innovatory conveying systems - freenguyen.hong.hai.free.fr/ebooks/science and... · 488 chapter 17...

17Innovatory Conveying Systems

1 INTRODUCTION

The motivation to use dense phase conveying technology arises generally from adesire to convey at low velocity in order to avoid a range of operational problems;in particular, the problems of erosive wear of pipelines and fittings or the attritionof the conveyed material. However, the ability or otherwise of a material to beconveyed in a dense phase flow regime depends on the particle and bulk propertiesof the material to be conveyed.

For materials that have natural dense phase performance in either of themajor modes of dense phase flow, no special equipment is required. For these ma-terials a standard pipeline and feeder may be used. In general, dense phase systemstend to use a blow tank to feed the conveying line since this device can operateover a very wide range of pressure conditions. For materials which do not exhibitnatural dense phase capability, there is often a need to use specialized techniquesand equipment to encourage the material to convey reliably in a dense phase modeof flow.

Three basic approaches are used in order to condition the material in theconveying system. The first method involves a form of plug creating device at thefeed point which aims to control the plug or slug formation in order to limit thesize of plug which is initially fed into the conveying line. The second approach isto use an air addition system, commonly known as boosters, to inject additional

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

486 Chapter 17

conveying air at various points along the pipeline in an attempt to ensure that thematerial in the pipeline is maintained in a fluidized condition. The third approachis to use an internal or external bypass l ine which aims to l imi t the maximum sizeof plug that wi l l form in the conveying line.

The exact design of these systems vary considerably, depending on the par-ticular manufacturer, but they have all been used with varying degrees of success.There is, however, a distinct lack of detailed technical literature underpinningthese systems, and hence the aim of this chapter is to approach these systems in ageneric manner and to explain their operation as far as is possible from a technicalpoint of view, rather than to review the systems of specific manufacturers.

2 PLUG CREATION SYSTEMS

In general, plug creation systems involve the use of a blow tank as a feeder inwhich the supplementary air supply is controlled in order to artificially createplugs of material of a given length. Typically, the supplementary air injectionpoint is located in the conveying pipeline just downstream of the blow tank dis-charge valve. The exact positioning is important since if the injection point is lo-cated too far downstream, the pressure drop across the extruded flow in the dis-charge pipe will lead to unacceptably high pressure drops across the dischargepipe.

This type of system was first developed following research undertaken bythe Warren Spring Laboratory in the UK [ I ] in the early 1970's. A sketch of atypical system, as originally developed, with a bottom discharge blow tank feedingdevice, is shown in Figure 17.1.

2.1 Principle of Operation

The pulse-phase system consists of a pressure vessel feeding a conventional pipe-line. The air supply to the blow tank is supplied both to the top of the vessel and toan aeration ring located around the conical section. The aeration ring providesfluidizing air which ensures that the material remains in a fluid-l ike state. Thisensures that, for powdered products, the material flows in a reliable manner intothe pipeline.

The aerated state of the material also ensures that the material can be moreeasily split-up into plugs. At the start of the conveying line, an 'air knife' device islocated. The air knife is essentially an annular device with a ring of small holesequally spaced around the pipeline. The air supply to the air knife is controlled tobe either on or off using a timer and a solenoid valve. When the air knife is operat-ing, a series of air cushions are created between the material plugs. The frequencyof the solenoid switching wi l l provide a degree of control over the plug length.Although this concept was originally created to handle fine powdered materials,the device has been used successfully for a wider range of materials includinggranular materials.


Innovatory Systems 487

Air

110/240 v

Solenoid Valve Air Knife

Figure 17.1 Schematic of the 'Pulse-Phase' System.

This technology has been licensed to a number of vendor companies aroundthe world who have also developed and refined the basic concept. Many systemshave been operating around the world with solids loading ratios exceeding 300 invery short systems. Operating velocities have been reduced to values betweenabout 300 and 600 ft/mm.

2.2 Stress State in Slugs During Feeding

Despite the use of 'pulse-phase' type systems, there is much evidence to suggestthat the plug formation for many granular materials occurs quite naturally and thatfor coarse granular materials, with a high degree of permeability, no such condi-tioning is necessary. Hitt [2] found that for most free-flowing materials, no specialconditioning was required at all, and that material plugs formed spontaneously andsettled to a steady conveying condition during the steady state period of the blowtank cycle.

Considerable discussion has also taken place regarding the stress state exist-ing in the plug at the beginning of the pipeline and whether this is influenced bythe method of feeding and/or any conditioning of the material that takes place atthe feed point. Research undertaken by Li et al [3] suggests that when a ful l boreplug of material is formed at the feed point, the condition of the material duringthe plug formation does, in fact, influence the stress state in the slug and hence


488 Chapter 17

may influence the subsequent behavior of the material in the conveying line. Fur-ther work is required in this area to fully understand the operation of such systems.

3 AIR ADDITION SYSTEMS

Air addition along the length of the conveying pipeline is a method of condition-ing the gas-solid mixture during conveying. Two approaches are generally used.The first involves the continuous addition of small quantities of air at regular in-tervals along the length of the pipeline. The second approach aims to prevent apipeline blockage from occurring by injecting air into the pipeline at the pointwhere a blockage is about to occur.

The principle behind the first approach is to ensure that the material remainsin a fluidized condition and hence can be conveyed in the fluidized mode of densephase along the entire pipeline. In the second approach, air is only injected at thetime and position it is required in order to prevent or clear a blockage. Usually, thecontrol of this type of operation is based on a pressure signal.

3.1 Continuous Air Addition

The motivation to provide continuous air addition along the pipeline is generallyan attempt to keep the material in the pipeline in an aerated state. In practice, thisis very difficult to achieve and in general leads to velocities which are significantlyhigher than necessary.

The most critical velocity in pneumatic conveying is the pick-up velocity orthe velocity at the point where the material is fed into the pipeline. In most cases,and certainly for single bore pipelines, the velocity at the feed-point will be thelowest throughout the pipeline. Therefore, it is essential that the required mini-mum transport velocity is maintained at this point. In conventional systems, the airflow rate required for conveying is based on the m i n i m u m transport velocity forthe material concerned.

As the air expands along the pipeline, with the fal l in static pressure, a natu-ral consequence is for the air velocity to increase in proportion to the ratio of thestatic absolute pressures. Clearly the addition of air at various points along theconveying line will lead to a further increase in air velocity beyond that due to theexpansion of the air. Clearly, this is not desirable with the potential for increasederosion and/or attrition, in addition to the higher specific energy requirements forconveying.

Figure 17.2 shows the relationship between air velocity and pipeline length,both with and without air addition. The graph is based on a 4 inch single borepipeline and an air supply to the start of the conveying pipeline of 216 fV/min offree air (scfrn). In both cases the air velocity at the pick-up point is 1500 ft/minand the conveying line pressure drop is 10 Ibf/irr. At this low velocity, it is clearthat the mode of flow would be dense phase. In the case of no air addition, the airexpands to an air velocity of 2500 ft/min at the end of the pipeline.



Pressure Drop = 101bf/in2

10,000

8,000

I 6,000£I

'g 4,000

2,000

With Air Addition

No Air Addition

100 200 300

Length - feet

Figure 17.2 Air velocity versus pipeline length with and without air addition.

In the air addition case, boosters are located every 10 feet along the entirelength of the conveying pipeline and the air flow rate to each booster is 20 scfm. Inthis case, the air velocity reaches 9500 ft/min by the end of the pipeline. An impor-tant point to note is that in the case of a booster system of this type, the velocity atall points in the pipeline will be higher than in the case where the boosters are notused.

It is clear that the use of continuously operating boosters makes velocitycontrol in the pipeline very difficult. In the case illustrated above, it can be seenthat even if the system operates in dense phase initially, the velocity at the end ofthe pipeline indicates that the system wil l be operating in dilute phase by thatstage.

Even if the air flow rates to each booster are halved, the exit velocity wil lstill reach 6000 ft/min. A degree of control over the air velocity could be achievedby careful stepping of the pipeline at appropriate positions along the pipelinelength. This is always a useful technique for controlling the air velocity, asconsidered in Chapter 9.

A further consideration for continuous air injection systems is the level ofgas flow rate required at each injection point to ensure reliable conveying. Clearly,the quantity of air wi l l depend on the material being conveyed. Very littleinformation is available in the literature to provide guidance on this point withmost manufacturers of systems treating any information they have on this point ascommercially confidential.


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3.2 Non-continuous air addition systems

Many of the disadvantages of continuous air-addition systems can be overcome byusing non-continuous systems. In general, these systems are aimed at eitherpreventing a blockage from occurring, or breaking up a blockage that is in theprocess of forming. These systems generally consist of a main conveying pipelineand a parallel air only pipeline with a series of connections at some predeterminedspacing.

Associated with each of these 'boosters' is some form of pressuremeasurement. In general, the pressure measurement is aimed at determining whena significant pressure drop is detected between two boosters, which is assumed toindicate the location of a blockage, or a potential blockage. Once detected, theboosters inject air into the pipeline. The quantity of air injected, the injectionperiod and the overall control of the injection vary from manufacturer tomanufacturer and, once again, this information is generally considered to becommercially confidential.

The action of the air injection under these circumstances has manysimilarities to the action of the bypass system which is considered in the nextsection. However, a critical difference between the two generic systems is that, inthe case of the air addition systems, the air injected is additional to the air supplyprovided at the feed point to the system.

4 AIR BYPASS SYSTEMS

The air bypass system consists of two pipes; a main conveying pipeline and a sec-ond small bore pipeline which may be internal or external to the main pipeline.The small bore pipeline has openings into the main conveying pipeline at prede-termined intervals which allow the conveying air to move between the two pipe-lines. A schematic of an internal bypass arrangement is given in Figure 17.3. Fig-ure 17.4 shows a schematic of an external bypass arrangement.

Figure 17.3 Schematic of internal bypass arrangement.



Figure 17.4 Schematic of external bypass arrangement.

The work of Barton [4] is probably the most recent work dealing specifi-cally with bypass systems. The work was predominantly a global approach to theproblem which involved a direct comparison between two pipelines of the samegeometry; one containing an internal bypass pipeline and the other without. Arange of bypass lines were used, including copper pipelines with varying flute (orhole) spacing, as well as a totally porous pipeline constructed from a permeablepolymer. The conveyed material was alumina, and various grades were used in thetest program.

The test pipeline was 160 ft long with 6 x 90° bends each having a benddiameter to pipe bore ratio of 6:1. The pipeline was two inch nominal bore and themajority of the pipeline was in the horizontal plane, with only 12 ft being vertical.This pipeline could be operated as a conventional pipeline with no bypass line.Alternatively, an internal bypass pipeline could be inserted. Various designs ofbypass pipeline were tested during the course of this work.

To establish the effect of the bypass pipeline, Barton used a macro approachto pipeline testing, whereby the performance of various bypass systems werecompared directly with a conventional pipeline of the same geometry. The generaleffect of the internal bypass was two-fold; firstly, conveying at lower minimumconveying velocities was achieved when the bypass line was fitted; and secondly,the stability of the material flow was significantly enhanced.

However, as expected, the conveying rate for a given air flow rate and pres-sure drop was reduced, largely due to the reduction in flow channel cross-sectionalarea due to the presence of the internal bypass pipeline. In the case of the sandygrade of Alumina, the min imum superficial gas conveying velocity in the conven-tional pipeline was about 2000 ft/min. In the case where a bypass line was in-stalled, minimum conveying velocities were as low as 220 ft/min.

However, whereas maximum conveying rates of about 37,000 Ib/h wereachieved with a conveying line pressure drop of 45 lbf/in2 in the conventionalpipeline, maximum rates of about 28,000 Ib/h were achieved under similar con-veying conditions where the bypass line was installed. This is due to the reduction


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in cross-sectional area of the main pipeline when the internal bypass line is in-stalled.

4.1 Analysis of the Operation of a Bypass Line

The primary function of the bypass pipeline is to prevent blockages of the mainconveying pipeline from occurring. The bypass line achieves this in two ways.Firstly, the bypass l ine provides an alternative route for the air to flow when ablockage occurs. Secondly, the provision of an alternative route for the air to flowprevents a build up of pressure behind the blockage, which reduces further com-paction.

The total air (or conveying gas) flow rate in the system is the sum of the gasflows in the main pipeline and in the bypass line. The ratio of these flow rates willdepend on the comparative resistance in each of the two flow channels. Understeady state conditions, the pressure profiles in each of the two flow channels willbe the same.

Clearly, if an increase in the flow resistance occurs in the main pipeline, agreater flow rate will occur in the bypass line in order to balance the pressures. Itis clear, therefore, that the design of such a system must be based largely on theresistance of the bypass pipeline.

The most likely reason for an increase in pressure in the main pipeline willbe due to the formation of a plug or slug of material. Hence, the pressure requiredto move a plug of material in the main pipeline is of critical importance to the de-sign of bypass systems. Barton [4] approached the problem by first attempting toestablish the relationship between the length of the material plug and the pressurerequired to move the slug. An example of the results reported in Barton's thesis isgiven in Figure 17.5 for alumina.

Knowing the relationship between slug length and the pressure required tomove the slug, Barton chose a critical slug length which corresponded to themaximum pressure available to move the slug. The bypass pipe diameter and theflute spacing was then selected to balance the resistance across a slug of criticallength. To establish the relationship between the slug length and the pressure re-quired to move the slug, Barton carried out experimental trials and compared theresults with the relationship developed by Konrad [5] given in equation I :

Ap

L D D ' " ° '"' D

- - - - - - ( 1 )

A comparison between the calculated values and the measured values ofpressure drop against plug length for sand are also given in Figure 17.5.



uu -

Ist;•g

Sp 40-CLoo5̂co" 20-QJ •*->-<

3LO[/]<L>L-Cu

0 -

/////

/

/0 10

/a\ Plug Length -

//

KonradModel

• Barton'sTest DataII

20 3(ft

Figure 17.5 Pressure drop versus plug

T 30-OBD

essu

re to

Mov

e I

—

to0

0

OH^/

/

/"

/

/

mKonradModel

" Barton'sTest Data

(b)

10 20

Plug Length - ft

30

Once the relationship between the plug length and the pressure required tomove the plug is determined, a decision can be made regarding the maximum de-sired plug length that wil l be allowed to occur in the pipeline. This decision wouldbe made based on the system pressure available with some margin of safety.

Barton's analysis focuses on determining the diameter of bypass pipelinerequired to ensure that a plug never exceeds the critical plug length. The analysisis based on a constant mass flow rate of gas supplied to the pipeline system suchthat:

or

m m<atal

(2)

(3)

Assuming a plug of critical length is created and the critical pressure is ap-plied across the plug, then the gas velocity through the plug can be determinedusing the Ergun Equation [6]:

TL 7^"T (4)

In quadratic form, this equation becomes:


494 Chapter 17

• 7 5 - 150-A,

(5)

This quadratic can be solved for the mean gas velocity, Ugm. The mass flowrate of gas passing through the bed is obtained as follows:

(6)

where A is the cross-sectional area of the plug and the mean gas density is givenby:

'" (1\~RT - - - - - - - - - - - - - - ( I )

Knowing the mass flow rate of gas passing through the plug and the totalgas mass flow rate supplied to the system, equation 3 wil l give the mass flow rateof gas passing through the bypass line. Barton then determines the diameter ofbypass line required to provide the same pressure drop across the bypass line usinga differential form of the Darcy equation:

Pdp 4f 32fm2

KRT

dL D p 7T2 d5

Integrating between points 1 and 2 gives:

(8)

P\ ~ Pi64fm2

gRTL(9)

Re-arranging for bypass line diameter as the subject:

d =64/ m-RTL

(10)

The Darcy equation is for isothermal, incompressible flow, however, butprovided the Mach number does not exceed about 0-2, and mean density and ve-locity conditions are used, a reasonable approximation can be obtained.



Barton suggests that if equation 1 is evaluated using the maximum desiredplug length (which corresponds to the plug length used to calculate the mass flowrate of gas through the plug) this will yield the minimum bypass pipe diameterrequired to ensure that a plug or blockage wil l not exceed the critical plug length.That is, the resistance of the bypass pipeline balances the resistance through theplug of maximum desired length for the specified value of total gas mass flowrate. The effect of using a larger bypass pipe diameter would be to balance theresistances between the plug and the bypass line at a shorter plug length.

Figure 17.6 shows the air only pressure drop for different diameters of by-pass line compared to the pressure required to move a plug of material of variousplug lengths for a gas mass flow rate of 0-009 Ib/s. The diagram shows that, in thiscase, a bypass line diameter of 0-31 in (5/16" approx) or smaller is required inorder to ensure movement of the slugs of length up to about 30ft. To prevent slugsforming in excess of about 13 feet in length, a larger bypass line diameter of 0-35in (3/8" approx) would be required.

The work carried out by Barton provides a guide to the sizing of bypasspipelines. However, it does not quantify the effects of the flute (or hole) spacing,but Barton states that "The spacing between holes is important since spacing of theflutes 20 in apart gives better results than 40 in spacing." He goes on to say that "Itis unlikely, however, that this is the ideal distance between holes."

Although the plug lengths tested, and hence pressures generated, are greaterthan those encountered in commercial systems, the experimental work undertakenand modeling employed has helped to provide a better understanding of the flowmechanisms involved.

200 r

160

D.O

120

80

% 40cx

0-23 in

0-31 in

0-35 in0-39 in

10 15 20

Plug Length - ft

25 30 35

Figure 17.6 Air only pressure drop for different bypass line diameters against the pluglength for a gas mass flow rate of 0-009 Ib/s.


496 Chapter 17

The work also shows that the system has the potential of being modeled sothat engineers should be able to evaluate pipeline sizes and air flow rates for agiven material in due course.

Work carried out by Jones and Soloman [7,8] argued that the bypass pipe-line has more of a transient effect whereby blockages are broken as they form. Themechanism behind this theory is that there will always be a residual pressure in thebypass l ine when a blockage is beginning to form. It is argued that this residualpressure wil l destroy blockages as they form.

The immediate effect will be to significantly reduce the air flow rate in themain conveying pipeline, as this will now be governed by the permeability of thematerial plug. This will lead to a very rapid change in the pressure profile in themain conveying line. However, the pressure profile in the bypass line will changemore gradually and will depend on how easily the gas can flow into the bypassline upstream of the blockage. This will be governed by the hole or flute diame-ters, their spacing or pitch and the diameter of the bypass pipeline. This situation isillustrated in Figures 17.7 and 8. Note that the exact shape of the pressure curve inthe main pipe is very dependent on the properties of the material being conveyed.

Bypass Pipe Pressure

— Main Pipe Pressure

Distance

Figure 17.7 Steady state flow conditions.



Distance

Figure 17.8 Instantaneous blockage conditions.

The dotted lines show the extremes of the possible pressure states in themain pipe. The upper limit of the pressure relationship is the situation where thepressure in the blockage decays linearly as indicated by the upper dotted line. Thiswould occur for materials that are highly permeable. The lower l imit of the pres-sure relationship would be where the pressure in the blockage drops off rapidly asindicated in the lower dotted line. This would occur in materials that have a lowpermeability. Most materials would display a pressure relationship in the blockagesomewhere between the upper and lower limits as indicated by the solid line.

It is argued that the pressure before the blockage in the main pipe will be-come constant as indicated by the flat line in Figure 17.8. This pressure will in-crease and rise to the limit of the air supply. The pressure after the blockage in themain pipe will also become constant and is indicated by a flat line. This pressurewil l decay to atmospheric pressure.

Clearly, air will flow from regions of higher pressure to regions of lowerpressure. In this case, air will flow from the main pipeline into the bypass pipelineupstream of the blockage and will flow from the bypass line into the main convey-ing pipeline at some point along the length of the blockage depending on the posi-


498 Chapter 17

tion of the cross-over of the pressure curves. The position of the pressure profilesand the magnitudes of the air flow rates into and out of the bypass line will dependon the diameter of the bypass line and the diameter and spacing of the flutes.

5 CONVEYING CHARACTERISTICS

It was mentioned earlier that when a sandy grade of alumina was tested in the 160ft long pipeline of two inch bore, a maximum flow rate of 37,000 Ib/h wasachieved and that 28,000 Ib/h was achieved when a fluted pipe was inserted in thesame pipeline. These two flow rates were obtained with a conveying line pressuredrop of 45 Ibf/in2 in each case and the difference was attributed to the reduction inpipeline material flow cross section area due to the presence of the internal pipe-line, since this should only transfer air.

It was mentioned above that in the conventional pipeline the minimum con-veying air velocity for the alumina was about 2000 ft /min and that with the flutedpipe it was as low as 220 ft/min. In terms of overall performance, however, con-veying characteristics are required. Those for the conventional pipeline were pre-sented in Chapter 12 on 'Aluminum Industry Materials' with Figure 12.Ib and aretypical of materials that can only be conveyed in dilute phase suspension flow.They are shown alongside those of a floury alumina in Figure 12.la to illustratethe differences between dilute phase and sliding bed dense phase modes of con-veying. Floury alumina naturally has very good air retention properties.

Conveying data for the alumina conveyed through the fluted pipeline waspublished in Reference 9 and this shows that the conveying characteristics are verysimilar to those for the polyethylene pellets presented in Figure 4.12b. This meansthat as the air flow rate is reduced, in the dense phase flow region, the materialflow rate reduces. Polyethylene pellets are naturally conveyed in plugs, and thepurpose of the fluted pipeline is to convey materials with no natural dense phasecapability in plugs, and so perhaps it is not to be unexpected that the conveyingcharacteristics should be very similar.

This does mean, of course, that at low velocity a very much reduced mate-rial flow rate wi l l be achieved. Thus a larger bore pipeline wi l l be required to con-vey a material at a given flow rate in the fluted pipel ine system at low velocity,than will be required to convey the material in dilute phase at high velocity in aconventional open pipeline. Many of the materials that are conveyed with flutedpipeline systems are highly abrasive and so the choice is possibly a decision inwhich the problems of pipeline wear and conveyed material contamination aretaken into account.

In the systems in which air is added to the material along the length of thepipeline, such as with 'boosters', it is often considered that this is an artificialmeans of giving a material a degree of air retention [10]. Air retentive materials,such as the floury alumina in Figure 12.la, suffer little or no reduction in convey-ing performance with reduction in air flow rate. It is not known, however, whether



the conveying characteristics for air addition systems are similar to those for mate-rials that are capable of being conveyed in dense phase in the sliding bed mode offlow. The constant addition of air, as mentioned above, may have an over-ridingeffect. It is hoped that an answer to this question will be provided soon.

6 CONCLUDING REMARKS

There is little doubt that the innovatory systems discussed in this chapter have apart to play for those materials that do not have natural dense phase capability.However, it is essential to ensure that the material to be conveyed is suitable forthe class of system under consideration. At present this is still an empirical processand the only way to adequately determine the suitability of a material for a particu-lar class of system is by pilot scale testing.

It should be noted that the air only pressure drop curves for the various by-pass line diameters are theoretical and based on the Darcy equation. The curverepresenting the pressure required to move a plug of material of various lengths isbased on experimental data. Barton's experimental work involved plug lengths ofup to about 22 feet and pressures up to approximately 90 Ibf/in2.

It should also be noted that those materials that have natural dense phasecapability do not require the additional complexity and expense associated withthe innovatory systems described. However, commercial pressures in the marketplace can lead to these systems being offered for applications where they are reallynot required. It is clearly important for the user to ensure that, if these systems arebeing offered by vendors, they really are necessary for reliable operation.

NOMENCLATURE SI

A Plug section area in2 m2

c Inter-particle cohesion lb/in2 kg/ms2

d Bypass line diameter in mdp Particle diameter in mD Pipe bore in m/ Friction coefficientF Frontal Stress Ibf/in2 N/m2

g Gravitational acceleration ft/s2 m/s2

= 32-2 ft/s2 = 9-81 m/s2

KH. radial stress/axial stressL Pipeline length ft mm Mass flow rate Ib/min kg/s

p Pressure Ibf / in 2 N/m2, kN/m2, bar(1 bar= 100 kN/m2)

R Characteristic gas constant Btu/lb R kJ/kg KT Absolute temperature R K

= t + 460 = t + 273


500 Chapter 17

U Velocity ft/min m/s

Greekt] Viscosity Ib/ft h kg/m ss Voidage(/> Angle of Internal Friction degrees degrees</>„. Angle of Wall Friction degrees degreesu Included angle on Mohr's Stress Diagramp Density Ib/ft3 kg/m3

Subscriptsb Bulk value

m Mean valuep Particlew Wall1 , 2 Reference points along pipeline

REFERENCES

1. J.H. Aspcy. The pneumatic pulse phase powder conveyor. Proc Joint Symposium onPneumatic Transport of Solids, SAIMechE, SA Institute of Materials Handling. 1975.

2. R.J. Hitt, A.R. Reed, and J.S. Mason. The effect of spontaneous plug formation indense phase pneumatic conveying. Proc 7th Powder & Bulk Solids Conf Chicago.1982.

3. J.Li and M.G. Jones. Towards the control of slug formation in low-velocity pneumaticconveying. Powder Handling & Processing. Vol 14. No 3. 2002.

4. S. Barton. The effect of pipel ine flow conditioning on dense phase pneumatic convey-ing performance. PhD Thesis. Glasgow Caledonian University. Scotland. 1997.

5. K. Konrad, D. Harrison, R.M. Nedderman, and J.F. Davidson. Prediction of the pres-sure drop for horizontal dense phase pneumatic conveying of particles. Proc 5th IntConf on the Pneumatic Transport of Solids in Pipes. BHRA Fluid Engineering Centre.Paper No El, pp 225-244. 1980.

6. S. Ergun. Fluid flow through packed columns. Chemical Engineering Progress. Vol48. No 2. pp 89-94. 1952.

7. M.G. Jones, X. Zhang, T. Krull, and R. Pan. Bypass systems in pneumatic conveying.Proc 15th Hydrotransport. BHR Group Conf. Banff Canada. June 2002.

8. M. Solomon. Bypass pneumatic conveying systems. Final project report. School ofEngineering, The University of Newcastle, Australia. 2002.

9. D. Mason and S. Barton. The use of air-bypass pipel ines to enable low velocity gas-solids flow in pneumatic conveying systems. Proc 8lh Int Freight Pipeline Soc Symp.pp 109-1 16. Pittsburgh. September 1995.

10. D. Mills. The artificial modification of material properties to achieve dense phasepneumatic conveying. Proc CJF-7. pp 249-254. 2000 China-Japan Symposium on Flu-idization. Xi 'an University, China. October 2000.


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