System selection considerations
From the foregoing analysis it will be seen that with so many variables to consider, and with many alternative combinations of parameters capable of meeting most conveying requirements, a comprehensive logic diagram for the selection of a pneumatic conveying system would be too complex to be practicable. At a very basic level, however, systems can be considered simply in terms of dilute and dense phase and this is dictated either by material type or by conveying distance. This can be illustrated by reference to Figures 8.35 and 8.36.
In Figures 8.35 and 36 three values of each of the requirement parameters, material flow rate and conveying distance are considered. The design parameters, in terms of conveying line pressure drop and pipeline bore, for each combination of material flow rate and conveying distance, are presented in the boxes beneath. The boxes are in direct line from the appropriate values of material flow rate and conveying distance.
The design parameters in Figure 8.35 are for a material having very good air retention properties and in Figure 8.36 they are for a material having very poor air retention properties. Values of conveying line pressure drop and pipeline bore correspond to the combination resulting in the lowest operating power requirements, for each material, over the range of the variables considered.
Materials capable of dense phase conveying
From Figure 8.35 it will be seen that for a material having very good air retention properties the choice of system is dependent upon conveying distance. For short distance conveying a dense phase system would be preferred, and for long distance conveying only dilute phase conveying is possible, with a conventional system. The transition from dense to dilute phase conveying occurs at a distance of 300–400 m with this material and with the limit of 3 bar on conveying line pressure drop that has been considered.
This represents the maximum, for the transition will occur at a shorter distance with a material not having such good air retention properties, or poorer conveying characteristics. For the material with very poor air retention properties in Figure 8.36 only dilute phase conveying is available over the range of conveying distances considered.
Although dense phase conveying is recommended when this is possible for a material, personal factors may dictate the selection of a dilute phase system. The decision presented here is simply based on operating costs. Plant capital costs, or a desire to use low pressure air movers or certain feeding devices, may result in a dilute phase system being preferred. As demonstrated in this chapter, a wide range of conveying parameter combinations are generally available for a material capable of being conveyed in dense phase, and so the material could be conveyed quite successfully in either dilute or dense phase in many cases.
Alternatives to dilute phase conveying
When the choice of conventional conveying system is restricted to dilute phase, either because of the distance conveyed or the properties of the material, the use of an alter- native innovatory system, such as pulse phase, plug control or air addition system, may be well worth considering. If the material is capable of being conveyed in such a system over the distance required, and this is clearly the first point to establish, such a system should enable the material to be conveyed at much lower velocities, which will help with respect of plant erosion and particle degradation.
The next point to establish is whether the operating costs are lower, for the plant capital costs will almost certainly be higher. Operating costs will depend upon the conveying characteristics for the material. If these are similar to those of material A in Figure 8.2 then the operating costs will be significantly lower, but if they are similar to those of material C in Figure 8.2 any such benefit is likely to be marginal. The ultimate decision, therefore, needs to be made in relation to these various points.