Outline of Pneumatic Conveying Systems
A pneumatic conveying system transfers powders, granules, and other dry bulk materials through an enclosed horizontal or vertical conveying line. The motive force for this transfer comes from a combination of pressure differential and the flow of air (or another gas) supplied by an air mover, such as a blower or fan. By controlling the pressure or vacuum and the airflow inside the conveying line, the system can successfully convey materials.
Advantages of Pneumatic Conveying
Pneumatic conveying provides several advantages over the mechanical conveying. A pneumatic conveying system can be configured with bends to fit around existing equipment, giving it more flexibility than a mechanical conveyor with its typically straight conveying path. This also means the pneumatic conveying systems occupy less space than a comparable mechanical conveyor. The pneumatic conveying system is totally enclosed, unlike many mechanical conveyors, which enables the pneumatic system to contain dust. The pneumatic conveying system typically has fewer moving parts to maintain than a mechanical conveyor, as well.
Possible Disadvantages
Pneumatic conveying also has some disadvantages compared with mechanical conveying. One is the pneumatic conveying system’s typically greater use of horsepower than a mechanical conveyor, resulting from the pneumatic system’s need to change air pressure to produce conveying power. The pneumatic conveying system also uses a comparatively larger dust collection system than a mechanical conveyor because the pneumatic system has to separate the materials from the conveying air at the system’s end. Some materials also have characteristics that make them difficult to convey in a pneumatic system. Examples are materials with a large particle size and high bulk density, such as gravel or rocks, and extremely sticky materials, such as titanium dioxide, which tend to build a coating on material-contact surfaces and can eventually block the conveying line.
Pneumatic Conveying System Types and Applications
Pneumatic conveying systems are classified by their operating principle into two basic types: dilute phase and dense phase. Either can run under pressure or vacuum.
Dilute Phase Conveying
In dilute phase conveying, particles are fully suspended in the conveying air and transported at low pressure and high velocity.
Dilute Phase Pressure Conveying
Dilute phase pressure conveying is one of the most common conveying methods for powders or granules. It’s most often used with non-abrasive, non-fragile materials that have a light bulk density (typically less than 62 lb/ft3); common examples are flour, sugar, corn starch, plastic granules, sodium bicarbonate, hydrated lime, activated carbon, and zinc oxide.
In this method, illustrated in Figure 1a, a blower at the system’s start supplies a high volume of low-pressure air to the system, and material is fed into the conveying line through a rotary airlock valve. The system relies
on the air stream’s velocity to pick up and entrain each particle, keeping the particles in suspension as they travel through the conveying line. The dilute phase pressure conveying system requires relatively little headroom and is simple to operate, economical, and ideal for conveying materials from a single source to multiple locations.
Dilute Phase Vacuum Conveying
Dilute phase vacuum conveying is suitable for conveying materials that tend to pack or compress under pressure, such as wood shavings and certain other fibrous materials, and for toxic materials that must not leak into the workplace air. This system is typically used to convey materials over short distances at low capacities. Dilute phase vacuum conveying requires minimal headroom at the feedpoint and is ideal for conveying material from multiple sources to a single destination.
Dense Phase Conveying
In dense phase conveying, particles aren’t suspended in the conveying air and are transported at high pressure and low velocity.
Dense Phase Pressure Conveying
Dense phase pressure conveying is suitable for gently conveying fragile or abrasive materials with particles ¾ inch and smaller over long distances (typically more than 250 feet). Commonly handled materials include silica sand, feldspar, fly ash, glass cullet, alumina, glass batch mix, carbon black, sorbitol, dextrose, candies, resins, cocoa beans, hazelnuts, and puffed rice cereal. The system conveys materials at a relatively low speed to reduce materials degradation, air consumption, and abrasion on pipeline, bend, and diverter contact surfaces. This system can also stop or start with the conveying line full of material.
Material conveyed by this method is loaded into a pressure vessel (also called a blow pot or transporter), as shown in Figure 1b. When the vessel is full, its material inlet valve and vent valve are closed and compressed air is metered into the vessel. The compressed air extrudes the material from the pressure vessel into the conveying line and to the destination. Once the vessel and conveying line are empty, the compressed air is turned off and the vessel is reloaded. This cycle continues until all of the materials required for the process have been transferred.
To overcome resistance in the conveying line, supplementary air injectors (also called air boosters or air assists) can be located along the conveying line (Figure 1b). These injectors provide additional air to help maintain conveying velocity, transfer materials over long distance, and minimize line plugging. They can also be used to gently restart flow when materials are left in the line after the conveying cycle. An air injector should be sued with a high pressure manifold to prevent backfeeding of material into the compressed air system.
Dense Phase Vacuum Conveying
Dense phase vacuum conveying is ideal for gently conveying fragile or abrasive materials short distances (typically 200 feet or less). This system is typically used to transfer powder and granules at a low rate (25 tph or less) in applications such as truck or railcar unloading.
A less commonly used semi-dense phase conveying system is configured like other dense phase systems, but uses a pressure vessel with a fluidizing bottom so it can handle semi-abrasive powders and fluidizable powders that need aeration to discharge into the conveying line. This method isn’t the ideal choice for fragile materials or materials containing large, variable, or both large and variable particle sizes. The largest particles handled by this method are approximately ¼ inch.
Fabio Novelli is vice president of sales and innovation specialist at Nol-Tec Europe, Gorgonzola, Milan, Italy. Mike Weyandt is corporate sales manager for Nol-Tec Systems, 425 Apollo Drive, Lino Lakes, MN 55014; 651-705-0108, ext. 244, fax 651-780-4400 www.nol-tec.com