12 1. Introduction
This research program has been undertaken at HiT (Telemark College, Department of Technology) and Telemark Technological Research and Development Centre, in collaboration with the University of Greenwich. The project has mainly been experimental in nature, and the aim of the investigation has been to establish a link between the physical characteristics of a particulate material, measurable on small samples in a laboratory, and the conveyability of the material in a pneumatic conveying line.
The flow in pneumatic transport systems is immensely complex. Unlike single phase systems (gas and liquid) particulate materials frequently are composed of entities which may have sizes ranging over several orders of magnitude. Current models for simulating such systems, by means of computers, resort to simplifying this to a small number of species of particles with different size, that may be viewed as separate "phases" in a multi phase system [1]. For certain materials encountered in "real life" this may be a very coarse simplification. One may hope that the "phases" are chosen so that each of them represent size classes that dominate the behaviour of the flow. Frequently the size classes are lumped together and called the disperse phase.
Several factors therefore have to be considered when investigating the conveyability of particulate materials. First of all it is necessary to define what is meant by conveyability. It is also necessary to consider which mechanisms influence the single particle and the collective behaviour of the material transported in an air flow. Furthermore it is desirable to determine which physical characteristics of the material influence these mechanisms
In the summary of the minutes of the first workshop-meeting on pneumatic conveying held in Karlsruhe in 1991 [2] questions regarding various aspects of conveyability are raised.
• Are there limitations in the conveying mode due to powder characteristics?
• Is there a limitation in the mass flow rate for different powders?
• Can pressure gradients be estimated on the basis of powder characteristics?
Ph.D. Thesis S.E.Martinussen Chapter 1, Introduction
• What is the minimum conveying velocity for fine materials?
These questions are all directly associated with conveyability. If the mode of flow is not suitable (for reasons of stability in the mass flow), the necessary flow rate can not be obtained, the pressure gradient is too high for the air supply with the given pipe length or, the air velocity falls below the minimum velocity, this has direct influence on the regularity of the transport of material in the pneumatic conveying pipeline. These aspects can be quantified in the form of a conveying characteristic, which provides a relationship between the pressure gradient, the mass flow of solids, and the conveying velocity. Other aspects of conveyability such as the explosivity, the abrasivity, or the cohesivity of the material may also be considered. But these aspects are difficult to incorporate into design equations for pneumatic conveying equipment, although such aspects may be investigated in separate tests. In this work, the prediction of minimum conveying velocities, based on material characteristics forms the main subject of the investigation.
The motivation for starting the work has been to enable evaluation, and improvement, of existing models for predicting pneumatic transport system performance. The design of pneumatic conveying systems is not only subject to the risk of under-dimensioning with regard to capacity, as for gas or liquid flow systems, it is also subject to the risk of total failure in the form of blockage. As will be shown in this thesis, the current state of the art in engineering formulae does not allow safe design to be carried out. Therefore the design of pneumatic conveying systems is, at present, largely based on experimentally obtained data, in the form of conveying characteristics, displaying the relation between mass flow of solids, air velocity and pressure drop for a given material. Obtaining a conveying characteristic is a time consuming procedure. The possibility of obtaining usable models for computing the design parameters from laboratory test data of the characteristic properties of the material transported, is therefore a major incitement for the work.
The existing design equations for pneumatic conveying systems can be divided into two
Ph.D. Thesis S.E.Martinussen Chapter 1, Introduction
14 particular. Both types of design equations are required to enable the design of a pneumatic conveying system. The conveying limit prediction is dependent on the start pressure in the pipeline, and the pressure drop depends on the conveying velocity chosen. The solution of the set of equations therefore results in an iterative process. Accurate prediction of the conveying limit is particularly important in that it enables the evaluation of the performance of existing transport systems to promote more economical operation by adjusting the air flow.
The approach taken in this investigation has been to carry out an extensive literature survey on minimum conveying velocity in horizontal pneumatic transport. Thereafter the main focus of the research program was on obtaining data for the seven different materials (polyethylene pellets, rape seed, sand, PVC granules, alumina, micronized dolomite and cement) included in the investigation. A series of measurements were then made to identify minimum conveying conditions for these seven materials. To eliminate the effect of pipeline geometry, these investigations were carried out in a straight horizontal pipeline with no flow hindrances. Several kinds of data analysis have been used to identify the nature of the mechanisms that govern the change in mode of flow, or blockage. This was considered to be necessary to improve modelling of the phenomena, and to identify the factors that influence the conveying limits of a material.
Ph.D. Thesis S.E.Martinussen Chapter 2, Review of Current Methods for Predicting Conveying Limits
2. A Review of the Influence of Material Characteristics and Solids Loading