YOURPNEUMATIC PROPEL MOVING 42 www.aiche.org/cep/ April 2001 CONVEYING SYSTEM TO HER HIG Herman Purutyan, Thomas G. Troxel, and Francisco Cabrejos, Jenike & Johanson, Inc. BULK GRANULAR SOLIDS and powders around chemical process industries (CPI) plants is always challenging. While fluids can be transported practically anywhere in a plant through a pipe, granular solids are usually moved by mechanical equipment such as belt, screw, aeromechanical, or drag conveyors. These methods impose severe limitations on layout and routing, provide (in most cases) only limited containment of material, and expose the product to direct contact with moving mechanical parts. Pneumatic and hydraulic conveying offers the containment and flexibility of pipeline transport for bulk solids. In principle, pneumatic conveying is simple: disperse a powder or granular solid into a moving fluid stream, send it through a pipe to the desired destination, and then remove the solid from the fluid. Benefits and drawbacks Both pneumatic and hydraulic conveying have been used to transport bulk solids for many years, but pneumatic conveying is far more common and is found in nearly every industry where bulk solids are handled. One of the earliest recorded uses was for unloading wheat from barges to flour mills at the end of 19th century in London (1). Grains, as well as other cargo, such as alumina, cement, and plastic resins are still unloaded using the same basic methods. Other common applications include unloading trucks, railcars, and barges, transferring materials to and from storage vessels, injecting solids into reactors and combustion chambers, and collecting fugitive dust by vacuum. In addition to the primary benefit of flexibility in routing, pneumatic conveying also offers these important advantages: 1. Cleanliness and containment — properly constructed and maintained pneumatic systems can be virtually dust-free. Vacuum systems offer the advantage that leakage is into the pipeline, so that CEP org/cep/ 43 .aiche. Here’s how to narrow down the options to make the appropriate selection for optimal performance.CY IEN FIC EF Pneumatic Conveying For new and existing installations. there are many system and component choices available. CEP April 2001 www. and to transport solids over much friability. vertical lifts have been most forms of contamination. Much larger ever. The range of materials that conveying lines. stringy materials such as chopped ing parameters. hardness. as well as metal particles from grinding operations. A few systems with solid. although this can sometimes be overcome matic conveying is by far the most expensive method of with temperature control or flexible pipelines. pneuis not practical. Moist substances that are wet enough to However. as veyed.000 ft (not simultaneously). gas stream.aiche. inward leakage and can use dry. Pneumatic conveying is also Particle characteristics hampered by the potential for Particle size. particles have been conveyed in straight runs. Anothimportant and practical form of transport with applications er rule of thumb is that. the higher cost Understanding pneumatic conveying systems requires a is justified. On the other hand. ics of solid particles in a gas stream. containment or cleanliness issues. are important variables in the design of a pneumatic longer conveying distances and conveying system. The largest common ones are generally restricted to about 300 tons/h Particle characteristics. rough ones having a wide size distribution. Particle size. Among the higher capacities have been key particle characteristics are built. hardness. and severe wear of equipment and Pneumatic conveying is simple in principle.Pneumatic Conveying even damaged or leaky systems contain all of the product. But. these models produce significantly different results for the Not surprisingly. density. there is no general agreement about which and cohesive properties are key parameters in determining provides the most reliable prediction (2). Pneumatic conveying systems are also limited in overall Material characteristics conveying distance and conveying capacity. concentrate. to prevent mechanical plugging of in almost every part of the CPI. and dynamwell as the low maintenance needed for these setups. pneumatic conveying is an angular. draulic conveying can be used shape. hydraulic conveying has been segregation tendency. in many cases. defining a 44 www. the pipe diameter must be at lar materials of nearly every type can be conveyed: finleast 3–5 times the maximum particle dimension. Recently. For a monoconvey ever larger and heavier particles extend to particles sized material. particularly when conveying materials can be transported is nearly unlimited. it is easy to define the particle size. and contact with moving meused to pneumatically convey ore containing 2–3 in. for most granular materials. cohesive strength. as well as bulk properties of the and 3. partichanical components is minimal. experience is often required in using a higher gas velocity to become entrained in an air stream. One of the primary successfully. as a fibers. and of plastic pellets and copper electrostatic effects. toxicity. For example. if the system is not properly designed affecting pneumatic conveying. sized round and smooth particles are easier to convey than Despite these limitations. larger particles of heavy material require same initial data. Materials with high oil or fat contents can drawbacks is high power consumption. uniformly and operated. Bulk properties that greater distances and has been are important include bulk denused for long-distance coal sity and compressibility. because the alternatives are not practical due to knowledge of the properties of the bulk solid to be conlayout limitations. One of the challenges is to express particle size in a The practical limits of simply increasing the air velocity to way that can be used in a predictive model. such as minimum conveying velocity. Taken on the basis also cause severe buildup in pipelines such that conveying of cost per unit weight per unit distance conveyed. Howof about 1–5 in. In some un2. Since many of whether a material is suitable for pneumatic conveying. meability. them. hyparticle size and distribution. While there are at least a dozen The limitations on what can be conveyed depend more models that include the effect of particle size on the miniupon the physical nature of the material than on its generic mum conveying velocity for a suspension of solids in a classification. and Much work has been done to characterize key conveymetal components. explosiused for long-distance transport bility. resistance to damage. pneumatic conveying has limitations and is stick to the walls of the pipeline usually cannot be handled not suitable for every application. reactivity. Pressure systems prevent cles hundreds of feet to the surface. per(slurry) transportation.org/cep/ April 2001 CEP . flow of compressible gas in a pipeline. distribution. candy. transporting materials. Powders and granucontaining large particles. Low contamination — Sealed systems can prevent derground mining applications. function of particle size. inert gas for conveying to Cohesive or sticky materials are often difficult to conexclude oxygen and moisture. vey pneumatically. ished products as diverse as bathroom tissue. for a material with a wide range of sizes. shape are known to be among attrition or degradation of the the most significant variables bulk solid particles. Particle density affects the minimum conveying velocity and pressure drop required for transport. This can result in flow problems in downstream bins and silos. Lastly. The compressibility and permeability of a bulk solid determine how readily the material will deaerate. such as compressibility. and how gas flowing through a bed or plug of material will affect the bulk solid. the Hardgrove grindability index for coal).particle size to use as a design parameter is more difficult. the exposure risk for workers. In this case. the time required to dissolve the soda ash increased dramatically and limited production. which is a direct function of the hardness of the particles conveyed. can give different results. irregular. Particle shape is another parameter that is difficult to define. affect how bulk solids behave in pneumatic lines. Work has been done to classify materials for comminution purposes (e. Pneumatic conveying was used to transfer the material to both the packaging area and to the reactor for cap- Particle hardness One of the disadvantages of pneumatic conveying is the potential for accelerated line wear. Materials typically become more difficult to handle as the particle size decreases. Fines generation can also affect the flow properties of the material. platelike. Most products are sold with a set of specifications that includes a particle-size range. If the size specification is achieved by screening. most materials created by crushing or grinding have a log-normal distribution. attrition in a pneumatic conveying system can only be accurately assessed by conducting a series of attrition tests that simulate conditions in the lines. by far the most common technique. Attrition can increase the propensity of some materials to cake.g. Overall.org/cep/ . The compressibility and permeability of a bulk solid determine how readily the material will deaerate tive use. Operators found that when they were forced to use unconveyed material in the reactor. ranging from 1 (talc) to 10 (diamond). particle-size data are highly dependent upon the measurement method. First. excessive fines result in a dustier material that can increase the risk of a dust explosion. to our knowledge. Particle friability cannot be defined easily by an index or value derived from tests. but these indices apply to a particular mechanism of comminution and. it is still difficult to capture all of the pertinent particle properties in a general model. particularly in systems operating at high-solids loadings and low conveying velocities. which does not capture the range of sizes. such as free-fall and flow through a silo. excessive fines could be a problem. as well as the conveying velocity. The study of particle properties is a science in itself and one of the ways particle shape has been incorporated into analytical models is by assigning a sphericity value to particles to account for their shape relative to spherical particles. for toxic materials. usually with undesirable results.. as well as other handling steps in the system. attrition of sugar crystals can expose fresh surfaces. Data obtained by sieve analysis. For example. While virtually all of the analytical and empirical models used to predict the behavior of pneumatic conveying systems include particle size and density. which describe the bulk density as a function of consolidation and gas permeability. which are more prone to caking. again making a standard definition of particle size difficult. On the other hand. coarse and fine particles may have different dissolution rates.aiche. Even measurements made by different machines. For example. the attrition that occurred in the conveying line was beneficial and essential in meeting production schedules. using the same method. whereas condensation processes produce Gaussian distributions. Particle size is often expressed as a mean diameter. Attrition may alter product performance. If the product conveyed is to be fed to a reactor. then any attrition that occurs during transfer from screening to packaging may make it difficult or impossible to meet the product’s specification. or fibrous. Fines generation can also impair product quality. Particle friability Attrition of particles in a pneumatic conveying system can affect the material being conveyed in several ways. In some cases. Shapes vary from near-spherical to angular. at a soda ash plant. or. 45 CEP April 2001 www. For example. some of the product was sold and some was used internally to make another product. One common way of classifying particle hardness is the Moh’s scale. are certain to differ from those generated using laser diffraction. this type of classification has never successfully transferred to particle attrition in pneumatic conveying systems. attrition can be both a blessing and a curse. the conveyed product was more difficult to handle and perceived as lower quality by customers who purchased it. For example. The nature of the size distribution may vary depending on the process used to produce the particles. Bulk characteristics Bulk properties. the gas velocity at the end of the line will be about 2. the increase in velocity from one end of the line to the other results in a difference in pressure drop per unit length of more than 2. This illustrates the significance of density changes in the gas as flow progresses from the beginning to the end of the conveying line.org/cep/ April 2001 CEP . as well as the gas temperature. the mass ratio of solids to gas. the gas velocity at the end of the line will be about 1. As Eq. D = pipe dia. in which any leak will be into the line. As solid particles are introduced into a moving stream of gas. segregation that occurs in a handling system. can impact pneumatic conveying. particles may be sliding on the bottom of the pipe. At low velocities. Combining Eq. however. it becomes evident that the mean gas velocity is a function of gas pressure: V = M R T/P A (3) Assuming that the gas mass-flow rate and the flow area are constant. such as in a bin. which separates the material into coarse and fine fractions. L = pipe length. However.. cohesive solids cause other serious problems. or neutralized. a function of Reynolds number and pipe roughness. If the system is designed for a mixture of fines and coarse particles. while at higher ones. as momentum is transferred to the particles to accelerate them to the conveying velocity. where ρ= gas density. In addition to the pressure drop.7)/14. at any two points in the line becomes proportional to the absolute gas pressure: V2/V1 = P1/P2 (4) where P1 and P2 are absolute pressures. The segregation potential of a given bulk solid can be determined by conducting tests (3). in which case the charge must be either dissipated by proper grounding. or erratic flow through equipment upstream of a conveying line. Indirectly. Materials that are toxic or need strict containment may require a vacuum system. Other bulk properties that must be considered during design include explosivity and toxicity. and 46 www. R = gas constant. M = gas mass-flow rate. Since particle size affects the pneumatic conveying characteristics of a material. the pressure drop is also a function of a number of other parameters including: • The amount of solids in the pipeline. rather than out to the environment. in fact. which is known as the solids loading ratio or phase density. Flow stoppages. as given by: ρ = P/RT (1) where f = friction factor. they will be fully suspended in the gas.aiche. In addition to the gas velocity. P = absolute pressure. Static electricity may be a source of ignition for materials prone to explosion. in a line with a 22 psig total pressure drop. a number of investigators have found that. such as newly reacted polyethylene or polypropylene powders. Example — In a line with an 8 psig pressure difference between the feed and end points. Materials that may contain residual hydrocarbons. 1 and 2.5 (5) Similarly. and its density is a function of pressure and temperature. typically.Pneumatic Conveying The direct effect of the cohesiveness of a bulk solid on pneumatic conveying can be buildup in the lines. the pressure drop in the line begins to increase. and A= flow area.5 times the beginning velocity. and T= absolute temperature. the pressure drop of a flowing suspension actually decreases slightly at low concentrations with small-sized particle materials.5 times the velocity at the beginning of the line: V2/V1 = P1/P2 = (8 + 14. The total pressure drop consists of two components: the pressure drop due to gas flow alone. Double-walled pipelines under positive pressure have been used to convey materials such as contaminated soils. the pressure drop in a pipe is approximately proportional to the square of the gas velocity. The relationship between gas velocity and pressure drop in a straight pipe is found by a simple calculation: ΔP = f (L/D) × (ρg U2)/2 (6) Single-phase flow Flow of gas in a pipeline is well understood. feeder or a chute. Even in systems with a modest pressure drop of 8 psi. The conveying gas obeys the ideal gas law. as well as the density and the flow area: V = M/ρA (2) where V= mean gas velocity. conveying only one of them can be a problem. ρ = gas density. given in Moody’s chart and others for turbulent flow. then velocity Two-phase flow While single-phase flow in a pipe is well understood.7 = ~1. Mean gas velocity in a pipeline is a function of mass-flow rate of the gas. may have to be conveyed using nitrogen to limit exposure to oxygen. The challenge is to be able to feed the conveying line uniformly. can be a roadblock to achieving the desired transfer rates. ρg = gas density. and U = gas velocity. changes in the gas velocity also affect the suspension of solids in the gas stream. adding solids into the moving gas stream complicates matters immensely. and that required for transporting the particles. 6 indicates. Empirical correlations have been developed based on tests conducted with a limited number of materials (4). Different correlations do not agree on predicting the minimum conveying velocity. saltation occurs at higher velocities at higher solids loading. entrain them in the moving gas. These forces are a function of gas velocity. what is most apparent is that some of the models predict an increase in the minimum velocity if the particle size is decreased. mm ■ Figure 1. As can be seen. A study done by Peter Wypych at the University of Wollongong in Australia (2) provides useful insight into the state of currently available correlations. In these cases. The range of calculated velocities is much wider here. and blow them downstream. This clearly illustrates that use of these correlations requires a thorough understanding of the basis on which they were developed. the most important velocity for the designer of a pneumatic conveying system is the minimum conveying velocity. the best solution is to find the minimum conveying velocities by conducting tests in a pilot conveying loop and then scaling up the results. Each line represents calculations of a particular investigator. Conveying velocities The moving gas stream applies drag and lift to the particles. One reliable method of determining minimum conveying velocities is to obtain data from an existing system conveying the same material. and frictional interaction with the pipe wall. the minimum calculated values can vary by as much as a factor of two. The velocity at which this occurs is referred to as pickup velocity. shape. Some investigators have suggested using the saltation velocity with a factor of safety. The minimum conveying velocity is calculated as a function of pipe diameter.01 Minimum Velocity. Terms such as pickup velocity. ■ Figure 2. Since scale-up is not without its challenges. the lowest velocity that must exist in a given system for a given material to prevent plugging the line. Perhaps. only the trend is important here.aiche. However. Consider a layer of particles lying at the bottom of a horizontal pipe. m/s 40 30 20 10 0 0 50 100 150 200 Internal Diameter of Horizontal Pipe. while others have developed empirical correlations. mm 50 40 30 20 10 0 0. In addition. there will be enough lift on some particles to pick them up out of the bed. m/s 0. Note that all of the models predict the same general trend that increasing the pipe size requires an increase in conveying velocity. In general. A number of investigators have shown that the minimum pickup velocity is a function of the density of the gas and the solids. as is so for Figure 2. Predicting minimum conveying velocity based on particle size gives conflicting results • The velocity of the solids relative to the gas Relevant particle properties are the size distribution. and minimum conveying velocity are used to describe the correlation of gas velocity to the behavior of solid particles in a line. using the same set of correlations.1 1 10 Particle Diameter. there is a direct relationship between the saltation velocity and solids loading ratio (SLR). however. At a certain velocity. as well as particle and line diameters (4).50 Minimum Velocity. while others predict a decrease. ensure that the test CEP April 2001 www. as well as particle and pipeline diameter. these correlations often predict widely differing velocities for the same set of conditions. saltation velocity. Figure 1 shows the predicted minimum conveying velocities for wheat calculated using nine different correlations. with the gas flow slowly increasing. Individual investigators are not noted. but. density. and still point at virtually no change over a wide range of particle sizes. This is also a function of particle and gas density. The velocity below which entrained solid particles begin falling out of suspension and start settling at the bottom of a horizontal pipe is the saltation velocity. and experience in applying them to real applications.org/cep/ 47 . Figure 2 shows minimum velocity calculations as a function of particle size. This is. this does present a chicken or the egg dilemma when such a system is not available. This requirement is closely coupled with the rate of solids and gas flow. as the solid particles continuously fall out of suspension and get reentrained by the conveying gas. However. the solids Inlet loading ratio is relatively low. ous range from fully suspended to a slow moving bed. solids parFeed Hopper ticles are generally suspended in air. the same amount of solids can be transported in a line using a number of velocity and pressure drop combinations.aiche. the designer must also know the required pressure drop. sugar. U Increasing Solids Flow ■ Figure 3. a further reduction in gas Blower velocity results in an increase in pressure. Under these conditions. which is a plot of pressure per unit length of pipe as a function of conveying gas velocity. the most efficient conveying can be achieved at the velocities that result in the lowest pressure drop. There are a number of references that outline various calculation methods in detail (5. This is best illustrated graphically in a general state diagram. Theoretically. The advantages and disadvantages of such systems are discussed below. as can be seen in the general state diagram for coarse particles. At high gas velocities. and fly ash. Multiple Delivery Points Storage Silo 48 www. and that test conditions in the pilot system cover an adequate range of velocities and solids loadings. lime. there is no distinct boundary that separates the dilutefrom the dense-phase regions. such as perlite. Coarser materials. For some materials. as particles begin to fall out of suspension and interparticle collisions increase. which can cover a wide range of conveying conditions.Pneumatic Conveying Pressure Drop. but including the effect of the conveyed solids is considerably more complicated. A large amount of work has been done to predict the pressure drop analytically. If the gas velocity is slowly decreased. After reaching a minimum. and plastic pellets. or at the pressure minimum points. where the solids loading ratio is typically higher than 15 and the velocity is below the saltation velocity. is that of dense-phase conveying. With other materials.■ Figure 4. and conveying can occur over a continu. It is relatively simple to calculate the pressure drop when there is only gas flow in a line. ΔP/L No Flow Stable DensePhase Unstable Dense-Phase Dilute-Phase Lines of Constant Solids Flow Rate Mean Gas Velocity. Typically. With many materials. Typical positive-pressure system. In fact. flow in this region becomes extremely erratic with severe pressure fluctuations. The most reliable method of determining the pressure drop requirement is to use experimental data derived from test loops with the actual material to be conveyed. the pressure reAirlock quired to convey a constant amount Feeder of solids also drops. the designer must decide whether the system will operate in the low-velocity or high-velocity region. fit into the other category that has a distinct region where conveying is unstable or impossible. This region. Generally. loop and the full-scale system are not too different. General state diagram for solids flow in a pipe. 6). Conveying pressures In addition to the minimum velocities required to convey a specified amount of bulk solids in a given system. very fine powders. followed by scaling up of the data. typiFilter cally below 15. such as cement. with constant solids flow rate as a parameter (Figure 3). This is dilute-phase conveying. very distinct boundaries define regions of stable and unstable conveying. but there is still significant disparity among various methods. fit into this category.org/cep/ April 2001 CEP . it is not always possible to get stable flow at the theoretical pressure minimums. Vacuum systems are ideal for hazardous solids. They can have higher capacities and longer conveying distances than negative pressure systems.and dense-phase. combined. Dust collectors are an example of conveying systems that operate at very low solids loading. operating pressure (positive. making it impractical to achieve useful results without employing a computer calculation procedure.org/cep/ . CEP April 2001 www. and at the other extreme by a completely full pipe where the solids are essentially extruded through the line. and magnitude of operating pressure. these correlations provide more reliable results when applied to fully suspended dilute-phase systems and are subject to the same caveats as the correlations described above for minimum conveying velocity. Positive-pressure systems These are above atmospheric pressure and ideal for a single feed point and multiple delivery points (Figure 4). In all cases. Combined systems often use vacuum for feeding and positive pressure for conveying over long distances. depending upon the relative solids loading and velocity of the system. or closed-loop). Most industrial conveying systems run somewhere in between these two extremes and are ranked broadly as either dilute.Storage Silos Vacuum Receiver Inlet Filter Filter Multiple Feed Points Airlock Feeder Filter In general. Conveying can occur over a wide range of conditions bounded on one end by gas alone with no entrained solids. or high-pressure dense-phase. dilute-phase. use of these correlations requires an iterative calculation procedure that must be performed in steps along the pipeline to account for the change in velocity. Exhauster ■ Figure 5. negative.aiche. Positive-pressure arrangements may be low-pressure. Storage Silos Filter Multiple Feed Points Filter Multiple Delivery Points Types of systems Systems can be configured and classified in a number of ways depending on their function.or dense-phase. where the performance is governed almost entirely by the gas flow. Negative-pressure systems Negative pressure or vacuum systems are ideal when the product must be picked up from a number of different lo49 Exhauster/Blower Storage Silos ■ Figure 6. The most common and often misunderstood categories are dilute. and leakage is into the line. and the separator. their basic functions remain the same. The gas is typically released into the atmosphere. the solids are decelerated and recovered from the gas stream and then stored in a silo or fed into another unit. A challenge for every designer is to combine the different types and models of equipment on the market so that the system operates efficiently over its specified range. In these cases. provisions are needed for makeup gas. while negative-pressure systems can call for feeders with a good seal to minimize leakage of gas into the pipeline. One common example is vacuum unloading of rail cars with transfer to a receiver. The benefit is that they capitalize on the ease of feeding into a vacuum and combine this with the higher capacity and longer conveying distance when using positive pressure. since the pressure in the conveying line is lower than atmospheric. Through-the-fan systems When the conveyed solid or the solids loading is relatively light. a closed loop is used. The required conveying velocity is often higher in a vacuum system than in a positive one. but with less equipment (Figure 8). conveying gas. Combined systems can be either pull/push or push/pull. Closed-loop systems When the conveying gas is other than air. Conveyor Filter Radial-Blade Open-Wheel Fan ■ Figure 8. negative-pressure systems are used as dust collectors for fugitive dusts. there may be good reason for recirculating the gas (Figure 7). A typical one is a pull/push system with a negative-pressure front end. safety equipment.aiche. argon. the advantages of each can be exploited (Figure 6). The pipeline consists of straight sections. Controls. it is possible to achieve highsolids loading and low velocity conveying over short distances (less than 200 ft). carbon dioxide. lowsolids loading. Also. The solids feeder introduces the solid particles at a controlled rate into the pipeline where they are mixed with the Heat Exchanger ■ Figure 7. In the separator. Reliable 50 www. If longer conveying distances are required. Pneumatic conveying system components A pneumatic conveying system consists of four basic components: the gas mover. Through-the-fan operation is simple. While their placement may vary depending on whether the system is in vacuum or pressure. and instrumentation are also required. An acceleration zone is required right after the feed point to speed up the solids to the steady transport velocity in the pipeline. the material is transported to the destination using positive pressure. the line can be extended by adding more fans. However. The gas mover provides the proper flow rate of gas required for the transport at the right velocity and pressure. From the receiver. such as nitrogen.Pneumatic Conveying cations and conveyed to a single destination (Figure 5). followed by a positive-pressure loop. Vacuum systems are usually limited to shorter distances than positive ones and typically operate with a dilute. all of the conveyed material travels through the fan. heat buildup requires heat exchangers to prevent overheating. the pipeline. since the gas will heat up as it is compressed. these schemes are better suited for toxic or hazardous materials.org/cep/ April 2001 CEP . Also. For the same reason. Filter Filter Makeup Gas Blower Storage Silo Receiver Airlock Feeder Combined systems By combining both types in the same setup. both horizontal and vertical. Closed-loop systems are employed when the gas must be recirculated. Since inevitably some leakage will occur. connected together with bends. Positive-pressure systems require devices to feed material from atmospheric pressure into a pressurized pipeline. requiring less equipment than a blower-based setup. because the gas density is lower in a vacuum system. Here. These arrangements consist of two sections. these systems offer the benefits of a combined system. or steam. the solids feeder. because it provides an economical source of gas flow that meets the pressure (or vacuum) and flow requirements of the largest category of systems. and eductors. of water (0. while others only provide a pressure seal. It is apparent from Figure 9 that a system driven by a fan may experience a significant change in gas-flow rate for small changes in pressure. while many pneumatic conveying suppliers often overlook the need to design or specify a bin or feeder to provide reliable material flow. double dump-valves. but as a pressure seal only. The ability of compressed gas and lobe-type machines to produce a nearly constant flow of gas over a wide range of pressures offers stability and allows these systems to recover from upsets that may cause momentary rises in pressure. This is important. presure < 2 psi). lowpressure applications (flow > 1. and require careful design because of their inherent operating characteristics. specially designed rotary valves exist that seal up to 100 psi. as well as meter solids into a line. Fans are generally limited to high-volume. Most compressed gas systems use a pressure receiver to Solids feeders and pressure seals For proper operation of a pneumatic conveying system. and their speed determines the solids throughput. sawdust. blowers. the gas flow through the system will change according to the characteristics of the supply system. Unfortunately. flow from the bin/silo through the solids feeder and into the pipeline is an absolute necessity as a starting point. and compressed gas stack up. are truly feeders. The most common fans used in conveying applications are radial blade machines. and other light nonabrasives. When handling cohesive solids. How fans. Some of these devices control the rate of solids flow into the line and. which have maximum pressure ratings of 20–40 in.7–2 psig).aiche. Fans are used for low-pressure systems. Gas movers By far. whereas they really originate in the upstream equipment (7). the flow rate is a function of the speed of the machine and the operating pressure. While they are primarily used for systems at less than 15 psig. Flow rate control can be accomplished by using a feedback flow-control device. In the case of fans and rotary blowers. recycled foam. and using a rotary valve not as a feeder. Since most conveying systems experience a range of operating pressures between the extremes of an empty line and a fully loaded network. A comparison of the typical relationship between flow rate and operating pressure for fans. Solids enter the rotary valve on the side of the 51 CEP April 2001 www. Rotary valves can be used to provide a seal. Rotary valves used as feeders can also cause flow problems by nonuniformly drawing material across the outlet of the hopper. they are operated as through-the-fan systems. it also restricts the opening of the hopper feeding the line to the size of the valve. Frequently. this may lead to flow stoppages due to arching and ratholing in the hopper above. but do not meter solids. and compressed gas systems is shown in Figure 9. since control of the gas flow in a pneumatic conveying system is critical for stable operation.% of Rated Discharge Pressure 100 80 60 40 20 0 40 50 60 70 80 90 100 110 % of Rated Flow Discharging to Atmosphere Radial Blade Fan Roots-Type Blower Compressed Gas/Nozzle ■ Figure 9.000 cfm. To use compressed gas for conveying requires some means to control both the flow rate and pressure. Devices used for this purpose include rotary valves. As feeders. A key feature is that the Roots blower delivers a nearly constant volume over its operating pressure range. or a static flowcontrol orifice or choked-flow nozzle. fans offer a simple and effective way of transporting material. and compressors (or plant compressed air or process gas) for higher pressures. store a volume of compressed gas and allow the compressor to operate intermittently.org/cep/ . For applications handling very light material. such as chopped textile fiber. In many cases. most bin/silo and feeder suppliers do not consider the effects of downstream equipment. the most common device for moving gas in a pneumatic conveying system is the Roots-type rotary lobe blower. blowers. namely those that operate at less than 15 psig for pressure systems and –6 psig for vacuum systems. the solids fed into the line must be controlled. but it is usually not cost-effective for pressures less than 20 psig. unless a source of “free” process gas is available that is suitable for the conveying requirements. they are placed at the outlet of hoppers of silos and bins. Feeding solids into a positive-pressure system requires a means of sealing against the pressure in the pipeline. hence. This blower is prevalent. This can be avoided by properly designing the hopper with an appropriate feeder that can provide uniform flow. Compressed gas can be used for any positive-pressure duty. While using a rotary valve both as a feeder and a pressure seal reduces the amount of equipment needed. specially designed screws. problems of nonuniform feed into the pneumatic conveying lines are thought to cause difficulties with pneumatic conveying systems. while one supplied with compressed gas may operate at an essentially constant flow rate over a wide range of pressures. as tween 2 and 4. rotary being conveyed. abrasives will quickly wear through an rate limitation.. ing. the type of bend makes relatively little lems can destroy mass flow in the hopper. continuous feed. the type of elbow is not a critical decision and should pressure sealing and venting at the feed point are critical. the bulk solid. ing velocity. Typically. as well as the valve operating at a 10 psi differimpact angle. the suring that the valve is properly vented and that the vent is use of hard. Wear can be reduced by the hopper outlet. vented and another batch of solids is transferred in. The answer. In addition. fully suspended conveying systems. wear resistant elbows may provide reasonable not blocked. In many cases very Perhaps the single most debated question regarding the fragile materials can only be successfully conveyed in low pipeline is what type of elbow is best. and can retard flow. A cerlow velocity. Fine powders often flow out of hoppers at rates much The key factors to consider in selecting pipe bends are: lower than do coarse granular materials. internal convergence of some rotary amount of debate is often out of proportion to the signifivalves can reduce the active hopper opening. where expensive wear-resistant components are not which is then sealed and pressurized. This amount a maximum for impact angles of gas going into a conical hopper around 20 deg. This flow can pipeline bends. both increase the pressure drop. Product attrition (left photo) takes place mainly at bows. smooth radius bends produce 52 www. ative hardness of the particles For example. For mildly abrasive materials. the pressure drop across the valve is not too wear life. of velocity systems. most wear is found near the pneumatic conveying line into the end of the conveying lines. When designing • Abrasive wear vessels that handle fine powders. then flow at the bends. handling nonabrasives that do not degrade during conveyIn positive-pressure systems handling fine materials. These probIn many systems. As with wear. or by using Pipeline components a low-velocity dense-phase scheme. well as the clearance between the Wear is also a function of relrotor tips and the valve body. This calculation is a strong Abrasive wear is by far the most critical element in function of the permeability and compressibility of the selecting an elbow. Product degradation also occurs primarily at bends in the While this results in a batch operation. particle attrition. probably be made based on cost. results in a gas the entrance of long-radius elvelocity of approximately 2 ft/s at ■ Figure 10. dia. Since virtually all of the wear occurs powder (8). independent of how fast a feeder is run.aiche. For systems per itself is properly designed. Pipeline wear is a strong function of the conveywill occur. blow tanks or transporters sons for using a low-velocity dense-phase system. using short-radius elbows. even these types high for the type and size of valve used. crease velocity. a 12–in. However. This can be done by minimizing and controlling the velocity in dilute. and perhaps more imporLeakage through a rotary valve can be reduced by entantly.Pneumatic Conveying outlet where the empty pockets are first exposed to the course. Abrasive wear is one of the primary reaIn high-pressure systems. hopper. Since the velocities in a line are typically This problem is exacerbated by gas leakage up from the higher near the discharge. the problem can be reduced to the point tain amount of solids is transferred into the transporter. wear in ential pressure could have as much steel and aluminum elbows is at as a 100 cfm leakage. With are often used to introduce the solids into the line.org/cep/ April 2001 CEP . but these options the particles trying to flow down. then the pressure is sive damage. and that the valve of elbows can be worn out quickly. decreasing the velocity tanks and alternating between them can provide near is the most effective way to minimize attrition of particles. and the size of the valve. The entire contents needed and standard bends can be used without excesof the blow tank are fed into the line. which occur at with a 12 in. using two blow pipeline (Figure 10). In general. If the hopper opening is too small. again resultcance of the difference between the various types of bends. elbow. or impose a significant body force on blind tees. ing in nonuniform discharge from the hopper. is properly maintained such that a tight fit is kept between The most effective means of reducing wear is to dethe rotor tips and the valve body. but for highly abrasive solids. even if the hopdifference in the performance and operation. depends upon the application. it is essential that the • Product degradation maximum discharge rate from an unrestricted outlet be • Product buildup calculated to ensure that the opening is large enough to • Pressure loss feed the downstream process. The amount of leakage Many investigators have shown through a rotary valve is a function wear to be proportional to veof the total pressure drop across the locity raised to a power bevalve. The efficiency of various types of separators for various size particles is given in the table below. The rough surface can be achieved by sand blasting or shot peening. a problem is formation of thin strips. When handling plastic pellets. not all are suitable for dense-phase conveying. provides diminishing returns. it is best to limit the number of bends and avoid placing them close together. separation actually occurs by a combination of inertia and filtration. depends upon the system layout. When material is delivered to several receiving bins. Answers to the following questions could be used in determining which type of conveying system is appropriate: • Can the material be conveyed pneumatically? While most materials can be conveyed in a dilute-phase system. % Inertial collector Medium-efficiency cyclone High-efficiency cyclone Shaker-type fabric filter Reverse-jet fabric filter 50 micron 95 94 98 >99 100 5 micron 16 27 42 >99 >99 1 micron 3 8 13 99 99 Disengagers. Since many of the materials conveyed encompass a wide range of particle sizes. hot conveying air. rather than slide. The selection of a gas-solids separator should be based on the material characteristics degree of separation required. Selecting a pneumatic conveying system Selecting the best system for your application depends on the process requirements and the characteristics of the material to be conveyed. often referred to as stringers or snakeskins. • What are the layout restrictions? Although pneumatic conveying allows more flexibility in routing than do mechanical conveyors. then the pneumatic conveying line will be susceptible to plugging due to loss of solids velocity. Most investigators have found that increasing the bend radius much beyond 4–6 pipe dias. to pulse-jet cleaned fabric filters. Also. of course. Certain types of buildup also can occur primarily at elbows (Figure 11). This. In most circumstances. Elbows are a place for product buildup. the contribution of each bend will be much more significant that in a long pipeline with only a few bends (say. long radius bends. periodic surface treatment is usually necessary to ensure roughness. changing the bends from blind tees to long radius sweeps may not make a significant difference in the operating conditions. it is not uncommon to use an inertial or cyclone separator at each delivery point and direct all of the conveying gas to a single fabric filter. CEP April 2001 www. Analysis based on the permeability and compressibility of the solids can be used as a first-pass determination of whether the material can be conveyed in dense phase. a 200 ft line with 9 bends). The efficiency of different types of separators varies with particle size. which periodically break loose. the concentration of solids and cost.aiche. If the layout requires a large number of turns.200 ft line with 4 bends). ambient conditions. In a short system with a large number of bends (for example. filter receivers Separating the solids from the gas can be accomplished in a number of ways ranging from inertial separation. but it does allow for cross-contamination between the bins. the larger particles separate as the gas stream enters the receiving vessel and the smaller ones at the filter surface. because the added length of pipe necessary to make the bend offsets any benefit of the more gradual turn. Since these surfaces are likely to become smooth again over time. lower attrition than blind tees or mitered bends. This is due to localized melting. Efficiency of separator. or by using specially made grooved pipe components. This is more economical than providing a fabric filter at each delivery point. it has generally been found that the difference in pressure loss between a long-radius bend and a blind tee is relatively small when compared to the total pressure drop in a system. can reduce the amount of heat generated.■ Figure 11.org/cep/ 53 . The sources of heat can be residual heat from the extrusion process. where the solids settle by gravity. In addition to determining the required conveying velocities and pressures. Pressure loss in pipeline bends is generally greater in blind tees and short radius bends than in smooth. a 1. and closely placed elbows. In the latter case. it is nearly impossible to discharge gas from a conveying system without using a high-efficiency fabric filter to meet environmental regulations. environmental regulations. Usually. These cause difficulties in downstream handling equipment. conducting lab-scale tests can answer questions such as whether particle attrition and line buildup will be problems. The strips build up as pellets deposit thin molten layers of plastic on the pipes (primarily at the elbows). Using pipe with a rough interior surface that causes the particles to tumble. or particle friction. However. such as nitrogen. • Is the material abrasive? Wear.aiche. Attendees will gain a better understanding of common problems. then a low-velocity system may be appropriate. Both pressure and vacuum systems can unload material received in trucks. because of the limited pressure differential available. similar to attrition. if your material is greater than a 5 on the Moh’s hardness scale. 2001 Houston # 032 Flow of Solids in Bins.org/cep/ April 2001 CEP . but the conveying distance becomes much shorter and the line size much larger at high transfer rates. John W. use a low-velocity system. Carson. To transfer materials over longer distances. You will learn how individual components join together to form a system that operates effectively.Pneumatic Conveying Also. project. barges. hence. #033 Pneumatic Conveying of Bulk Solids For those experiencing problems with existing pneumatic conveying systems or plan to design or specify a new system. Also. since some gas will inevitably be lost. as well as any plant operations personnel who are responsible for solving and preventing flow problems or purchasing solids handling equipment. there is no upper limit on the transfer rate for a vacuum conveying system. then an inert gas. and research engineers. rates of up to several hundred tons per hour can be achieved with positive-pressure systems. which can reduce line wear. evaluate your in-plant systems and determine their effectiveness. However. Craig. then it may be necessary to run pilot tests to determine the level of attrition expected. because it occurs slowly and it is often not practical to accumulate enough run time in a lab to determine wear life. therefore. understand system layout based on proper operation. If an inerting environment is needed. reducing the number of elbows will reduce system pressure. as well as particle attrition. • What is the maximum transfer rate required? The maximum vacuum in most conveying lines is 5–7 psig. pull/push. or push/pull system can be used. Expert you’ll hear from: Eric Maynard. it is limited to relatively free flowing materials that can be easily fed into the conveying line. if the material is abrasive. a heat exchanger is generally needed to prevent the buildup of heat in the line. Intended for design. a combination. In such systems it is important to ensure that there is a supply of makeup gas to prevent a drop in pressure. as well as transporting across only short distances. since any leaks that occur will be into the line. • How many feed and discharge points are required? As mentioned above. as a rule of thumb. is a strong function of velocity. then a vacuum system should be considered. and Feeders For those who want to keep all types of bulk solids flowing smoothly and reliably throughout the plant — with minimum downtime and maximum quality control. then a pressure system may be advantageous. Wear is often difficult to assess in pilot tests. Therefore. Vacuum systems are limited in general to less than 300 ft. a closed-loop system can be used. it is likely that abrasive wear will be significant in fully suspended dilute-phase systems. 54 www. a single silo feeding several receiving bins. Because of the availability of higher pressure drops. Longer distances are possible with combined systems. As a general rule. Two Related AIChE Courses April 23–27. If the process requires the product to be delivered to multiple locations. a vacuum system may be appropriate when feeding from multiple silos or bins to a day bin or a receiving hopper. Hoppers. Chutes. or ships. may be used as the conveying gas. since the gas mover is located at the discharge end. be able to identify problems in their plants — and apply various solutions. and David A. and learn how to choose system components. • Is a conveying gas other than air required? If the material reacts with oxygen. velocities. For more information: www. vacuum systems have the advantage of accommodating multiple feed points. since attrition is a strong function of velocity. Vacuum has the advantage of requiring little or no conveying components built into the transportation devices. since the air mover is located upstream of the solids feed point. To reduce the cost. but also the conveying distance. • Is the material friable? Is particle attrition a concern? If so. Experts you’ll hear from: Author Herman Purutyan. rail cars. Tom Baxter. • Is the material hazardous? If exposure to the material or release to the environment is a concern. then a pressure system is preferred to keep oxygen from entering the line. for example.aiche. whereas pressures of up to an order of magnitude higher than this can be found in positive-pressure systems. If you or your supplier do not have experience conveying this material. James Prescott. positive-pressure systems can provide higher conveying rates than do vacuum arrangements. In contrast.org/education (800) 242-4363 • What is the required conveying distance? The range of operating pressures for vacuum systems not only limits the conveying rate. Eric Maynard. K. Moody Award from ASME for the best paper in the Fluids Engineering Division. such as AIChE. is more important than ever. “The Ins and Outs of Pneumatic Conveying. If the material is sensitive to temperature. and successful operation will require all parts of the process to work efficiently.. “Fine Powder Flow Phenomena in Bins.. pp.net). H. flow problems may develop in the solids handling equipment downstream of the pneumatic conveying system. 94 (4). Web: www.. Boca Raton. and M. Reed. as well as at in-house courses to individual companies. go to the ProcessCity Discussion Room for CEP articles at www.” Bulk 2000. 1994). Prescott. 11–13. as well as to individual companies. 14 (3). and fluidization. 541–550 (July/Sept. Klinzing.” 2nd ed. and has undertaken more than 70 projects.• Is the material hygroscopic? Hygroscopic materials exposed to humid air often become more difficult to handle. San Luis Obispo. A. CEP April 2001 www. 4. Understanding the fundamental operating principles of pneumatic conveying. “Advances in the Design of Pneumatic Conveying Systems: A United Kingdom Perspective. Troxel has been involved in many aspects of the firm’s consulting and research activities on a wide range of projects. et al. FL (April 1997). he has designed reliable handling systems for a wide range of materials for the food. To sum up Processes that work as intended from the day of startup deliver the most economical value to the user.com). 2. prior to joining Jenike & Johanson.org/cep/ 55 . Klinzing. asking the right questions at the HERMAN PURUTYAN is vice president for Jenike & Johanson. S. Fax: (+56) 32 690 596. Since joining Jenike & Johanson in 1991. His doctorate was awarded for experimental research in gas/solids suspension and pipeline flow. When conveying materials that are sensitive to temperature. Adopting a systems view. J. blending. 3. the amount of heat generated can cause line buildup. extended production interruptions can mean not only a brief loss in revenue. and storage bins for a wide variety of applications. the pneumatic conveying system is only a part of a process. Fax: (978) 392-9980.com/cep. When upgrading systems. rather than troubleshooting in the field.. Pilot conveying tests can provide valuable insight into potential problems when other operating experience is not available. early design stages. He is also a part-time professor at the Universidad Santa María.” paper presented at AAPS-PDA 2000. W. mainly in the mining and materials handling industries. Tests can be run to determine how much moisture your material is likely to absorb during transit and while in storage. 11 (1) (Mar. Eng.. Perhaps. Chile. He has been a major force behind the firm’s expansion of services in the areas of mechanical design engineering and supply of custom built equipment. • Is the material temperature–sensitive? Heat is generated as the conveying gas is compressed... Bradley.” Bulk Solids Handling. T. being able to produce the target capacity and establishing a market position can mean the long-term viability of a business. 27–39 (Apr. CA ((805) 541-0901. with a BS in engineering science. careful planning upfront. F. Fax: (805) 541-4680. He has published numerous articles on the field of bulk solids handling. BINSERT‚ tumble blenders. “Solve Solids Handling Problems by Retrofitting. But. The latter includes mass-flow screw feeders.. and J. Hoppers and Processing Vessels. and holds MS and PhD degrees from the University of Pittsburgh. “Minimum Conveying Velocity in Horizontal Pneumatic Transport and the Pickup and Saltation Mechanisms of Solids Particles. THOMAS G. R. but also an opportunity for the competition to make inroads.processcity. Carson. Progress. W.. Perry.com). portable antisegregation bins for pharmaceuticals. A. pp. pneumatic conveying. 1998).com. and G. “Bench-Scale Segregation Tests as a Predictor of Blend Sampling Error. R. After graduating from college in 1981. MA. Reliable Flow of Particulate Solids III. Purutyan received his bachelor’s and master’s of science in mechanical engineering from Worcester Polytechnic Institute in Worcester. P... E. Wypych. Chapman & Hall. pharmaceutical. et al. He lectures frequently on the subject at AIChE’s continuing education series. Norway (Aug... modeling. Cabrejos. He is the holder of two patents. < Discuss This Article! > To join an online discussion about this article with the author and other readers. 8. then dry air may be needed. Royal.jenike. Cabrejos studied mechanical engineering at the Universidad Técnica Federico Santa María. MA ((978) 392-0300. New York pp. Purutyan.” Chem. he went to work at General Dynamics Co. He was named the Outstanding Senior in the Engineering Science program. Green. and his MBA from Babson College in Wellesley. London. including flow properties. 7. 1991). MA. McGraw-Hill. A. 5-48 (1984).” Proc. FRANCISCO CABREJOS is a consulting engineer for Jenike & Johanson Chile in Viña del Mar. E-mail: jenikechile@entelchile. He has been consulting with the firm since 1995. et al. Porsgrunn. Buildup in the lines may become a problem. If the material is hygroscopic.aiche. 1999). must be carefully considered. Chile ((+56) 32 690 59.. E. Troxel is a graduate of California Polytechnic State University (Cal Poly) in San Luis Obispo. Literature Cited 1. For new installations.. E-mail: tgtroxel@slo. and making decisions based on process requirements and the characteristics of the material will increase the potential for success. but also its interaction with the solids handling equipment upstream CEP and downstream. as well as downstream handling problems.” 6th ed. J. testing. H.. Inc. and lectures frequently on the subject both through professional organizations. Westford. 5-46. E-mail: hpurutyan@jenike. then it may be necessary to include a heat exchanger in the line. understanding not only the operation of the pneumatic conveying system. and D. He has published numerous articles and papers in the field of bulk solids handling. and chemical industries. more importantly.” Bulk Solids Handling. He helped to open the firm’s San Luis Obispo facility in 1982. He received the 1994 Lewis F.jenike. “Perry’s Chemical Engineers’ Handbook. “Pneumatic Conveying of Solids: A Theoretical and Practical Approach (Powder Technology Series). TROXEL is vice president for Jenike & Johanson. and is author of several technical papers in bulk solids handling and pneumatic conveying. Therefore. 5. 6. G. Inc. (1991).