Negative pressure (Vacuum) Conveying system:

A negative pressure system is employed to pick up material from multiple sources and deliver the same to one remotely located point.

Basic principle of negative pressure pneumatic conveying system:

The basic principle of operation of a negative pressure system consists of a vacuum pump which is in communication with the conveying pipeline via the separation unit (cyclone separator) and the bag filter unit. Because of the vacuum produced in the system, air is drawn in the conveying pipeline through the air inlet nozzle. The material from the storage hopper or silo is introduced in the conveying line in which a negative pressure is maintained, through suitable feeders. As the material is introduced in the pipeline at negative pressure, the feeders used in this type of system need not be complex in design. A number of storage silos may be connected with the same conveying line, although when one feeder deliver material to the conveying line, usually other feeders are cut out. However, the material is conveyed along the pipelines to the cyclone separator where the major portion of the solid is separated from the air. The relatively clean air from the separator is allowed to pass through the bag filter unit which further cleans the air. The cyclone separator and the bag filter both deliver materials to the receiving hopper where the material is stored for use.

The most notable point in favour of this system is that all air leakage is from the surroundings into the system, so that pollution of the surroundings by the material being conveyed is practically eliminated. Hence this type of system is particularly suitable for toxic, explosive and powder like materials. But a serious limitation of the system is that, as the vacuum pump is located at the discharge end, unless the solid air separation unit and beg filter unit is especially efficient, there is possibility of dust laden air entering the vacuum pump and thereby damaging the life of the same. It is due to this reason that in a vacuum system multiple receiving hoppers cannot be used. Another limitation of the negative pressure system is that in practice only about 0.7 bar pressure drop can be allowed in the whole system and hence long distance conveying and high tonnage of conveying rate is not possible with a negative pressure system.