Logic diagram for system design
A logic diagram for the design of a pneumatic conveying system based on the use of test data is presented in Figure 15.3. The process is traced from the specification of the fixed parameters, through the necessary scaling procedures, to the final specification of the most suitable pipeline bore and air requirements. Full details are given of all the individual stages, as indicated in Figure 15.3, together with an explanation of the various loops incorporated.
Specify mass flow rate of material required
This specification is essentially the same as that at stage 2 for the corresponding logic diagram in Figure 15.1, based on the use of equations. Account must be made of whether the system is to be continuous or batch operating, and the conveying line feeding device must be capable of meeting the flow rate requirements.
Although not specifically added as a stage to this logic diagram, it would always be recommended that comprehensive details of every material to be conveyed should always be kept on file for reference, as detailed in Section 15.2.1.1.
Specify conveying distance required
This specification is also the same as that for conveying distance at stage 3 in Figure 15.1. Pipeline bore is again an entirely separate variable and is not considered at this stage.
Conveying characteristics for material
The conveying data points or set of conveying characteristics for a material obtained from conveying trials form the starting point in a design based on experimental data. Conveying characteristics for a number of materials were presented in the previous four Chapters. These were included to illustrate the potential differences that can exist between materials with respect to minimum conveying air velocities, mode of convey- ing, material flow rates for given conveying conditions, and the slope of the constant conveying line pressure drop curves. All this information is embodied in the convey- ing characteristics, and so system design is simply based on the scaling of the conveying characteristics for a specified material from the test situation to the plant requirements. The scaling is in terms of the pipeline geometry.
Scaling is clearly critical in this process, and the closer the test line is to the plant situation the more accurate will be the analysis. However, scaling can be carried out with a reasonable degree of accuracy over a fairly wide range of pipeline bores and distances.
Scaling parameters for various aspects of pipeline geometry are presented in Chapter 14. Conveying characteristics are presented at numerous points throughout this Design Guide and in each case details of the pipeline through which the material was conveyed are also given. These conveying characteristics could, therefore, be used as the starting point for a system design for the pneumatic conveying of any of the materials presented.
Scale to conveying distance
Scaling the conveying characteristics for a material is best carried out in two stages. The first stage involves scaling to the required conveying distance, with allowances for vertical sections and bends. In the second stage the resulting data or conveying char- acteristics are scaled in terms of pipeline bore.
Scaling with respect to conveying distance is a fairly complex process and can result in marked differences in conveying parameters, as was illustrated between Figure 14.1 and 14.4. Significant changes can result in material flow rate, solid loading ratio, and air flow rate (in the case of materials capable of being conveyed in dense phase). In order to illustrate the order of magnitude of these changes, and to provide additional guidance at this point, the influence of conveying distance, pipeline bore and material type is specifically considered later in Section 4 of this chapter.
Once again it is recommended that when actual design data is extracted from the results of the scaling process a margin of 20 per cent is allowed with regard to air flow rate for the design point taken in relation to the minimum conveying conditions. This is summarized with Equation (15.1).
Can material flow rate be achieved?
This stage is essentially one of checking whether, for the given pipeline bore the material flow rate can be achieved. If the conveying characteristics for the material were deter- mined for a wide range of conveying line pressure drop values, it is probable that the required material flow rate would be achieved if a wide range of pipeline bores are considered. The decision here is essentially the same as that outlined at stage 14 for Figure 15.1. If a preference exists for a low pressure system or a particular pipeline bore, then the choice will be automatically restricted. If there are no restraints then a full survey could be carried out in order to determine the most economic combination of parameters.
Calculate power required
If, for a specified pipeline bore, the material flow rate can be achieved, then the power required can be determined. A model that can conveniently be used to determine the approximate power required was presented with Equation (15.7). The air mass flow rate is required for this model but it can be obtained directly from the conveying char- acteristics for the material.
Scale to different pipeline bore
If the required material flow rate cannot be achieved with a given pipeline bore, or if the power requirement for a certain pipeline bore is not satisfactory, the conveying characteristics should be scaled to another size of pipeline and the process repeated. The influence of pipeline bore on conveying parameters is also considered in Section 4 of this chapter.
Specify pipeline bore required
This specification is the same as that for pipeline bore at stage 15 in connection with Figure 15.1.
Specify air requirements
This specification is the same as that for air requirements at stage 16 in Figure 15.1, where an appropriate model for volumetric air flow rate was presented (see also Equation (15.9)). Allowances will also have to be made for air leakage and other com- ponent pressure drops as discussed at stage 16 for the corresponding logic diagram based on the use of equations.