Understanding the three different types of pneumatic conveying will enable you to keep material flowing at the best possible rate. By utilizing the right form of pneumatic conveying for your material and application, you will be able to move your product in a stable and continuous flow.
Lean phase pneumatic conveying uses a high volume of air at low pressure to move product. Most often a fan is used to create sufficient pressure to move product with an air to product ratio of less than 1:1. Lean phase conveying uses air velocities that are above 4000 FPM and often as high as or greater than 5000 FPM. This combination of high velocity/low pressure allows you to use air to do things other than just convey product. One example of this use of air is to control the temperature of the conveyed product. This could be as simple as cooling the product down to finely controlling the temperature going from one stage to the next. With high airflow velocities, the material being conveyed is actually kept in the center of the pipe with the main air flow in suspension flow. The lower pressures in lean phase systems also have a much less drastic impact on the conveyed material should pressure of any area of the line dramatically decrease.
Dilute phase pneumatic conveying often uses a PD blower to move less air but at a higher pressure than lean phase conveying. This higher pressure allows dilute phase conveying to move some materials at a product to air ratio as high as 7:1. The pressure is kept at less than 15 PSIG thus allowing this type of conveying system to utilize velocities of 3600 FPM or greater. Dilute phase also uses suspension flow similar to lean phase.
Dense phase pneumatic conveying often uses compressed air to push dense slugs of product along the line. It typically uses pressures less than 60 PSIG but pressures can potentially be much higher. Because compressed air is pushing material along versus keeping it suspended in the pipe, air velocities are typically less than 2000 FPM. Since the air is pushing the product in one slug, the product is not constantly hitting the inside of the pipe, thus dense phase conveying can keep more complex products intact over longer runs. With this type of conveying system, the product to air ratio is very high. In order to stay in dense phase the ratio must be kept above 20:1 and can be as great as 100:1 depending on the type of material being conveyed. If this ratio goes below 20:1, the product will likely enter dilute phase and break out of the neatly packed slug.
The materials you can convey with a dense phase system is limited. Products with particles that are all larger than a half inch in diameter will not work well in this type of system because the air tends to go around the particles instead of pushing them. However, products with particles of varying sizes may work even if some are larger than half an inch. Dense phase systems can utilize very high product to air ratios, but these ratios must be consistently maintained in order to prevent the system from slipping into dilute phase velocities. Though dense phase systems cannot handle many product mixes with larger particles, lean and dilute phase systems can handle almost any material particle size and shape. Also, since the pressure is so great with dense phase systems, the last slug of material going through a pipe will likely burst into its final destination if pressure is not carefully controlled and adjusted at the end.
With so many ways to convey product in a pneumatic system, knowing how each of these systems work as well as their capabilities and requirements will allow you to choose the best system for your application and material so you can move material faster and more efficiently.