Pneumatic conveyors are used to transport dry, free-flowing, grannular material in suspension within a pipe or duct. This is done by the use of a high-velocity air stream or by the energy of expanding compresses air within a comparatively dense column of fluidized or aerated material.
Principal uses of pneumatic conveyors are
(2) Conveying soft materials such as flake or tow, and
(3) Conveying hard materials such as fly ash, cement, and saw dust.
The primary advantages of pneumatic conveyor systems are the flexibility of piping configurations and the fact that they greatly reduce the explosion hazard. With exception of the primary driver, there are no moving parts that can fail or cause injury. However, when they are used to transport explosive materials, there is still some potencial for static charge buildup that could cause an explosion.
A typical pneumatic conveyor consists of Schedule -40 pipe or ductwork, which provides the primary flow path used to transport the conveyed material. Motive power is provided by the primary drive, which can be a fn, fluidizer, or positive-displacement compressor.
Pneumatic conveyor performance is determined by the following factors:
(1) Primary-driver output,
(2) Internal surface of the piping or ductwork, and
(3) Condition of the transported material.
Specific factors affecting performance include motive power, fricition loss, and flow restricitons.
The motive power is provided by the primary drive, which generates the air velocity to transport material within a pneumatic conveyor system. due to this the efficiency of the system is depends on the primary drive's operating condition.
Friction loss within a pneumatic conveyor system is a primary source of efficiency loss. To reduce these losses the piping or duct work must be properly size is used.
One of the disadvantage of this system is their potencial for blockage. To reduce this the inside surfaces must be clean and free of protrusions or other defects that can restrict or interrupt the flow of material. In some cases when the system is shutdown or the velocity drops below the minimum required to keep the transported material suspended, the product will settle in the duct, this settled material cause reduce flow and eventually result in a complete blockage of the system.
All piping and ductwork should be as stright and short as possible. Bends should have a radius of atleast three times the diameter of the pipe or ductwork. The diameter should be selected to minimize friction losses and maintain enough velocity to prevent settling of the conveyed material. Brach lines should be configured to match as closely as possible the primary flow direction and avoid 90-degree angles to the main line. The area of the main conveyor line at any point along its run should be 20-25% greater than the sum of all its branch lines.
Clean-outs and drop-legs, should be installed at regular intervals throughout the system to permit foreign materials to drop out of the conveyed mateerial. It is must to install adequate clean-out systems near flow restrictions and at the end of the conveyor system.
Pneumatic conveyor systems must be operated properly to prevent chronic problems. The primary concern being to maintain constant flow and velocity.
Constant velocity can be maintained only when the system is operated within its performance envelope and when regular clean-out is part of the normal operating practice. In addition, the primary driver must be in good operating condition. Any deviation in the primary driver's efficiency reduces the velocity and can result in partial or complete blockage.
The entire pneumatic conveyor system should be completely evacuated before shutdown to prevent material from settling in the piping or ductwork. In non-continuous applications, the conveyor system should be operated until all material within the conveyor's piping is transported to its final destination. Material that is allowed to settle will compact and partially block the piping. Over time, this will cause a total blockage of the conveyor system.
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