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Facts at your Fingertips: Pneumatic Conveying Flow Patterns

By Scott Jenkins, Chemical Engineering magazine |

Pneumatic conveying is a ubiquitous mode of conveying bulk solids in a wide range of industries, including chemicals, plastics, grain, food, agriculture, mining, power generation and cement, among others. The flow patterns within conveying lines depend on the properties of the materials being moved, the pressure drop across the conveying route and the velocity of the conveying gas. This one-page reference provides information about plotting relationships between these variables.

Zenz plot

The relationship between flow pattern (mode of conveying) and key operating variables (such as pressure drop, gas velocity and solids flowrate) for a given conveying layout is best represented by the system state diagram (Figure 1; often called a Zenz plot). Today’s versions of the Zenz diagram typically plot the overall pressure drop against terminal gas velocity on a linear scale. In any pneumatic conveying system, each combination of bulk material characteristics and specific pipe-routing schematics will have its own unique characteristics, hence a system-specific state diagram. Overall pressure drop is used for the ordinate ( y-axis). This diagram is independent of the feeding technology.

In the system state diagram, the lowest curve represents the pressure drop characteristics of single-phase (gas) flow in a conveying line. Here, the pressure drop is proportional to the square of gas velocity. When solids are introduced into the conveying line, additional energy is required to overcome losses due to friction, wall impacts, and to initially accelerate the particles and then reaccelerate them after bends or vertical lifts. These losses manifest themselves as additional pressure drop, which increases with rising solids flowrate.

Saltation velocity

If the conveying gas velocity is sufficiently high, then stable dilute-phase conveying conditions will prevail, where all particles are fully suspended in the conveying gas. As the gas velocity (or gas flowrate) is reduced, the pressure drop continues to decrease, even though the solids flowrate remains constant. Correspondingly, the flow pattern in the conveying line changes from fully suspended flow to stratified flow, with a higher concentration of particles in the lower section of the pipe. Eventually, the particles begin to fall out of suspension and begin to roll, slide and move along the bottom of the pipe. The gas velocity corresponding to this state of flow is called the saltation velocity, and the corresponding pressure drop shows a minimum in the characteristic curve. Operating conditions to the left of the pressure drop minimum will result in additional settling of particles (saltation), which leads to sluggish conveying behavior and may cause temporary plugging of the conveying line along with intense line vibrations.

 

Figure 1. The state diagram of pneumatic conveying shows the relationships between key operating variables

Figure 1. The state diagram of pneumatic conveying shows the relationships between key operating variables

Dense phase conveying

This unstable zone (shown in Figure 1) separates the dilute-phase from the dense-phase areas in the state diagram. If the conveying pressure required for stable conditions exceeds the available pressure from the blower, compressor or compressed plant air supply, then the conveying system will stall or plug. With further decreases in gas velocity, the zone of stable dense-phase conveying is reached — to the left of the unstable zone (that is, at lower gas velocities but significantly higher pressures).

The conditions that produce stable, dense-phase conveying are much more limited compared to those that produce dilute-phase conveying. Finally, the line shown furthest to the left in Figure 1 represents the termination of dense-phase conveying in the form of a stationary plug.

The zone of stable, dense-phase conveying is wedged between the unstable region and the conveying limit. The pressure required to move a slug of solids is significantly higher than that required for dilute-phase conveying at the same conveying rate — thus, most practical applications require the use of compressors.

Dense-phase conveying is often referred to as “slow-motion conveying.” Not all high-pressure conveying systems will actually operate in dense-phase mode or in slow-motion conveying. One must pay particular attention to the system design, line stepping (that is, increasing the line diameter along the conveying length to reduce local gas velocity) and velocity profile, and manage the conveying gas, to achieve dense-phase conveying.

Editor’s note: This “Facts at your Fingertips” column was adapted from the following article (Reference 1): Wilms, H. and Dhodapkar, S., Pneumatic conveying: optimal system design, operation and control, Chem. Eng., Oct. 2014, pp. 59–67.

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