The determination of conveying characteristics
Although the analysis of single phase flow is well established, that for the two phase flow of solid particles in a gas is not. Mathematical and empirical models have been derived to predict the influence of the many variables, but their use is generally very limited. Where models are available they are likely to be restricted to a narrow range of operating conditions, and nothing is currently available that will cover the entire range of the conveying characteristics shown in Figure 11.5b. It is necessary, therefore, to carry out tests with the actual material in a pneumatic conveying test facility.
The necessity for carrying out tests with the actual material for which the data is required is essential, for conveying characteristics can vary significantly from one mater- ial to another, and even between different grades of the same material. Carrying out tests in a similar pipeline, however, is not as critical as it is possible to use scaling parameters to scale the conveying characteristics from a test line to an actual plant pipeline. This aspect of system design is considered in Chapter 14.
Instrumentation and control
In order to determine the conveying characteristics for a material it is necessary to have a conveying plant that has sufficient controls and instrumentation to enable conveying trials to be carried out over as wide a range of conditions as possible. Air flow rate, material flow rate and conveying line pressure drop are the main parameters that have to be measured, and air flow rate and material flow rate need to be varied over as wide range as possible, within the limits imposed by the conveying air supply.
Rotameters, orifice plates and choked flow nozzles are among some of the devices that can be used for the measurement of air flow rate. The choice depends upon the mag- nitude of the flow rate, the pressure at which it has to be measured, and whether the flow is subject to pulsations.
Load cells are ideal for the measurement of material flow rate. These are used either on the supply hopper or the reception hopper. On the supply hopper to the feeding device, or on a blow tank if this is used, loss in weight will be measured. On the recep- tion hopper, gain in weight will be recorded. Whichever is more convenient can be used. Load cells on both hoppers would only be required if it were necessary to observe mate- rial deposition in the pipeline.
Conveying line pressure drop for a given pipeline system can be measured quite simply with a pressure gauge, although a pressure transducer would be preferred. If this is positioned in an air supply or extraction line, a bourdon type gauge can be reliably used. This is because there should be no material in the flow to interfere with the recording. It will also give a very reasonable indication of the pressure drop since losses prior to the conveying line and that across the filtration system will generally be negligible in comparison with that across the pipeline.
If individual elements of a pipeline need to be assessed in isolation, such as bends or straight horizontal or vertical sections of pipeline, however, recordings will need to be monitored from a series of pressure tappings in the conveying line itself. In this type of situation pressure transducers will be essential and pressure tappings will have to be carefully designed to eliminate the possibility of dust affecting the accuracy of the read- ings. Such recording techniques are particularly necessary for changes in section and direction in the pipeline, such as step changes and bends. These will result in a deceleration of the particles, and the effects of these will be seen for many metres of straight pipeline following the change.
If full controls are available on a conveying plant it should be possible to convey a mater- ial at any required flow rate, at any conveying air velocity and with any conveying line pressure drop within the capabilities of the system. Individual tests on this basis, how- ever, take a long time to carry out since, with so many variables, very precise conveying conditions have to be established and then maintained each time. Precise material flow rate is also difficult to achieve with a blow tank, if this is used, since their discharge characteristics are also dependent upon the properties of the material being conveyed (see Section 18.104.22.168.2).
The method usually adopted is to set the plant in operation and record the necessary results when steady state conditions are obtained. If material and air flow rates are each progressively changed over as wide a range as possible a large amount of data can be obtained very quickly. Subsequent analysis of the results is then reasonably straightfor- ward since so much information is available.
A few tests are generally conducted without the material so that the pressure drop for the empty line can be determined, and thereby establish a datum for the conveying line as illustrated in Figure 11.1. Tests should also be repeated periodically to provide a check on the condition of the conveyed material, if this is being re-circulated.
Presentation of results
Graphical representation of the results is probably the best method of displaying the inter- relating effects of the many variables in the problem. With a number of major variables and a large number of test results, the drawing of families of curves provides an ideal means of both handling the data and presenting results. If two of the variables are chosen for the x- and y-axes of a graph, all the test results for a third variable can be marked on this graph. They can be appropriately rounded for convenience, with the decimal point representing the actual location of the test results on the graph. Lines of constant value of this variable can then be drawn through the data to provide a family of curves.
Results obtained from tests carried out with cement conveyed through the 95 m long pipeline of 81 mm bore shown in Figure 11.2 are presented in Figure 11.6. Figure 11.6a is a graph of conveying line pressure drop against air flow rate and Figure 11.6b is a graph of material flow rate against air flow rate. In each case experimental values of the third variable are plotted. Lines of constant value of the given parameter have been drawn through the data and it will be noticed that the family of curves drawn can be clearly identified from the data, despite the fact that no two tests were carried out at the same pressure and with the same material flow rates.
In Figure 11.7a the curves have been drawn without hindrance of the test results and lines of constant solids loading ratio have also been superimposed to produce the conveying characteristics for the cement in the given pipeline.
It must be emphasized that these conveying characteristics relate only to this material in this pipeline. The conveying characteristics for another material, or for this material in
(a) material flow rate data and (b) pressure drop data.
another pipeline, could differ very significantly from that for the cement shown in Figure 11.7a. The lines of constant conveying line pressure drop could be in different positions relative to the material flow rate axis, have a different shape and slope, and terminate at totally different values of air flow rate. It is for this reason that it is necessary to deter- mine the conveying characteristics of the actual material to be conveyed.
From the conveying characteristics for the cement in Figure 11.7a the adverse effect of conveying the material with too high an air flow rate can be clearly seen. Although the cement can be successfully conveyed over the entire range that the conveying characteristics cover, and beyond at even higher air flow rates, the trend for this particular material, in the pipeline tested, is to decrease in material flow rate with increase in air flow rate for a constant value of conveying line pressure drop. This applies over the entire range of air flow rates investigated. A more detailed analysis of the influence of air flow rate, and hence the choice of conveying parameters, is considered in more detail later.
Determination of minimum conveying conditions
In order to determine the minimum conveying conditions for the cement, a graph of conveying line inlet air velocity drawn against solids loading ratio is presented in Figure 11.7b. On this graph some of the low velocity test results have been plotted. The spread of results was obtained because a wide range of conveying conditions was required for the characteristics to be drawn, but they do show a distinct trend, and a curve representing the possible minimum conveying conditions is drawn.
It has been found that the minimum conveying conditions for most materials can be correlated in this manner. This is a major parameter in system design, and although
the data can be obtained from Figure 11.7a, it is a much more complex relationship in this form because of the additional influence of pressure on conveying air velocity. Plots such as those in Figure 11.7b, therefore, provide a very useful means of identifying minimum conveying conditions for materials. The exact position of the curve on Figure 11.7b, which represents the minimum conveying conditions, is rather difficult to locate. If the pipeline is blocked no experimental results are obtained for the test, although in some cases it might be possible to estimate the approximate location on the graph from tests which preceded it.
As this is the design parameter that dictates the air requirements in terms of volumetric flow rate for a conveying system it would obviously be expedient to specify an air mover having a capacity with a reasonable margin, in order to allow for any differences in this relationship that might occur if a material with a slightly different specification has to be conveyed. Although an optimum design would normally be based on the sys- tem operating as close as possible to the minimum conveying conditions, a margin in air flow rate would be advisable in case the solids loading ratio specified in the design was, for some reason, on the low side.
The use of conveying characteristics
With conveying line pressure drop, solids loading ratio and both material and air flow rates all represented on the one graph, all the data necessary for the design of a pneumatic conveying system is available. If a system has to be designed to achieve a given flow rate, a point on the conveying characteristics must be chosen just above the minimum convey- ing conditions to ensure that the pipeline will not block. This point gives the compressor rating required, in terms of delivery pressure and volumetric flow rate (evaluated from the air mass flow rate), and the solids loading ratio of the conveyed material.
Alternatively, if a compressor or conveying system is already available with a given air supply pressure, the conveying characteristics can be used to determine the volumetric flow rate required to achieve optimum conveying conditions. They will also give the expected material flow rate and solids loading ratio. A particularly advantageous feature of presenting design information in this form is that it can be scaled quite easily. Conveying characteristics for a given material in one pipeline can be scaled to that of another pipeline of a different length, bore and configuration. The conveying characteristics themselves are scaled and so design data is obtained directly for the new pipeline. This process is considered in Chapter14.