This article takes an in depth look at pneumatic conveying.
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Pneumatic conveying is a method for transferring bulk materials, like powders and granules, using compressed gas or air, 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.
The process of pneumatic conveying can be seen in the image below, where plastic flakes are being conveyed in a dilute phase pneumatic system using an eductor valve.
In order to function, all systems need some form of energy. For most types of machinery, energy is provided by electricity. A pneumatic system uses compressed air to create energy using air from a storage unit and depends on the pressure created when the air is pulled into an enclosed space.
The principle of a pneumatic system is built on the concept that bulk materials such as grains, cement, powders, and chips can be moved through a pipeline using air as the power and vehicle. The enclosed pressurized air creates a propulsion force, which moves bulk materials.
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.
The two most common forms of pneumatic conveying are pressure and vacuum where pressure conveying moves materials through the line using pressurized air. A pressure system necessitates the use of a feed line such as a rotary gate valve and pressure containers. A pressurized air system is used for large materials that have to be moved over long distances.
A vacuum conveying, or suction system, uses a vacuum pump to create a centralized or decentralized vacuum that moves the sucked up material to a loader where the vacuum air is separated from the material. When the vacuum air reaches the pump, it is filtered and released into the environment. Vacuum systems are able to supply multiple processing units with different materials due to their flexibility.
In some vacuum systems, the vacuum is created through the use of pressurized air and a venturi valve. Material is sucked in by the venturi valve and moved to its destination by pressurized air. It is a system that can be defined as a suction system and is restricted in its use to small loads of materials.
The forms of pneumatic conveying are the dilute phase and 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.
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). A necessity for dilute system design is a pressure vessel and inlet valve that cycles less and is not in constant motion.
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.
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 can use "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.
There is a downside to using a booster. Although they increase the velocity of the material being moved, they increase maintenance costs, downtime, and are a selling gimmick. Boosters should only be used out of necessity and when conditions warrant.
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.
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.
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:
Knowing the characteristics of the material is essential since each pneumatic system is designed to handle only certain types of materials.
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.
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 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.
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.
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:
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.
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.
Eductors use compressed air to transport solids. Compressed air is expanded across the nozzle of the eductor to create a vacuum in the suction chamber, which draws in the material and moves it through the diffuser section of the eductor into the conveying system. Eductors are used to avoid blowback, airlock wear, jamming, and damage to products.
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.
In a rotary airlock valve, air is sealed between the valve’s inlet and outlet ports. Vanes of the valve rotate and form a pocket where the material enters through the inlet valve, where the vane rotates to allow the material to exit through the outlet and move downward. The movement of the vane in a rotary valve restricts airflow but allows for continuous movement of the material. For a rotary valve to function properly, pressure or vacuum must be maintained.
Conveying line contains the material and method of transport, as can be seen in the diagram below.
Diverter valve prevents contamination and provides line switching as can be seen in the diagram. It is designed to reroute product from one conveying line to another and can be made of cast iron, aluminum, or stainless steel. Diverter valves are sealed with polyurethane rubber seals and have a parabolic shape that has without gaps.
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 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 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 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.
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.
When dealing with bulk materials, mechanical systems, such as conveyors, can be costly and lead to losses. They are not built to handle bulk materials regardless of how they are configured with troughs, sides, and dividers. Their design does not address the issues of dust, contamination, and potential harm to workers.
Conveying and movement of materials is a part of a wide range of industries from automobile production to package delivery. Each of those systems is designed to serve the needs of a product. In most cases, they are engineered specifically to meet a product's needs. Efficient movement of materials is essential for on time production and delivery. It is the reason that the use of conveying systems is growing.
In instances where bulk materials need to be moved over long distances, pneumatic systems can be economical and more efficient. Their enclosed design and tightly sealed tubes make material movement efficient and economical. Workers can be redirected to other applications with the knowledge that materials will be instantly delivered on time and in excellent condition.
As companies expand and grow, they need to change and redesign their method of conveying raw materials. With mechanical systems, the process involves work stoppages to re-engineer the system, which is costly and time consuming. Such procedures are not necessary for pneumatic systems.
If a company is moving bulky materials, such as powders, flakes, chips, or granules, a pneumatic system is the ideal choice. Pneumatic systems can be configured to deliver supplies and materials to a location and easily be adjusted if the needs of the location increases or changes. The first consideration is the consistency and type of material, which has to meet the criteria of being some form of bulk material. Once that determination is made, the next logical step is to install a pneumatic conveying system.
Since pneumatic conveying systems consist of pressurized tubing, they are relatively easy to move and change positioning. They increase the choices of routes and locations and can easily be expanded with the growth of the supply chain. Pneumatic systems can transport bulk materials to any point and are capable of going through, over, around, and under any parts of the infrastructure.
All manufacturing operations work tirelessly to lower production costs to be able to remain competitive in the marketplace. Companies are constantly looking for steps that can lower maintenance costs, repair costs, and costs of the supply chain. These factors are some of the advantages of a pneumatic system.
Unlike other forms of mechanical conveying, pneumatic systems are easy to install and require little maintenance. Once in operation, they provide outstanding flawless performance without the need for increased labor costs or other means of conveying. It is the cost factor that has convinced so many bulk material operating companies to choose pneumatic conveying.
As operations have become more complex and intricate, there is a growing concern for the safety of workers. Normal forklift and manual handling of materials can lead to injuries, time off, and delays in production, which is not the case with pneumatic systems.
With a pneumatic conveying system, bulk materials are isolated and contained away from workers and possible damage to the operation. The tight seal on a pneumatic system guarantees that dust and contaminants are contained, filtered, and controlled.
Mechanical conveying systems are ideal for moving materials over short distances such as within a manufacturing facility where materials can be moved on overhead and floor conveyors. When it is necessary to move materials outside the facility over long distances, it is more than likely that some other form of transport is used.
There are very few restrictions regarding the distance that pneumatic conveying systems can move materials. They are capable of moving bulk materials between buildings, towns, and distant locations without concern for loss of materials or damage. This particular aspect of pneumatic systems has been a major selling point for large operations that have several widely dispersed sites.
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.
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 pressure drop.
Pneumatic conveying is required to meet a set of requirements and regulations regarding material handling. Most of the regulations relate to safety procedures and specific processes for certain materials. The National Fire Protection Association (NFPA) and the Occupational Safety and Health Administration (OSHA) are the main bodies for developing the regulations. Below is a short description of some of the regulations and requirements.
NFPA 61 protects lives and property from fires and dust explosions in facilities handling, processing, or storing bulk agricultural materials, their byproducts, or other agricultural related dusts and materials.
This standard provides the minimum requirements for safety to life and property from fire, flash fire, and explosion posed by agricultural and food processing combustible dust and represents the industry and commodity specific requirements for agricultural and food processing.
Dust collector explosion venting, also called deflagration venting, lets pressure safely escape during a fire. The force of the deflagration or explosion needs to vent away from people. Vent design must avoid material being thrown into other buildings.
NFPA 91 provides minimum requirements for the design, construction, installation, operation, testing, and maintenance of exhaust systems for air conveying of vapors, gasses, mists, and particulate solids as they relate to fire or explosion prevention.
Organizations that process or convey dry bulk materials should have a combustible dust plan. Combustible dust is any material at risk of catching fire and causing explosions when mixed with oxygen and exposed to an ignition source.
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