Conveying air velocity

Minimum conveying air velocity was also determined during the test programme. It was found that this velocity is influenced by system pressure, with its value decreasing from 10.5 m/s at atmospheric pressure to 6m/s at a pressure of 20 bar gauge. This effect was expected due to a significant increase of gas density at higher pressures. 

Control conveying air velocity, optimize air-to-solid ratio, install new parallel blowers, replace existing blower with bigger capacity, replace rotary valves, increase speed of blowers, screw feeders and rotary valves... 

Using the above models and method of feeding, the following optimal operating conditions were predicted for the extreme case of maximum pipeline length and maximum throughput. Note, a stepped-diameter pipeline was selected to minimize pressure drop, air mass flow rate and hence, conveying air velocities. 

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. 

Figure 4.33 The influence of solids loading ratio on the minimum conveying air velocity for the pnemnatic conveying of ordinary portland cement.

Conveying air velocity is clearly important and that at the material feed point into the pipeline is critical. A problem comes in evaluation of this velocity, for compressors are specified in terms of a given quantity of air being delivered at a given pressure, and the reference point for each is different. The volumetric flow rate of the air delivered, Vo, is that at free air conditions (standard atmospheric pressure and temperature) and this will generally be close to that at the pipeline exit. The delivery pressure will be close to that at the pipeline inlet. Compressibility, therefore, must be taken into account. 

Since pipeline bore comes in incremental sizes, fine tuning and spare capability need to be considered in terms of reserve pressure available. With first approximation values for pressure and pipeline bore, the available conveying data can be scaled more precisely to take account of differences between pipeline geometries. Conveying air velocities and the solids loading ratio can be evaluated so that differences between air only pressure drop and acceleration pressure drop values can also be taken into account. This is an iterative process, as there are many inter-dependent variables, and so in the initial stages approximations can be made. 

If data on minimum conveying air velocities is to be used in the design process, such as that shown in Figure 4.41, it would generally be recommended that a 20% margin be allowed for the value of conveying line inlet air velocity to be employed ... 

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