System selection considerations
The selection of a pneumatic conveying system for a particular application involves the consideration of numerous parameters associated with the conveyed material, the conveying conditions and the system itself. The basic specification is usually that a material should be conveyed at a specified flow rate over a given distance. Unfortunately the conveying potential of a pneumatic conveying system is not easily defined or evaluated.
The influence that conveying distance has on material flow, for example, is particularly complex. For any given situation, however, a wide combination of pipeline bores and conveying line pressure drop values are usually available that will adequately meet the requirements. There is rarely a problem of not being able to achieve a given duty, therefore, but getting it right first time is a common problem.
Power consumption, and hence system operating costs are an obvious factor in the decision-making process. This, however, is not straightforward either, for problems of material and system compatibility also have to be taken into account. The inter-relating effects of all of these parameters are considered, both to provide information on the potential capability of pneumatic conveying systems, and as an introduction to the next section of the book, Part B, on System Design.
Generally the most economic system is required that will convey a material satisfactorily, with as few operational problems as possible. System economics are based on a combination of plant capital cost and operating costs. The operating costs take into account costs of power for operation, maintenance and staffing. Maintenance and staffing costs are partly dictated by the capital cost for the plant. Choice of components, such as feeders, filters and pipeline, and automatic controls and instrumentation will dictate to a certain extent the potential level of plant maintenance and manpower that will be required.
Capital costs of plant are generally provided as part of a tender, and so comparison can be made between competing pneumatic conveying systems, and possibly with alternative mechanical conveying systems, for a given duty. Operating costs, in terms of power requirements, however, are relatively easy to evaluate. These costs can often play a dominant role, particularly with pneumatic conveying systems, and so as it is such an important parameter, power requirements are also considered.
With so many different systems, requirements and possible variables to take into account it will only be possible to consider a relatively narrow range in this review. In order to illustrate the potential influence of as many of the main parameters as possible, several series of graphs are included. They are all concerned with continuously operating conventional systems, but they will provide a basis for comparison with other systems. The main emphasis is on conveyed material influences in both dilute and dense phase flow. For dense phase flow only sliding bed flow is considered in detail but reference to plug flow is made for comparison. In dilute phase flow the upper and lower extremes of conveying capability are considered which will cover almost any material.
When selecting a pneumatic conveying system for a particular application it is generally the conveying potential of the system that is of primary importance. The number of factors that have a potential influence on material flow rate, however, is quite considerable. They can be grouped into three broad categories: those associated with the conveyed material, the conveying conditions and the pipeline geometry.
The conveyed material
Properties of the conveyed material that can influence the conveying capability and potential flow rate that can be achieved include mean particle size, particle size distri- bution, particle shape, particle and bulk densities, air retention and permeability. The influence of material properties are dealt with in Chapter 12, with test methods for the determination of material properties relevant to pneumatic conveying being detailed in Appendix 1.
For the purpose of this introductory chapter, two representative materials are considered. These are materials that, in the experience of the author, cover the extremes of conveyability of powdered and granular materials. One is typical of powdered materials, such as bentonite and fly ash that have very good air retention properties. These materials are capable of being conveyed in dense phase and with low air velocities in conventional conveying systems, and are presented here in terms of material type A. The other material is typical of coarse granular materials having poor air retention properties, such as sand and granulated sugar. These materials are only capable of being conveyed in dilute phase suspension flow in conventional conveying systems, and are presented here in terms of material type B.
Material conveying conditions that have a direct influence on material conveying potential include solids loading ratio, conveying line pressure drop, and air flow rate or conveying air velocity. Of these conveying line pressure drop is the only fully inde- pendent variable since both solids loading ratio and conveying air velocity are additionally material dependent.
Conveying line pressure drop
Conveying line pressure drop is a primary variable. It is one of the main variables associated with the energy imparted to the conveying air by the air mover for the system. In order to show the influence of conveying line pressure drop on the flow rate that can be achieved for a given material in a given pipeline, values of conveying line pressure drop up to 3 bar are considered. This adequately covers the operating range of the majority of pneumatic conveying systems, and is sufficiently wide to illustrate the potential influence that higher values of pressure drop can have.
All the data presented here are based on continuously operating systems, since the main objects are to show the potential of pneumatic conveying systems and the relative effect that changes in system parameters can have on operating performance. If a choice is ultimately to be made between a system capable of continuous operation and one based on the intermittent conveying of batches, however, the relationship between the steady state flow rate achieved during batch conveying and the time averaged mean will have to be taken into account (see Figures 2.7 and 4.9).
The solids loading ratio at which the material can be conveyed and the minimum conveying air velocity that can be employed are both dependent upon the properties of the material being conveyed. The influence of material properties features prominently and so the effect of solids loading ratio and minimum conveying air velocity are also considered in detail. Both conveying line pressure drop and conveying distance have an inter-relating effect on these parameters and so these influences are also incorporated.
Pipeline geometry can be varied principally in terms of the length of the pipeline, the bore of the pipe and the number of bends in the pipeline. The influence of pipeline geometry is dealt with specifically in Chapter 14. For the purposes of this introductory chapter a basic pipeline geometry has been selected, and all pipelines considered are geometrically similar so that the influence of changes can be clearly seen.
Pipeline length has to be considered in terms of its orientation, and account must be taken of the individual lengths of horizontal, vertically up and vertically down sections. For this introductory chapter all conveying distances are essentially for horizontal pipeline. Bends are automatically taken into account as discussed below. Any elements of vertical lift in a pipeline can be approximated by taking double the vertical rise and adding this to the total length of horizontal pipeline. Conveying distances that have been considered, on this basis, in general range from about 50 to 500 m, in order to cover as wide a range of applications as possible, and to show the potentials and limitations of pneumatic conveying for long distance conveying.
All pipelines considered here are single bore lines. When high air supply pressures are used for conveying a material the pipeline bore is often increased to a larger size part way along its length. This is particularly the case where high conveying air pressures are employed, when several such changes may be made. Stepped pipelines are con- sidered in Chapters 9 and 14. The range of pipeline diameters considered here is from 50 to 250 mm.
Pipeline bends can have a significant influence on the performance of a pneumatic conveying system pipeline. The number of bends in a pipeline, therefore, is particularly important. In the data presented here the proportion of bends to pipeline length considered is approximately in the ratio of one bend to every 15 m of pipeline.
Bend geometry is another important factor and this is considered in detail in Chapter 14. This is usually considered in terms of the ratio of the bend diameter, D, to the pipe bore, d. Bends can range from those having a very large radius, to elbows and blind tees. In the data presented here the bends in the pipeline have a D/d ratio of about 8:1.