Dilute Phase Pneumatic Conveying
Blowing & Vacuum Systems
Dilute phase pneumatic conveying, sometimes also referred to as lean phase or suspension flow, is a mode of conveying where the material is kept in suspension in the air stream of the pipeline it is being conveyed in. This mode of conveying can be in either in a positive pressure system (blowing system) or a negative pressure system (vacuum system). The air stream must maintain a velocity high enough to suspend the product particles throughout the length of the blowing system or vacuum system. The minimum value of the conveying line air velocity to suspend a particle in the air stream is known as the “Saltation Velocity” and can be easily determined and tested for through pneumatic conveying trials. If the air velocity falls below this value the particles will drop out of the air stream and have a tendency to settle in the bottom of the conveying pipe.
This type of blowing or vacuum system is the most common and elementary type of pneumatic conveying system used around the world today.
These type of systems comprise:
- A motive air force – a blower/fan or vacuum pump/ exhauster that delivers a flow of air to the pipeline
- A feed device that entrains the product into the conveying air stream.
- A filter or air separator / cyclone to remove the product from the air stream at the receiving end /collection hopper.
Dilute phase pneumatic conveying systems normally operate at positive or negative pressures up to 1.0 barg and have a low product to air ratio, which is usually up to a maximum of 15:1 with conveying velocities from 15 to 35m/s.
Designing such a system firstly requires specific product knowledge gained through conveying trial data and then requires a series of complex calculations to size the system to determine: the pipe diameter; conveying air volume; conveying line pressure; and size the blower / vacuum pump, feed device and filters for the given conveying line duty and configuration.
For any of the these types of dilute phase conveying systems, it is possible to have complex systems that can convey to any one of multiple delivery points through the use of diverter valves or collect product from multiple collection points. It is also possible to design the systems in a form of a closed loop conveying system limiting the introduction of atmospheric air and providing containment, or introduction of dried air for hygroscopic products, or, an inert conveying gas such as Nitrogen where there is an explosion risk.