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.
No comments:
Post a Comment