Low pressure and vacuum:Air leakage

Introduction

All pneumatic conveying systems, whether they are of the positive or negative pressure type, conveying continuously or in a batch-wise mode, can be considered to consist of the basic elements depicted in Figure 3.1.

In terms of pneumatic conveying system components, a considerable number of devices have been specifically developed to feed materials into pipelines. The material feeding device is particularly critical to the successful operation of the system and so with a large number of devices from which to select, two chapters are devoted to this topic. A number of devices have also been developed to disengage materials from the conveying air at the reception point and these are considered separately in Chapter 7.

Air movers are equally important components and these are considered in Chapter 6. With air movers, however, it is more a matter of selection from existing machines, but an early choice for pneumatic conveying systems was made with the positive dis- placement blower. These can be used as either compressors or exhausters, but they are limited in operation to a pressure ratio of about 2. This means that their maximum positive pressure capability is about 1 bar gauge, and for vacuum duties it is about 0.5 bar. The conventional rotary valve has been a very commonly used feeding device and this has a very similar upper pressure limitation, and hence the division between these two chapters with regard to feeding devices.

Air leakage

In vacuum systems the material feeding is invariably at atmospheric pressure and so the pipeline can either be fed directly from a supply hopper or by means of suction

Pipeline feeding devices Part 1- Low pressure and vacuum-0026

nozzles from a storage vessel or stockpile. The main point to bear in mind, however, is that there will be no adverse pressure gradient against which the material has to be fed. The feeder, therefore, does not have to be designed to additionally withstand a pres- sure difference. With no adverse pressure drop to feed across it also means that there will be no leakage of air across the device when feeding material into the pipeline. Separation systems in these cases, therefore, by necessity, do have to operate under vacuum conditions.

In positive pressure systems, separation devices invariably operate at atmospheric pressure. Pipeline feeding in positive pressure systems represents a particular problem, however, for if the material is contained in a storage hopper at atmospheric pressure, the material has to be fed against a pressure gradient. As a consequence of this there may be a loss of conveying air. The feeding device in this case has to be designed to withstand the pressure difference in addition.

In certain cases this air flow can hinder the downward gravity flow of material into the feeder and hence interfere with the feeding process. Also, if the loss is significant, the volumetric air flow rate will have to be increased to compensate, for the correct air flow rate to the pipeline must be maintained for conveying the material. This loss, therefore, represents a loss of energy from the system.

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