Pneumatic Vacuum Conveying Systems | Pneumatic Powder Transfer System | Negative Pressure Vacuum Pump

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.

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Positive pressure Pneumatic Conveying system | Material Conveying with Pneumatic Conveyors | Pneumatic Conveyor Advantages and Disadvantages

Positive pressure:

Positive pressure system is normally employed to pick up material from one source and deliver the same too many position.

Basic Principle of Positive Pressure Pneumatic Conveyors:

The essential component of a positive pressure system consists of a blower which sucks atmosphere air through an air inlet filter and delivers the air at higher pressure to the conveying line. The material which is stored in a storage silo or hopper is fed into the conveying line through a rotary air lock feeder. A basic problem in all the pressure system is how to introduce the material into the conveying pipeline which carries air under pressure. It is feeding device that has to cater for this and hence the special type of rotary air lock feeder is normally used with this type of systems to minimize leakage of air into the storage silo which is at atmosphere pressure. In general, a positive pressure system is not suitable for multiple pick up points because the air leakage through several rotary valves may be considerably high as compared to total air requirements. The material is then carried in suspension along with air through the conveying pipeline and delivered to one or more than one receiving hoppers. Receiving hoppers are nothing but a kind of silo or bin where the material is stored before use. If more than one receiving hoppers are to be served by the same conveying line, diversion valves are used in the pipeline to deliver the supply of material to the desired receiving hopper.

Pneumatic Conveying Cyclone Separators

Separation of solid from the air is usually achieved with the help of a cyclone separator and a bag filter unit which are fitted on the top of the receiving hoppers. A cyclone separator is essentially a cylindrical vessel with its axis vertical and the conveying pipeline is connected with it in such a way that the mixture of solid and air enters the cyclones tangentially and receives a circular motion within the vessel. This produces a vortex flow of the mixture of solid from air is achieved in cyclone separator, still the air contains certain amount of dust particles to be filtered in bag filter unit. The relatively cleaner air is taken out from the separator from the top and led into another cylindrical vessel where a bank of vertical bags, of cotton, cambish or other artificial fibres, are arranged in such a way that the air is made to pass through these bags before going out into the atmosphere. The dust particles carried by air from the cyclone separator are now filtered before being discharged into atmosphere. The filter bags are frequently shaken mechanically to drop the solid particles to the receiving hoppers.

Advantages and Disadvantages of Positive Pressure Conveying System:

An important merit of the positive pressure system is that the blower or fan, which is the heart of the conveying system, is always free from the dust of the material to be conveyed. On the other hand, a serious concern in the case of positive pressure system is the elimination of leakage from the system to the surroundings, a factor which becomes especially important when toxic material is to be conveyed.

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