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Influence of the Entrance Configuration on thePerformance of a Non-Mechanical Solid Feeding Devicefor a Pneumatic DryerCibele Souza Lopes a , Thiago Faggion de Pádua a , Maria do Carmo Ferreira a & José TeixeiraFreire aa Drying Center of Pastes, Suspensions and Seeds, Department of Chemical Engineering –Federal University of São Carlos , São Carlos, BrazilPublished online: 27 Jun 2011.

To cite this article: Cibele Souza Lopes , Thiago Faggion de Pádua , Maria do Carmo Ferreira & José Teixeira Freire (2011)Influence of the Entrance Configuration on the Performance of a Non-Mechanical Solid Feeding Device for a Pneumatic Dryer,Drying Technology: An International Journal, 29:10, 1186-1194, DOI: 10.1080/07373937.2011.575495

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Influence of the Entrance Configuration on thePerformance of a Non-Mechanical Solid FeedingDevice for a Pneumatic Dryer

Cibele Souza Lopes, Thiago Faggion de Padua, Maria do Carmo Ferreira,and Jose Teixeira FreireDrying Center of Pastes, Suspensions and Seeds, Department of Chemical Engineering – FederalUniversity of Sao Carlos, Sao Carlos, Brazil

The solids feeder is an important component of a dryer, since it isresponsible for introducing the moist material at controlled, speci-fied rates. The purpose of this paper is to investigate the effects ofsolids feeding configuration on the fluid dynamic behavior of a53-mm-diameter vertical pneumatic conveyor with a loop of 180�,aiming at further applications in drying granular materials. Anon-mechanical solid feeding system constituted by a hopper con-nected to an inclined pipe was applied to feed type D particles inthe conveying line. This simple feeding apparatus was modifiedthrough the insertion of different flow restriction devices at the airinlet, namely a reduction nozzle and a Venturi device. This wasaimed at studying how the solids flow rates and the fluid dynamicsof the whole conveying line are affected by the entrance configur-ation and inlet devices. The use of inlet devices combined with thenon-mechanical inclined valve affected significantly the performanceof the valve when operating with type D particles in a pneumaticconveying line. When using inlet devices, an increase in the conveyedsolid flow rates at a given air velocity was observed. The reductionnozzle yielded a range of solids loading ratios similar to that of theinclined valve with no inlet device, and introduced some pressureinstabilities at the entrance region. The Venturi device allowed oper-ation at a wider range of solids loading ratios and no pressure insta-bility was detected in the conveying line. For the conditionsinvestigated, neither the gas velocity nor the loading ratio affectedthe extent of entrance length. The inlet devices may be successfullyapplied to modify and improve the performance of the inclined valveas a solids feeder in pneumatic dryers.

Keywords Entrance length; Pneumatic conveying; Pressure pro-files; Solid feeding systems

INTRODUCTION

The introduction of moist material in a dryer is con-trolled by the solids feeder, which needs to be selected con-sidering various criteria to enhance the feeder performance.The latter depends strongly on the properties and flow

characteristics of the solids and on the type of dryingequipment. The most used feeding devices in industrialdryers and some practical guidelines regarding selectionand specification were listed by Jumah and Mujumdar,[1]

who observed that mechanical feeding devices are predomi-nantly applied in pneumatic and recirculating fluidized beddryers. Nevertheless, the use of non-mechanical valves isemerging as an appealing alternative for they have poten-tial advantages over the mechanical systems. They use onlyaeration and pipe geometry to control the solid flow rates,provide stable and continuous feeding, avoid mechanicaldamage to the product, and minimize feeding blockagecaused by mechanical failures. It is well known that inpneumatic conveying of particles with simultaneousparticle-gas heat transfer, much of the interphasemomentum, heat, and mass transfer takes place over theacceleration region, where the slip velocities are high.[2]

Therefore, the assessment of mechanisms that governgas-solid interactions at the entrance region of pneumaticdryers is important to accurately predict the performanceof drying equipment. Additionally, experimental data areoften required to validate gas-solid flow modeling in pneu-matic and fluidized beds.

The entrance effects in gas-flow conveying of fine pow-ders has been investigated by a number of researchers.[3–5]

However, few studies exist focusing on conveyors operat-ing with coarse particles (belonging to class D onGeldart’s classification map), which are representative ofthose typically pneumatically dried, such as agriculturalgrains,[6–8] ore particles,[9] and granular materials.[10,11]

In previous studies, we evaluated the performance of apneumatic dryer operating with a spouted bed type solidsfeeding system in which the riser may be considered as avery long draft tube.[12,13] Based on results reported byOlazar et al.,[14,15] who applied inlet devices of differentgeometrical configurations in conical spouted beds operat-ing with a draft tube and coarse particles, the use of areducing nozzle at the air inlet of the dryer has been

Correspondence: Jose Teixeira Freire, Drying Center ofPastes, Suspensions and Seeds, Department of Chemical Engin-eering – Federal University of Sao Carlos, PO Box 676, 13565-605, Sao Carlos, SP, Brazil; E-mail: freire@ufscar.br

Drying Technology, 29: 1186–1194, 2011

Copyright # 2011 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2011.575495

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tested by Sousa et al.[13] It was found that the nozzleimproved the performance and stability of the conveyor.The results pointed to the possibility of modifying theentrance configuration and even enhancing the fluiddynamics behavior in gas-solid transport by changingthe design of this inlet device.

This paper is aimed at investigating the effects of solidsfeeding configuration on the fluid dynamic behavior of avertical pneumatic conveyor with a loop of 180� operatingwith glass spheres with diameter of 1mm. The feeding com-prised a hopper from which the particles were dischargedtowards an inclined pipe with a slide valve that could bemoved to allow the restriction of the available areafor solids discharge. This system operated like a non-mechanical feeding device and will be referred to here asinclined valve. The particles were discharged from this valveinto the bottom of a riser, and then pneumatically con-veyed along the line. To modify the entrance configurationin this simple feeding apparatus, inlet devices of differentgeometries were placed at the riser entrance. The purposewas to investigate how the solids entrainment featuresand fluid dynamics of the whole conveying line are affectedby the entrance configuration and inlet devices.

EXPERIMENTAL METHODOLOGY

The study was conducted as a set of experiments formeasuring pressures, solid and gas flow rates at the differ-ent entrance configurations. The tests were carried out in aconveying line with internal diameter of 53mm, consistingof a 0.305m solids insertion section (2 in Figure 1), a 3.540-m-long riser (3), a loop of 180� (4), and a 2.095-m-longstandpipe (5), as shown in Figure 1. The whole line is madeof galvanized iron. The standpipe is connected to a solidsreservoir of cylindrical section and conical basis (6) withintrinsic angle of 60�, holding a capacity for 50 kg of solids.An orifice of 53mm located at the center of the conicalbasis allows the solids to be discharged through a 42�

inclined pipe (7), also with a diameter of 53mm, which isconnected to the riser entrance at position (1). A slide valve(8) located at the bottom of the solids reservoir was used tochange the area of the orifice for the solids discharge, whichmight be kept 100% open or partially blocked. In thisarrangement, characterized as a non-mechanical feeder,the solids were fed into the riser by gravity, and thenconveyed by the upward air flow. The conveying air wasprovided by a 7.5HP blower (ERBELE, Sao Paulo,Brazil), with maximum capacity of 380m3=h. The air flo-wed through a horizontal pipe (53mm in diameter, 3mlong), in which two globe valves (9) were used to adjustthe air flow rate, and at the end of this line a short elbowwas placed to change the flow direction before enteringthe riser. A previously calibrated Venturi type flow meter(10) was employed for measuring the volumetric airflow rate.

The particles conveyed were glass spheres, with meandiameter of 1.0mm, particle density of 2,500 kg=m3, andterminal velocity Ut¼ 7.8m=s (belonging to class D on Gel-dart’s diagram).[16] They were initially placed into the reser-voir, from where they were discharged into the inclinedvalve and then into the riser. At steady operation, the solidswere entrained by the air flow, conveyed along the riser,loop and standpipe and finally returned to the reservoir.

A solids sampler was located at the standpipe (number11 in Figure 1) that allowed the measurement of solid flowrates by deviating the solids flow direction during a timeinterval, causing minimum disturbance to the normal flow(a detailed description is given in Costa et al.[12]). Twenty-eight pressure taps were distributed along the line. Sevenof these taps were connected to pressure transducers(AutoTran 600 s series) operating in the range of 0-35kPa,at heights 0.03, 0.26, 0.36, 0.41, 0.46, 1.10, and 6.69m.A NI9205 module (DP Union, Sao Paulo, Brazil) and dedi-cated software based on Labview1 (National Instruments)

FIG. 1. Pneumatic conveying line. (1) solids insertion; (2) inlet device; (3)

riser; (4) loop; (5) standpipe; (6) solids reservoir; (7) inclined pipe; (8) slide

valve; (9) globe valves; (10) air flow meter; (11) solids sampler (all dimen-

sions in mm).

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were employed for data acquisition. The pressures werecollected at a sampling frequency of 2,000Hz. At theremaining positions the pressure taps were connected towater manometers.

Two different custom-made inlet devices were used atthe inlet of the riser. Each inlet device was placed at theposition identified by number (2) in Figure 1 and reducedthe diameter of the air inlet orifice. The first device was areduction nozzle (Figure 2a), in which the air inlet orificediameter was abruptly reduced from 53mm to 33mm ina short length of 80mm. At this configuration the solidswere inserted upstream the device, just above this diameterrestriction. The second device was designed to provideidentical reduction of the air inlet orifice diameter as inthe previous case, but the reduction was gradual andfollowed by an expansion, at angles of 6.5� and 3.6�

(Figure 2b). This design followed the classical design of aVenturi device, as described in Delmee.[17] It was con-structed as a forged piece of galvanized iron, with a totallength of 305mm. In this configuration the solids were

inserted into the device’s throat, at the position in whichthe diameter was minimal. Each device was tested fortwo different solids orifice discharge areas, equal to2.2� 10�3 or 0.29� 10�3m2. The configurations evaluatedare summarized in Table 1.

At each configuration, a given air flow rate was set andthe solids feeding initiated. Once steady-state conveyingwas established, the pressures along the line were recordedand the gas and solid flow rates were measured. The pro-cedure was repeated at a new air flow rate, until themaximum blower capacity was reached. The mean air tem-perature at steady conditions was about 70�C for all theconditions tested.

RESULTS

The goal of the experiments was to compare the per-formance of the six different solids feeding configurationsdescribed in Table 1. The comparison was based on thevalues of solid flow rates, pressure profiles, and on thecurves of dimensionless pressure drop versus solids loadingratio obtained at each configuration. First, the perform-ance of the inclined valve operating with no restrictiondevice is analyzed, and then the alterations owing to usingeach inlet device are addressed.

Inclined Valve

The performance of inclined valve operating with nointernal device is presented in this section. The solids orificedischarge areas tested, viz. 2.2� 10�3m2 and 0.29� 10�3m2,correspond, respectively, to 100% and 13% of the totalcross-section area of the inclined pipe.

Solid Flow Rates

Curves of solid flow rates as a function of the dimen-sionless air velocity are shown in Figure 3. In a range ofdimensionless air velocity (U=Ut) from 1.5 to 5, theinclined valve yielded solid flow rates (Ws) from 70 down

TABLE 1Inlet configurations tested in the experiments

CodeInternaldevice

Solids orificedischargearea (m2)

Openingstatus

N-1 None 2.2� 10-3 100%N-2 None 0.29� 10-3 13%R-1 Reduction

nozzle2.2� 10-3 100%

R-2 Reductionnozzle

0.29� 10-3 13%

V-1 Venturi 2.2� 10-3 100%V-2 Venturi 0.29� 10-3 13%

FIG. 2. Inlet devices. (a) Reduction nozzle; (b) Venturi.

FIG. 3. Solid flow rates versus dimensionless air velocity in operation of

inclined valve.

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to 14 kg=h. It is well known that the drag force exerted onthe solids in pneumatic conveying increases with the airvelocity. The patterns of curves in Figure 3, with the solidsflow rate decreasing as the air velocity rises, contradict thisexpected behavior. However, the dependence can beexplained by the configuration of solids feeding system,as follows: the solids discharged from the hopper into theinclined valve are entrained up in the riser by the upwardflowing air. Because the inclined valve is connected to theriser, some air tends to escape from there to the valve.A dense packed bed of solids is formed in the valve, butthe resistance offered by the solids is not high enough tototally prevent this flow.

As can be seen in Figure 4, in which Qdev is plottedagainst Q for the same conditions in Figure 3, the rate ofair deviated to the valve increases with the increase of inletair flow. The gas flow rates through the valve were esti-mated from the Forchheimer equation, with the pressuredrop measured between two positions of the inclined pipe.Qdev ranged from 0.6 to 2.4m3=h, which was not a relevantloss because the volumetric rates were always less than 1%of the rates of inlet air. The mean velocities in the inclinedpipe section for this range of air flow rates varied from 0.07to 0.30m=s. These are small velocities, particularly con-sidering the high value of particle terminal velocity. Never-theless, as can be seen in Figure 5, they cause significantpressure drops through the packed-bed of particles in theinclined valve. These pressure drops increased by a factorof 4 as Qdev was raised from 0.60 to 2.4m3=h. For example,for Qdev¼ 1m3=h, the mean air velocity in the inclinedvalve was only 0.13m=s, but the measured pressure dropreached 3,000 Pa=m. Therefore, the remarkable reductionin the conveyed solid flow rates as the inlet air velocitywas raised is not surprising.

It is worth noting that the values of Qdev and solid flowrates did not change when the solids discharge area wasreduced in 87%. This allows one to infer that the solids flow

rates yielded by the inclined valve were being controlledonly by the conditions at the riser entrance.

Pressure Profiles

For single- or two-phase flows inside pipes, a non-linearvariation of pressure along the pipe length is evidence of anot-fully-developed flow velocity profile, characterizing anentrance length or acceleration region.[13,18] Commonly, itappears as a result of a tube entrance or due to the presenceof fittings or accessories (such as curves or elbows) in theline. The analysis of pressure profiles along a conveyingline is useful for identifying not-fully-developed flowregions, where momentum, heat, and mass transfer coeffi-cients vary locally and the transfer phenomena are intensi-fied. The variation of local pressures along the conveyingline is depicted in Figure 6, at U¼ 22m=s, for single-(air) and two-phase (air-solids) flows. The axial distancein Figure 6 is represented by a variable f, which is the lineardistance measured from the air inlet distributor (see indi-cation in Figure 1). The results obtained at U¼ 22m=sare representative of other conditions investigated.

As would be expected, for single- or two-phase flow thelocal pressures decreased along the conveying line as aresult of the pressure losses due to wall-friction and, intwo-phase flow, of solids weight as well. A comparisonbetween the curves for single- and two-phase flow inFigure 6 shows qualitatively similar pressure profiles,except that the curves are slightly shifted up in the two-phase flow, which is due to the contribution of solidsweight to the pressure drop in the line.

In the analysis of the pressure variation patterns, oneshould consider the line configuration. As described inthe methodology section, the air flowed through a long,straight, horizontal section of pipe, which was connectedto the riser by a short elbow. The insertion of solids wasdone at the connection located after this elbow (number1 in Figure 1), slightly above f¼ 0. By examining the

FIG. 4. Deviated air flow rate versus inlet air flow rate, configurations

N-1 and N-2.

FIG. 5. Pressure drops per unit length in the inclined valve as function

of Qdev.

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pressure profiles in Figure 6, a deviation from linear beha-vior is observed from f¼ 0 up to 1.6m in both single- andtwo-phase flows. This length characterizes the acceleratingregion where the gas and solids velocity profiles aredeveloped along the riser. Above this length of 1.6m,the pressure varied linearly with f, indicating that theflow achieved a fully developed profile. When the flowapproached the loop, located between f¼ 3.85m andf¼ 5.10m, the pressure profile was disturbed again, andanother entrance length was observed, extending fromf¼ 3.80 to 5.15m. From this distance, a linear pattern isobserved, which is maintained along the standpipe. Itshould be expected that the presence of solids would affectthe extension of entrance lengths, but data in Figure 6show that they are quite similar in single- and two-phaseflows.

As may be observed in Figure 6, the restriction in thesolids discharge area did not affected the pressure profiles,as the curves show practically the same behavior for config-urations N-1 and N-2.

The pressure gradients measured at the riser in fullydeveloped conditions were previously plotted against Uto identify the minimum pressure gradient and the air velo-city at this condition (Umin). These plots, though notshown here, were used to classify the flow regime, accord-ing to the classical procedure described in Marcus et al.[19]

In vertical conveying, the gravitational and wall-frictionforces are the main forces acting on the suspension. At aconstant Ws when U is raised, the solids hold-up decreased(and so did the solids weight), while the fluid-wall frictionincreased proportionally to the square power of U. Thedilute flow was limited to the region in which U>Umin.It was characterized by pressure gradients increasing withU, which was a result of the predominance of wall frictionforce over the gravitational force. The plots obtained here

showed typical behavior of dilute conveying for all con-ditions investigated. It is worth noting that the pressuregradient maps are usually applied for systems in whichthe solid flow rates do not depend on the variation ofair velocity. Though this is not the case for the inclinedvalve used here, previous studies from the authors[12,13]

have shown that the same analysis may be applied fornon-mechanical systems in which Ws is not constant whenU is changed. For the sake of concision, this analysis willnot be presented here, as they have been well-discussed inthe aforementioned papers and the results of the presentstudy were not qualitatively different from those alreadyreported.

Figures 7(a) and 7(b) show the pressure profiles for sin-gle- and two-phase flows, respectively, where the effect ofincreasing air flow rates on the pressure profiles may bevisualized. It is noted in both cases that the local pressuresincreased as the air velocity was raised and the curves show

FIG. 7. Static pressure versus axial distance at different values of U=Ut;

(a) single-phase flow; (b) two-phase flow.

FIG. 6. Static pressure versus axial distance in the conveying line at

U¼ 22m=s.

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very similar patterns for single- and two-phase flows. Sucha behavior reflects the predominance of wall-friction forceover the gravitational one in the transport of particles,characteristic of dilute conveying. The acceleration lengthswere not affected by increasing of air velocity or by theinsertion of solids.

The fluid dynamic data presented so far are consistentwith those reported in the literature for pneumatic con-veyors operating with non-mechanical devices.

Inclined Valve with Flow Restriction Devices

In this section, the effects of flow restriction devices onthe dynamic behavior of conveying are discussed.

Solid Flow Rates

The performances obtained with the inlet devices may becompared in Figure 8, which depicts the results for thesolids discharge orifice totally opened (configurations N1,R-1, and V-1). The results for no inlet device are alsoshown for comparison. The dependence of solid flow rateson the air velocity is different in each case, with inletdevices yielding larger values of solids flow rates (Ws).While configuration N-1 provided Ws from 50 to 10 kg=hin a range of U=Ut from 1.5 to 5.0, R-1 and V-1 yieldedWs from 80 to 25 kg=h, and from 140 to 100 kg=h, respect-ively, in a similar range of U=Ut.

The increase in Ws observed at configuration R-1 incomparison with N-1 may be attributed to the significantdecrease of Qdev noted in the former configuration, ascan be seen in Figure 9. The air inlet diameter was reducedfrom 53.4mm to 33mm with the nozzle, and the solidswere introduced into the riser just above the restriction.Due to the restriction of the flow area, the local pressuredecreased and the air was accelerated, creating a regionof low pressure and high velocity, which minimized theair escape from the riser towards the inclined valve. A

smaller air flow rate in the inclined valve means lowerpressure drops and minor resistance to solids downflow,thus the global effect was an increase in the rate in whichthe solids were fed into the riser. It is interesting to notein Figure 8 that the concavity of the curve obtained at con-figuration R-1 is different from that of N-1 – the rate ofdecrease of Ws is smaller at lower values of U=Ut andbecomes steeper as U=Ut was raised. This phenomenonneeds further investigation to be consistently explained. ACFD-based model (using both, Euler-Euler and Euler-Lagrange approaches) is being developed to describe thegas-solid flow in the feeding region in an attempt to clarifythese behaviors.

From the configurations evaluated, the Venturi deviceprovided the higher values of solid flow rates. The depen-dence of Ws on U=Ut shows a similar pattern to thatobserved for configuration N-1. Figure 9 shows higher Qdev

using the Venturi device than those observed using the noz-zle but, depending on the range of inlet air velocity, theywere equal or lower than those obtained with no restrictiondevice. The remarkable increase in the solids flow rateobserved with the Venturi may be attributed to the designof this device. As depicted in Figure 2b, the air inlet diam-eter is gradually reduced from 53mm to 33mm and thenprogressively expands to 53mm over a 305-mm-lengththrottle. The solids are discharged from the inclined valveinto the strangulated section of this throttle, meetingthe upflowing air at a position in which the velocity ismaximum, which enhanced the gas-solids momentumtransfer. It may be inferred from these results that the dragforce was probably high enough to overcome the resistanceoffered by the deviated air flow in the inclined valve, result-ing in quite high values of Ws. However, a thorough com-prehension of the mechanisms that led to this behavior stillrequires further investigation and consistent modeling ofgas-solid flow at the solids feeding region.

It is worth noting that, as opposed to that observedwhen no inlet device was employed, the variation of solids

FIG. 8. Solid flow rates versus dimensionless air velocity in the operation

of inclined valve with inlet devices.

FIG. 9. Deviated air flow rate versus air inlet air flow rate for the differ-

ent configurations tested.

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discharge area affected the valve performance. Figure 10shows the solid flow rates versus dimensionless air velocityobtained using the inlet devices at different solids dischargeareas.

At configuration R-2, Ws decreased from 65 to 5 kg=has U=Ut was raised from 1.5 to 4.8, while at R-1 thesolid flow rates were shifted up, with Ws decreasing from85 to 20 kg=h as U=Ut was increased from 1.7 to 4.5.The change is much more impressive when the Venturidevice was employed. At configuration V-1, the solidflow rates decreased from 140 to about 100 kg=h as U=Ut was raised from 1.2 to 3.7, and from then on theyremained constant until U=Ut¼ 5. At configurationV-2, in which the solids discharge area is only 13% oftotal open area, the solid flow rates varied in a narrowerrange, decreasing from 40 to 20 kg=h at 1.2<U=Ut< 3.0,staying constant at 3.0<U=Ut< 3.8 and finally increas-ing to 40 kg=h as U=Ut varied from 3.0 to 5.0. The dra-matic change in the patterns of the curves forconfigurations V-1 and V-2 suggests that the interactionbetween the Venturi and the solids discharge area withthis device is stronger than the observed with the nozzle.

Additional investigation is needed to clarify the reasonsfor this behavior.

Pressure Profiles

Pressure profiles along the conveying line obtained atU¼ 22m=s for configurations N-1, R-1, and V-1 may becompared in Figure 11, where a dimensionless pressure,defined as the ratio of the local pressures in two- andsingle-phase flows (P=P0), is plotted against the axial dis-tance. The effect of solid feeding device on the developmentof pressure profile, particularly at the pipe entrance, is evi-dent in this figure. With no inlet device, the dimensionlesspressure is practically constant over the whole tube length.When the inlet devices are used, P=P0 is high at f¼ 0m,and rapidly drops between f¼ 0 and f¼ 0.5m. From thisdistance on, the pressure profiles vary depending on thedevice geometry. With the Venturi device, a slight increaseof P=P0 was observed up to f¼ 1.5m, after which itdecreased continuously along the tube length. For the noz-zle device, the dimensionless local pressure shows an oscil-lating pattern from f¼ 0.5 up to 2.0m. After this distancethe behavior is similar to the other situations. It is notedthat for developed flow, the higher values of dimensionlesspressures were observed for the Venturi device, which is aconsequence of the higher solid flow rates conveyed atthis configuration. As the flow reached the standpipe, atf¼ 5.10m, from where the suspension flows by gravity,the local pressures became practically identical for all theconfigurations. The entrance lengths observed at the tubeentrance and at the loop vicinity were not affected by theinlet devices, since their values are essentially the same asin Figure 7.

It is noteworthy that the values of dimensionless pres-sures were close to 1 for most conditions, except by the first0.5m from the tube entrance at configurations R-1 and

FIG. 11. Dimensionless pressure versus axial distance along the convey-

ing line at U¼ 22m=s for different inlet configurations.

FIG. 10. Solid flow rate versus dimensionless air velocity: (a) reduction

nozzle; (b) Venturi device.

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V-1. Regarding the effect of inlet device on the pressuredrops in the inclined valve, the major change was observedwith the use of the reduction nozzle. At U¼ 22m=s forexample, DPv was about 750 Pa for configurations N-1and V-1, and 490 Pa for R-1 (see Figure 12). These pressuredrops are associated to the magnitude of air flow ratesdeviated to the valve, equal to 1.08, 0.98, and 0.31m3=hfor configurations N-1, V-1, and R-1, respectively. It isclear in Figure 12 that the Venturi device has not signifi-cantly modified the pressure drop in the inclined valve incomparison to configuration N-1. Nevertheless, at thisparticular condition, Ws obtained at configuration V-1was equal to 113 kg=h, which is about fivefold the valueobtained with no inlet device at a similar U=Ut.

Total Pressure Drops

The total pressure difference measured between f¼ 0and 6.69m was estimated for the different transport con-ditions in order to analyze the effects of entrance configur-ation on the whole conveying line. The pressure drop perunit length was obtained by dividing the total pressuredrop by the linear length. To work with dimensionlessvalues, the pressure drops obtained in two-phase flowswere divided by the equivalent value in a single phase flow.The dimensionless pressure drop per unit length (DP=DP0)was plotted as a function of solids loading ratio Ws=Wf, forthe different configurations and are shown in Figures 13aand 13b for operation with the solids orifice discharge par-tially and completely opened, respectively. Quite differentconveying rates could be obtained just by changing theentrance configuration. In addition, the range of operationat each configuration depended on the available area forsolids discharge at the entrance of the inclined valve. Whena restricted area was adopted (see Figure 13a), the inclinedvalve operating either with no inlet device or with thereduction nozzle yielded solids loading ratios from 0.02

to 0.60. Using the Venturi device, the range of solids load-ing ratios varied in a narrower range, from 0.10 up to 0.38.When the discharge orifice was completely open (seeFigure 13b), however, the Venturi reached solids loadingratios from 0.2 up to 1.4, while the performances of theinclined valve with no device and with the nozzle changedlittle in comparison to the previous ones.

The values of dimensionless pressure drops at a givenWs=Wf were not quite different at each configuration andgenerally the dimensionless pressures increased linearlywith the increase of Ws=Wf.

CONCLUSIONS

The use of two types of inlet devices combined with anon-mechanical inclined valve for solids feeding in a pneu-matic conveying line affected significantly the performanceof the valve when operating with type D particles. For bothinlet devices evaluated, an increase in the solid flowrates conveyed at a given air flow rate was observed. By

FIG. 12. Pressure difference along the inclined valve at different inlet

configurations.

FIG. 13. Dimensionless pressure drop versus solids loading ratio for the

different configurations. Solids orifice discharge area: (a) 0.29� 10�3m2;

(b) 2.2� 10�3m2.

NON-MECHANICAL SOLID FEEDING DEVICE 1193

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evaluating the solids loading ratio obtained at each con-figuration, it is possible to conclude that the reductionnozzle yielded a range of solids loading ratios similar tothat of the inclined valve with no inlet device. This device,however, introduced some pressure instabilities at theentrance region. The Venturi device, on the other hand,allowed operation at a wider range of solids loading ratiosand no pressure instability was detected. For the conditionsinvestigated, neither the gas velocity nor the loading ratiohad influence on the extent of entrance length. The resultsreported here indicate that the inlet devices may be success-fully applied to improve the performance of the inclinedvalve as a solids feeder for pneumatic dryers.

NOMENCLATURE

P Static pressure in gas-solid flow, PaP0 Static pressure in gas flow, PaQ Inlet volumetric air flow rate, m3=hQdev Volumetric air flow rate deviated to the

inclined valve, m3=hU Inlet air velocity, m=sUmin Inlet air velocity at minimum pressure

gradient, m=sUt Particle terminal velocity, m=sWs Solids flow rate, kg=hWf Air flow rate, kg=h

Greek letters

DP Pressure difference, Paf Axial distance in the riser, m

ACKNOWLEDGMENTS

The authors thank FAPESP and CNPq for financialsupport.

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