There is essentially no limit to the capability or a pneumatic conveying system for the conveying of dry bulk particulate materials. Almost any material can be conveyed and high material flow rates can be achieved over long distances. There are, however, practical limitations and these are mainly imposed by the fact that the conveying medium, being a gas, is compressible. The limiting parameters are then mainly the economic ones of scale and power requirements. Conveying capability depends mainly upon five parameters. These are pipe bore, conveying distance, pressure available, conveying air velocity and material
properties. The influence of many of these variables is reasonably predictable but that of the conveyed material is not fully understood at present.
4.4.1 Pipeline bore
The major influence on material flow rate is that of pipeline bore. If a greater material flow rate is required it can always be achieved by increasing the pipeline bore, generally regardless of the other parameters. In a larger bore pipeline a larger cross-sectional area is available and this usually equates to the capability of conveying more material.
4.4.2 Conveying distance
In common with the single phase flow of liquids and gases, conveying line pressure drop is approximately directly proportional to distance. Long distance conveying, therefore, tends to equate to high pressure, particularly if a high material flow rate is required. For the majority of conveying applications, however, it is not convenient to use high pressures. As a consequence, long distance, with respect to pneumatic conveying, typically means about 11/2km, and dilute phase conveying.
4.4.3 Pressure
Although air, and other gases, can be compressed to very high pressures, it is not generally convenient to use air at very high pressure. The reason for this is that air is compressible and so its volumetric flow rate constantly increases as the pressure decreases. In hydraulic conveying, pressures in excess of 100 bar can be used so that materials can be conveyed over distances of 100 km with a single stage. With water being essentially incompressible, changes in the velocity of the water over this distance are not very significant. In pneumatic conveying, air at a pressure above about 1 bar gauge is generally considered to be ‘high pressure’. Although the air expansion can be accommodated to a certain extent by stepping the pipeline to a larger bore, as illustrated in Figure 4.30, this is a complex design procedure.
As a consequence, air pressures above about 5 bar are rarely used for pneumatic conveying systems that deliver materials to reception points at atmospheric pressure.
4.4.4 Conveying air velocity
The main design parameter with respect to pneumatic conveying is conveying air velocity, and more particularly, conveying line inlet air or ‘pick-up’ velocity. Since the air expands along the length of the pipeline it will always be a minimum at the material feed point at the start of the pipeline, in a single bore pipeline, regardless of whether it is a positive pressure or a vacuum conveying system. In a single bore pipeline the velocity will be a maximum at the end of the pipeline. It is the value of the minimum velocity of the air that is critical to the successful operation of a pneumatic conveying system. In dilute phase conveying the particles are conveyed in suspension in the air and this relatively high value of velocity is due, in part, to the large difference in density between the particles and the air. In hydraulic conveying typical velocities for suspension flow are only about 11/2m/s, but the difference in density between water and particles is very little in comparison.
4.4.5 Particle velocity
In dilute phase conveying, with particles in suspension in the air, the mechanism of conveying is one of drag force. The velocity of the particles, therefore, will be lower than that of the conveying air. It is a difficult and complex process to measure particle velocity, and apart from research purposes, particle velocity is rarely measured. As a consequence it is generally only the velocity of the air that is ever referred to in pneumatic conveying. This is effectively the ‘superficial air velocity’, because the presence of the particles is disregarded in evaluating the value of velocity. The velocities quoted on Figure 4.30, therefore, are conveying air velocities.
In a horizontal pipeline the velocity of the particles will typically be about 80% of that of the air. This is usually expressed in terms of a slip ratio, defined in terms of the velocity of the particles divided by the velocity of the air transporting the particles, and in this case it would be 0.8. The value depends upon the particle size, shape and density, and so the value can vary over an extremely wide range. In vertically upward flow in a pipeline a typical value of the slip ratio will be about 0.7 in comparison.
These values relate to steady flow conditions in pipelines remote from the point at which the material is fed into the pipeline, bends in the pipeline and other possible flow distur-bances. At the point at which the material is fed into the pipeline, the material will essentially have zero velocity. The material will then be accelerated by the conveying air to its slip velocity value. This process will require a pipeline length of many metres and this distance is referred to as the ‘acceleration length’. The actual distance will depend once again on particle size, shape and density. The process was illustrated earlier in Figure 4.29 in relation to the pressure drop across a bend.
4.4.6 Material properties
The properties of the conveyed material have a major influence on the conveying capability of a pneumatic conveying system. It is the properties of the material that dictate whether the material can be conveyed in dense phase in a conventional conveying system, and the minimum value of conveying air velocity required. For this reason the conveying charac-teristics of several different materials are presented in order to illustrate the importance and significance of material properties.
Although it is the properties of the bulk material, such as particle size and size distribution, particle shape and particle density that are important in this respect, at this point in time it is the measurable properties of materials in bulk that are more fully understood. These include air–material interactions, such as air retention and permeability, and are more convenient to use. In general, materials that have either good air retention or good permeability will be capable of being conveyed in dense phase and at low velocity in a conventional conveying system. Materials that have neither good air retention nor good permeability will be limited to dilute phase suspension flow.
4.4.7 Dense phase conveying
As mentioned earlier, there are two main mechanisms of low velocity dense phase flow. For materials that have good air retention, the material tends to be conveyed as a fluidised mass.
Air mass flow rate (kg/s)
Figure 4.32 Conveying data for ordinary portland cement.
In a horizontal pipeline the vast majority of the material will flow along the bottom of the pipeline, rather like water, with air above but carrying very little material. For materials that have good permeability the material tends to be conveyed in plugs through the pipeline. The plugs fill the full bore of the pipeline and are separated by short air gaps. As the conveying air velocity is reduced, the air gap between the plugs gradually fills with material along the bottom of the pipeline and the plug ultimately moves as a ripple along the top of an almost static bed of material. As the air flow rate reduces, to give very low conveying air velocities, the material flow rate also reduces.
Materials composed entirely of large mono-sized particles, such as polyethylene and nylon pellets, peanuts and certain grains and seeds, convey very well in plug flow. In dilute phase conveying, nylons and polymers can suffer damage in the formation of angel hairs, and grains and seeds may not germinate as a consequence of damage caused at the high velocities necessary for conveying. Because of the very high permeability necessary, air will readily permeate through the material while it is being conveyed and so maximum values of solids loading ratio will typically be about 30.
4.4.8 Sliding bed flow
Pneumatic conveying data for cement is presented in Figure 4.32. This is essentially a performance map for the material in a given pipeline and is a graph of material flow rate against air mass flow rate, with lines of constant conveying line pressure drop plotted.
The pressure drop lines are derived from experimental data obtained from conveying the material in the given pipeline, which was 53 mm bore, of 50 m length, almost entirely in the horizontal plane and included nine 90◦bends having a bend diameter to bore ratio of 24:1. Since solids loading ratio is the ratio of the material to air flow rates, these can simply
Solids loading ratio (dimensionless)
Minimum conveying air velocity (m/s
) 12
8
4
00 20 40 60 80 100
Transitional relationship Dilute phase conveying limit
Ultimate dense phase conveying limit
Figure 4.33 The influence of solids loading ratio on the minimum conveying air velocity for the pneumatic conveying of ordinary portland cement.
be added as straight lines through the origin. The lines of constant conveying line inlet air velocity can be plotted from basic thermodynamic relationships.
It will be seen from Figure 4.32 that the cement could be conveyed at high values of solids loading ratio and with low values of conveying line inlet air velocity, and hence in dense phase. These ‘conveying characteristics’ are typical of those for most fine powdered materials that have very good air retention. At very low values of conveying line pressure drop it will be seen that the minimum value of conveying line inlet air velocity for the cement is about 11 m/s. Everything to the right of this line, at higher velocities, is entirely dilute phase suspension flow. There is a natural transition between dilute and dense phase flow, and because of the air expansion the flow can change from dense to dilute phase along the length of a pipeline when velocities are in the transition zone.
The locus of the line defining the ‘no go area’ for the cement is given in Figure 4.33.
The minimum value of conveying air velocity required for the reliable conveying of the cement is dependent upon the solids loading ratio, or the concentration of the cement in the pipeline. As the solids loading ratio of the material increases, the conveying line pressure required also increases, and so much higher pressures are required to convey a material in dense phase than in dilute phase. At low velocities, however, problems of erosive wear with abrasive materials, and particle degradation with friable materials, are significantly lower. For materials such as cement, power requirements are also very much lower.
With a limit on air pressure used for conveying, because of the problems associated with air expansion, dense phase conveying, as a consequence, is additionally limited by conveying distance. If conveying distance is doubled, for example, for the same air supply pressure, the value of solids loading ratio will fall by about half. With the same pressure there will be no need for an increase in air flow rate, but the material flow rate will have to drop by about half to compensate, since there is no increase in energy to the conveying system. Since conveying capability is dependent upon both parameters, pressure gradient is a more convenient term to use and the effect can be shown in Figure 4.34.
30 25 20 15 10 5
00 40 80 120 160 200
Pressure gradient (mbar/m)
Solids loading ratio
Figure 4.34 Approximate influence of solids loading ratio on conveying line pressure gradient for the hori-zontal conveying of cement.
For materials that are capable of being conveyed in dense phase, therefore, this capability will only be possible if the air supply pressure is relatively high or the conveying distance is relatively short. For low pressure or long distance conveying it will only be possible to convey a material in dilute phase, even if the material has dense phase conveying capability.
Pressure gradient is simply the ratio of the pressure drop available for conveying, divided by the equivalent length of the pipeline. For convenience it is expressed in mbar per metre of horizontal pipeline.
Conveying distance is in terms of an equivalent length in order to take account of vertical sections and the number and geometry of bends in the pipeline. The reference for equivalent length is that for straight horizontal pipeline. For flow vertically up a scaling parameter of two approximately holds, such that the equivalent length is double the actual vertical lift in terms of straight horizontal pipeline. The equivalent length of bends depends very much upon the value of the conveying air velocity. A correlation derived for cement and fine fly ash is presented in Figure 4.35 (Mills 2004; Mills et al. 2004).
4.4.9 Plug flow
Pneumatic conveying data for polyethylene pellets is presented in Figure 4.36. The pellets were conveyed through the same pipeline as the cement in Figure 4.32 and so a direct comparison of performance is possible, as the data was obtained over the same range of air flow rates and air supply pressures. Although conveying is possible at low values of conveying air velocity, there is a marked change in material flow rate for low velocity conveying. An optimum maximum value of material flow rate occurs at a conveying line inlet air velocity of about 15 m/s, which corresponds closely with the minimum value of conveying air velocity for the dilute phase conveying of the material.
As with the cement, there is a natural transition from dilute to dense phase conveying for the material. With the lines of constant pressure drop being so close together in the dense phase conveying region, however, care must be exercised with controlling material flow rate in the region close to the conveying limit. Solids loading ratio values are significantly
Conveying line inlet air velocity (m/s)
Figure 4.35 Equivalent length of long radius 90◦bends for the conveying of cement.
lower because of the high permeability of the material and the very different form of the conveying characteristics with low velocity dense phase conveying of the material. For the dense phase conveying of this type of material, therefore, solids loading ratio does not have the same significance as it does for sliding bed flow.
4.4.10 Dilute phase conveying
Pneumatic conveying data for granulated sugar is presented in Figure 4.37. Once again this material was also conveyed through the same pipeline as that for the cement and polyethylene pellets and so further direct comparison of conveying performance is possible.
Granulated sugar has very poor air retention capability, and poor permeability, and so can
Air mass flow rate (kg/s)
Figure 4.36 Conveying data for polyethylene pellets.
Air mass flow rate (kg/s)
Figure 4.37 Conveying data for granulated sugar.
only be conveyed in dilute phase suspension flow in a conventional conveying system. The pipeline was relatively short and the air supply pressure was quite high, but despite the fact that the pressure gradient was very high, the sugar could not be conveyed in dense phase and hence at low velocity. High pressure, therefore, is not synonymous with dense phase conveying.
It will be seen from Figure 4.37 that the minimum conveying air velocity for the sugar was about 16 m/s. The maximum value of solids loading ratio achieved was just over 15 and so this is quite clearly dilute phase suspension flow conveying. From Figure 4.31, it will be seen that the cement could be conveyed at a solids loading ratio of about 40 in dilute phase flow. This is because the minimum conveying air velocity was only 11 m/s and the cement could be conveyed at a very much higher material flow rate.
With no dense phase conveying capability the operating envelope for the sugar is signif-icantly smaller than that for either the cement or the polyethylene pellets. For any given pressure drop and air flow rate the material flow rate achieved for the sugar was also sig-nificantly lower than that for either the cement or the polyethylene pellets. A comparison of the performance of a number of different materials conveyed through this same pipeline is presented in Figure 4.38.
It will be seen from Figure 4.38 that the conveying capability of different materials can vary widely, and not only in terms of dilute and dense phase conveying. Every material was conveyed in dilute phase suspension flow and even here there was a 2:1 variation.
Differences are clearly evident between sliding bed and plug modes of dense phase flow but particularly wide differences exist between powdered materials, having good air retention, at low air flow rates, and hence low velocities. Copper concentrate might be classified as
‘medium phase’ but this capability was due to the fact that the air retention capability was not as good as the other finer materials. The coke fines had a lower value of minimum conveying air velocity than the granulated sugar due to the fact that the material had a very much wider particle size distribution. It can also be recorded that particle density does not correlate very well with these materials.
0 0.02 0.04 0.06 0.08 0.10 Air mass flow rate (kg/s)
Conveying line pressure drop1.5 = 1.5 bar
Material flow rate (tonne/h)
Figure 4.38 Comparison of the pneumatic conveying performance of different materials conveyed under identical conditions.