Conveying characteristics
Introduction
The capability or a pneumatic conveying system for conveying bulk particulate materials 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, at present. For this reason the conveying characteristics of many different materials are presented and featured in order to illustrate the importance and significance of material properties.
Conveying characteristics
If a pneumatic conveying system is to be designed to ensure satisfactory operation, and to achieve maximum efficiency, it is necessary to know the conveying character- istics of the material to be handled. The conveying characteristics will tell a designer what the minimum conveying velocity is for the material, whether there is an optimum velocity at which the material can be conveyed, and what pipeline diameter and air mover rating will be required for a given material flow rate and conveying distance.
Alternatively, for an existing pneumatic conveying plant, the appropriate conveying characteristics will tell a designer what flow rate to expect if it is necessary to convey a different material, and whether the air flow rate is satisfactory. Conveying characteristics can also be used to check and optimize an existing plant if it is not operating satisfactorily. This aspect is considered in Chapter 21.
In order to be able to specify a pipe size and compressor rating for a required duty it is necessary to have information on the conveying characteristics of the material. If sufficient previous experience with a material is available, such that the conveying characteristics for the material are already established, it should be possible to base a design on the known information.
If previous experience with a material is not available, or is not sufficient for a full investigation, it will be necessary to carry out pneumatic conveying trials with the material. These should be planned such that they will provide data on the relationships between material flow rate, air flow rate and conveying line pressure drop over as wide a range of conveying conditions as can be achieved with the material.
The trials should also provide information on the minimum conveying air velocity for the material and how this is influenced by conveying conditions. This is particularly
important in the case of dense phase conveying, for the differences in conveying characteristics between materials can be very much greater than those for dilute phase conveying.
If the investigation is to cover the entire range of conveying modes with the material, then the previous experience must be available over a similar range or conveying condi- tions. Scale up in terms of air supply pressure, pipe bore, conveying distance and pipeline geometry from existing data is reasonably predictable, provided if the extrapolation is not extended too far. Scale up in terms of mode of conveying, into regions of much higher solids loading ratios and lower conveying air velocities, however, should not be attempted unless evidence of the potential of the material for such conveying is available.
Conveying mode
If the pressure gradient available is sufficiently high, conveying is possible in the dense phase mode, provided that the material is capable or being conveyed in this mode. It is the influence of material properties on the possible mode of conveying, as well as differences in material flow rates achieved for identical conveying conditions that makes it essential for conveying trials to be carried out with an untried material. In conveying tests where the operating pressure gradient is high there is an additional need, therefore, to establish the limits of conveying and this may be over a very wide range of conveying conditions.
In addition to material properties, conveying distance can have a significant influence on the solids loading ratio, at which a material can be conveyed, and hence mode of conveying that is possible. The influencing factor here is simply pressure gradient, and this will limit conveying potential regardless of the capabilities of the material. Pressure gradient is simply the conveying line pressure drop available divided by the equivalent length of the pipeline. This was introduced in Chapter 1, and in Figure 1.1 an approximate relationship was presented between pressure gradient and the solids loading ratio that might be achieved with a material capable of dense phase flow in a sliding bed mode of flow.