Pressure dilute-phase conveying system

Dilute Phase Conveying. Dilute conveying systems, sometimes called disperse conveying or stream conveying, operate as positive pressure systems at pressures up to 100 kPa (14.5 psig), or as negative pressure systems (vacuum conveying) at pressures up to —50 kPa (—500 mbar). 

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... 

Figure 3.32 shows the four main types of dilute-phase conveying systems previously mentioned. The pressure system, also called positive-pressure, or push system operates at super-atmospheric pressure and is used for delivery to several outlets from one inlet (Figure 3.32a). Although most applications of these systems lie within the scope of dilute-phase conveying, under certain arrangements they can also operate as high-pressure, dense-phase...

Positive displacement blowers Positive displacement blowers are probably the most commonly used type of compressor for dilute phase conveying systems. They provide an ideal match, in terms of pressure capability, with the conventional low pressure rotary valve, and are a typical working combination on many plants. They can be used as vacuum pumps, or exhausters, as well as blowers. The principle of operation is illustrated in Figure 4.26. Twin rotors are mounted on parallel shafts within a casing, and they rotate in opposite directions. As the rotors turn, air is drawn into the spaces between the rotors and the casing wall, and is transported from the inlet to the outlet without compression. As the outlet port is reached, compression takes place when the air in the delivery pressure pipe flows back and meets the trapped air. 

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)... 

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. 

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... 

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. 

Vacuum (negative-pressure) dilute-phase pneumatic conveying. Fig. 1, often is selected for reasons of convenience (e.g. several feed points, any leakage inwards), but without consideration of the relatively high rates of velocity increase and high pick-up velocities (i.e. compared to positive-pressure systems. Fig. 2). The fundamental differences between these two types of transport are demonstrated by tiie following examples. 

The differences in behaviour of different sized particles in a dilute phase (fully suspended flow) pneumatic conveying pipeline was illustrated by the use of a model. The model treated the system as a multi-phase continuum, with the conveying air as one phase and the solids as two other phases. The velocity and pressure profiles for two different particle sizes were illustrated graphically [3]. The model ran with gas and solids parameters typical of a dilute phase pneumatic conveying line. 

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... 

The blow tanks used vary in size up to 50 m or more, generally depending upon the material flow rate required as well as a need to maintain a reasonable frequency of blow tank cycling. The material can be conveyed in dilute or dense phase, depending upon the capability of the material, the pressure available and the conveying distance, as with continuously operating systems. 

---