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\225-002 Book1 Section 8.doc-10/2/2003 6:08:00 PM

8.0 PNEUMATIC CONVEYING

8.1 INTRODUCTION

8.2 GAS-SOLIDS FLOW THEORY

8.2.1 General

8.2.2 Vertical Upward Flow

8.2.3 Horizontal Flow

8.2.4 Material Characteristics

8.2.5 Design Calculation Methods

8.3 TYPES OF PNEUMATIC CONVEYING SYSTEMS

8.3.1 Dilute Phase Systems

8.3.2 Dense Phase Systems

8.4 SYSTEM SELECTION AND DESIGN

8.4.1 System Type

8.4.2 Pipeline Design

8.4.3 Mode of Operation

8.4.4 Solids Feeder

8.4.5 Air Mover

8.4.6 Gas-Solid Separation Equipment

8.4.7 Solids Storage

8.4.8 Factors Affecting System Design

8.5 SAFETY CONSIDERATIONS

8.5.1 Introduction

8.5.2 Dust Explosions - General

8.5.3 Sizing of Vents - Basic Methods

8.5.4 Factors Affecting Estimation of Vent Size

8.5.5 Venting Considerations for Pneumatic Conveying Equipment8.5.6 Control of Ignition

8.5.7 Inerting

8.6 REFERENCES, CODES AND STANDARDS


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8.7 APPENDICES

8.7.1 Appendix I: Design Calculation Methods

8.7.2 Appendix 2 :

8.7.3 Appendix 3: Fluor Daniel Shortcut Calculation Method

8.7.4 Appendix 4: Fluor Daniel Modified Allied Flotronics Method

8.7.5 Appendix 5: Fischer-Gerchow Method

8.7.6 Appendix 6: Fan Engineering Method

8.7.7 Appendix 7A: Konno and Saito Correlation (FPS Units)

8.7.8 Appendix 8

8.7.9 Appendix 9: Bulk Solid Material Characteristics

8.7.10 Appendix 10: Not Used.

8.7.11 Appendix 11

8.7.12 Appendix 12

8.7.13 Appendix 13: Sieves

8.7.14 Appendix 14: Not used



8.7.17 Appendix 17A: Airlock Size and RPM Calculation

8.7.18 Appendix 18A: Diverter Valve Application Chart(a)

8.7.19 Appendix 19A: Filter Air-to-Cloth Ratio Selection A:C = (AxBxCxDxE):1

8.7.20 Appendix 20: Properties of Common Vapors and Gases

8.7.21 Appendix 21: Altitude - Pressure - Temperature - Density Table of air

8.7.22 Appendix 22: Economics

8.7.23 Appendix 23: Fundamental Burning Velocities of Selected Gases and Dusts

8.7.24 Appendix 24: Fire Hazard Properties of Selected Liquids, Gases and Volatile Solids

8.7.25 Appendix 25: Defining the Limits of Hazardous (Classified) Locations For Compliance withNational Electrical Code

8.7.26 Appendix 26: Explosion Properties of Dusts

8.7.27 Appendix 27: Equipment Data Sheets - Process Input

8.7.28 Appendix 28: Sample Specification

8.8 INDEXES TO FIGURES AND TABLES (NARRATIVE AND APPENDICES)

8.8.1 Index of Figures

8.8.2 Index of Tables


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8.0 PNEUMATIC CONVEYING

8.1 INTRODUCTION

Pneumatic conveying is widely used in the process industries for the handling of dry bulksolid materials, in powdered, granular or pelletized form.

Pneumatic Conveying vs. Mechanical Systems

Advantages over Mechanical

Fewer moving parts

Compact layout in product section

Multiple pickups/discharges Completely enclosed

Heat/cool/dry/blend

Disadvantages

Low efficiency

High velocity attrits and erodes

Inert or dry gas needed to prevent explosions or moisture pickup

There are two broad types of systems, dilute phase and dense phase.

Dense Phase vs. Dilute Phase

Advantages of Dense Over Dilute Phase

Reduced wear from abrasive products

Reduced breakage for friable products

Reduced skins and fines for polymers

Disadvantages

Requires multiple systems for multiple pickups

Pneumatic conveying systems have a variety of applications including unloading ofstockpiles, feeding raw materials to process units and transferring product to or from storage

bins.


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This manual is intended to provide a guide towards establishing a logical basis for thepreliminary selection and specification of a conveying system, and to enable informed

evaluation of alternative tenders by conveying system vendors.

A valuable published work on the subject is the "Pneumatic Conveying Design Guide" byDavid Mills. It contains an exhaustive treatment of conveyer design and a wealth ofexperimental data. This document has drawn upon Mills' work and other published andunpublished data. A few selected articles are listed at the end of this manual and provideadditional insight into the design and operation of pneumatic conveying systems.

Stokes' Law states that the terminal velocity of a particle falling through a fluid isdetermined by the particle density, diameter, shape, and fluid properties such as densityand viscosity. Translated into pneumatic conveying terms, a flowing gas will dragparticles with it above a gas velocity which is characteristic of the solid particle and gas

physical properties, and particle shape. This characteristic gas velocity is known as thesaltation velocity. Particles traveling above the saltation velocity are suspended in streamflow with the gas, or are entrainedin the gas stream. System pressure drop is the sum ofthe energy losses in the system. These losses are described by an energy balance, andinclude terms for gas acceleration, solids acceleration, gas friction loss, solids frictionloss, and static losses in vertical flow. In pneumatic conveying systems, this energybalance describes a two-phase compressible flow system, and is therefore usually a trial-and-error calculation procedure. All available procedures are approximations, havedependence upon average solids material characteristics, which can vary widely, makingdesign calculations difficult to make with certainty. Inexperienced engineers shouldapply these methods with caution. The approaches presented in this manual will yieldsuitably conservative estimates, but must be verified by either direct experience with thematerial in question, or laboratory tests.

8.2 GAS-SOLIDS FLOW THEORY

8.2.1 General

An appreciation of the nature of two-phase gas-solid flow within an enclosedduct is needed to understand the flow regimes in pneumatic conveying. Apneumatic conveying system is generally made up of sections of straight pipe,some of which are vertically oriented and normally carry solid material in anupward direction, and some which are horizontally oriented and provide for flowin lateral, horizontal directions.

There is a distinct difference in the flow and transport characteristics of gas-solidsystems between vertical and horizontal flow. This can be seen in the analysespresented below. It is also recognizable in plant operations in the form of linevibrations which may occur at too high a solids loading or too low a gas flowrate.


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Figure 8-1

VERTICAL CONVEYING PHASE DIAGRAM


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8.2.2 Vertical Upward Flow

The phase diagram in vertical conveying is illustrated in Figure 8-1. It consistsof a plot of pressure drop per unit length versus superficial gas velocity, with thespecific solid flow rate as a parameter, and is most conveniently drawn on log-log coordinate paper. Such a phase diagram is specific for a given conveyingfluid density and viscosity, for a given pipe size and for a given density andparticle size of the conveyed material. Representation on such a plot indicatesthe bounds within which a vertical conveying line may operate and the attendantpressure drop and required gas rate. The curve for the empty pipe represents alower bound; the dilute suspension "fluidization" curve represents anotherboundary (essentially the free fall or terminal velocity of the largest particle inthe material to be conveyed), and the available pressure drop represents an upperbound. Within this area vertical pneumatic conveying may be carried out, with

the following main types of flow being identified: Dilute Phase Flow - At low solid-gas ratios the particles are carried upwards

in the flowing gas steam as a uniform suspension.

Dense Phase Flow - Occurs at higher solid-gas ratios and may be eitherslugging or non-slugging. Heavy/coarse particles tend to be carried upwardas a series of slugs. Small/light particles may be transported upward withoutslugging but with a large amount of internal recirculation occurring.

Moving Bed Flow - The product is transported upwards as a packed column,with very little internal circulation.

The transition from dilute phase to dense phase conveying is not always clear,particularly when dealing with materials of wide particle size distribution inwhich the largest particles might slowly accumulate at a bend near the bottom ofa vertical line (if the velocity is only sufficient to carry up the fines in dilutephase flow) until they form a slug, bridging the pipe, and are then blown upmomentarily as another slug begins to accumulate at the bottom. Such operationmight go undetected if the slugs form rapidly enough or if the total line pressuredrop is large enough to overshadow the fluctuation it would cause in thedischarge pressure of the air mover.

Ideally the transition from dilute to slugging dense phase vertical flow for auniform particle size material would appear as illustrated in Figure 8-1where W1,

W2 etc., represent increasing specific solid flux rates in units of mass flowratetimes the total pipe cross sectional area. At some high gas velocity representedby Point A, the introduction of solids at a rate W2 results in a pressure dropgreater than that necessary to push the gas alone through the pipe. As the gasvelocity is lowered, the pressure drop decreases, following a path nearly parallelto that of the curve for the empty pipe. When the velocity has decreased toaround Point B there is a slower decline in pressure drop with further reductionsin gas velocity. This is a consequence of the slowing down of the particles andof the resulting increase in the density of the suspension in the pipe. The


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particles travel up the pipe at a lower velocity than the gas. This velocitydifference, or "slip", is related to their free fall or terminal velocity in the gas

medium. If gas were passing up the pipe at a superficial velocity equal to theparticle's free fall or terminal velocity then the particle could (theoretically) beheld in suspension, moving neither upward nor downward. Thus, as thesuperficial velocity is reduced from Point A to Point B, the particles slow downsignificantly. Since the net mass flowrate W, remains constant, the flowingdensity or holdup must increase. This increased particle holdup, or inventory, orsuspension density, is reflected in the pressure drop; the frictional pressure dropbecomes negligible at low velocity, but the holdup or inventory pressure dropincreases, and predominates as the superficial velocity decreases from Point B toPoint C.

As the suspension density increases, the distance between particles decreases.

When, as illustrated in Figure 8-2, this distance decreases to the point where adownstream particle gets into the wake of its following neighbor, it drops intothis wake and falls, touching its upstream neighbor, and thus effectively presentsa larger binary to the flowing gas stream. The stream cannot support this largerparticle and hence the entire suspension collapses to the bottom of the pipe. Thevelocity at which this collapse of the dilute suspension occurs is referred to as thechoking velocity. Choking velocity, as illustrated in Figure 8-1is a function ofthe solid flowrate W; the greater the mass flowrate the higher the velocity neededto maintain the particles sufficiently distant from each other to avoidprecipitating the choking condition.

If choking occurs while a continuous feed of solids is maintained at a rate W1,the solids build up, starting at the lower end of the vertical pipe, until theinventory reaches a point where slug flow (dense phase) becomes the steady statemode. This sequence of events is illustrated schematically in Figure 8-3.

No good correlations for dense phase flow in vertical pipes exist (especially for"dune" type flow), although the Particulate Solids Research Institute (PSRI) isinvestigating this area.

8.2.3 Horizontal Flow

The phase diagram for horizontal conveying is more complex than that describedfor vertical conveying, because it is dependent on the deaeration characteristicsof the solids being conveyed. In a vertical pipe when the solids slow down orapproach choking, they cannot fall to rest; they can only fall head-on into theoncoming gas stream. In a horizontal pipe when the solids slow down, they cansink to the bottom of the conveying line and either remain there as stationarysolids, still pushed along by the conveying gas as an aerated mass, or be pushedthrough the pipe as deaerated slugs. As particles drop out, a layer of materialbuilds up, which moves in wave or "dune" flow along the bottom of theconveying pipe, with particles in stream flow in the gas stream above the saltedlayer. As velocities drop lower, the dunes fill the pipe forming pistons. Since


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gas density decreases and therefore velocity increases as the gas flows throughthe conveying system, it is possible to transition from dense to dilute phase flow

in the system. The flow regime is dependent upon the solids flowrate, the gasvelocity and the solids' deaeration characteristic. The pressure drop per unitlength of pipe length differs depending upon the mode of the conveying, whetheras a dilute suspension, a dense aerated mass, or slug flow.

The various types of flow regimes as well as pipeline pressure drop versus airvelocity for horizontal and vertical pipe are shown in Figure 8-4A. Additionally,Figure 8-4B presents five modes of gas-solids flow in horizontal pipes.

Consider first a simple situation involving conveying a relatively coarse materialof uniform particle size with air through a horizontal line; the correspondingphase diagram, again on a log-log grid, is illustrated in Figure 8-4. The

Curve AB represents the pressure drop for the gas only and the accuracy andreliability of prediction of the conveying pressure drop depends on the reliabilityin predicting the Curve AB. If at some relatively high gas velocity solids areconstantly introduced into the line at a rate W

1, an increased pressure drop will be

necessary to propel the gas-solids mixture through the line, as represented byPoint C in Figure 8-4. As gas velocity is reduced, the flowing frictionalresistance decreases and the observed pressure drop decreases along the CurveCD. However, as gas velocity decreases the particle velocities also decrease,until at some sufficiently low gas velocity, represented by Point D, the particles"salt" out, or settle out, on the bottom surface of the pipe. The velocity at whichthis occurs is termed the "saltation velocity"; it is a function of the gas and solidscharacteristics and also of the pipe size.

When dealing with relatively coarse and uniform particle sizes, saltation isgenerally accompanied by a rapid filling up of the pipe to nearly half its crosssection. Thereafter, steady state conveying proceeds in the open space above thesalted layer. As gas velocity is further reduced, the salted layer becomes deeper,thereby further restricting the pipe area and resulting in a rising pressure drop asalong Curve EF.

Comparing Figure 8-4 for horizontal flow, with Figure 8-1 for vertical flow, itbecomes evident that in the case of vertical flow the particle free fall or terminalvelocity represents an ultimate lower velocity limit below which essentially nodilute phase vertical conveying can occur; in the case of horizontal flow there

must also exist some similar lower limit. The lower limit in horizontal conveyingmust be the minimum velocity necessary to convey a single particle through thepipe without having it salt out; i.e., the single-particle saltation velocity or thesaltation velocity at zero loading.


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Figure 8-2

CHOKING VELOCITY PHENOMENA


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P

D


Figure 8-3

SCHEMATIC OF SOLID BUILD-UP FROM DILUTE TO DENSE PHAS


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Figure 8-4A

FLOW REGIMES & PRESSURE DROP FOR HORIZONTAL AND VERTICAL P


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Figure 8-4B

MODES OF COCURRENT GAS-SOLIDS FLOW IN HORIZONTAL PIPE


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Figure 8-4

HORIZONTAL CONVEYING PHASE DIAGRAM


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As applicable to pneumatic conveying, the effective single particle saltationvelocity is that velocity at which the particle will travel through the pipe,

occasionally hitting the walls, such that the contact is minimal and not normallydetectable. Measurement of single particle saltation velocities reveals that thereare several distinct velocity criteria applicable to concurrent fluid particle flowwhen a particle is dropped into a stream flowing through a pipe:

a) The minimum velocity needed to move the particle, though withouttransporting it an appreciable distance before it finally comes to rest(presumably related to the particles' orientation in its most stable position ofrest).

b) The minimum velocity required to transport a particle by rolling or bouncingalong the bottom of the pipe.

c) The minimum velocity required to transport an injected particle, withoutsaltation, in fully suspended flow.

d) The minimum velocity required to pick up a particle from rest on the bottomof the pipe and transport it.

e) The minimum velocity required to pick up a particle from a layer of particlesand transport it through the pipe.

f) Conditions a to e correspond to increasing velocities in that order. Practicalconsiderations suggest that criteria c and e are the most significant inhorizontal conveying. In general, criterion e corresponds to a superficialvelocity 2 - 2 times that of criterion c. Criterion c is considered tocorrespond to the single particle saltation velocity which, as illustrated in

Figure 8-4, represents the minimum conveying velocity in horizontal pipes,analogous to the choking velocity in Figure 8-1.

The factor of 2 - 2 between criteria c and e is in agreement with observationsthat when saltation occurs the pipe fills up nearly half full (doubling the velocityin the space above the salted layer) before steady state conveying is restored.

8.2.4 Material Characteristics

There are two primary considerations in determining the practicability of and thedesign of pneumatic conveying system; first is the material's characteristics, andsecond is the system's design parameters.

Material characteristics can vary widely in the same material in ways which cansignificantly impact pneumatic conveying systems. Bulk or apparent density isthe uncompressed apparent density of the solids. True density is the actualdensity of the material without void space in between the particles. Bulk densityincludes the void space, which lowers the density of the powder when comparedto the solid itself. If the bulk density is variable (aeration is greater or lesser), thefeed rate into a pneumatic conveying system can vary greatly, particularly insystems which are fed volumetrically. Feed rate variation can cause surging,


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which if extreme could plug the system.

Particle size and distribution can also cause the bulk density to vary since finematerials become aerated more readily, lowering the bulk density. Fine materialsmay work in one conveying system, but not in another. For example, finematerials may not perform well in piston type dense phase systems over longdistances. Some materials readily break into smaller particles (i.e., are friable).This tendency may reduce the value of the material or cause excessive losses.Low velocity dense phase systems can be used to reduce this type of degradation.Particle shape will affect system selection as well. Efficiency of conveying andseparation equipment is affected by particle shape. Long, thin particles such asfibers cannot be separated efficiently using a cyclone. They are carried throughwith the gas. These particles must be filtered.

Materials with a high moisture content can stick inside piping causing plugs, clogrotary valves and blind filters such as dust collectors. Cohesive powders can actlike moist powders since the particles may form large agglomerates withpressure.

Some powders, especially refractories, are highly abrasive. Abrasive powdersare typically handled in dense phase systems, which have low velocities. Lowvelocity reduces wear. Refractory liners, and special fittings such as vortexelbows or blinded tee elbows are used to control wear in dilute phase systems.

Other considerations include whether the material is toxic, carcinogenic, anirritant, flammable, hygroscopic, or explosive. Most organic and metal powdersare explosive or flammable when fine enough.

A summary of design problems, the principle effects of a materialscharacteristics, and the design approach to solve the design problem follows.


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Design Problem Principle Effects Approach to Solution

FlowCharacteristics

Power consumption; uniformityof operation.

Velocity control; use offeeders.

Attrition(Degradation)

Product damage; change in flowcharacteristics; increase inexplosive hazard.

Reduce bends; lowervelocity; dense phaseconveying.

Bulk Density Power consumption; tendencyto aerate.

Velocity control.Component sizing.

Particle Size Power consumption; build-up in

ducts. Filter efficiency.

Velocity control. Filter

design.

Abrasiveness Accelerated component wear. Velocity control.

MoistureSensitivity

Caking in storage; productspoilage.

Dry conveying medium;ventilation in storage.

Toxicity Personnel hazard. Vacuum systems.

TemperatureSensitivity

Product damage. Cool conveying medium.

ChemicalActivity

Corrosion; contamination. Material of construction.

Odors Spoilage of foods. Special filters.

This table is taken from the lecture notes by Hendrik Colijn, ConsultingEngineer, Transportation & Material Handling Services, for a "PneumaticConveying Systems" course.

8.2.5 Design Calculation Methods

Dilute phase design calculation methods include the Zenz-Othmer method, theFischer-Gerchow method, the Fan Engineering method, the short-cut methodused at Fluor Daniel, the Modified Allied Flotronics method and the Konno-Saitocorrelation recommended by PSRI. All of these methods involve some form ofenergy balance equation analogous to the Bernoulli equation in fluid hydraulics.The Fischer-Gerchow and Fan Engineering methods focus on a momentumequation which use empirical material friction factors. These material factors areusually proportional to the tangent of the angle of repose. The Kenz-Othmer andKonno-Saito methods use the gas frictional loss and a material to gas loadingratio, avoiding the empirical factors, but producing conservative solutions:


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a) Fluor Daniel Short-cut Method

b) Modified Allied Flotronics Method (Modified by Fluor Daniel Houston)

c) Fischer-Gerchow Method

d) Fan Engineering Method

e) Konno-Saito Method (PSRI)

f) Zenz-Othmer Method (Solt)

Dense phase design calculation methods include various graphical phase diagrammethods, the Zenz-Other method (for two-phase "dune" or "wave" flow), and thePSRI method (for "slug" or "piston" flow).

a) Phase Diagram Method (Graphical)

b) Zenz-Othmer Method (Solt)

c) PSRI Method

The three basic parameters calculated for pneumatic conveying systems areconveying line size, system pressure drop, and gas mover horsepower. Thevarious calculation methods as well as example problems are included inAppendix 8.7.

8.3 TYPES OF PNEUMATIC CONVEYING SYSTEMS

In pressure systems a source of pressurized gas is positioned at the supply end of thesystem. Pressure is used to push gas through the conveying system through the pick-uppoint, and a cyclone or dust collector which disengages the solids from the flowing gas atthe solids destination. The gas is discharged directly to the atmosphere. Pressuresystems may operate in dilute, dense, or some combination flow regime. Pressuresources include fans, rotary lobe blowers, centrifugal blowers and various types ofcompressors The solids flow capacity and ultimate conveying distance will be limited bythe pressure the source is able to supply. The conveying gas may be air or some inert gassuch as nitrogen, carbon monoxide, carbon dioxide, or argon.

In vacuum systems a fan or blower is positioned on the discharge end of the system. A

vacuum is pulled on the conveying system through the pick-up (material feed) point, anda cyclone or dust collector which disengages the solids from the flowing gas at the solidsdestination. The gas is exhausted from the flowing gas at the solids destination. The gasis exhausted from the fan or blower to atmosphere. Vacuum systems are typically dilutephase systems using fans or rotary lobe blowers to provide the vacuum. Small systemsmay use regenerative blowers as well. Dense phase vacuum conveying may be used overshort distances.


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Closed systems are used to limit the make-up of inert gas or conditioned air required forsome systems. These systems can be operated in pressure or vacuum, but are typically

operated with a minimum system pressure just over ambient atmospheric pressure withinert atmospheres. Setting a slightly positive minimum system pressure ensures that thesystem will leak out, keeping air (oxygen) from entering the system. These systems canbe treated the same as the pressure system, except that the fan, blower, or compressordischarges through the system, ending at the suction, instead of exhausting toatmosphere. System pressure is controlled by bleeding excess gas and adding make-up atthe system minimum pressure point, typically at the fan, blower, or compressor suction.Temperature is controlled by an aftercooler at the discharge of the fan, blower orcompressor. It is important in designing closed loop systems that the design pressures ofbins, hoppers, silos, and solids disengagement equipment such as cyclones and dustcollectors be considered carefully. Typically such equipment is a very low designpressure (-4" W.C. to +12" W.C.). Locate bins, hoppers, and silos at or near the system

low pressure point in order to minimize the required design pressure. Dust collectors andcyclones are readily available with design pressures up to +100" W.C., but typically arelimited to 30" W.C. All these vessels and equipment may be designed for much higherpressures at much greater expense.

Combination (vacuum/pressure) systems use vacuum on the feed end of the system, andpressure on the discharge end. Low pressure systems using fans may at times pass solidsalong with the gas through the fan. Material handling fans are prone to high maintenancedue to wear. Most combined systems require a rotary lobe blower, which cannot tolerateparticulates. The material is filtered through a dust collector, and then re-fed to thepressure side of the system.

Pneumatic conveying systems are broadly divided into dilute and dense phase systems.

8.3.1 Dilute Phase Systems

In dilute phase systems a material feeder introduces solid particles into a gasstream, which is either created by a source of positive air pressure, or induced bya source of vacuum. The kinetic energy of the airstream is converted intodynamic pressure and aerodynamic lift, and the particles are fluidized andaccelerated to form a suspension. The mass ratio of solid-gas in the suspensiondefined as the phase density, is less than 10:1. At the destination the particlesmust be separated from the gas stream.

A variety of mechanisms may be used for feeding the material into the gasstream. Rotary valves are the most common, although blowing seals, venturifeeders and screw feeders have also been used. Material feeders are potentialsources of gas leakage from the system and their influence upon system selectionand design is discussed in Section 8.4.4.

The gas-solid separation devices used include cyclones, fabric filters and, insome applications, elutriators. The selection of separation devices is primarilydependent upon the product characteristics, as discussed in Section 8.4.6.


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The minimum conveying velocities required to achieve dilute phase flow aretypically in the range 13-15 m/sec. Volumetric expansion with declining

pressure along the pipe may therefore yield conveying velocities of the order of40 m/sec at the outlet. Most bulk solids can be conveyed in the dilute phasemode; the effect of particle characteristics and size distribution upon thesuitability for dilute and dense phase conveyance is critical.

Figure 8-5a shows dilute phase flow at velocities slightly above the minimumconveying velocity; a strand of particles skips along the bottom of the pipe,whilst the particles above this region are in fully suspended flow. Figure 8-5billustrates flow at higher velocities where the particles have formed a completelyuniform suspension.

Dilute phase systems may be broken down into the following categories:

Positive pressure systems

Vacuum systems

Combination vacuum-pressure systems

They may be further divided into open and closed systems.

a) Positive Pressure Systems

Positive pressure systems involve a gas mover forcing gas through a pipe

into which the product is introduced, fluidized and accelerated. At thedestination a gas-solid separator removes the bulk solid from the gas.Positive pressure systems usually have a pressure not exceeding14.5 psig/1 barg and utilize either:

Axial or centrifugal fans; or

Twin lobed or positive displacement blowers

Air mover selection is discussed in Section 8.4.5.

Positive pressure systems are especially suited for delivery to multipledestinations. Diverter valves may be used to select the direction of flowfrom several alternative routes. Positive pressure systems are notrecommended where several sources feed the same conveying line via rotaryvalves, because the air leakage (and energy loss) through the valves can besignificant compared to the total air volume required for conveying.

A simple positive pressure system is depicted in Figure 8-6.


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Figure 8-5


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b) Vacuum Systems

Vacuum systems operate according to the same principle as positive pressuresystems, except the solids are conveyed by an air flow induced on the suctionside of the air mover see Figure 8-7. Centrifugal fans or twin-lobed rotaryblowers are usually used in such systems. The conveying line pressure dropfor vacuum systems is limited to 7 psia/0.5 bara max. (there will beadditional pressure drop due to the gas solid separation equipment). As aresult they cannot achieve the throughputs or distances possible for anequivalent positive pressure system. The lower air density in vacuumsystems means piping and equipment are generally larger than for pressuresystems with the same conveying rate. Feed hopper walls are thicker whenthey are subjected to vacuum. Properly feeding the conveyor from thehopper reduces the potential for hopper wall collapse. Vacuum systems need

complex pipework and isolation valves. Vacuum systems are less commonlyfound in multipoint discharge systems because they are more prone to "makeup" than positive pressure systems.

Despite such disadvantages vacuum systems are ideally suited for a varietyof uses, such as vacuuming up material from stockpiles, ship unloading, andcleaning up product spills. Vacuum systems have been successfully used inmultipoint discharge systems in batching applications such as with dryingredients or micro-ingredient blending. Each receiver is manifolded to acommon vacuum source and has its own vacuum valve. The number ofreceivers that can be on-line simultaneously is limited only by thevacuum source size. They are superior to positive pressure systems for

transferring product from several sources to a single destination. Leakageacross rotary valves is relatively insubstantial when compared with positivepressure systems because of the small pressure differential across the valveswhen in vacuum service. The fact that leakage is inward is alsoadvantageous, enabling the handling of toxic, odorous or radioactivematerials.

Air ingress must be prevented if it at all possible. However, at many points itis probably unavoidable (e.g., at flexible piping sections used in shipunloading). Air ingress will alter the balance of conveying air velocities andmust be accounted for in the specification of the air mover.

c) Combination Vacuum-Pressure Systems

Combined vacuum-pressure systems have the advantage of being suitable fortransferring product from multiple sources to multiple destinations. Thesource hoppers may be isolated by knife gate valves, the destinations selectedby diverter valves. There are several types. The air mover serves as both anexhaust and blower. Particle degradation and erosion make it unwise toconvey the product through the air mover, although this has been done in


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Figures 8-6 and 8-7

POSITIVE LOW PRESS & VAC SYSTEM


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some applications. As single blower "pull-push" systems tend to beundersized, they have a history of being heavy maintenance items. On no

account should product be passed through a Roots type or a Gardner Denver(PD lobe type) blower. Instead it should be bypassed using an intermediatestorage hopper with its own filter and feed device as depicted in Figure 8-8and Figure 8-8A. Conservatively size the air mover, especially when usingfans, and also ensure low air-to-cloth ratios in the filters.

Dual combined systems separate the positive and negative pressure systemby means of an intermediate vessel and enable the optimum equipment itemfor each service to be specified. Selection of a liquid ring vacuum pump anda screw or reciprocating compressor, instead of the single twin-lobe rotaryblower usually used in single systems, would enable transport over a greaterdistance. A schematic of a dual combined system appears in Figure 8-9and

Figure 8-9A.

The different pressures in the two parts of the system influence the airvolume and therefore velocity; the different air densities influence theminimum conveying air velocities. Therefore, for an equivalent solidsflowrate, different pipeline diameters may be required in the two differentparts of the system.

d) Closed Circuit Systems

Most pneumatic conveying systems draw air from the atmosphere anddischarge it to the atmosphere (via appropriate filtration equipment to protect

the air mover from damage, the product from contamination and theenvironment from pollution). This arrangement is adequate for mosttransport duties because the product itself is enclosed, and pollution may beeliminated by correct design of gas-solid separators and vents.

In a closed system the discharge gas is recycled from the vent back to the airmover suction. This recirculation of the conveying medium to (generally airor nitrogen) reduces the demand to a small makeup supply compensate forleakage.

Where the product characteristics dictate the use of a conveying mediumother than air, economic considerations will favor conservation of the gas in

a closed system. If the product is explosive in air, or would becontaminated/degraded by exposure to air, an alternative medium must beused. Nitrogen is the most common alternative medium. Othercircumstances which may necessitate the use of a closed system include thetransport of radioactive, toxic or odorous products. The effect of the productcharacteristics and of the conveying medium on system selection is discussedfurther in Section 8.4.


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Figure 8-8

"PULL-PUSH" CONVEYING, ONE PRIME MOVER


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Figures 8-8A

"PULL-PUSH" CONVEYING SYSTEM VACUUM PRESSURE WITH ONE PRIME MOVER


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Figure 8-9

"PULL-PUSH" CONVEYING, TWO PRIME MOVERS


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8.3.2 Dense Phase Systems

The British Standard draft definition is as follows:

"Dense phase conveying occurs when products are conveyed throughall or part of the pipeline with air velocities lower than those requiredfor dilute phase conveying and at phase densities equivalent to thosefound in fluidized flow". The phase density is defined as the solids-to-gas loading ratio by weight.

Dense phase systems usually operate at pressures in the range of 15-90 psig/1-6 barg, with gas velocities of typically 3-33 ft/sec/1-10 m/sec. Initial gasvelocities and velocities at the exit greater than 10 m/sec have been observed.Streams with phase densities of 40 and above are considered to be in dense phaseflow. Systems operating at phase densities up to 300:1 have been designed.

Dense phase systems have several advantages over dilute phase systems. Theyare generally more efficient, achieving higher product throughputs at lower gasflowrates and thereby reducing energy costs. The tendency for particle breakupis also reduced at lower gas flowrates. The lower volumetric flowrates enable theuse of smaller air movers, piping sizes and separators. Higher pressure operationenables conveying over much greater distances (than the few hundred metersattainable by dilute phase systems) with some dense phase systems transportingproduct as far as 3,000 m.

In most dense phase systems solids are fed to the conveying pipe using a vesselcalled a "blow tank" or transporter. Blow tanks/transporters usually operate atpressures above 1 barg; in such cases they must be designed according to theASME code for Pressure Vessels, Section VIII, Division 1. They, together withthe required instrumentation and control, are therefore a relatively expensivecomponent. Similarly, the higher pressure means that the Roots type blowerscommon in dilute phase applications are usually inadequate for dense phasesystems. Instead more expensive compressors must be used, unless the gasconsumption is low enough to be accommodated by the plant air system. Thetransport mechanism at such low velocities is shown in Figure 8-10. Inhorizontal flow (a) Particles are metered into the pipe and remain on the bottombecause the air velocity is too low to overcome the frictional resistance, R. (b) theparticle dune increases in cross section as more particles are fed into the pipe. Asthe height of the dune increases so does the air resistance force, W. (c) the dunemoves in the direction of air flow and spreads out and other dunes collide with itforming a larger dune. The pipe cross section is reduced, the velocity increasesand the dune moves along the pipe.

In vertical flow (a) an individual particle settles when the air velocity v fallsbelow the terminal velocity WSof the particle. (b) the pressure of more particles

in the same cross section of pipe reduces the gas flow area and thereforeincreases the velocity. (c) when a sufficient number of particles are present theeffective air velocity between the particles exceeds the terminal velocity, W

S, and

the group of particles is lifted. (d) in effect, when conveying bulk granular


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solids, a slug flow pattern develops.

Unlike dilute systems, many materials cannot be successfully conveyed in thedense phase. The particles most suitable for dense phase conveying are thosewith a narrow particle size distribution and good air retention properties.Granular products, especially those with a high percentage of fines, generallycannot be transported in the dense phase because their low permeability leads toblockages. Theoretical modelling of dense phase flow behavior is extremelydifficult and for reliable design the use of test rigs and scale up techniques isessential.

a) Blow Tank/Transporter Systems

1) General Principles

The most common type of dense phase system is based on the blow tankor transporter. Essentially a blow tank/transporter is a pressure vesselwhich is charged with material, pressurized and discharged batchwiseinto a pipeline. The filling and discharging cycle must then be repeated.While it is inherently a batch process it may be adapted for continuousoperation by using twin pressure vessels either in series or parallel, asdiscussed below. When specifying batch systems, however, state theaverage pseudo-continuous rate required as such to the vendor who willrecommend the approximate blow tank system size and cycle time.

2) Single Plug Blow Tanks

3) The simplest form of blow tank, Figure 8-11only has valves to isolatethe tank from the supply hopper and the vent line. The blow tank startsto pressurize as soon as the vent line is closed, and both the tank and linemust be pressurized before any material is delivered. The material ispushed into the line as a single plug, usually via a bottom discharge. Noseparate conveying air is used and fluidizing air is not usually supplied tothe vessel. Towards the end of the cycle the tank and line must bedepressured to enable charging of the tanks for the next cycle. The timespent pressurizing and depressurizing the system reduces the proportionof the cycle that is spent actually conveying product. Therefore, to

achieve a given time-averaged transfer rate the actual transfer rate mustbe higher (Figure 8-12). The ratio of the mean to peak transfer rates maybe increased (thereby reducing the peak transfer rate required to achievea given mean transfer rate) by:

Increasing vessel size

Fitting valves to the discharge line and (if fitted) supplementary airline


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It is desirable to reduce the peak transfer rate required to meet a givenservice because it is this rate that provides the sizing basis for the

equipment.

By increasing the vessel size and hence the amount of product conveyedper cycle also increases the proportion of the cycle spent conveying. Abalance must be met between the rate cycling and the blowtank/transporter size.

The fitting of valves to the discharge line and supplementary air supplyline (if fitted) enables rapid pressurization and depressurization. Thetank may be rapidly pressurized if all the air available is used anddischarge is prevented until the required steady state pressure is reached.Depressurization time is reduced by isolating the tank from the

conveying line, closing the discharge line valve and opening the vent linevalve immediately upon its emptying. This is also advantageous becauseit prevents the large volume of air in the blow tank from rapidlyexpanding through the conveying line once the plug has been dischargedfrom the pipeline. The very high air velocities that otherwise resultcould cause severe erosion problems during this pipework ventingprocess and subject bends (especially blank tee-pieces) to very highforces; pipework must be well supported in such circumstances.The"rapid expansion" problem does not exist in systems that have beendesigned to maintain product in the line between blow tank/transporterfillings.

The air supply used to pressurize the blow tank is usually also used tofluidize the tank contents and thereby facilitate discharge. The fluidizingmembrane is usually porous plastic, porous ceramic or filter clothsandwiched between two perforated metal plates or rubber "pulsers." Asecondary air supply is frequently fed into the conveying line justdownstream of the tank. More recently, however, 80-90 % of the air isfed along the total length of the line. This supplementary air is useful ifthe material has poor air retention properties, and is essential for goodcontrol. Where the secondary air supply is fed into the conveying linejust downstream of the tank, the discharge rate may be controlled byproportioning the air supply between the fluidizing and supplementaryair lines (Figure 8-13

).

Increased product flow is obtained by increasing the fraction of thetotal gas rate that is supplied to the blow tank.

Reduced product flow is obtained by increasing the fraction of thetotal gas rate that is supplied as supplementary air.


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Blow tanks/transporters may be classified as top discharge or bottomdischarge, depending upon the direction in which the product is

discharged. Top discharge tanks have an internal discharge pipepositioned above the fluidizing membrane (typically by 1.2 in/30 mm forpowdered materials) as shown in Figure 8-14. Top discharge tanksachieve the highest feed rates and enable better control; they are bestsuited to fluidizable powders with low air permeability and good airretention properties. However, with top discharge tanks the contents arenever completely discharged. If complete discharge is essential (e.g., forconveying accurately weighed batches or if contamination betweendifferent batches must be avoided) bottom discharge operation isnecessary.

Bottom discharge tanks used not to have fluidizing membranes; the

material being gravity fed into the pipeline. Current designs, however,have fluidizing membranes. They are recommended for granularmaterials for which top discharge is unsuited because the highpermeability may preclude build up of sufficient lift.

The pressure drops across the discharge section of blow tanks must beaccounted for in specification of the air mover. In general,

Bottom Discharge Tanks < 1.5 psi/0.1 barTop Discharge Tanks > 1.5 psi/0.1 bar

Large top discharge tanks may have the pressure drop reduced byremoving the mixture from the side.

4) Pulse Phase ("Air Knife") Systems

A pulse phase system involves a blow tank discharging a stream ofmaterial into the conveying line. Intermittent timed air injection from an"air knife" at the pipe entrance divides the stream into a series of discreteplugs as illustrated in Figure 8-15. For powdery materials with poor airretention properties a long plug will tend to block the pipeline. Bychopping the material into shorter plugs the friction between the particlesand the pipe wall is reduced and blockage may be avoided. The mainproblem with conveying materials of this type occurs in vertical pipes

where the material does not form plugs, and the air velocity is below thechoking velocity; the material builds up and chokes the pipe. Layout isparticularly influential in this situation. The use of short risers willenable plugs to build up and then be conveyed as a mass-flow slug whenthe pressure differential exceeds the frictional forces and gravity. Someauthorities (e.g., Krambrock, 1983) have questioned the utility of thismode of conveying, asserting that the transport of compact plugs requiressignificantly larger forces than for the transport of material in duningflow mode.


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Figure 8-10

DENSE PHASE SYSTEM TRANSPORT MECHANISM AT LOW VELOCITY


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Figure 8-11

SINGLE PLUG BLOW TANK


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Figure 8-12

MATERIAL FLOWRATE AGAINST TIME FOR A SINGLE PLUG BLOW TANK SYSTEM


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Figure 8-13

AIR SUPPLY PROPORTIONED BETWEEN THE FLUIDIZING ANDSUPPLEMENTARY AIR LINES TO IMPROVE CONTROL


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Figure 8-14

TOP DISCHARGE BLOW TANK SHOWING INTERNALDISCHARGE PIPE POSITIONED ABOVE FLUIDIZING MEMBRANE


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This use of timed pulse air injection has been recommended for productswith the following characteristics:

Granular/plastic pellets

Narrow size distribution

High air permeability

Low air retention

It should be noted that fine materials with very low air retentioncharacteristics may be unsuitable for dense phase flow altogether.

In designing such a system, the air knife must be located sufficientlyclose to the blow tank to ensure that the discharge line pressure drop isnot excessive, but far enough from the tank to avoid impeding the plugformation.

5) Plug Control Systems

Numerous proprietary dense phase systems have been developed fortransport of solids at very low velocities over long distances, based uponvarious means of controlling plug formation to avoid blockage.

Three approaches have been used in the design of plug control systems:

Plug Prevention with Injection of Secondary Air

Plug Elimination using Bypassing Air

Plug Prevention using Controlled Secondary Air

Plug Prevention with Injection of Secondary Air

Secondary air may be supplied along the length of the conveying lineeither via a perforated tube or via a bypass (Figure 8-16

). This mayfluidize the product and help prevent plugging. If however a plug doesmanage to form, the air will follow the path of least resistance into theline, entering downstream of the plug without affecting it. Over longerconveying distances the air velocity increases excessively due toexpansion of the gases resulting in a higher pressure drop and airconsumption unless the pipeline size is stepped up appropriately.

This method is best suited to transporting readily fluidizable materials athigh solid- gas ratios.


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Plug Elimination using Bypassing Air

The conveying pipe may be fitted with either an internal or externalbypass, as shown in Figure 8-17. During normal operation little air flowsthrough the branch pipe. When a plug forms air will bypass it until it hasreached the point in the plug where the air pressure exceeds theresistance by the downstream section of the plug. In this way the plug issplit into sections, disintegrating progressively from the downstream toupstream end.

This system enables very low velocities to be used for the transport offree flowing bulk materials. It is especially useful for powdery andpulverized materials with low air permeability and high air retention. Anexternal bypass may be used if the material is damaged by an internal

bypass. It cannot be used for fine cohesive product because the bypasswould become plugged. Abrasive solids also create problems byseverely eroding the bypass, in which air velocities are relatively high.

Figure 8-15

PULSE PHASE ("AIR KNIFE") SYSTEMDIVIDES THE STREAM INTO DISCRETE PLUGS


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Plug Prevention by Controlled Secondary Air

This system is designed for the transport of fine, adhesive/caking bulkmaterials at very low air velocities. To keep the pressure and conveyingvelocities as low as possible, solids plugs must be quickly detected anddestroyed without increasing the conveying gas volume. Air boosters arepositioned either at strategic locations (i.e., bends) along the conveyingline, or at regular intervals 10-50 ft/3-15 m apart depending upon thematerials being conveyed (Figure 8-18). They sense the pressure at eachstage and adjust the booster pressure downstream to keep the materialflowing and prevent back pressures in the system from developing.Booster valves-unlike bypass systems - add air to the line and thereforeincrease the conveying air velocity. The valves only admit air when andwhere it is required.

Such systems, if properly designed for an appropriate product, offer lowmaintenance and long service life despite their relative sophistication.

6) Continuous Operation Using Dual Blow Tanks

Single blow tank systems operate in a batch mode. As discussed above,to achieve a given time-averaged product flowrate, a higher rate mustprevail during the steady-state section of the cycle. The pipelinediameter and air requirements must be based upon this higher rate. Theuse of dual blow tanks enables almost continuous operation and the time-average flowrate approaches the steady state flowrate. As a result, the

pipeline diameter, air requirements etc., are lower than for single blowtank systems - although a second pressure vessel is required. The costmay still be competitive with that for single blow tank systems becausethe continuous nature of the operation may enable the duty to beachieved with smaller blow tanks. In single tank systems the blow tanksize tends to be larger in order to increase the ratio of average to peakconveying rates.

Dual blow tank systems may be configured with the pressure vesselseither in parallel or in series.

Parallel Blow Tanks

A parallel blow system is depicted in Figure 8-19. Note that eachblow tank requires a dedicated set of discharge, vent and isolationvalves. Whilst one blow tank is being discharged, its twin is beingdepressured, filled and repressured, ready for discharge when thefirst tank is empty. An automatic control system is required toensure correct timing and sequencing. In this way almost continuousconveying is achieved through the shared pipeline.


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Figure 8-16

PLUG PREVENTION WITH INJECTION OF SECONDARY AIR


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Figure 8-17

PLUG ELIMINATION USING BYPASSING AIR


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Figure 8-18

PLUG PREVENTION BY CONTROLLED SECONDARY AIR


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For a given mass of material conveying "continuously" uses less airas efficiency is increased, provided adequate control logic is in place

to keep the material flowing.

Series Blow Tanks

Continuous blow tank operation may also be achieved by twopressure tanks vertically in line beneath a supply hopper as shownin Figure 8-20. The intermediate vessel is used as an airlock fortransferring material between them. This vessel is filled from thehopper and pressurized to the same pressure as the blow tank(usually by a pressure balance line from the blow tank). Theisolation valve to the blow tank is opened, the blow tank topped upwith product, and the valve closed. The transfer tank is then vented

and refilled. In this way a continuous flow of material is maintained.

The plot plan may influence the choice between parallel and seriesblow tanks, with parallel systems occupying the most floor space butseries systems requiring substantial headroom.

b) Air Mixing Systems

Gas mixing systems handle fluidizable, pulverized, powdered and granularmaterials. Gas and product are mixed at the entrance to the conveying line,to yield high solid-gas ratios. Two types of these systems have been

developed:

1) Screw Feeder with Air Jet

A variable pitch screw feeds material from a hopper to a mixing chamberinto which high pressure air jets are directed. Material is then dischargedinto the conveying line (Figure 8-21A). Pressures up to 40 psig/2.8 bargmay be achieved.

2) Air Swept Double Entry Rotary Feeders

Product trapped in the vaned pockets of a rotary feeder is mixed directlywith high pressure air entering each pocket through air ports built intothe end bells of the feeder (Figure 8-21B). The pocket of material is thenblown into the pipeline by the trapped air. The system operates atpressures up to about 20 psig/1.4 barg.


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Figure 8-19

PARALLEL BLOW TANKS


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Figure 8-20

CONTINUOUS BLOW TANK OPERATION WITH TWOPRESSURE TANKS IN LINE BENEATH A SUPPLY HOPPER


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Figure 8-21A

SCREW FEEDER WITH AIR JET


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c) High Pressure Rotary Valve Systems

At least one vendor offers a dense phase pneumatic transport system basedupon high pressure rotary valves instead of the usual blow tank-type feeders.Rotary valves are available rated for differential pressures of up to 50 psi/3.5bar, with special attention given to minimizing the air leakage that usuallyprecludes the use of high pressure systems.

Operational differences with dense phase blow tank systems include:

Product flowrate is controlled by setting the valve rotation speed, not bysplitting the conveying air into two parts.

Rotary valve systems allow continuous conveying from a single vessel.

Air leakage occurs from rotary valves but not from blow tanks. This airleakage must be compensated for to ensure that the average velocity atthe end of conveying line remains constant, even as the leakage ratechanges with changing differential pressure across the valve.

8.4 SYSTEM SELECTION AND DESIGN

The objective in undertaking the selection and design of a pneumatic conveying system isto provide the means for the reliable and economical transfer of a given bulk material at aspecified rate over a given distance.

In selecting the most suitable system for a given service numerous interrelated issuesmust be resolved - the system type (open or closed), system pressure (positive ornegative), mode of flow (dilute or dense phase), type of operation (batch or continuous),and the types of feed and gas-solid separation systems. The key parameters influencingthose issues are the properties and conveying characteristics of the product to betransferred and the conveying distance and layout involved.

In the "Pneumatic Conveying Design Guide" by David Mills, a method is presentedwhich should yield the most economical and suitable system in circumstances wherethere are no constraints on selection. Client preferences and constraints such as spacelimitations may limit the choices available. The guidelines below borrow heavily fromMills' treatment of conveyor selection.

The stages in the specification of a pneumatic conveying system are as follows:

a) Select Basic Type of System (8.4.1)

b) Design Pipeline (8.4.2)

c) Select Mode of Operation (8.4.3)

d) Select Feeder (8.4.4)


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e) Select Air Mover (8.4.5)

f) Select Gas-Solid Separation System (8.4.6)

g) Design Solids Storage (8.4.7)

h) Factors Affecting System Design (8.4.8)

The order of the decision stages will change as external constraints dictate that aparticular type of equipment must be included in or excluded from the selection. Thedecision stages are discussed below in the preferred order (i.e., assuming there are noconstraints), although in all cases they involve a degree of iteration.

A summary of the advantages and disadvantages as well as process conditions for thevarious types of systems is shown in Figures 8-22A through F.

Pneumatic Conveying Systems are being used today for products which a few years agowould have been handled exclusively by mechanical means.

In spite of this, however, it is still the "conventional" products which make up the bulk ofthe systems installed and consequently provide the most extensive design information.

Most Pneumatic Conveyor manufacturers favor a certain method of conveying or acertain component applied to a variety of methods. This is understandable as it is at leastan attempt at partial standardization.

We try to be impartial in our selections, but we do have a tendency to stay away fromrotary air locks unless they are definitely indicated. The following comments shown in

Figure 8-22Fmust, therefore, reflect our preferences and prejudices and should be usedas a guide rather than an indictment!

If the product you are interested in is not listed, ask the mechanical department forassistance.

8.4.1 System Type

The first choices are concerned with whether an open or closed system isrequired, and whether a positive or negative pressure system should be used.

a) Open and Closed Systems

The material properties usually decide whether the system should be open orclosed. Open systems are preferred because of their lower capital cost andminimal complexity. In many cases the proper design of gas-solid separatorsand vents is sufficient to prevent pollution. Open systems should thereforebe used except where a closed system is necessary for economic,environmental or safety reasons.


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Closed systems usually involve recirculation of the discharge gas back to theair mover suction. This recirculation of the conveying medium reduces the

demand to a small makeup supply to compensate for leakage. The volume ofexhaust requiring filtration is substantially reduced, with only a small bleedstream required. Closed systems are best suited to continuous operation.

Closed systems are used where a closely controlled environment is required.For instance, hygroscopic materials must be transported in dry air and maybe conveyed in a closed system to minimize the air drier duty.

In other cases the material may react (sometimes explosively) with air,necessitating the use of an alternative, inert conveying medium. Nitrogen isthe most common gas for this purpose. Economics usually dictate thatconveying media other than air are conserved in a closed system. Materials

with an excessive dust content may be transported in a closed system tominimize the size and cost of the exhaust filtration system. Toxic orradioactive materials must be transported in closed systems andcomprehensive measures taken to ensure that leakage to the environmentdoes not occur.

b) System Pressure

The choice between the following systems must be made:

Positive pressure systems

Negative pressure (vacuum) systems

Combined negative-positive pressure systems

Dual combined systems

The distinctions between those systems were described in Section 8.3. Tobriefly summarize:

Positive pressure systems are:

Suited to the widest range of solids feeders. Capable of long distance conveying at high operating pressures.

Ideal for feeding multiple destinations from a single source via divertervalves.

Inadequate for feeding from multiple sources in series because airleakage across several solids feeders may be excessive.


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Figure 8-22A

SYSTEM DESIGN AND SELECTION


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S

P

D


Figure 8-22B



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S

P

D


Figure 8-22C



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S

P

D


Figure 8-22D



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S

P

D


Figure 8-22E



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Figure 8-22F

DESIGN CONDITIONS FOR CONVEYING VARIOUS SOLIDS MATERIALS

PRODUCT CONVEYINGMETHOD

ABRASIONPROBLEM

CORROSIONPROBLEM

FRIABILITYPROBLEM

EXPLOSIONHAZARD

CEMENT

USUALLYPRESSURE

Yes No None

ALMOST NIL

FLOUR Usuallypressure

No Food product -

watch for

contamination

None Yes, in someair/materialconcentrations

WHEAT &CORN

Vacuum orPressure

Mild No Yes Yes, in someconcentrations

SAND Vacuum orpressure

Severe No Some, if

coated

None

PLASTIC

PELLETS

Vacuum orpressure

No Watch for

contamination

None Yes, usually dueto static build-up

ALUM Vacuum orpressure

No Some None Slight

AMMONIUM

NITRATE

Vacuum or

pressure

No Yes Yes, if in

pellet form

Yes

MALT Vacuum orpressure

Mild No Yes Yes, in some

concentrations

SALT Vacuum orpressure

Moderate Yes With some

Grades

No


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Figure 8-22F


SEGREGATIONPROBLEM

LONG RADIUSBENDSRECOMMENDED

TYPICALCONVEYINGRATE 100 FT.

USE OF ROTARYAIR LOCKS

DUSTINGPROBLEMS

None Steel sch. 40 pipeor rubber hose

40 T.P.H. thru 4"line @ 15 P.S.I.

Not recommended Surprisinglylittle, if min.req'd. air flowused

None Same mat'l. asconveying line


Recommended ifvented adequately

Yes, should betrapped in dustcollector

Some Steel sch. 40 pipe 15 T.P.H. thru 4"line @ 8 P.S.I.

Recommended Not excessivebut dustcollectors usual

Some if coated Heavy wall rubberhose or wearpocketelbows


Not recommended Can benoticeable

None Same mat'l asconveying line


Recommended ifproperly designed

Minimal

Possible Same mat'l asconveying line


Not recommended Yes

Possible As for conveying

line

10 T.P.H. thru 4"

line @ 6 P.S.I.

Can be used Some

Some Steel sch. 40 pipe 7 1/2 T.P.H. thru4" line @ 4 P.S.I.

Recommended Not excessive

With somegrades

Stainless steel min.1/8" wall


Can be used Yes


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Figure 8-22F


REMARKS

High density (.2 to .4 S.C.F.M. 1b. of material) pressure systems usual. Pumps like viscous fluid whenaerated. Systems in use operating at 100 P.S.I. Look out if line plugs - usually must be disassembled.

Use of Rotary air locks precludes high pressure systems. If R.A.L.'s used, should be blow thru, type forsatisfactory clean out. Pressure pod systems for flour operate similar to cement systems but less air req'd.

Free flowing product. Feeds & conveys well. Does not fluidize - must be blown thru line (1 to 2S.C.F.M./lb. of material common). Handles equally well in pressure or vacuum system. R.A.L. systemsmost common. Maximum pressure usually 8-10 P.S.I.

Abrasion biggest problem. Heavy wall pipe should be used for straight runs. High air velocities (6000ft./min.) req'd. for sharp sands. Hose or wear pocket elbows will combat abrasion.

Air locks should have controlled feed inlet. Pressures should be kept low to minimize velocity and temp.gradients. Special conveying lines often used to eliminate streamer formation. Conveys well. Usuallycan be stopped in line and re-started.

Alum. hardens and glazes on rubbing surfaces. Air locks, if used, need large clearances. Often coatsinside of pipelines.

Usually handled in pellet (prill) form. Must be kept from contact with oil (explosion hazard). Air locks

should, therefore, have outboard bearings.

Handles well, but air flows usually kept low to minimize breakage. Otherwise, conveys similar to wheat.

Corrosion main problem. Air locks, if used, should be stainless steel. Conveying lines usuallyaluminum. Requires min. 5000 ft./min. air velocity for steady flow. Fine salt harder to convey thancoarse grades.


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SECTION 8.0

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Figure 8-22F


PRODUCT

CONVEYING

METHOD

ABRASION

PROBLEM

CORROSION

PROBLEM

FRIABILITY

PROBLEM

EXPLOSION

HAZARD

SUGAR Vacuum or

Pressure

Yes Food prod. -

watch for

contamination

Some Yes, in some

dust concen-

trations

LIME

(pulverized)

Usuallypressure

Moderate Some No No

LIMESTONE

3/4" / 1 1/4"

Usually

pressure

Yes Some No No

FERTILIZER Vacuum orpressure

Moderate Yes Yes Yes

SODA ASH

(light)

Usuallypressure

No Yes No No

SODA ASH

(heavy"

Vacuum orpressure

Slight Yes Yes No

PLASTIC

RESIN

Vacuum orpressure

Slight Watch forcontamination

No Yes

CLAY Usuallypressure

No No No No


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SECTION 8.0

PAGE 56

DATE 8-94


Figure 8-22F


SEGREGATION

PROBLEM

LONG RADIUS

BENDS

RECOMMENDED

TYPICAL

CONVEYING

RATE 100 FT.

USE OF ROTARY

AIR LOCKS

DUSTING

PROBLEMS

Some Stainless steel min.

1/8" wall

30 T.P.H. thru6" line @ 6P.S.I.

Can be used but notrecommended

Yes

No As for conveying

line


Possible Yes

No Sch. 40 steel pipe 28 T.P.H. thru5" line @ 6P.S.I

Not recommended Yes

Yes Sch. 40 pipe 18 T.P.H. thru4" line @ 12P.S.I.

Not recommended Yes

No As for conveying

line


Possible Yes

Yes As for conveying

line

12 T.P.H. thru

4" line @ 7P.S.I.

Possible Yes

Some Stainless steel min.1/8" wall

7 1/2 T.P.H.thru 3" line @ 9P.S.I.

Not recommended Yes

No As for conveying

line


Not recommended Yes


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SECTION 8.0

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Figure 8-22F



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Advanced dense phase systems (e.g., pulse phase and plug controlsystems) are capable of transferring many products that are unsuited to

conventional dense phase transport. They are especially suited to lowvelocity transport of friable/abrasive products, offering lower operatingcosts but at a higher initial capital cost.

Negative pressure (vacuum) systems are:

Ideal for drawing product from multiple sources because air leakageacross the feeders is minimal.

Inward leakage prevents the escape of fines, dust etc., an essentialrequirement when transporting toxic or radioactive products.

Limited in pressure differential and conveying distance.

Combined negative-positive pressure systems are:

Ideal for transferring a product from multiple sources to multipledestinations.

Limited in pressure differential and conveying distance.

Dual combined systems are:

Able to convey over greater distances than normal combined systems.

More efficient than combined systems because the duty is split into apositive and negative half, enabling the optimum blower and exhausttypes to be specified for each half.

c) Other Design Considerations

Other considerations include system leakage, and system layout. Leakage,either into vacuum systems or out from pressure systems, must be added tothat required for conveying when sizing the gas supplier. Diverter valves androtary valves all leak through the seals. Additionally, rotary valves displacegas as they rotate. Leakage in very high pressure (>40 psig) becomes verylarge. Typically blow pots are used at pressures greater than 40 psig to

reduce leakage. Other leak points (such as at flanges) also occur.

Equipment arrangement is typically dictated by the process. However, somechoices can be influenced by the requirements of the pneumatic conveyingsystem. Plant Design System 3-D modeling enables the process engineer tooptimize process layout including the conveying system to minimize runlengths and numbers of sweeps, lowering the ultimate system pressure drop,and therefore gas and horsepower requirements. Typical guidelines forpiping layout include allowing for a minimum straight run before the first


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sweep in a system, or between horizontal-to-vertical sweeps, avoidance ofinclined lines, and minimization of the number of sweeps.

A minimum distance of 200 pipe diameters or 15 feet (whichever is smaller)is required to allow the gas and the solids to reaccelerate after a bend or a

pick-up point. Diverter valves are counted as 30 obends. Sub-90 obends

are ratioed directly as a fraction of the 90 obend. While maximum verticalruns are limited by the available pressure drop, typical limits are about 100 fttotal vertical distance for medium pressure systems. Inclines are morecomplex than either vertical or horizontal runs due to solids recycle. The

maximum pressure drop (and recycle) occurs at an incline of 45 o.

While maximum vertical runs are limited by the available pressure drop,typical limits are about 100 ft total vertical distance for medium pressure

(


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7) Dust collector undersized for peak gas flow rate, especially for the tankand line clearing step of a blow tank dense phase system cycle.

8) Failure to provide control interlocks to ensure that the blower isoperating when the solids feed is begun, and remains operating for aperiod after the solids feed has ended.

9) Installation of upward inclined sections of pipe resulting in saltation,high pressure drop, and plugging.

10) Taps not provided to permit checking of conveying gas line pressure orvelocity during system troubleshooting.

11) The system has too many diverter valves, feed points, or discharge pointsmaking the system difficult to troubleshoot due to complexity.

12) Centrifugal blower has a head vs. capacity characteristic that overloads

the motor when no solids are flowing, pressure drop is small, and gasflow is large.

13) Poorly sealed diverter valves leak gas into or out of the in-servicebranch, or leak solids into the out-of-service branch.

14) S

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