31-01-2013, 11:55 AM
A Seminar presented on Pneumatic Conveying System
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INTRODUCTION
A pneumatic conveying system is a process by which bulk materials of almost any type are transferred or injected using a gas flow as the conveying medium from one or more sources to one or more destinations. Air is the most commonly used gas, but may not be selected for use with reactive materials and/or where there is a threat of dust explosions.
Types of Pneumatic Conveying
Dilute-phase conveying
Dense-phase conveying
Air-film conveying
Dilute-phase conveying
This process uses a relatively large amount of air to convey a relatively small amount of material and at lower pressures than dense phase systems. The material is transported at high velocities through the system while being suspended in air.
It is often referred to as suspension flow because the particles are held in suspension in the air as they are blown or sucked through the pipeline. To keep the material in suspension, it is necessary to maintain a minimum conveying air velocity that, for most materials, is of the order of 2500 – 6000 ft/minute.
Components of dilute – phase system
Major pneumatic system components include:
1. Pressure blowers and vacuum pumps with integral sound enclosures
2. Rotary airlock valves
3. Transfer line including piping, elbows; divert valves (flex-tube diverters, wye-diverters, plug diverters and other line diverter configurations).
4. Filter receivers
5. Cyclone separators
6. Gain-in-weight and loss of- weight batching systems
7. Dust collectors and bin vents
8. Controls and electrical equipment
9. Silos, day bins and other storage vessels[2]
DENSE PHASE CONVEYING
1.The main principle of a dense phase conveying system is to slow down the velocity of the product in the pipe to a point that is below the speed at which the product breaks or degrades. At low velocities, the product lies for periods of time in the bottom of a horizontal line and it is blown under pressure to the discharge point in slugs or plugs
2.The velocity should not exceed 4200 ft/minute
Advantages of dense phase conveying
1. First, because the conveying pipe line is so dense with the bulk material, the air cannot “slip” past the bulk material, which is a common inefficiency in dilute pneumatic conveying systems. If we eliminate the slip, we can improve efficiency.
2. Second, when the conveying pipe is at maximum density, only a small percentage of the particles are in contact with the conveying pipe at any given time. The majority of the particles are in the interior of the pipe, therefore not abrading the pipe. So, this significantly decreases pipe wear.
AIR-FILM CONVEYING
This method of pneumatic conveying uses a film or cushion of air to move items such as cans, boxes, or plastic containers through a plant. Used primarily in the packaging industry, air film conveying usually requires fan static pressures of no more than 8" WG. In most cases, the system utilizes several smaller fans as opposed to one large fan
CONCLUSION
Essentially, the two most distinct categories of pneumatic conveying can be described as either low pressure (dilute phase) or high pressure (dense phase) systems. The choice between dilute and dense phase operation is typically dependent on the material properties. In general, bulk materials with poor air retention capabilities are less suited for dense phase system and granular products with narrow particle size distribution have good air permeability and are well suited for dense phase conveying. In addition, dense phase operation (with its relatively low velocities) is usually preferable if the conveyed material degrades easily or is highly abrasive. Dense phase conveying can handle high throughputs over long distances while requiring smaller line sizes than either dilute-phase or vacuum conveying. A drawback of the dense phase option is that Polystyrene presented a non-uniform particle distribution in horizontal pneumatic transport in pipes. Transport of cylindrical polystyrene particles is mainly in the lower sections of the pipe, even at low solid loads and high air velocities. The best distribution was found when the system was operated at high air velocity and low polystyrene loads. The results show the importance of studying the dispersion of particles and the characteristic curve of the system.