Classification of Pneumatic Conveying Systems
Pneumatic conveying systems can be classified on different basis of consideration. These basis are listed below:
- On average particle concentration (modes)
- On air pressure (Types of systems)
- On air supply arrangement
- On solid feeder type
Of these, the first two are very important and often influence the choice of the specific design for a given material, loading and delivery condition and the distance to be covered.
Classification based on Average Particle Concentration – Modes of Conveying:
Depending on the mass flow ratio, defined as the ratio of mass of particles conveyed to the mass of fluid used to convey, pneumatic conveying system may be classified into two modes; namely
- Dilute phase
- Dense phase
If the mass flow ratio is low, the system is said to operate in dilute phase. Whereas, if the mass flow ratio is high, the system is said to operate in dense phase. If the mode of operation of a system is in dilute phase, the probable range of mass flow ratio is 0 – 15. The dense phase system operates at mass flow ratio over 15.
In a dilute phase system the material is carried through the pipeline by a large volume of air having high velocity but relatively low pressure. The stream of air or gas carries the material in suspension in the pipeline as discrete particles owing to lift and drag forces acting on each particles. The distribution of particles over the cross section of the pipe is fairly uniform. In order to keep the particles in suspension, the air must possess a minimum velocity, called the pick up velocity. The pick up velocity for a particular material depends on many factors, like, the shape, size, specific weight of material and inclination of the pipeline.
If the velocity of air is gradually lowered down below what is required to keep the particles in suspension, the particles gradually settle down and form dunes at the bottom of the pipeline all along the length of the pipe. In this condition the material is about to choke the pipeline and if the pressure is increased these dunes and plugs of materials may move along the pipeline causing a dense phase flow. In a dense phase flow usually the velocity of fluid is much lower than the minimum velocity required for a dilute phase flow. The distribution of particles over the cross section of the pipe is non uniform. At the lower part of the pipe there is slow moving dunes or plugs of material and the upper part of the cross section of the pipe is filled with certain proportion of finer particles in suspension in a state of dilute phase.
The maximum mass flow ratio achievable for a dense phase flow depends on many factors like the nature of materials and air velocity. It is usually greater than 30
Classification based on Air Pressure:
Based on air pressure, Pneumatic conveying systems may be classified as follows:
- Low pressure systems, in which the operating air pressure is about 1 atmosphere (760 mm Hg). This type of systems may be further sub classified into
- Positive pressure system
- Negative pressure system
- Combined positive – negative pressure system
- Medium pressure system
- High pressure system