Review of pneumatic conveying systems:System requirements and Multiple pick-up

System requirements

The uses, applications and requirements of pneumatic conveying systems are many and varied. A number of system requirements were highlighted at various points with regard to the systems. Some of the more common requirements of systems can be identified and are detailed here for easy access and reference, since these may feature prominently in the choice of a particular system.

Multiple pick-up

If multiple point feeding into a common line is required, a vacuum system would generally be recommended. Although positive pressure systems could be used, air leakage across feeding devices such as rotary valves represent a major problem. The air leakage from a number of feed points would also result in a significant energy loss. The air loss could be overcome by adding isolation valves to each feed point, but this would add to the cost and complexity of the system.

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Multiple delivery

Multiple delivery to a number of reception point can easily be arranged with positive pressure systems. Diverter valves can be used most conveniently for this purpose. The problem with vacuum systems performing this function is equivalent to the problem of using a positive pressure system for the multiple pick-up of materials.

Multiple pick-up and delivery

The suck–blow, or combined vacuum and positive pressure, system is ideal for situ- ations where both multiple pick-up and delivery is required. The pressure available for conveying is rather limited with this type of system and so if it is necessary to convey over a long distance, a dual system would be more appropriate. In this the vacuum and positive pressure conveying functions are separated and a high pressure system can be used to achieve the distant conveying requirement.

Multiple material handling

If it is required to handle two or more materials with the one system, reference must be made to the conveying characteristics for each material to be conveyed. It is quite likely that the air requirements for the materials will differ to a large extent. In this case it will be necessary to base the air requirements, to be specified for the air mover, on the mater- ial requiring the highest conveying line inlet air velocity. Consideration will then have to be given to a means of controlling the air flow rate, to lower values, for the other mater- ials, if this should be required. It is also likely that the flow rate of each material will be different. The feeding device, therefore, will have to meet the needs of every material, in terms of flow rate and control. These issues are dealt with at length in the Guide.

Multiple distance conveying

If it is required to convey a material over a range of distances, such as a road tanker sup- plying a number of different installations, or a pipeline supplying a number of widely spaced reception points, consideration will again have to be given to differing air requirements and material flow rates. For a given air supply, the material flow rate will decrease with increase in conveying distance, and so the material feeding device will need to be controlled to meet the variation in conveying capability. For materials cap- able of being conveyed in dense phase there is the added problem that the air flow rate will also need to be increased for longer distance conveying.

Conveying from stockpiles

If the material is to be conveyed from a stockpile, then a vacuum system using suction nozzles will be ideal. The type of system required will depend upon the application and conveying distance. For a short distance a vacuum system will probably meet the demand on its own. Where access is available to a free surface, as in ship off-loading, vacuum nozzles can transfer material under vacuum to a surge hopper. If this is not the final destination for the material it could be the intermediate hopper in a combined positive and negative pressure conveying system, or the supply hopper for the second part of a dual system, from where the material can be blown to a distant reception point.

For clearing dust accumulations and spillages, and surplus material deposited in stockpiles, mobile units are particularly useful. These are generally suck–blow systems with a vacuum nozzle. Although they can be small versions of a continuously operating suck–blow system, they are more usually batch conveying systems with the transfer hop- per acting also as a blow tank. Material is first drawn into the hopper/blow tank under vacuum, and when it is full it is pressurized and conveyed on to the reception point.

Start-up with full pipeline

If there is likely to be a need to stop and start the conveying system while it is conveying material, a system capable of doing this will need to be selected. This is rarely possible in conventional systems, unless a large air receiver is installed specifically for the pur- pose, and so consideration will have to be given to innovatory systems. Many of these systems are capable of starting with a full pipeline, although their capabilities on vertical sections may need to be checked, particularly if the stoppage is for a long period. The possibility of power cuts, from whatever source, should also be taken into account here.

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