A well designed pneumatic conveying system is often a more practical and economical method of transporting materials from one point to another than alternative manual or mechanical systems. This article outlines some of the fundamental principals of pneumatic conveying systems and explains various special considerations for fan selection.
Pneumatic conveying encompasses numerous different system designs, technologies, and pressure ranges; however, there are only three basic methods for moving material with air. These can be categorized into the following system types:
- Dilute-phase conveying is the process of pushing or pulling air-suspended materials from one location to another by maintaining a sufficient airstream velocity to capture and convey the suspended particles.
- Dense-phase conveying relies on a pulse of air to force a slug of material from one location to another. This form of conveying usually requires positive displacement blowers or compressors to generate the necessary pressure of 1.5 to 30 psig or more.
- Air-film or air-float conveying is a means of moving product along a conveyor on a cushion of air.
The use of fans for pneumatic conveying is generally limited to dilute phase conveying and air film conveying.
In this method of conveying, material is suspended in the airstream. Suction or vacuum are not factors in this type of system and fan static pressures are no greater than 60" WG. If the system uses a fan on the exhaust end and the material is collected or separated from the airstream before it reaches the fan, the fan itself can be of a more efficient type such as backwardly inclined. If the system is designed so that the combined material and air mixture passes through the fan, selection is limited to the more rugged but less efficient fan types intended for material laden airstreams. A number of radial-blade wheel designs are available to handle various concentrations, sizes, and types of airborne particles. Radial-tip wheel designs are tolerant of airborne contaminants, but radialtip fans are not generally thought of as bulk material handling designs. In all cases, the fan manufacturer should be consulted to determine the most appropriate fan type available to handle the specific material quantity and type, but it must be understood that the fan manufacturer can neither control the variables in pneumatic conveying systems nor provide any guarantee of the service life of the fan itself.
Applications requiring fans for dilute-phase pneumatic conveying fall into one of three basic categories: dust collection, fume removal, or material conveying.
Although the differences between dilute-phase material conveying systems and dust collection or fume removal systems might appear to be minimal, there are certain distinctions that are critical to the successful operation of material-conveying systems. These differences include the method of introducing the material to the hood, the velocity requirements, the duct configuration, and the fan type.
The introduction of material into a material conveying system can be difficult. The most important criterion is to feed the material into the airstream evenly. This can be accomplished by means of gravity or by a mechanical device. A hood or hopper can be used as a gravity feeder. Use of these components is limited to dry, free-flowing materials. It is important to remember that it is the velocity moving around and past the material that induces it to flow. If the entry becomes plugged with material, the required velocity cannot be maintained, significantly impeding air and material flow. A venturi feeder can be used to introduce material into the airstream. Like the hood, it has no moving parts so there is virtually no maintenance. However, the design of the venturi must be tailored to each application and even the best ones can be rather easily blocked if system conditions vary.
Rotary valves and screw-type (auger) feeders are the most common mechanical devices used to introduce material into the airstream. Both types offer a controllable flow rate and are readily available in a number of standard designs to handle pressures common to dilute phase conveying. However, there are some precautions. Both are typically more expensive than gravity-feed alternatives. Rotary valves can experience internal air recirculation which causes a reduction in material through-put. The screw-type feeder is a relatively high maintenance device. In either case, the manufacturer of the specific feeder should be consulted for selection, equipment recommendations, and system limitations.
Since the purpose of a conveying system is to move quantities of material suspended in air, the ratio of material to air (by weight) is critical. The most common form of reference is to state the ratio according to the combined weight in pounds per hour. A conservative design approach is to keep the ratio of material-to-air below a 1:2 proportion. However, successful systems have been designed using material loadings of 1:1 or more when the system components are well-designed and eliminate sharp turns, abrupt junctions, or other potential points of binding, clogging, or drop-out and the material being conveyed is well-defined and consistent. Certain minimum conveying velocities must be maintained to keep the material in suspension and flowing. To some extent these velocities are dictated by, or at least related to, the material-to-air ratio.
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 opposed to one large fan. Because the air is clean, various fan types can be used in these systems, including backwardly inclined and radial-bladed designs. Selection is based on pressure and flow, but configuration is equally important.
Either positive pressure or vacuum can be used to move the containers. In a pressurized system, air is directed through a drilled or slotted surface, where the air is discharged at a slight angle in the direction of flow. The greater the discharge angle, the higher the velocity from one station to the next. Vacuum elevators are used to raise or lower containers to different levels in the system by holding them to a moving, perforated belt. Vacuum transfer devises allow fallen or damaged product to drop out of the system, thereby reducing downtime and maintaining efficient high-speed processing. Both techniques may be employed in different portions of complex conveying systems.
The benefits of air film conveying over conventional mechanical conveying include:
- Increased process speed.
- Lower maintenance costs (fewer moving parts).
- Reduced energy consumption.
- Reduced noise and safety hazards.
- Reduced downtime from jamming.
- Gentler handling of the product.
Many companies in the packaging industry use a combination of air and mechanical conveying systems in their manufacturing processes.
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