Optimizing and up-rating of existing systems
There is often a need in industry for an existing plant to be up-rated to meet a demand for increased output or production. If part of a process plant includes pneumatic con- veying facilities it is not always obvious as to how this might be achieved. It may be possible to increase capacity simply be optimizing the existing system. Alternatively it might be necessary to add or replace some of the plant.
All too often, when it is required to increase the capacity of an existing pneumatic conveying system, an attempt is made at improving output by increasing the amount of air used for conveying the material in a pipeline. This is usually done by adding another blower in parallel, by changing the existing blower for one with a much higher volu- metric flow rate, or by changing the drive such that the rotor speed of the blower is increased. In nearly all cases, however, the net result of these simple modifications is that the material flow rate through the pipeline does not increase at all, but decreases, and often by a considerable amount.
There are many different solutions to the problem and consideration is given to some of the alternatives that are available. An explanation is also given as to how an existing system could be tested in order to check whether it is operating under optimum conditions. Various methods are compared, in terms of potential material flow rates, and the effect that the changing of one plant item can have on the rest of the system are considered. Although low pressure continuously operating systems using positive displacement blowers are generally used for reference purposes in this chapter, the underlying principles and points considered will generally apply equally to any other type of system.
Optimizing conveying conditions
Engineers asked to undertake the design of a pneumatic conveying system are usually aware of the situation with respect to air flow rate, but are often not certain of the relationship between conveying air flow rate, or velocity, and material flow rate, air supply pressure and pipeline layout. The problem, of course, is that different materials can have totally different conveying characteristics and that conveying distance can also influence these characteristics, as illustrated at many points in Part B of this ‘Design guide’.
Unless conveying trials are carried out with a material, or previous experience with the material is available, it is unlikely that the plant could be built to achieve the required output without over-design in certain areas. They will know that a dilute phase conveying system will not operate if the velocity is below the saltation or choking velocity. They are equally aware, however, that the system will operate reasonably well, although inefficiently, if the velocity is on the high side.
The tendency, therefore, is to ‘play safe’ and either install a pipeline with a larger bore than necessary, or to install a blower having a capacity much greater than is actu- ally necessary. If, on commissioning the plant, the design flow rates are not achieved, the situation can usually be rectified by changing the V-belt drive gear ratio to achieve a lower volume output. It is quite likely, therefore, that the output of an existing conveying plant could be increased quite simply by adjusting the air flow rate and optimizing the conveying of the material in the existing pipeline.