Pneumatic conveying dilute-phase systems

Pneumatic conveying dilute phase pressure continuous nominal gas velocity 5 to 35 m/s with usual 11 to 25 m/s. Solids loading 3.5 to 15 kg solid/kg air with usual 6 to 15 kg solids/kg air or 1 to 7 m solids/m air. Power 7 to 11 kJ/kg. Problems about 30% air leakage out of the system. Rotary/star valve problem/bridging overcome with bin... 

Pneumatic conveying dilute phase for pressure use pressure at the outlet of the blower as prime indicator. "Ap across blower > design or 2 1 ratio restriction in downstream conveying line/check valve jammed closed/dirty intake filter/ plugged discharge silencer/increase in feed to the system/length of pipe... 

This case study demonstrates that dilute-phase systems are not straightforward , as well as the importance of designing not only the drop-out box (i.e. interface), but also the attached pneumatic conveying system, to suit the matetM and application. 

The accurate determination of minimum transit conditions for a particular product and application is an equally essential requirement for many areas of dilute-phase pneumatic conveying (e.g. system ign optimisation of operating conditions minimisation of product damage, pipe nd wear and power consumption). Consequently, a considerable amount of research work also has been undertaken for many years resulting in a wide selection of correlations. Also, based on various comparadve- pe studies, e.g. [7-9], several models have been labelled as best bi s . 

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

Pneumatic conveying systems and in particular dilute phase conveying systems are known to create a high stress on particulate solids leading to significant attrition. In contrast to fluidized beds, it is not the material loss which is the main problem. Depending on the application, problems may rather occur in a number of different areas. Attrition may, for example,... 

Salman, A. S., Verba, A., and Mills, D., Particle Degradation in Dilute Phase Pneumatic Conveying Systems, Proc. of the 1992 Powder Bulk Solids Conf. and Exhibition, Rosemont, USA (1992)... 

Knowlton, T., The Effect of the System Pressure/Pipe-Diameter/Mass Flux Interaction on Pressure Drop in Dilute Phase Pneumatic Conveying, Proc. of Pneumatic Conveying Workshop, Powder Technol. Forum, AIChE, Denver (1994)... 

Pneumatic Conveying Pneumatic conveying systems can generally be scaled up on the principles of dilute-phase transport. Mass and heat transfer can be predicted on both the slip velocity during acceleration and the slip velocity at full acceleration. The slip velocity increases as the solids concentration is increased. 

P. Maijanovic, Determination of performance characteristics of dilute phase pneumatic conveying system, Proceedings of the 3rd Symposium of Process Industry Applications, Belgrade, Yugoslavia, 1988, pp. 215-223 (in Serbian). 

Pneumatic conveying systems can be classified on the basis of the angle of inclination of pipelines, operational modes (i.e., negative- or positive-pressure operation), and flow characteristics (i.e., dilute or dense phase transport steady or unsteady transport). A practical pneumatic conveying system is often composed of several vertical, horizontal, and inclined pipelines. Multiple flow regimes may coexist in a given operational system. 

Pneumatic Conveying Dryers A gas-solids contacting operation in which the sohds phase exists in a dilute condition is termed a dispersion system. It is often called a pneumatic system because, in most cases, the quantity and velocity of the gas are sufficient to hft and convey the solids against the forces of gravity and friction. (These systems are sometimes incorrectly called flash dryers when in fact the moisture is not actually flashed off. True flash dryers are sometimes used for soap drying to describe moisture removal when pressure is... 

Rotary, star valve feeder used especially as solids feeders for dilute-phase pneumatic conveying to provide an air lock and to feed solids. Seal/wear depends on Ap and abrasiveness of powder. For pressure systems keep Ap <80 kPa for vacuum systems Ap <40 kPa. Provide an air vent to take the air loss away from the gravity flow of the solids and to control the filling of the star. 

Dense-phase conveying, also termed "nonsuspension" conveying, is normally used to discharge particulate solids or to move materials over short distances. There are several types of equipment such as plug-phase conveyors, fluidized systems, blow tanks, and, more innovative, long-distance systems. Dilute-phase, or dispersed-phase conveyors, are more versatile in use and can be considered the typical pneumatic conveying systems as described in the literature. The most accepted classification of dilute-phase conveyors comprises pressure, vacuum, combined, and closed-loop systems. 

For proper operation of a dilute-phase, pneumatic conveying system the solids fed into the pipeline must be carefully controlled. Two common types of feeders are the venturi feeder and the rotary valve or star feeder, illustrated in Figure 3.31. The venturi feeder is only suitable for low-pressure systems. The rotary valve feeder, also known as star feeder as described previously in this chapter is widely used for feeding medium-pressure conveyors. This feeder is efficient and simple in principle, but requires careful design in order to minimize air leakage. Excessive air loss from feeders, wastes power, causes dust, and leads to system instability. Rotary feeders work well on free-flowing, nonabrasive powders and special types are available for more difficult materials. 

Dilute-phase pneumatic conveying systems (a) pressure system, (b) vacuum system, (c) combined system, and (d) closed-loop system. 

Basically, dilute-phase pneumatic conveying systems can be broken down into three categories based on the physical principle used for conveying. 

In order to complete the design of a dilute-phase pneumatic conveying system, other components of the systems must be determined. 

The layout of the piping systems has many important factors in pneumatic conveying. One should keep the flow path as the most direct between two points. Bends should be eliminated as much as possible. Care should be taken in the design that the distance after a feed point before the first bend is inserted in a minimum of 3 meters (10 feet) when two-phase conditions are present. If the flow is dilute or dense, this distance is not crucial. The two-phase condition tends to cause a sloshing of the solids in the bend in an unsteady condition. This sloshing behavior causes plugging and other upsets in the operation of the pneumatic conveying systems. As noted before, one should at all costs avoid more than two bends in quick succession. 

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