Pneumatic Conveying Systems by Kathy Hunter
Monday, October 26, 2009
Pneumatic conveying is a clean and efficient way for food processors to transport raw ingredients from point A to point B in a completely contained manner. Pneumatic conveyors use compressed air to either push or pull material through a fully enclosed horizontal or vertical conveying line. The conveying pipe or tube can be constructed of a variety of materials, most frequently stainless steel for food operations, and can vary in diameter—the wider the diameter of the conveying line, the higher the conveying rate can be.
A vacuum conveying system applies a vacuum source at the delivery end to pull or suck the material through the conveying line. A pressure conveying system applies compressed air at the system inlet to push the material through the line. Both vacuum conveying and pressure conveying systems can operate using either dilute phase (low pressure, high velocity) or dense phase (high pressure, low velocity) technology.
Vacuum Conveying Systems
The basic components of a vacuum conveying system are a vacuum generator to move the air, a pick-up nozzle, the conveying line, and a receiver. In vacuum conveying, no moving parts are in contact with the materials. No dust escapes into the atmosphere, just as no contamination can enter the conveying system. In a vacuum conveying system, the vacuum creates suction that allows the pick-up nozzle to pick up the material from a hopper or other container. The vacuum pulls the material though the conveying line and into the receiver, where it drops in by gravity. A valve allows the material to be discharged from the receiver to the next process vessel, perhaps a feeder or a packaging line.
Vacuum conveying is used for transporting free-flowing non-abrasive materials short distances, generally 200 ft or less. The advantages of vacuum conveying include safety, hygiene, efficiency, flexibility, cost effectiveness, and dust-free operation. Because the air stream is sucking rather than blowing, there is little chance of the conveyed material escaping from the system.
Pressure Conveying Systems
The basic components of a pressure pneumatic conveying system are a blower, fan or air compressor system to serve as the air mover, a rotary airlock feeder, pressure vessel, the conveying line, and a receiver. The rotary airlock feeder charges the material into the pressure vessel. When it is full, the vessel’s inlet valve closes and seals, and compressed air is introduced into the vessel. The high- pressure air sends the material into the conveying line and on to the receiver. A filter is generally used at the receiver to separate the material from the air. Sensors and valves control the system pressure and velocity. The conveying cycle ends when the desired low-pressure setting is achieved, the air mover is turned off, and the remaining air volume purges the material from the vessel and the line.
Pressure conveying systems are generally used for conveying heavier material over a longer distance, as far as 1,500-1,600 ft. The system can be costly compared to a vacuum conveying system because of the need for special equipment. It also requires more space because of the need for the rotary airlock feeder to introduce the material into the airstream at the inlet, as well as extra components to remove the air at the discharge end, such as a bin vent or dust collector.
Dilute Phase Conveying
The difference between dilute phase and dense phase conveying is twofold: 1) air velocity 2) pressure differential within the conveying line. In dilute phase conveying a large amount of air or gas is used to convey a small amount of material at a high velocity. Depending on the material particle size and density, the material is most often fluidized or suspended in the airstream. The dilute phase system constantly supplies the material without interruption, with no waves or plugs of material and no air pockets. Since the material is actually suspended in air while conveyed, the total amount of material that can be conveyed is limited. The material-to-air ratio in dilute applications is no more than 10:1 by weight.
Dense Phase Conveying
In dense phase conveying a small amount of air or gas is used to convey a larger amount of material at low velocity. The material-to-air ratio is generally 30-50:1 by weight. The conveying velocity is below the saltation level, which is the critical velocity at which particles fall from suspension in the airstream. The dense phase system moves the material through the line in batches of material waves or plugs, separated by air pockets. Adjusting the system’s valves to add less material increases the air pocket size, while adding more material reduces the air pocket size. Dense phase conveying handles a relatively small range of applications, but the low velocity results in less damage to the material and less segregation in the material flow.
The Four Basic Types of Pneumatic Conveying in Brief
Dilute phase vacuum conveying is used where material degradation is not a concern for coarse/lightweight particles or for fibrous, fine, granular, nonabrasive materials with a low or high bulk density. It has a high air-to-material ratio and can be used for short distances of up to about 200 ft. The conveying velocity is 1,000-4,000 fpm initially and up to 4,000-9,000 fpm at terminal.
Dense phase vacuum conveying is used for materials that are abrasive, coarse, cohesive, fine, and sticky, as well as for fragile materials where material degradation is a concern. It has a low air-to-material ratio and can be applied to even shorter distances than dilute phase vacuum. Optimal performance is generally limited to about 100 ft. The conveying velocity is 0-50 fpm initially and up to 50-1,000 fpm at terminal.
Dilute phase pressure conveying is used for all dry bulk materials and is best for adhesive and cohesive materials that are hard to fluidize, as well as nonpermeable, sticky, and very fine materials. The air-to-material ratio is high for dilute pressure conveying and it can convey material a good distance, around 650 ft. The conveying velocity is about 2,000 fpm initially and up to 4,500 at terminal.
Dense phase pressure conveying is used where material degradation or the erosion of the conveying line are factors. It is best for abrasive or non-abrasive materials that are free-flowing, fluidizable, granular, noncohesive, noncompressible, and uniformly sized. The air-to-material ratio is low for dense phase pressure conveying and it can convey the longest distances, up to around 1,600 ft. The conveying velocity is about 50 fpm initially and around 500 fpm at terminal.
Selection of the correct pneumatic conveying system for use in the manufacture of food products is generally based on the nature of the material to be conveyed, the distance the material must be conveyed, concerns for material degradation or line erosion, space available at inlet and terminal, the materials of construction, and the ease of cleaning of the system.