Year

1989

Degree Name

Doctor of Philosophy

Department

Department of Mechanical Engineering

Abstract

The pneumatic conveying of bulk solids through pipelines has been used in industry for several decades. With the introduction in recent years of new techniques and more efficient hardware, there has been a considerable increase in the use of this method of transport (e.g. dense-phase, low-velocity and long-distance conveying). Unfortunately, the technology available to assess the relative merits of the large number of commercial systems that now compete for a particular application is lacking sadly, especially when efficient and reliable dense-phase or long-distance transportation is required. The main objective of this thesis is to provide industry with some of this technology in relation to fine powders (e.g. pulverised coal, fly ash, PVC powder, fly ash/cement mix) and some coarser products (e.g. screened coke, crushed bath, granulated aluminate).

A convenient method for presenting the variation of major steady-state conveying parameters is needed for efficient design, system evaluation and optimisation. One technique based on other work and extended to include saltation and minimum transport behaviour is established. A standardised-test procedure comprising three different types of pneumatic conveying experiment also is developed to generate efficiently the data required for this purpose.

The method of scaling up test rig data to full-scale installations, previously used quite extensively in the design of pneumatic conveying systems, is investigated and found to be inadequate in particular applications. Two popular forms of definition and three existing empirical correlations for the solids pressure drop are modified to demonstrate the possible extent of this inadequacy. Steady-state pipeline conveying characteristics of three products are used in the development of an improved scale-up procedure. Methods to predict the air-only pressure drop for both single- and stepped-diameter pipelines and to generalise the conveying characteristics of a particular material (applicable to other combinations of length and diameter) also are formulated and verified.

Pulverised coal conveyed over 25 m and fly ash over 943 and 293 m (utilising three different configurations of blow tank) are used to investigate the effect of blow tank air injection on the performance of a pneumatic conveying system. The addition of supplementary conveying-air to a blow tank incorporating a top-air supply and transporting a good dense-phase material (pulverised coal) is shown to achieve higher values of mass flow ratio and/or conveying rate and also provide smoother and more consistent transportation. The installation of a fluidising discharge cone to the outlet of a blow tank conveying a cohesive fly ash is found to improve the discharge characteristics of the blow tank, as well as decrease pressure and flow rate fluctuations.

The method of air injection also is found to have a significant impact on the plug-phase mode of conveying. Experiments on three different products are carried out to demonstrate the advantages of this method of transport (i.e. to handle conventionally difficult dense-phase materials, such as crushed bath) but also its sensitivity to changes in material property (viz. particle size). However, it is shown further that this may be compensated to some extent by selecting a different method of air injection.

Two powder classification techniques based on physical properties are evaluated and found useful in explaining and indicating the minimum transport (dense-phase) behaviour for a wide range of materials. The steady-state pipeline conveying characteristics (dilute- and dense-phase) and the fluidisation behaviour of ten products are compared for this purpose.

Various mathematical models utilising numerical integration and analytical approximations are formulated to predict blow tank performance characteristics. Despite the lack of good accurate data for the experimental verification of these models (i.e. due to certain difficulties in measurement technique), preliminary results still are obtained and presented in graphical format. Five existing pipeline theories also are investigated and reviewed. One particular model is found useful in predicting the dense-phase conveying parameters of fine powders, and a worked example is presented.

The applicability of generalised solids friction factor correlations to the design of pneumatic conveying systems is reviewed. The resulting degree of uncertainty is considered too great for applications involving relatively high operating pressures (e.g. long-distance and/or large-throughput conveying). Test rig data obtained from pulverised coal, a fly ash/cement mix and various fly ash samples are used to identify certain areas of improvement. Based on this work, a test-design procedure is developed to determine an accurate solids friction factor correlation (i.e. for a given material and a wide range of diameters). Results from recent investigations into the long-distance pneumatic conveying of pulverised coal are used to demonstrate the good accuracy and reliability of this improved approach.

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