15 System Design Using Conveying Data

1

INTRODUCTION

The design of pneumatic conveying systems is still very much based on the scaling of conveying data. Such data is generally obtained from a purpose built test facility, similar to the data presented in the previous five chapters. It is not practical, of course, that the plant pipeline should be replicated for test purposes. For convenience of testing different materials, the reception point at the end of the pipeline is generally located above the material feeding device to provide a convenient loop, so that material can be re-circulated, where possible. As a consequence the test loop is likely to be mostly in the horizontal plane and may contain a disproportionate number of bends. 1.1

The Use of Scaling Parameters

Scaling parameters, however, are available, as considered in Chapters 7 and 8, that will enable data obtained from one pipeline to be scaled to that for another pipeline. In Chapter 7 scaling parameters were presented that will allow for differences in pipeline bore and conveying distance to be taken into account, between that of the test facility and that of the plant pipeline to be designed. In Chapter 8 similar consideration was given to the influence of pipeline bends, both in terms of number and geometry, and to pipeline orientation, including vertically up and vertically down routings. The influence of pipeline material was also considered. It is

Copyright  2004 by Marcel Dekker, Inc. All Rights Reserved.

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Chapter 15

for these reasons that full details of all pipelines have been given, for which conveying data has been presented in this Handbook. 1.2

Manufacturers'Approach

Most manufacturers of pneumatic conveying systems have built such test facilities in order to obtain data for design purposes. They will generally ask for a representative sample of the material to be conveyed and demonstrate the conveying of the material to the client. This, of course, provides an element of protection for the vendor, should the client manufacture or source a slightly different grade of material when the system comes into use. Manufacturers, however, are unlikely to derive entire performance maps for a material, such as those presented here. The cost of running and maintaining such a test facility is very high, and most companies will generally advertise the fact that they will undertake conveying tests at 'no charge'. Only a couple of tests are likely to be undertaken, therefore, in order to establish the conveyability of the material. The vendor will know what type of system it intends to offer a client and so only limited data will be required. 1.2.1 Previous Experience Alternatively data obtained from previous experience of designing and building a system may be used, provided that the data relates to exactly the same material. Extreme caution must be exercised here, for different grades of exactly the same material can have very different conveying characteristics. It is also important to note that data should never be scaled either to lower values of conveying air velocity, or to higher values of solids loading ratios, than have previously been achieved with the material. 1.3

Cases Considered

Since the geometry of the test facility, or a previous system installed, is unlikely to be the same as that of the plant pipeline to be built, scaling parameters are used. Account may additionally have to be taken of changes in solids loading ratio, and hence conveying line inlet air velocity, particularly if the scaling is to a longer pipeline. A review of appropriate scaling parameters is presented, and these are illustrated with two case studies. One is for a material conveyed in dilute phase, suspension flow. Another is for a material capable of being conveyed in dense phase, non suspension flow. 1.4

The Future

For single phase fluids, as is well known, the problem of analyzing the flow was ultimately solved empirically by the use of a friction coefficient in conjunction with Reynolds number and pipe wall roughness. The parallel problem with twophase, gas-solid flows will no doubt be solved one day, and it is also likely to be

Copyright  2004 by Marcel Dekker, Inc. All Rights Reserved.

Design Using Conveying Data

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an empirical solution, but that day is not yet in sight. Meanwhile companies that manufacture pneumatic conveying systems are reluctant to publish any data that they have because it is of too high a commercial value, and there are very few research groups around the world that are working in this area f I]. 2

SCALING PARAMETERS

Scaling parameters for system design can be split into two main groups and the scaling process can be undertaken in two stages. The first stage relates to horizontal conveying distance. This is expressed in terms of an equivalent length and incorporates pipeline bends, pipeline orientation and pipeline material. The second stage relates to pipeline bore. In each case allowance must be made for the difference in air only pressure drop for the given pipeline. This is because the scaling parameters relate only to the conveying of the material in the air. Scaling relates to individual test points or data. If a complete set of conveying characteristics require to be scaled, a considerable number of data points will need to be scaled and so it is a time consuming process. The best way of doing this is to place a grid on the conveying characteristics and scale for every grid point. The most convenient grid for this purpose is probably one based on lines of constant conveying line inlet air velocity and lines of constant conveying line pressure drop, at regular increments of each. Both equivalent length and pipeline bore can have a very significant influence on material flow rate through a pipeline. It must be recalled, however, that any required material flow rate can generally be achieved, over any given conveying distance, with an appropriate combination of pipeline bore and air supply pressure or vacuum. The limitation, for any extreme value of either material flow rate or conveying distance, is generally power requirement. 2.1

Equivalent Length

The equivalent length of a pipeline, as mentioned above, incorporates straight pipeline sections and pipeline bends. The scaling parameter for equivalent length is an inverse law model. This was presented earlier in Chapter 7 and is reproduced here for reference. mp