EXPERIMENTAL STUDY OF PARAMETERS INFLUENCING THE PICKUP VELOCITY IN PNEUMATIC CONVEYING SYSTEMS

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1 Coyright 009 by ABCM EXPERIMENTAL STDY OF PARAMETERS INFLENCING THE PICKP VELOCITY IN PNEMATIC CONVEYING SYSTEMS Luiz Moreira Gomes, André Luiz Amarante Mesquita, Lab. de Transorte Pneumático - Faculdade de Eng. Mecânica FPA - R. Augusto Corrêia, s/n CEP Marcelo de Oliveira e Silva, marcelo.silva@solveengenharia.com.br Pedro Henrique da Silva Macias, edro.macias@solveengenharia.com.br Solve Engenharia Ltda. Rua Curuçá, 60 s Belém, PA Abstract. The transort velocity is one of the most imortant arameters in neumatic conveying of articles. The success of conveying articulate material deends on the rediction or the determination of the minimum transort velocity. The high conveying velocity will result in greater energy consumtion due an increased ressure dro of the system, solids degradation and ie erosion. On the other hand, conveying velocities that are very low can result in saltation and blockage of the ieline. In this research work, the article entrainment is investigated by measuring the velocity required to ick u articles from rest, also known as icku velocity. An analysis of the influence of several arameters on the icku velocity, such as article diameter and density and ie diameter, is made. A correlation is develoed and the numerical results evaluated show good concordance with exerimental data. Keywords: icku velocity; minimal velocity; gas-solid flow; neumatic conveying. 1. INTRODCTION In the neumatic transort of solid articles in ies, it is imortant to choose an air velocity that is as low as ossible in order to reduce ower consumtion, ie wear and article breakage. In accordance to Cabrejos et al. (199), the minimum transort velocity is one of the most imortant arameters in the neumatic transort of solids. A velocity greater than the minimum necessary to the steady transort of solid articles leads to a great energy consumtion due to the system s ressure dro, solids degradation and ie erosion. Oositely, the velocity below this value threshold will certainly result in the deosition of solid articles on the ie bottom and blockage. If this gas velocity is set at the beginning of a neumatic transort system (at the feed oint), the rest of the ieline will oerate well above this lower velocity threshold, since the gas velocity will increase along the ieline due to comressibility effects. Keeing gas velocity above minimum conveying velocity in all horizontal sections of a ieline ensures no deosition of solids in the system and a continuous and steady conveyance of solids. In literature, several terms are emloyed to refer to the minimum transort velocity: icku velocity, saltation velocity, susension velocity, critical velocity, rolling or sliding velocity, initial mixing velocity, velocity at the ressure minimum oint of the general state diagram etc. All these minimum transort velocities have similar numerical values and are based on visual observations and ressure dro measurements. These velocities are characteristic to an alteration in the flow regime, changing from a steady to an unstable hase. It also indicates some transition in the way that the articles are moving or beginning their movements (Herbreteau et al., 000). The icku velocity is relevant in a wide range of alications. For examle, some harmaceutical industries are focused on dry owder inhalers for drug delivery, the movement of sand dunes and soil deosition in river and ocean flows (Kalman et al., 005) and understanding erosion of silt on riverbeds (Kalman and Rabinovich, 007). Likewise, icku velocity is an imortant arameter in dust control alications. A good estimate of the minimum conveying velocity must be known in order to otimize any solids conveying system. This research work evaluated the influence of diameter and density of the article and the ie diameter at the icku velocity.. LITERATRE REVIEW Cabrejos et al. (199) develoed a technique to determine the icku velocity of solid articles in horizontal neumatic conveying. It was based on visual observations of articles in rest at the bottom of a transarent ie when the air velocity was gradually increased. sing a structure consisting of a transarent, 7-meter-long, 5-millimeter diameter ieline with a removable section, a comressed regulated air suly, and a collector of solids, the researchers measured the icku velocity exerimentally. In order to carry out the measurements, the researchers created, inside a removable and transarent section of the ie, a 1-meter-long layer of articles with aroximately half of the cross-sectional area of the ie left free. A constant gas flow rate was set through the ieline and the layer started to erode slowly as the gas stream icks u the to articles. As the free cross-sectional area increased, the gas velocity over the layer decreased and so did the caacity of the stream to ick u more articles. The described henomenon takes

2 Coyright 009 by ABCM lace continuously, and a final equilibrium was automatically reached when no more erosion occurred. By measuring the volumetric flow rate of air (Q air ) and the free cross-sectional area (A free ) remaining over the articles, the minimum icku velocity ( u ) could be easily calculated using the equation: u Q A air free (1) Cabrejos and Klinzing (1994), alying dimensional analysis, found that the following π grous can describe the icku mechanism of the coarse articles in horizontal tubes f g d D d,,,, 0 d () They also analyzed the effect of several arameters that affect the icku velocity of coarse articles in horizontal ies. By changing one arameter and keeing the others arameters constant, the following relationshis were found: 1/ 3/4 1/ 1/4 u d ; u ; u ; u D (3) The authors also verified that the influence of the conveying gas viscosity on the icku velocity is minimal. Finally, they ostulated the following relationshi to determine the icku velocity: u g d 0,5 0,75 D 0,175 0, 048 Re d (4) This relationshi is valid for 5<R e <5000, 8<(D/d )<1340 e 700<(ρ /ρ)<440. Kalman et al. (005) used a rectangular wind tunnel (which consisted of 10 square ducts) to measure the icku velocity of the articles initially in rest. The layer of articles was created by filling a rectangular shallow bath attached to the bottom of the wind tunnel (Fig. 1).The to surface of the article layer matched the bottom surface of the tunnel. The technique used by them consisted in measuring the icku velocity by lotting the amount of entrained articles (weight reduction of the layer) as a function of oerating gas velocity. The icku velocity was determined at the intersection of the extraolated curve assing through the measured oints and abscissa. Figure 1. Schema of exerimental setu of the Kalman et al. (005) They also develoed a relationshi for the icku velocity in terms of modified Reynolds number as a function of modified Archimedes number. They modified the Reynolds number emirically to take the ie diameter into account: Re * u d D/ D50 1,5 1, 4 0,8. e (5) Then, they resented the following correlation: u u50 D/ D50 1,5 1, 4 0,8. e (6)

3 Coyright 009 by ABCM By this correlation, all measured icku velocity found in the literature can be converted to the icku velocity in a -inch ie for further investigation. In accordance with the authors, this equation agrees well with the measurements conducted with ie diameters ranging from about 1 u to 6 in. By using their own exeriments and data of others researchers, they develoed three relationshis, each one alied to a secific flow situation. The three-zone model was reasonable according to the Geldart classification. The first zone alies to large articles and the cohesive forces are negligible. 3 * 7 Ar Re 5, for Ar >16.5 (7) In the second zone, the cohesive forces are considerable, but still the articles are icked u individually. * Re 16, 7, for 0.45 < Ar < 16.5 (8) In the third zone, the cohesive forces are very strong, the articles are very small and the cature occurs in agglomerates instead of individual articles. 1 * 3 Ar Re 1,8., for Ar < 0.45 (9) The relationshis are valid for 0.5<Re * <5400,.10-5 < Ar < , 0.53 < d < 3675 μm, 1119 < ρ < 8785 kg/m 3 and 1.18 < ρ <.04 kg/m MODEL FOR INCIPIENT MOTION OF A SINGLE SPHERE The significant forces acting on a single shere in rest at the bottom of a horizontal tube are: drag force, friction force, cohesive force, buoyancy force, lift force and gravitational force. Cabrejos and Klinzing (199) showed that the horizontal movement occurs before the vertical lift-off. According to Hayden et al. (003), the icku velocity is considered to be the velocity at which the article becomes entrained in the moving fluid, which imlies vertical movement. In this aer, the force balance is made for the motion article in vertical direction. To achieve the vertical icku, the resulting force in this direction is equal to zero. Fg Fa Fl Fb Fd (10) The lift, buoyancy and adhesive forces are not areciated in this demonstration. Thus v Fg Fd CD A (11) Since v (friction velocity) and cv f f g D D F C A C A cv (1) where c is a arameter related the turbulent fluctuations. Rewriting Eq. (16), in terms of friction factor, f P f P C Acv D CD Acv (13) sing Blasius relation, f 1/4 (14) Re

4 Coyright 009 by ABCM Making some algebraic maniulations we obtain the relationshi, uo g d D C c /3 /3 /3 1/3 /3 1/3 /3 1/3 D (15) We adjust the arameter c emirically. Then we get uo C /3 / 3 / 3 1/3 /3 1/3 /3 1/ 3 D ln( d ) 90.5 g d D (16) This relationshi shows us the deendency of the icku velocity of the roerties of the article and conveying gas as well as that of the duct diameter. 4. THE GENERAL SEMI-EMPIRICAL CORRELATION By combining the single article model with exerimental results for icku velocity of a ile of articles in a ie on the horizontal direction, it was ossible to determine a general semi-emirical correlation which was simle and ractical to determinate the icku velocity of a layer of articles. The semi-emirical correlation develoed is valid over the range of articles sizes from 30 to 4000 µm and can be written as follows: 1/4 1/14 1/ Ar Ar Ar u uo (17) The Archimedes number was used to correlate the article-article interactions and article shae. It characterizes the article-gas roerties and is related with the Froude number, the Reynolds number and the ratio of gravity to buoyant force. The next section describes briefly the exerimental setu used and the adoted rocedures in order to measure the icku velocity of solid articles in a horizontal ieline. 5. EXPERIMENTAL SETP An exerimental setu used to determine the icku velocity of articles was built in the Pneumatic Conveying Laboratory - LTP - FPA. Exeriments were carried out using air at ambient conditions as the conveying gas. The setu consists basically of a 1.5-meter-long, 50mm in diameter. horizontal steel ieline, three horizontal PVC ielines (6-meter-long each and 50, 75 and 100mm in diameter) with a butterfly valve on the end of each ieline, three transarent sections (laced in the middle of the PVC ielines) where the visual observations were carried out, a roots blower (controlled by a frequency inverter) that rovides the gas velocity and ressure necessary to ick u the articles and a solids collector with a aer filter bag laced on to of it. Figure shows schematically the setu designed to carry out icku velocity exeriments. Figure.The exerimental setu-schematic reresentation.

5 Coyright 009 by ABCM The exerimental method used to measure icku velocity starts with a stationary layer of articles laced in the center section of a transarent ie. Air flow is initiated at a constant volumetric flow rate through the ie. As the free cross-sectional area of the ie increases due to removal of articles, the air velocity decreases. Eventually, when the velocity is no longer sufficient to entrain any additional articles, a final equilibrium is automatically reached. This rocedure is reeated until nearly 95 er cent of the whole material has been catured. Plotting the amount of entrained articles (weight reduction of the layer) as a function of oerating gas velocity made it ossible to determine the icku velocity by the intersection of the extraolated curve assing through the measured oints and abscissa (Fig. 3). Obviously, a higher quantity of measurements imroves the accuracy, esecially if measurements with very small weight losses can be achieved. Figure 3. Schematic reresentation of icku velocity measurement. The main difficulty of this method arises when the initial layer consists of very fine owders. This layer induces a comression load in the acked material samle, affecting the measurements.. Therefore, most of the measurements were conducted with coarse articles. This roblem was also observed by Cabrejos and Klinzing (199), Hayden et al. (003) and Kalman et al. (005). In order to assess the effect of ie diameter in the icku velocity, three PVC ielines were used in an alternate way. Air flow was initiated at a constant volumetric flow rate through the ieline with one of the three butterfly valves oened. The layer started to erode slowly as the gas stream icked u the to articles. The icku velocity was measured with the use of our technique. This rocedure was also carried out with the second and third valves and then we analyzed the effect of the ie diameter in the icku velocity. 6. RESLTS AND DISCSSION In order to assure the accuracy of the methodology develoed, the exeriments were reeated six times. Figure 4 shows the measures of weight loss as a function of oeration gas velocity. In this resent research work and in that of Kalman et al. (005), the articles icku velocity is obtained with the intersection of the extraolated curve, assing through the measured oints, and abscissa. Two indicators used for comarison, the relative deviation and the average absolute deviation, were found to resent the highest values equal to 7.1% and 0.68 m/s, resectively, for all measured oints. Figure 4. Weight loss as a function of the average air velocity.

6 Coyright 009 by ABCM In Fig. 5, the effect of article diameter is illustrated when keeing the others arameters constant, i.e. article density, conveying gas density and viscosity, and ie diameter. Several measurements were erformed with sand grains in the article diameters of, 48.5, 63.5, 89.5, and 53.5 µm. Each exeriment was reeated three times and then a curve of average icku velocity as function of the article diameter was lotted. An imortant result obtained is the existence of a minimum oint in the curve of minimum icku velocity as a function of article diameter, which corroborates Cabrejos and Klinzing (199), Hayden et al. (003) and Kalman et al. (005). This minimum oint aears at article diameter of 89.5 µm. As exected, bigger articles required higher mean gas velocities to be icked u due the inertial effects to be dominated. For the smaller articles, the icku velocity is also high due to the article-article interactions to be significant in this region. Figure 5. Picku velocity as a function of the article diameter Figure 6 also shows the result of exeriments made with sand grains with sizes ranging from 100 to 800 µm. The exeriments were made in 50, 75 and 10 mm ies. We can see the influence of the duct diameter in the icku velocity, i.e., higher seeds for larger ies diameters. Figure 6. Picku velocity as a function of the article diameter. Pies Diameters: 50, 75 and 10 mm. Figure 7 shows in a semilog lot the icku velocity as a function of the ie diameter. The article diameters used were 0.1, 0.34, 0.73 and 1.68 mm. It can be seen that ie diameter affects the icku mechanism of solids articles in horizontal ies, an imortant fact to be considered in designing scale -u rocedures. This results are in good concordance with the Cabrejos and Klinzing (1994). These researchers investigate the influence of the ie diameter at the icku velocity, reorting that this velocity varies with the 0.5 ower.

7 Coyright 009 by ABCM Figure 7. Picku velocity as a function of ie diameter. In order to illustrate the relative significance of article density, Fig. 8 shows a lot of the icku velocity as a function of the article diameter for alumina and sand articles. The densities are 3750 and 636 kg/m 3, resectively. As we have seen, the icku velocity for alumina is higher, thus corroborating Cabrejos and Klinzing (1994). This is so because the inertial effects dominate and require higher flow rates to entrain articles with higher densities. Figure 8. Picku velocity as a function of the article diameter. Alumina and sand articles. The Fig. 9 shows the icku velocity as a function of the article diameter of the sand grains obtained exerimentally in this resent work and the correlations develoed by Cabrejos and Klinzing (1994) and Kalman et al. (005). The vertical bars lay the exerimental error (difference between the higher and the lower measured value) and the black square reresents the mean value. The results are good, mainly in the diameter range of µm, which is a critical region. The accordance with the correlation of Cabrejos and Klinzing (1994) is reasonable for the measured values, being that with the increased mean diameters of the articles, the values evaluated by Cabrejos and Klinzing (1994) rise u more quickly.

8 Coyright 009 by ABCM Picku Velocity (m/s) Kalman et al, 005 (Correlation) Cabrejos et al, 1994 (Correlation) Present work (Exerimental) Material: Sand Density =700 kg/m 3 Pieline Diameter: 50 mm Mínimum Picku Velocity Particle Diameter (m) Figure 9 Picku velocity as function of the article diameter It is imortant to oint out that the roosed technique resents good accordance with the techniques roosed by Cabrejos and Kinzing (199), Haiden et al. (003) and Kalman et al. (005). Fig. 9 shows that the minimum transort velocity (in this research work) occurs in the mean diameter of 89,5 µm. Therefore, the transort of sand articles to minimum ower consumtion would have to use this size of article. In industrial alications, sand grains with a secific diameter are not used; however, this result is very imortant because it gives one better henomenological understanding of the hysical henomenon involved. This minimum oint corresonds to the transition between the effect of the electrostatic forces and inertial forces. Thus, at the larger articles diameter, inertial effects dominate and require higher flow rates to entrain larger articles, and for the smaller article sizes, article article interactions are significant such that higher velocities are needed to searate smaller articles. In order to evaluate the accordance of the correlation develoed in this work, the icku velocity was calculated and lotted against the exerimental result of Cabrejos and Klinzing (1994), Hayden et al. (003), Kalman et al. (005) and Gomes and Mesquita (006). The redicted values using the resent correlation showed a very good concordance with the exerimental data (Figs ). 0 P (m/s) Glass sheres - = 480 kg/m 3 - Cabrejos et al Sand - = 636 kg/m 3 - Present Correlation Non-Sherical Alumina - = 3750 kg/m 3 - Cabrejos et al Sand - = 636 kg/m 3 - Gomes & Mesquita Sand - = 636 kg/m 3 - Cabrejos & Klinzing (Correlation) Glass sheres - = 480 kg/m 3 - Present Correlation Alumina - = 3750 kg/m 3 -- Present Correlation d (m) Figure 10. The icku velocity D = 5 mm.

9 Coyright 009 by ABCM P (m/s) Glass sheres - = 480 kg/m 3 - Cabrejos et al Non-Sherical Glass - = 480 kg/m 3 - Cabrejos et al Salt Irregular - = 34 kg/m 3 - Kalman at al Iron Oxide - = 5000 kg/m 3 - Cabrejos et al Zirconium Sheres - = 5964 kg/m 3 - Kalman at al Zirconium Sheres - = 5964 kg/m 3 - Kalman at al Glass sheres - = 834 kg/m 3 - Kalman et al Salt - = 34 kg/m 3 - Present correlation Glass sheres - = 834 kg/m 3 - Present correlation d (m) Figure 11. The icku velocity - D = 5 mm. Our correlation resents good results for article densities ranging from 1500 to 6000 kg/m 3. Another interesting asect is the fact that it also shows very good results for articles sizes ranging from 30 to 4000 µm, i.e., a large interval in densities and sizes. Almost all articles handled by industry, in a general way, are within this size range, making evident the great alicability of this correlation. 7. CONCLSIONS There is a strong correlation between article size and the dominating forces that determine the magnitude of the icku velocity. Preliminary data investigating icku velocity as a function of article size indicate the existence of a minimum icku velocity. For larger article sizes, the mass of the article demands a greater fluid velocity for entrainment, and for smaller article sizes, greater fluid velocities are required to overcome article article interactions. The measured icku velocity for the different ie diameters of sand articles indicates a countable effect of the ie diameter. The icku velocity for different solids cannot be easily redicted because this arameter is influenced by many variables. Among these, the characteristics of the material itself, such as article size, density and shae, the coefficient of sliding friction, and the article interaction with other articles are the most imortant variables that affect icku velocity. The ractical significance of the minimum icku velocity is that it can be used as the safe gas velocity for the horizontal conveyance of solids. The usefulness of the concet of minimum icku velocity is that one of the most imortant arameters for the design of neumatic conveying systems, i.e., the minimum transort velocity, can be redicted by erforming relatively simle and inexensive exeriments with the technique develoed, which rovides a new alternative for engineers and designers to redict the minimum transort velocity of solids. 8. ACKNOWLEDGEMENTS The authors areciate the financial suort from CNPq (roc. no /008-7), and CAPES (doctorate scholarshi), without which this research work would be imossible. 9. REFERENCES Cabrejos, Francisco J., Klinzing, G. E., 199, Inciient motion of solid articles in horizontal neumatic conveying, Powder Technology,Vol. 7, Cabrejos, Francisco J., Klinzing, G. E., 1994, Picku and saltation mechanisms of solids articles in horizontal neumatic transort, Powder Technology, Vol.79, Gomes and Mesquita, 006, Análise exerimental de velocidade de catura em sistemas de transorte neumático, IV Congresso Nacional de Engenharia Mecânica, Recife-PE.

10 Coyright 009 by ABCM Hayden, K. S., Park, K, Curtis, J. S., 003, Effect of article characteristics on article icku velocity, Powder Technology, Vol.131, Herbreteau, C., Bouard, R., 000, Exerimental study of arameters which influence the energy minimum in horizontal gas solid conveying, Powder Technology, Vol. 11, Kalman, H., Satran, A., Meir, D., Rabinovich, E., 005, Picku (critical) velocity of articles Powder Technology, Vol. 160, Kalman, H., Rabinovich, E., 007, Picku, critical and wind threshold velocities of articles Powder Technology, Vol. 176, RESPONSIBILITY NOTICE The authors are the only resonsible for the rinted material included in this aer.