Pneumatic conveying is the transport of material from one location to another using air as the conveying medium with which the material is moved. Energy is needed to transport the material from one place to another. This energy can be supplied by a mechanical method (such as conveyor belts, buckets, etc), fan, blower, or a compressor. This newsletter will focus on the use of a fan to convey material. |
There are several methods of transporting materials using pneumatic conveying. In general, they seem to fall into three main categories: air conveying, dense phase, and dilute phase. Air conveying is the use of air to create a thin film of air between the material and the surface it is being conveyed on. Air film or air cushion is the use of air injected from below a porous surface such that the conveyed material "floats" along the system. This is somewhat like an air hockey table. Air can be directional causing the material to be transported along a desired path. The advantage to a system such as this is that it requires very little force to move the conveying material and it can be moved or rotated in any direction easily.
Another type of air conveying uses air nozzles with ball valves such that the air only flows through the valves when the conveying material makes contact with the valve depressing the ball. This, of course, reduces the operating costs of the system. However, the first cost of such a system would be higher.
Dense phase conveying is the transport of a slug of material by using high pressure (15 psig and nore) to essentially push the material along the desired path. Compressors or pressure blowers are typically used in this type of conveying system. It is similar to extruding and material is conveyed at relatively low velocities.
Dilute phase conveying 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. Dilute phase systems are the most commonly used method of transporting materials. Dilute phase will be the focus for the remainder of this article.
Positive pressure systems are normally used for transporting material from one entry point to one or more exit points. The orientation of components in this type of system is fan, feeder, and then separator. Cement, fly ash, and dry chemicals are examples of products that have been conveyed using this method. There are several different methods to introduce the material into the air stream for positive pressure systems. Screw feeders compress the material and force it into the air stream. Rotary valves drop material into the air- stream a little at a time. Using the positive pressure layout, one has to be careful of high velocity impact of introducing material into the air stream. Depending on the material being handled, this could be damaging to the material. Also, if dust is a potential problem with the material, leaks must be eliminated. Since it is pressurized, leaks will be out of the system, so dust can escape through any leak. In this type of system, the material does not go through the fan. There are two advantages to this. First, the fan wheel does not damage the material. Second, the fan does not experience any wear and tear from the material.
Pneumatic conveying is one of several methods of moving material from one location to another. Each individual case must be evaluated for multiple methods of transport before deciding on the best method. Pneumatic conveying, when designed correctly, can provide many benefits over other methods of material transport. The space required for a pneumatic conveying system is typically less than a mechanical method of transport. It can be modified without significant cost. Also, the amount of material lost can be minimized.
Selecting the fan for pneumatic conveying can range from easy to very involved depending on the location of the fan in the system. Special construction may be required to handle the additional loading and wear caused by the material being handled. If you are in need of help in determining what type of fan to use or if special construction is required, consult your fan manufacturer.
For additional information please refer to http://olegsystems.com/index.html.
Oleg Chetchel Systems Designer Oleg Systems Co. http://olegsystems.com http://www.olegsystems.com/industrialventilation/Index.html
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