Some points you might to know on Pneumatic Conveying

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Introduction

Everything you ever wanted to know about pneumatic conveying.

You will learn:

• What is pneumatic conveying?
• The Three Types of Pneumatic Conveying Systems
• How to Choose a Pneumatic Conveying System
• Pneumatic Conveying Design
• And much more&#;

Chapter One &#; What is Pneumatic Conveying?

Pneumatic conveying uses compressed air, or gas, to transfer bulk materials, like powders and granules, from one processing center to another. Material is moved through an enclosed conveying line or tube using a combination of pressure differential and airflow from a blower or fan. The positive or negative pressure, inside the conveying line, moves materials safely with little damage or loss.

Pneumatic conveying systems transport cement, fly ash, starch, sugar, salt, sand, plastic pellets, oats, polymers, lime, soda ash, plastic resin, plastic powder, dry milk, and feeds in a cost and energy efficient way from  railcars, trucks, or silos. Other uses of pneumatic conveying include intermodal or transloading, in plant transfer, and dust control.

The process of pneumatic conveying is a combination of well-engineered components that work together to move substances and materials safely, efficiently, and economically.

Chapter Two &#; Types of Pneumatic Conveying

The forms of pneumatic conveying are dilute phase, dense phase, pulse dense phase, and semi-dense phase. Each type moves materials using air pressure and an enclosed line or tube. The difference between them is their method of creating air pressure and how the material travels through the system.

Types of Pneumatic Conveying

Below are brief descriptions of each method.

Dilute Phase

There are two methods for moving materials using dilute, suspension, or lean phase pneumatic conveying, which are positive and negative. A positive or blowing phase system uses a fan or blower to create pressure in the line and suspend the material. At the end of the line, the material is removed using a separator or filter. Materials travel between 15 meters per second to 35 meters per second (m/s).

Negative pressure systems, or vacuum systems, work in reverse. Instead of pushing or blowing material through the line, they create a vacuum and pull material through.

The difference in the two systems can be seen in the diagram below. In the vacuum system, on the left, the blower is located on the right with the hoppers on the left. In the pressure system, on the right, the blower is to the left of the feed tube pushing the material to the storage silo.

Vacuum Conveying vs.                     Pressure Conveying (from doetechnologies.com)

Dense Phase

Dense phase pneumatic conveying uses a small amount of air to move a large amount of material in slugs, much like extruding. A dense phase system pushes a denser concentration of bulk solids at low velocities, as can be seen in the diagram below.

The Dense Phase pneumatic conveying uses &#;booster pulsers&#;, known as &#;air saver boosters&#; or &#;air fluidizers&#;, to move the product and free it from the piping walls. Multiple controls or fluidizers are installed through the wall of the hopper section and the wall of specific pipe runs to loosen the material and direct the airflow, which inputs small shots of air to control the solidity of the product and maintain pipeline velocity.

Since the conveying pipeline is densely packed, air does not get past the material, which improves efficiency. Very few particles of the material make contact with the piping or tube, which reduces pipe abrasion and wear. The density decreases the transfer rate and pipe diameter, which increases velocity control.

Dense phase is perfect for fragile materials and mixtures because of its low speed and air volume, which prevents materials from breaking down.

Dense Phase Pneumatic Conveying (from www.xavier-meyrigne.info)

Semi-Dense Phase

Semi-dense phase is an alternative between dense and dilute phase conveying. Materials travel at a medium rate velocity of to ft/min, which is higher than dense phase but lower than dilute phase. It is the perfect solution for transferring aeratable abrasive and friable materials. Semi-dense pneumatic conveyors provide increased speed with protection of the material.

Pulse Dense Phase

In pulse dense phase, material moves in a plug flow fashion. It is ideal for abrasive materials. A pulse pump separates the product into &#;plugs&#; using an air knife. The plugs slide through the pipeline, as can be seen in the pipe diagram below. The benefits of a pulse dense phase system are a smaller pipeline, low air consumption, and less degradation of products.

Pulse Dense Phase System (from turbokopar.com)

Chapter Three &#; Choosing a Pneumatic Conveying System

Pneumatic conveying is a safe and efficient method for transporting materials and is used in thousands of industries. Entire books, journal articles, and white papers with diagrams and complicated equations have been written to understand its intricacies.

When exploring pneumatic systems, as a transportation method, there are factors that have to be considered. The equipment and method of conveying has to match the material to be transported. Below are a few guiding points to consider when choosing a pneumatic conveying system.

Characteristics of the Material

The first consideration when choosing a conveying system is the type of material to be handled. Bulk density is the first consideration since it determines vacuum receivers and sources of air. The bulk density determines how many cubic feet per minute is needed in the conveying line. Other factors to be considered are:

• Particle size/shape
• Friability
• Moisture content
• Abrasiveness
• Toxicity
• Explosive properties

Dry Bulk Materials

Knowing the characteristics of the material is essential since each pneumatic system is designed to handle only certain types of materials.

System Size

It is important to understand if the system will be able to handle the distance, vertical or horizontal, the material will travel and if it will fit inside the facility. The size of the system has to leave room for maintenance and oversight. The system picture below is a complex vacuum system connected to container storage units.

Other factors include the number of pickup points, the size of the receiving container, if the process is continuous, feed rate, and if it is a batch process how often the batches will arrive and the size of the batches.

Vacuum Pneumatic Conveying System with a Container System

Vacuum vs Pressure

A vacuum system sucks materials through the pipeline and is ideal for pressure sensitive non- abrasive materials. A pressure system pushes material through the pipeline and can move abrasive materials. Quartz and copper sulfate are the types of abrasive materials that a pressure system will move.

Non-abrasive powders conveyed by a vacuum pneumatic conveying system are moringa powder and powdered metals.

Dilute Phase vs Dense Phase

Dilute phase moves materials at high velocities under pressure and has some breakage during transport. Dense phase is used for fragile materials since it operates at low pressure. Dilute phase has a high air to product ratio, while dense phase has a low ratio. The image below shows the difference between dilute phase on the left and dense phase on the right.

Dilute and Dense Phase Pneumatic Conveying (from iedco.com)

Cost

The initial cost of a pneumatic conveying system can vary between several hundred thousand dollars for a complex system designed to move huge amounts of material to tens of thousands of dollars for a simple system. The first image below is a complex food conveying system designed to move tons of raw materials for food production. The second image is of a less complex straight material handling system for conveying powdered materials.

Unloading Conveying System for Powders for the Food Industry

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Dry Material Handling

Chapter Four &#; Pneumatic Conveying Design

All pneumatic systems use pipes or ducts to transport materials on a stream of air. An air mover generates pressure or a vacuum and is located in the system at the beginning to push materials through the line or at the end to pull materials through.

The basic components of a pneumatic system are:

1. Pressure blower or vacuum pump
2. Rotary airlock valves
3. Conveying line
4. Diverter valves
5. Filter receiver
6. Cyclone separators
7. Batching systems
8. Hoppers
9. Controls

Components of a Pneumatic Conveying System

Pressure blower

Pressure blower is one of the two methods for moving material using a pneumatic conveying system. When designing a system, a major factor is the amount of pressure drop throughout the system, which is the most extensive at the end of the system. The force of the blower determines the flow rate at cubic feet per minute (CFM), velocity in meters or feet per second, and the pressure. It pushes the material down the pipeline

The working principle: The side channel blower will be used for vacuum conveying or discharge conveying the raw material. 

Vacuum pump

Vacuum pump is another method of moving material. Vacuum systems are often used as part of a dilute phase system but can also be applied to dense phase systems. An electric driven vacuum pump is the most efficient and recommended method for producing the vacuum. As can be seen in the diagram, the vacuum conveyor is located to the right at the end of the pneumatic pipeline and pulls the material through.

Lean Phase Vacuum Conveying (from technolinksglobal.com)

Rotary airlock valves

Rotary airlock valves are used for handling solids when it is necessary to separate two areas under pressure, while moving the material from one condition to the next. They are used at the beginning and end of a pneumatic conveying system.

Conveying line

Conveying line contains the material and method of transport, as can be seen in the diagram below.

(from unitedstatessystems.com)

Diverter valve

Diverter valve prevents contamination and provides line switching as can be seen in the diagram.

Filter receivers

Filter receivers contain and separate dust from material in a pneumatic conveying system. The unwanted airborne material is trapped in the filter in the receiver unit as seen in the diagram on the right.

Cyclone separator

Cyclone separator uses centrifugal force to remove particles from air in a pneumatic conveying system. Air enters the cyclone whose rapid rotation separates the particles, which are blown against the wall and fall to the bottom of the container.

Batching system

Batching system uses pneumatic conveying for mixing ingredients that are injected into the conveying line and forwarded into a mixture. Batching operations dramatically increase the size of a pneumatic system.

Hopper

Hopper contains the material before it is introduced into the conveying line. A rotary airlock valve releases the material from the hopper as can be seen in this diagram of a chip hopper feeding a conveying line.

Dense Phase Pneumatic Metal Chip Conveying System (from nationalconveyors.com)

Controller

Controller is a necessary part of a pneumatic conveying system and is designed to fit the needs and configuration of the system. Common controllers are microprocessors or PLC based. They are designed to control blower and filter operations, valves, receiving hopper control, loaders, and filter pulsing using a PC.

Chapter Five &#; Pneumatic Conveying Control Systems

Pneumatic conveying systems require immediate responses to changes in the conveying line. When a storage container is full and the system has to switch to another container, the system has to react and adjust the volume and airflow. Maintaining these changes, and adjusting to them, is the reason for pneumatic control systems.

The Benefits of a Pneumatic Conveying Control System

• Selection of conveying modes
• Cost savings
• Quicker startup
• Intuitive operation
• Connectivity
• Remote diagnostics
• Control air consumption
• Batching and weighing

An airflow controller continually assesses the demands of the system and provides instant feedback. The system measures material mass flow rates and makes adjustments to keep it within the optimum range. Data collected from the system is used to analyze overall efficiency.

Closed loop controllers combine flow rate and process controls. The system stores data and calculates where leakage and errors will occur. Using the accumulated data the system makes adjustments and compensates for any problems. Using the pressure difference, density, and the temperature of the medium, the system calculates the best flow rate. The volume flow is adjusted by changing the stroke of the control valve creating less

Conclusion

• Pneumatic conveying systems use compressed air, or gas, to transfer bulk materials, like powders and granules, from one processing center to another.
• The positive or negative pressure in a pneumatic conveying line moves materials safely with little damage or loss.
• Pneumatic conveying systems use pipes or ducts to transport materials on a stream of air.
• Materials transported by pneumatic conveying systems include cement, fly ash, starch, sugar, salt, sand, plastic pellets, oats, polymers, lime, soda ash, plastic resin, plastic powder, dry milk, and feeds.
• Pneumatic conveying systems are widely used because of their efficiency, cost effectiveness, and flexibility.

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As Figure 2 shows, for the pressure gradient versus gas flow rate plot, it is best to operate near but not on or too close to the saltation flow line. This is where the pressure gradient is the lowest, which means less capital and lower power consumption. In addition, for dense phase flow, which is left of the saltation flow line, gas flow that is too low can result in a packed bed that plugs the line and does not move. The pressure gradient and pressure fluctuations in dense phase mode are higher than in dilute phase mode and manifest as vibration in the conveying line. For dilute phase flow, which occurs at high gas velocities and to the right of the saltation flow line, the pressure fluctuates less, creating fewer concerns about mechanical stress from vibration. 

Dilute versus dense phase conveying 

Dilute phase conveying occurs when the gas flow rate (gas velocity) is sufficiently high to fully entrain the transported solids. Typically for dilute phase conveying, the superficial gas velocities tend to be over 4,000 ft/min, with solids loading less than 15 mass of solids to the mass of gas (i.e., 15 lbm of solids/lbm of gas). The particles tend to be fully suspended. There is no stationary layer, moving dunes, strands, or plugs at the bottom of a horizontal conveying line section, or recirculation patterns in a vertical section. As previously noted, slightly above the saltation velocity should be considered the absolute minimum gas velocity to operate a dilute phase conveying line. 

Several references in the literature define dilute phase conveying by the conveying pressure drop (less than 1 bar) or solids loading ratio (mass of solids/mass of gas) less than 15. As noted, such operating characteristics are typical, but dilute phase conveying is not bounded by these operating limits. For instance, a long-distance dilute phase conveying system can operate at a pressure drop of more than 3 bars. 

Dense phase conveying occurs at much higher solids concentrations but lower gas velocities &#; around 200 to 600 ft/min at the pickup point, with end-of-line velocities not exceeding 1,600 to 2,000 ft/min. This tends to be below the saltation velocity, and as expected, particles are not in suspension. Various flow patterns may develop depending on the nature of the particles (fine versus coarse) and the conveying line orientation. As shown in Figure 1, for horizontal lines and fine powders, the solids flow just below the saltation velocity is dune flow. As the gas flow decreases further, dune flow transitions into slug flow. Many materials can be conveyed in stable slug flow (such as plastic pellets), but such systems require additional design features to achieve stable and reliable conveying. The airflow control in the line must be &#;smart&#; to prevent the flow regime from becoming unstable, and shock-absorbing line supports must added to prevent line failures due to high dynamic loads. The operating window for a stable dense phase conveying is narrow, and the control system must modulate constantly to keep the operating point within the stable zone. 

Since conveying velocity strongly affects particle breakage (attrition) and abrasive wear of the conveying line and system components, dense phase conveying mode is desirable where erosion or attrition is problematic. The lower gas flow rates also reduce the size of the filters and gas conditioning system, especially in systems that need to use gas other than air. However, dense phase conveying is not always possible, feasible, reliable, or economical. Consider the following factors when selecting the most suitable conveying mode. 

Material Factors. Dense phase conveying may not be suitable if: 

• The particles are fibrous, sticky, moist, wet or cohesive.
• The particles are very dense.
• The particles are very soft, pliable and elastic/rubbery.
• The particle size distribution is very broad, with 10-25 wt% fines fraction smaller than 425 mesh.
• The particles have no natural slugging ability (e.g. Geldart B, high permeability). Such materials may require special gas injection systems or bypass systems along the conveying line, which requires access to the line for maintenance.
• The maximum particle size is smaller than 50 millimeters.
• The incoming feed material properties are highly variable. 

Operational Factors. Dense phase conveying may not be suitable if: 

• A significant turn-down (Max/Min > 5) in conveying rate is desired.
• A high-pressure conveying gas source is not available.
• Vacuum conveying is required to convey solids from multiple sources to a single destination. Vacuum-based dense phase conveying systems have a limited feasible conveying distance.
• The conveying and drying steps are combined. The total gas flow rate may not be sufficient for drying. 

Configurational Factors. Dense phase conveying may not be suitable if: 

• The required capacity is very high, or the conveying distance is very long.
• Conveying line supports cannot be properly supported along the conveying length due to structural considerations. This is a common problem for retrofit situations.
• Cross-contamination is an issue.
• The system size is tool large or the pressure limit too high for available components. 

While dilute phase conveying systems are more common, reliable, inexpensive and simple to design and operate, dense phase conveying systems offer the significant benefits of lower product degradation and system wear. For instance, dense phase conveying of polymer pellets dramatically reduces fines and streamers/floss generation.  Table 1 provides a list of the pros and cons of dilute versus dense phase conveying. 

Designing a conveying system 

Conveying lines can be designed in a push (pressure) or pull (vacuum) configuration, as shown in Figure 3. Dense phase conveying systems primarily use the push configuration, but dilute phase systems can use either configuration depending on the processing requirements. A push configuration is where the gas mover is located before the conveying line and is recommended for conveying from a single source to multiple destinations. A pull configuration is where the gas mover is located after the conveying line and may be the correct choice for conveying from multiple sources to a single destination. Note that conveying lines in the pull configuration operate under a vacuum, so all components must be compatible with that vacuum, especially the hoppers. 

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