|In this type of conveying technology, the solid is carried by a gaseous stream, which imposes a pressure gradient along the conveyor line.
Depending on the material¿s chemical-physical properties (hygroscopic behaviour, pyrophoric behaviour, etc.), the conveying fluid may be air, de-humidified air or inert gas.
A pneumatic conveying system basically comprises the following
- a machine for generating a gaseous flow;
- a feed system (Venturi-type ejector, rotary valves, powder pumps, etc.) that can insert the solid in the gaseous stream, while preventing passage of the gas into the storage equipment;
- a conveyor line, both the main conduct (the pipe) in which the material is carried, and the accessories (flanged couplings, curves, flow dividers etc.) of which it is made up;
- a solid-gas separation system (in the form of the classical centrifugal separators or sophisticated reverse jet bag filters type)
The most frequently used pneumatic solid movement systems are:
- negative-pressure system;
- positive-pressure system;
- combined positive- an negative-pressure system.
In negative-pressure systems the material moves in a gaseous stream with a pressure lower than atmospheric pressure. The advantages of this type of system are that all the pumping system energy is used for moving the product and that the product is sucked into the line without rotary valves or other sealing devices to prevent infiltration of gas at the pick-up point.
They are suitable for low rates of solids and for paths not excessively long, so as to avoid using machines with a high vacuum level. This type of movement is suitable for pneumatic circuits with pressure drops not greater than 300 mm Hg.
This type is used for picking up material from several points and feeding it to a single point.
It is ideal for toxic material, where even a single hole in the duct could lead to dangerous leaks.
In positive-pressure systems, the material falls through a rotary sealing valve into a positive pressure gaseous stream. The speed of the gaseous stream keeps the material in suspension until it reaches the apparatus provided to contain it, where it is separated from the carrying gas by a filter or a centrifugal separator.
Depending on the surface velocity of the conveying gas, two different flow regimes are possible, generally described as dense phase flow (solid mass/gas ratio higher than 10) or dilute phase flow (solid mass/gas ratio lower than 5).
Compared with the negative-pressure system, the pressure drops along the circuit may be higher; generally up to 1 bar for dilute phase flow and about 6 to 7 bar for dense phase.
Generally, dense phase flow (which requires a considerably lower gas flow rate) is much more efficient than the dilute phase; this means, however, that it is necessary to construct more expensive circuits, as the systems are working in pressure and require high pressure compressors.
The gas velocities in dilute phase are about 25 m/s, compared with 1 m/s for dense phase systems.
High velocities give rise to high degradation of the solids, due to collision between the particles and, above all, between the particles and the circuit walls; this may lead to considerable abrasion on the conveying line; therefore, it is often necessary to provide an anti-wear coating.
The combination of positive- and negative-pressure systems provides the best technological solution for many processes.
It is common practise, for example, to employ a vacuum system for unloading solids in powder form from trains, which cannot be pressurised, and convey the product to a filter fitted with a collector. Subsequently, a positive-pressure system can be adopted to convey the product from the collector filter to the storage point.
ADVANTAGES OF PNEUMATIC CONVEYING
The advantages of a pneumatic system compared with a mechanical conveyor system are:
- absence of dust dispersal in the environment;
- geometric flexibility of the conveyor system;
- simplicity of designing circuits that have several pick-up and unloading points;
- low maintenance costs.
DISADVANTAGES OF PNEUMATIC CONVEYING
- Makes it necessary to purchase a suitable machine, with high power consumption for achieving the necessary pressure gradient;
- solid conveying capacity lower than that of mechanical systems;
- limitated distance along which the product can be conveyed;
- possible wear along the circuit in case of dilute phase flow;
and - since complex fluid-dynamic phenomena are involved in pneumatic conveying systems, require very careful design of the conveyor circuit.
Designing a conveyor circuit is quite complex and requires a theoretic-experimental approach based mainly on the adoption of a pilot plant with which to carry out the flow tests. A positive-pressure conveying system, in dense or dilute phase, is generally selected by evaluating the characteristics of the material to be conveyed, and the head provided by the blowing machine (compressor or fan).