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…
This diagram shows the process of pneumatic conveying
image by: www.vac-u-max.com
Chapter One – What is Pneumatic Conveying?
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.
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.
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.
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.
Semi-dense phase is an alternative between dense and dilute phase conveying. Materials travel at a medium rate velocity of 1500 to 3500 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.
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
- Moisture content
- Explosive properties
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.
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.
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.
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:
- Pressure blower or vacuum pump
- Rotary airlock valves
- Conveying line
- Diverter valves
- Filter receiver
- Cyclone separators
- Batching systems
Components of a Pneumatic Conveying System
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.
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 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.
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.
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
- 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 pressure drop.
Chapter Six – Pneumatic Conveying Certification
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 - Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities
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.
NFPA 68 - Standard on Explosion Protection by Deflagration Venting
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 - Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids
NFPA 91 provides minimum requirements for the design, construction, installation, operation, testing, and maintenance of exhaust systems for air conveying of vapors, gases, mists, and particulate solids as they relate to fire or explosion prevention.
NFPA 652 - Standard on the Fundamentals of Combustible Dust
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.
OSHA 1926.555 - Conveyors
- 1926.555 (a)(1) - Means for stopping the motor or engine shall be provided at the operator's station. Conveyor systems shall be equipped with an audible warning signal to be sounded immediately before starting up the conveyor.
- 1926.555(a)(2) - If the operator's station is at a remote point, similar provisions for stopping the motor or engine shall be provided at the motor or engine location.
- 1926.555(a)(3) - Emergency stop switches shall be arranged so that the conveyor cannot be started again until the actuating stop switch has been reset to running or "on" position.
- 1926.555(a)(4) - Screw conveyors shall be guarded to prevent employee contact with turning flights.
- 1926.555(a)(5) - Where a conveyor passes over work areas, aisles, or thoroughfares, suitable guards shall be provided to protect employees required to work below the conveyors.
- 1926.555(a)(6) - All crossovers, aisles, and passageways shall be conspicuously marked by suitable signs, as required by Subpart G of this part.
- 1926.555(a)(7) - Conveyors shall be locked out or otherwise rendered inoperable, and tagged out with a "Do Not Operate" tag during repairs and when operation is hazardous to employees performing maintenance work.
- 1926.555(a)(8) - All conveyors in use shall meet the applicable requirements for design, construction, inspection, testing, maintenance, and operation, as prescribed in the ANSI B20.1-1957, Safety Code for Conveyors, Cableways, and Related Equipment.
- The three types of pneumatic conveying systems are dense phase, dilute phase, and semi-dense phase.
- 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.