Pneumatic conveying system: Is transportation of dry bulk particulate or granular materials through a pipeline by a stream of gas. Normally air is used for conveying and occasionally nitrogen is used in situations where there is a fire or explosion risk.
Basic parts of pneumatic conveyor system:
- Blower or compressor
- Feeding device
- Gas/solids disengaging device (Filters)
- Pipe line
Principle of operation:
The pneumatic conveyor system consists of a source of compressed air, a means of feeding the product into the pipeline, and a receiving hopper fitted with a means of separating the conveyed product from the conveying air. The blower or compressor supplies the compressed air to the conveying system. This compressed will travel towards the less pressure side (positive pressure system). The solid material is fed into the air stream by means of feeder device like a rotary valve or screw feeder or venturi feeder etc. There by the solid particles are conveyed to another location by the travelling air. The product is separated from the conveying air by means of filter at the destination.
Classification of pneumatic conveyors:
a. Based on method of carrying:
Dilute- phase: In the case of dilute phase flow, the bulk solid is conveyed in suspension with the particles more or less uniformly distributed over the cross section of the pipe. To keep the particles in suspension in the pipe line it is necessary to ensure that the conveying velocity does not fall below a certain minimum value which for the majority of bulk solids, is about 13 – 15 m/s.
Dense-phase: When the conveying velocity is less than that required to keep the bulk solid in suspension and particles begin to settle to the bottom of the pipe, the flow is said to be in a dense-phase mode.
b. Based on pressure:
1. Low pressure conveying systems
i. Positive-pressure systems.
ii. Negative – pressure (Vacuum) system: In vacuum conveyor system bulk solid is picked up at inlet end of the conveying line and transported by the flowing gas to the discharge end. The basic difference is the air mover is at the discharge end of the pipeline. The other differences are the components required to feed the bulk solid into the conveying line and to separate it from the gas at the discharge point. Since the conveying gas finally has to pass through the fan or blower, it is important to ensure that the solid material is adequately separated from the gas. Thus a high-efficiency gas/solid disengaging device is an essential requirement. Since the solid material does not have to be fed against an adverse pressure, the feeding mechanism is simple.
2. High pressure systems: The conveying method will be in dense – phase. The blow tank provides the means for feeding the bulk solids to be transported into the pipeline. In this system product is delivered to the pipeline in batches as the blow tank is filled and emptied. The blow tank is essentially a pressure vessel, which is gravity fed with product from the top and then, after closing the feed valve, and with the valve on the conveying line closed, is pressurized. With the compressor still operating, the outlet valve is opened and conveying starts
c. Based on velocity:
1.Low velocity conveying
2.High velocity conveying