Pneumatic conveying dense-phase systems

Pneumatic conveying dense phase No jJow or flow < design plugged line/mal-function of line boosters because of stuck check valve/high humidity. Solids fed to conveying line < design ratio of air to fluidize in the blow tank relative to convey is too small/fault in control system. 

Figure 1. Breakdown of dense phase pneumatic conveying systems. 

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. 

The book is arranged in two parts Part I deals with basic relationships and phenomena, including particle size and properties, collision mechanics of solids, momentum transfer and charge transfer, heat and mass transfer, basic equations, and intrinsic phenomena in gas-solid flows. Part II discusses the characteristics of selected gas-solid flow systems such as gas-solid separators, hopper and standpipe flows, dense-phase fluidized beds, circulating fluidized beds, pneumatic conveying systems, and heat and mass transfer in fluidization systems. 

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. 

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. 

The properties of the conveyed material have a major influence on the conveying capability of a pneumatic conveying system. It is the properties of the material that dictate whether the material can be conveyed in dense phase in a conventional conveying system, and the minimum value of conveying air velocity required. For this reason the conveying characteristics of several different materials are presented in order to illustrate the importance and significance of material properties. 

A model has been developed to simulate the non-suspension moving-bed type of flow in pneumatic conveying systems. The flow is modelled as two layers a dilute gas-solids mixture flowing above a dense gas-solids mixture. For each layer the conservation equations for mass, momentum and energy were solved for both the gas and solids phases. In addition mass, momentum and energy transfers between the two layers were modelled. The prediction of pressure profile and the depth of the dense layer showed good agreement with experimental observations. 

Crowther J.M. et al, The Design of a Gamma Ray Tomograph for the Study of Pneumatic Conveying Systems in Dense Phase, Nondestr. Test. Eval., 14,143-162,1998. 

A commercial twin-plane ECT system was cormected to two 12 electrode rings on a horizontal length of pipe, as shown in Figure 2 and cormected in the pneumatic flow loop as shown in Fig 3. The loop is designed for dense phase conveying, 5-200 kg solids/kg air, at velocities of 1 to 30 m/s. 

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