope which channel operated about 12°. 12°. Wh ilst th is desig conveyor apparently requires much less t h a n m o s t of the o t h e r s o p e r a t i n g h o r i z o n t a l l y , a n d o n u p ward inclin es, this type p r e ss s s u r iz iz e d c o n v e y i n g c h a n n e l is not so v e r s a t i l e , and it loses s o m e of its s i m p l i c i t y d e s i g n . order operate pressure, some form lock type feeder needed. Control valves r e q u i r e d on the air s u p p l y to the plenum chamber compartments, these have to be c a r e f u l l y a d j u s t e d . T h e y a l s o h a v e to be readjusted if the material flow rate needs to be changed. Because of the c o n t i n u a l r e d u c t io io n pressure conc o m p a r t m e n t a l nature o f t h e device, s e q u e n t e x p a n s i o n of the air means that distance over which conveyor c o u l d p r a c t i c a l l y o p e ra ra t limited. 8.4

The Stepped Stepped Conveyor

Although novel forms air-float conveyor described a b o v e interesting, and may be useful in certain specialized applications, they all f a i l to take f u l l vantage of the ma jor ch aracteristi aracteri sti their liquid-like fluidized m a t e r i a l s , w h i c h behavior f l o w i n g f ro r o m h i g h e r o l o w e r l e v e l u n d e r g r a v i t y . It is this feature that makes air-gravity conveyor such a t t r a c t i v e p r o p o s i t i o n for the econ o m i c tr tr a n s p o r t b u l k particulate materials high rates over long distances, provided that c o n t i n u o u s d o w n w a r d s lo lo p e c a n b e m a i n t a i n e d . been suggested [18] that wh ere there It insufficient headroom for the installation f single long conveyor adequate slope, distance required might achieved using short lengths inclined conveyor, joined risers s u p p l i e d w i t h air at a s l i g h t l y h i g h e r p r e ss ss u r e . S u c h arrangement illustrated F i g u r e 18.21.

Porous Distributors

(Increased Pressure)

ir Inlets Figure 18.21

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u l t i - s e c t i o n s t ep ep p e d c o n v e y o r .


Plenum Chambers

Chapter

534

8.5

Potential Fluidization

H a n r o t [19] d e s c r i b e s a p r e s s u r i z e d h o r i z o n t a l c o n v e y i n g s y s t e m d e v e l o p e d b y A l u m i n u m Pechiney a l u m i n a w a s t o b e conveyed from convey a l u m i n a . single s u p p l y p o in in t more than h u n d r e d o u t l e t s . E l e c t r o l y s i s pots n m o d e r n a l u m i n u m sm s m e l t e r w e r e r e q u i r e d t o b e f i l le le d a n d t h e d i s t a n c e f r o m t h e s i l o t o furthest hopper a b o u t 600 ft. Air at a pressure psig used. sketch of the system given Figure 18.22 this illustrates principle operation. conventional c h a n n e l material. e m p l o y e d , but the channel runs f u l l Balancing columns are positioned on the conveying duct and are used for dedusting. This is not a c o n t i n u o u s l y o p e r a t i n g s y st s t e m in the a p p l i c a t i o n d e s c r ib ib e d . is a batch type system and its object is to meet the demands of the intermittent filling of the pot hoppers. With several hundred such pot hoppers to f i l l , h o w e v e r , the system syste m ust be op erating on a sem i-contin uo us basis at leas least. t. Of all the systems future presented, t h i s pressurized system probably has the greatest p o t e n t i a l development. With c h a n n e l r u n n i n g f u l l , flow p slight incline s logical extension developm ent, but this will start start to t o re qu ire a significant increase in air a ir pressure, and the c h a n n e l w i l l have to be designed to be more pressure-tight meet these demands. Simple modeling, based on static fluid mechanics, can be applied here to illustrate increase pressure required. Supply Hopper

De-dusting Duct

^•"
£&«£*>

I'jiiftTJ

Blower Hoppers

Figure 18.22

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Principle

potential fluidization ducts.



The hydrostatic r e l a t i o n s h i p is:

2-^-

Ap

where Ap

an

lbf/in

-

-

-

-

-

-

-

pressure drop r e q u i r e d

-

-

-

Ibf7in

g

Ib/ff b u l k density of f l u i d i z e d m a t e r i a l g r a v i t a t i o n a l acceleration - ft/s

H

vertical lift

-

gravitational constant

-ftlb/lbfs"

Thus for a typical m a t e r i a l h a v i n g

b u l k density of about 50 lb/ft

pressure drop r e q u i r e d to l i f t the m a t e r i a l a vertical distance of say about

-

the

ft, would be

!/ l b f / i n

REFERENCES C.R. Woodcock J .S .S . M a s o n . B u l k S o l i d s a n d l i n g I n t r o d u c t i o n to the Practice T e c h n o l o g y . B l a c k i e a n d S o n L t d . 1987. 2. J. D o d g e . P r o c e d u r e transportation materials c o n v e y i n g c h a n n e l s u s in i n g press u r i z e d a i r . D R P 8 8 4 0 2 , 1 8 95 95 . e r m a n P a t e n t . R.E. L e i t z e l a n d W . M . M o r r i s e y . A i r - f l o a t c o n v e y o r s . B u l k M a t e r i a l s H a n d l i n g , 307-325, M.C. Hawk, Univ Pittsburgh, Mech Eng. 1971. 4. E. Bush ell and R.C. askell. Fluidized han dling of alu ina powder. Mech Hand ling, 4 7 , N o 3 , p p 126-131. March 1960. C.R. Woodcock and J.S. ason. Fluidized bed conveying - art or science? Proc Pncumotransport 3, B H R A C o n f paper A p r i l 1976. Bath, .R. Woo dcock and J.S. ason. The flow ch aracteristics of fluidized PVC powder inclined ch ann el. Proc 466-475. Chicago. Powder Bu lk Solids Conf, 1977. C.R. W o o d c o c k J.S. Mason. modeling air-assisted bulk particulate solids Flow Inclined Channels. Proc Pneumotransport Paper BHRA Conf. Calif. June 1978 G e l d a r t . T y p e s of gas fluidization. P o w d e r T e c h n o l , V o l 7 , p p 285-292. 1973. Leitzel W.M. Morrisey. Air-float conveyors. Bulk Materials Handling, 307-325. E d M C H a w k , U n i v Pittsburgh Mech Eng. 1971. M.N. Kraus. Pneu matic Co nveying of Bu lk Materials. The Ronald Press Co. New Y o r k . 1968. a r t in i n . N o - t r a n s f e r -p -p o i n t o p e n c o n v e y o r . P r o c e s s E n g i n e e r i n g , p 39. J u l y 1 9 7 4 . R . E . F i l t e r . C o n v e y i n g s o l i d s w i t h c o - o p e r a t i n g s e r i e s of air jets. A S M E p a p e r - 3 1 . 1 96 96 8 K o v a c s a n d S . V a r a d i . C o n v e y i n g g r a n u l a t e d su s u g a r t h r o u g h a e ra ra te te d c h a n n e l s . P r o c 2nd Conf on Pneumatic Conveying, paper p p 1 3 1 - 1 3 9 . Pecs, H u n g a r y . German. arch 1978

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S t e g m a i er er . H o r i z o n t a l l y c o n v e y i n g p n e u m a t i c c h u t e s . F o r d e n Hebcn, Vol 26. 621-624. German. 1976. No 6, Shinohara and T a n a k a . A n e w d e v i c e f o r p n e u a t i c t r a n sp sp o r t particles. Chem Japan, o l 5 , N o 3 , p p 279-285. 1972. K. Shinohara, K. Hayashi, and T. Tanaka. Residence time distribution of particles w i t h p n e u a t i c e s c a la la t o r . J n l h e m E n g o f J a p a n , o l 6 . N o 5 . p p 4 4 7 - 4 5 3 . 1 9 7 3 . air-slide type conveyor horizontal u p w a r d inclined transport. Islcr. Zement-Kalk-Gips. 482-486. G e r m a n . 1960. 0, EEUA Handbook Pneumatic handling pow dered m aterials. onstable L o n d o n . 1963. J -P -P . H a n r o t . M u l t i p o i n t f e e d i n g hoppers, mounted a l u m i n u m s m e l t e r pots, potential fluidization piping. Proc 115th An Mtg The Met Soc of A I M E , means Orleans. M a r c h 1986. 103-109.

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Air Slide DKE292_Ch18

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