Pneumatic conveyors are driven either by creating air pressure behind conveyed material or by creating a vacuum in front of it. Vacuum systems are chosen for a variety of reasons. One is that they allow a variety of material pick-up options: inlets can be moved from hopper to hopper, or there can be a variety of inputs. Another is that pipelines containing low pressures will not readily leak into the outside environment.
A vacuum conveyor can also be designed to have either a high or low ratio of material to air. In practice, this is usually a choice between making air move at high speed but low pressure (called “dilute phase”) or under high pressure but at moderate velocity (“dense phase”).
In dilute phase systems, material often becomes airborne, but the usual objective of a dense phase system is to produce a gentle and regular flow. In this relatively fluidised state, frictional and abrasive forces are reduced, while the state of the material also helps to conserve the pressure.
In practice, the way particular materials move also depends on the properties of the material, so the advice of specialist installers is always valuable when designing any kind of pneumatic conveyor. Contact firms like http://www.aptech.uk.com/pneumatic-conveying-systems/vacuum-conveying/ or see the design guide here: http://www2.hcmuaf.edu.vn/data/dh04cc/file/Pneumatic%20Conveying%20Design%20Guide.pdf.
It is possible to design dense phase vacuum conveyors that feed into continuous processes, but batch delivery systems are more common.
Typically, the vacuum is allowed to draw material from its silo into a vacuum hopper. Once this is full, one vacuum isolation valve closes as the discharge valve is opened, gradually emptying the vacuum hopper into the conveyor pipeline. Once empty, the valves reverse while the hopper refills.
A similar cycle occurs at the delivery point. A dense phase vacuum conveyor generally terminates in a purpose-designed receiving vessel where air is first separated from material by filters. Then, a vacuum isolation valve opens to divert the pressure away while a discharge valve opens to take delivery of a batch of material.
Throughout the pipeline, a variety of manual and automatic control valves and sensors can be located to regulate the flow and optimise the fluidisation state of the material. Another advantage of dense phase vacuum conveyors is that it is straightforward to centralise control of these valves and sensors or fully automate the supply line.