Conveying Methods: Features and Limitations of Installation Options - Report Example

Feasibility Report: Conveying Methods: Features and Limitations of Installation Options [Client’s Name] [School/University/Affiliation] Abstract Feasibility Report: Conveying Methods: Features and Limitations of Installation Options A pneumatic conveyor is the equipment that moves solids typically suspended in or forced by streams of gas through horizontal and/ or vertical pipes (Zenz & Othmer, 1960). Pneumatic conveyors, once successfully installed, are highly useful in transporting solid materials to and from different points and are relatively cheaper to maintain and operate (mechanical and hydraulic conveyor systems). Some other reasons why pneumatic conveyor systems are sought instead of mechanical and hydraulic conveyor systems are the geometric flexibilities involved in such kind of conveyor system and the simplicity involved in designing circuits that have several pick-up and unloading points (Zhang & Wang, 2006). But the installation procedure of a pneumatic conveyor system carries significant challenges. The flow of materials in an extensive pneumatic conveyor system is complex. The performance of a pneumatic system is typically subject to the physical constraints like friction, altitude of the pipeline, the materials used in the pipes, pressure differences, power requirements, and the likes. Moreover, design complexities like the number of valves involved, the choice of solid/air separator, the type of feed system, the maximum attainable surface velocity of the conveying gas, and the likes has to vary from one project to another that it is considerably difficult to perform the design this type of system. Any slight variations of these elements can significantly alter the performance of the conveyor system (Yasuharo et. al., 1999). Designing the pneumatic conveyor system requires skills in both art and science (Zenz & Othmer, 1960). This means that a series of tests using different equipment set up and material specifications must be done in order to arrive at the best approximate of the dependent variables. The keys to achieving an optimal performance of pneumatic conveyor systems are (a) thorough understanding of the system requirements, (b) application of the capacity of the system to the system requirement, and (c) allowances for system upgrades in the general design of the system (Kimbel, 1998). However, the system requirements alone are daunting and most of the time, the capacity of the conveyor system does not match with the system requirements, and if they do, there are a lot of physical constraints that must be overcome in order to successfully install and operate the pneumatic conveyor system. Description of the Problem The problem involves the design of a pneumatic conveyor system that would carry solid materials at three receiving silos at 125 m, 150m, and 175 meters from the finishing plant. Each silo is required to receive 10 tons of materials within an 8-hour period. Before the first diverter valve, the pipeline will go through a vertical rise of 8m. The pipeline will also have to go through five 90-degree bends strategically located to ensure that full re-acceleration of materials is achieved in the straight sections of the pipeline following the bend. There are two methods considered for this pneumatic conveyor system – batch and continuous methods. Each method has its own advantage and disadvantages. This report evaluates the most appropriate method to use for design under consideration. There are four elements that must be evaluated for each method – air mover requirements, pipeline design, feeder type and specifications, and power requirements. Recommendations will be made based on a comparative assessments of these elements for all the methods under consideration. Outline of Elements for Evaluation Air mover requirements One of the most important considerations in pneumatic conveyor systems is the requirements for the air mover. Once the requirement for pneumatic conveyance is outlined and understood, the layout for the rest of the system can be generated. As a general rule, pneumatic conveying can be used for particles up to 2 inches in diameter of typical density and in bulk densities with a range of 1lb/ft to 200 lb/ft which qualifies the materials under consideration for this design (flour). There are three main categories for pneumatic conveying: dilute phase, dense phase, and air conveying. In dilute phase conveying, air-suspended materials are pushed or pulled from one location to another by controlling the air stream velocity in the pipes (Agarwal, 2005). The main characteristics of dilute phase conveying are high air speed, low pressure, and low product to air ratio as it is a continuous process. There are three ways to design a dilute phase conveying system. These are by positive pressure system, negative pressure system, and a combination of positive and negative system. Positive pressure system operates above atmospheric pressure. This system is aptly used for bulk materials from single or multiple sources to single or multiple locations on medium distances and with greater capacity. Convey distance is up to 600 feet or longer. Negative pressure system operates on a vacuum. The main advantage of a negative pressure system is its superior leak containment where no materials make contact with moving parts and nothing can escape into the atmosphere. Convey distance is lower at 300 feet because of the distance sensitivity of vacuum. Dense phase conveying on the other hand is used in batch process because of its low air velocity, high pressure, and high product to air ratio which allows pulses of air to move a slug of material from one location to the other (Zenz & Othmer, 1960). Dense phase is typically chosen to transport fragile materials or materials that are sensitive to abrasion. Because of the low speed of conveyance, material degradation is significantly reduced as well as the air consumption and the possible abrasions on the pipeline, bends, and contact surfaces of the diverter. Pipeline Design The main considerations involved in designing the pipeline include the total length of the pipeline, the velocity of the solids along the pipeline, the velocity of the solid along the radius of the bends on the pipeline, the solid to air ratio, gas density and pressure loss (or pressure drop), the relationship between the air and pressure along the pipeline, the air-volume and velocity relationship, pressure and volume relationships in the pipeline for changes in altitude, material velocity, energy balance over the length of the pipe, and minimum conveying velocities in the pipeline. Feeder Type and Specifications Feeder types depend on the pressure of the system. For low pressure systems, feeders are usually powered by centrifugal fans. On the other hand, high pressure systems use high displacement blowers. The type of feeders used (as well as the geometry of the tube and the velocity of the air) determines the grain flow (and grain pattern) in the pipe. The type of feeder can help control the total volume of suspended air mass, the volume of grains that settles at the bottom of the pipe, and hence the formation of blockages in the bends. Optimal capacity of the conveyor is achieved when the following energy losses can be eliminated with the aid of the type of feeder: losses due to air friction at intake entrance and in the main pipe, losses on wall friction produced by interaction of the grain with the walls, losses due to the energy requirement in accelerating the grain, and air friction losses on separation devices such as valves. Power Requirements The power requirements of the system depend largely on the first three factors considered. Subsequent calculation for the power requirements can be obtained once a specific pneumatic conveyance system is chosen. Methodology, Results and Analysis A 40 set test experiment was done to determine the mass flow rate of air, mass flow rate of solid, and the change in pressure from the rotary air lock to the destination valves were considered. From there, the values of pressure drop rate, air velocity, frictional force of air, frictional force of the solid particles, density of the mixture, drag coefficient, pressure of air in the bends, power, and pick up velocity are mathematically obtained. A simplistic design approach for the system appears below. The pipeline layout is the most likely layout of the system that will be followed (with 5 bends). The first and second bends in the pipeline are the 8-meter change in elevation before the first valve. The air is separated from the solid (flour) particles through an air vent filter. The primary consideration in creating the pneumatic design and in the selection of the appropriate features is the type of material or substance that the pneumatic conveyor system needs to transport. In this case, the material that needs to be transported is flour. It is apparent that for this type of materials to be transported that the positive pressure dilute phase conveying system will be used. The choice for the pneumatic system is obvious; the total distance that the material must be transported is within the range for both the dilute phase and the dense phase pneumatic systems. The requirement left to assess is the optimal capacities of both phases to transport the required material which is the flour. However, dense phase pneumatic system will not be appropriate for the design of the pneumatic conveyor system for two reasons: (a) the materials that will be transported is neither fragile nor sensitive to abrasion and (b) the transport rate requirement is 10 tonnes per 8-hour for each receiving silo which means that a continuous process must be selected since dense phase pneumatic system transport materials at low speeds. Comparison between the results of the experimental flow rates of the air mass and of the solid mass and the relationship between air speed and mass are shown in the graphs below. The graphs show that the air mass flow rate remained almost constant for the rest of the experiment whereas the mass flow rate of the solid changed relative to the pickup velocity. It is easy to see that the solid mass flow rate is inversely proportional to the pickup velocity which means that heavier particles require higher air velocities inside the pipe system. The average pickup speed in the pipe system is 7.71m/s whereas the range of speed (max speed – min speed) is 10.89. Hence, the design requires a blower that can have a maximum blowing speed of 20m/s, the excess velocity is to account for the loss of speed in the pipeline due to drag and the particle’s interaction with the internal walls of the pipes, and minimum air speed of 10 m/s. The graph below shows the models of friction considered in the design. The frictional force exerted by the air alone is smaller compared to the frictional force exerted by the solid particles on the inner surface of the pipeline, which is already predicted theoretically. These frictional forces dissipate the initial energy of the air pressure from the rotary point to the each of the exit valves; the rate of dissipation of the energy is dependent on the distance of the pipe from the air pump. This graph below shows the disparity between the experimental value of the pressure drop with its theoretical (or expected value). The theoretical values of the drop in pressure for a given solid and air masses shows high pressure fluctuations when the solid mass involved is relatively heavy. This conforms with common sense as bigger grains require stronger energies to transport, hence energy dissipates at a higher rate when the grains are significantly larger. However, the same theory could not be said about the experiemental observations on the pressure drop from one location to the other. Although the experimental data shows fluctuation with respect to the mass flow rate of the flour grains, it is apparent that theory and experiment do not agree on this term. This graph shows the ratio between the theoretical value for pressure change and the experimental values obtained. The peaks in the graph shows the instances (of the 40 experiments) that theoretical and experimental values (almost) match. The troughs on the other hand shows the instances where theory and experiment do not agree or do not correlate at all. References Agarwal, A. (2005). Theory and Design of Dilute Phase Pneumatic Conveying Systems. Power Handling and Processing. 17(1). Kimbel, K. (April 1998). Troublefree Pneumatic Conveying. Chemical Engineering Magazine. Van Belle, J, . Saint Germain, B., Verstraete, P., et.al. (2009). A Holonic Chain Conveyor Control System: An Application. Lecture Notes in Computer Science. 5696; 234-243 Yasuharu, H., Hiroshi, T., Tomohiro, A., et. al. (1999). Continuous Belt Conveyor Systems for Tunnels. Hitachi Zosen Technical Review. 60(2); 99-103. Zenz , F. & Othmer, D. (1960). Fluidized and Fluid Particle Systems. Reinhold Publishing Company. Zhang, Y. & Wang, C. (2006). Particle Attrition Due to Rotary Valve Feeder in a Pneumatic Conveying System: Electrostatics and Mechanical Characteristics. The Canadian Journal of Chemical Engineering. 84(6). 663-689. Read More

Dense phase conveying on the other hand is used in batch process because of its low air velocity, high pressure, and high product to air ratio which allows pulses of air to move a slug of material from one location to the other (Zenz & Othmer, 1960). Dense phase is typically chosen to transport fragile materials or materials that are sensitive to abrasion. Because of the low speed of conveyance, material degradation is significantly reduced as well as the air consumption and the possible abrasions on the pipeline, bends, and contact surfaces of the diverter.

Pipeline Design The main considerations involved in designing the pipeline include the total length of the pipeline, the velocity of the solids along the pipeline, the velocity of the solid along the radius of the bends on the pipeline, the solid to air ratio, gas density and pressure loss (or pressure drop), the relationship between the air and pressure along the pipeline, the air-volume and velocity relationship, pressure and volume relationships in the pipeline for changes in altitude, material velocity, energy balance over the length of the pipe, and minimum conveying velocities in the pipeline.

Feeder Type and Specifications Feeder types depend on the pressure of the system. For low pressure systems, feeders are usually powered by centrifugal fans. On the other hand, high pressure systems use high displacement blowers. The type of feeders used (as well as the geometry of the tube and the velocity of the air) determines the grain flow (and grain pattern) in the pipe. The type of feeder can help control the total volume of suspended air mass, the volume of grains that settles at the bottom of the pipe, and hence the formation of blockages in the bends.

Optimal capacity of the conveyor is achieved when the following energy losses can be eliminated with the aid of the type of feeder: losses due to air friction at intake entrance and in the main pipe, losses on wall friction produced by interaction of the grain with the walls, losses due to the energy requirement in accelerating the grain, and air friction losses on separation devices such as valves. Power Requirements The power requirements of the system depend largely on the first three factors considered.

Subsequent calculation for the power requirements can be obtained once a specific pneumatic conveyance system is chosen. Methodology, Results and Analysis A 40 set test experiment was done to determine the mass flow rate of air, mass flow rate of solid, and the change in pressure from the rotary air lock to the destination valves were considered. From there, the values of pressure drop rate, air velocity, frictional force of air, frictional force of the solid particles, density of the mixture, drag coefficient, pressure of air in the bends, power, and pick up velocity are mathematically obtained.

A simplistic design approach for the system appears below. The pipeline layout is the most likely layout of the system that will be followed (with 5 bends). The first and second bends in the pipeline are the 8-meter change in elevation before the first valve. The air is separated from the solid (flour) particles through an air vent filter. The primary consideration in creating the pneumatic design and in the selection of the appropriate features is the type of material or substance that the pneumatic conveyor system needs to transport.

In this case, the material that needs to be transported is flour. It is apparent that for this type of materials to be transported that the positive pressure dilute phase conveying system will be used. The choice for the pneumatic system is obvious; the total distance that the material must be transported is within the range for both the dilute phase and the dense phase pneumatic systems. The requirement left to assess is the optimal capacities of both phases to transport the required material which is the flour.

However, dense phase pneumatic system will not be appropriate for the design of the pneumatic conveyor system for two reasons: (a) the materials that will be transported is neither fragile nor sensitive to abrasion and (b) the transport rate requirement is 10 tonnes per 8-hour for each receiving silo which means that a continuous process must be selected since dense phase pneumatic system transport materials at low speeds.

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