For positive pressure conveying systems air extraction provides a viable alternative to the stepping of a pipeline if a high air supply pressure is employed. For an existing conveying system air extraction is potentially a quicker and more convenient method of improving pipeline performance than stepping the pipeline. It is also a very much more precise method and one that is capable of control. Pipeline stepping is dependent upon the availability of pipeline in different bore sizes and the range available is necessarily limited. For a given range of pipe sizes the lengths of the individual sections must be carefully evaluated, for if the steps are incorrectly located, the pipeline could block.

With the alternative air extraction method such problems need not arise because the quantity of air extracted can be controlled. The number of steps in a stepped pipeline is limited by the availability of standard pipe sizes, but no such restraint applies to the air extraction system because the rate of air extraction can be infinitely variable. This also means that air extraction points can be located at convenient points along the length of the pipeline. Stepped pipeline systems also present problems with regard to pipeline purging, particularly when materials are conveyed in dense phase and at low velocity. There are no such problems with the air extraction system since there is no change in pipeline bore.

For materials such as polyethylene pellets the air extraction system is a natural choice, since air filtration requirements at the extraction points are minimal. To provide some indication of the capability and flexibility of the system, together with the design requirements of the air extraction system, a case study is presented in this paper. A direct comparison with an alternative stepped pipeline system is included in order to illustrate the differences between the two systems. The performance of the equivalent single bore pipeline is also included for comparative purposes. A first approximation method of design analysis is presented for the purpose.