Solids loading ratio

Solids loading ratio, 4 , as mentioned earlier, is the dimensionless ratio of the mass flow rate of the material conveyed divided by the mass flow rate of the air used to convey the material. Air flow rate is almost exclusively expressed in terms of a volumetric flow rate and so air mass flow rate is most conveniently derived from the conveying line inlet air conditions. A further re-arrangement of the above equations gives  

This then gives the solids loading ratio as  

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The discharge limit of the blow tank used was about 26 tonne/h with the 53 mm bore pipeline and approximately 50tonne/h with the 81 mm bore pipeline. Within this capability of the conveying facility, however, tests were carried out with conveying line pressure drop values of well over two bar and the materials were all capable of being conveyed at solids loading ratios of well over one hundred. A minimum of fifty individual tests were undertaken with every material/pipeline bore combination, in order to draw the various families of curves required. 

There was no lower limit on pressure drop, material flow rate or solids loading ratio that the test facility could operate,... 

Conveying data presented in this form clearly show the capability of pneumatic conveying systems and the inter-relating effects of pressure, material concentration and pipeline bore, as well as air flow rate when designing a system to convey a material at a given flow rate, over a specified distance. Since there is generally a limit on air supply pressure, a compromise has to be made between solids loading ratio and pipeline bore. 

In Fig. 18, the results are presented in terms of a normalised pressure gradient, which is the ratio of the pressure gradient for a particular angle of inclination divided by that for the horizontal. Once again, solids loading ratios of 5, 10 and 20 were considered. 

Because of the high gas velocities, solids loading ratios, and momentum loss in the collision zone, the pressure loss in ISDs is much greater than in pneumatic dryers, but it is comparable with that of fluidized and spouted bed dryers [44,45], The impinging stream configurations can, however, compete in various aspects with the classical systems for drying of particulates and pastes (Table 21.11). 

Because of the high gas velocities, solids loading ratios, and momentum loss in the collision zone, the pressure loss in ISDs is much greater than in... 

With moving bed flows, solids loading ratios of well over 100 can be achieved if materials are conveyed with pressure gradients of about 20 mbar/m of horizontal pipeline. For plug-type flows the use of solids loading ratio is not as appropriate, for as the materials have to be very permeable, maximum values are only of the order of about 30. Despite the low value of solids loading ratio, materials can be reliably conveyed at velocities of 3 m/s and below in plug-type flow. 

Figure 4.33 The influence of solids loading ratio on the minimum conveying air velocity for the pnemnatic conveying of ordinary portland cement.

Figure 4.34 Approximate influence of solids loading ratio on conveying line pressure gradient for the horizontal conveying of cement.

For material flows vertically down the pressure gradient can be positive or negative, depending upon the value of solids loading ratio at which the material is conveyed. For... 

For the cement the increase in power with increase in pipeline bore can also be explained in terms of velocity profiles, but in this case it is values of conveying line inlet air velocity that are relevant. Since cement is capable of being conveyed in dense phase, the relationship between minimum velocity and solids loading ratio, as shown in Figure 4.41, dictates. In an 81 mm bore pipeline the inlet velocity is only 4.2 m/s, since the solids loading ratio is 109. In the 200 mm bore pipeline the solids loading ratio is reduced to 14 and so the inlet air velocity is 12.0 m/s. 

Since pipeline bore comes in incremental sizes, fine tuning and spare capability need to be considered in terms of reserve pressure available. With first approximation values for pressure and pipeline bore, the available conveying data can be scaled more precisely to take account of differences between pipeline geometries. Conveying air velocities and the solids loading ratio can be evaluated so that differences between air only pressure drop and acceleration pressure drop values can also be taken into account. This is an iterative process, as there are many inter-dependent variables, and so in the initial stages approximations can be made. 

The value required depends very much upon the materials being conveyed and the mode of conveying. Typical values for Cmin were presented in Figure 4.41 with a recommendation that Cl should be about 20% greater than this (Equation (4.12)). For fine powders capable of being conveyed in dense phase the value of the solids loading ratio also needs to be... 

The variation in the lower layer volume fraction is due to a tenfold increase in the gas mass flow rate in the layer, which acts to fluidise the layer. The balance of mass flow rates through the pipe cross section 20m downstream of the inlet is shown in Table 4. From this table it can be seen that there is a significant change in the nature of the flow as the solids loading ratio is increased ... 

Conv ng line inlet and outlet air velocities have also been added for completeness, and from this it will be seen that the pressure minimum point occurs at a conveying line inlet air velocity of about 10 m/s, over the entire pressure range. The maximum value of solids loading ratio of only 30 is typical for the low velocity dense phase conveying of this type of material, because of its extremely high value of penneability. 

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