Pneumatic Conveying System
A pneumatic conveying system is a process by which bulk materials of almost any type are transferred or injected using a gas flow as the conveying medium from one or more sources to one or more destinations. Air is the most commonly used gas, but may not be selected for use with reactive materials and/or where there is a threat of dust explosions..
A well designed pneumatic conveying system is often a more practical and economical method of transporting materials from one point to another than alternative mechanical systems (belt conveyors, screw conveyors, vibrating conveyors, drag conveyors and other methodologies) because of three key reasons:
1. First, pneumatic systems are relatively economical to install and operate
2. Second, pneumatic systems are totally enclosed and if required can operate entirely without moving parts coming into contact with the conveyed material. Being enclosed these are relatively clean, more environmentally acceptable and simple to maintain.
3. Third, they are flexible in terms of rerouting and expansion. A pneumatic system can convey a product at any place a pipe line can run.
Pneumatic conveying can be used for particles ranging from fine powders to pellets and bulk densities of 16 to 3200 kg/m3 (1 to 200 lb/ft3). As a general rule, pneumatic conveying will work for particles up to 2 inches in diameter @ typical density. By "typical density" we mean that a 2 inch particle of a polymer resin can be moved via pneumatic conveying, but a 2 inch lead ball would not.
Types of Pneumatic Conveying
There are several methods of transporting materials using pneumatic conveying. In general, they seem to fall into three main categories: dilute phase, dense phase, and air conveying.
1. Dilute-phase conveying is the process of pushing or pulling air-suspended materials from one location to another by maintaining a sufficient airstream velocity. Dilute phase conveying is essentially a continuous process, characterized by high velocity, low pressure and low product to air ratio.
Dilute phase conveying is the most common used method of transporting materials.
Dilute phase system is characterized by:
• High velocity conveying @ 3,200 to 8,000 feet per minute
• Operating pressures in range of 5-12 PSIG (positive) or negative pressures of 4- 12” Hg
• High air to solids loading ratios (> 2.0)
2. Dense-phase conveying relies on a pulse of air to force a slug of material from one location to another. Dense-phase system is essentially a batch process, characterized by low velocity, high pressure and high product to air ratio unlike dilute phase which is a low product to air ratio.
Dense phase conveying is characterized by:
• Convey rates is high upto 100 tons/hr or higher.
• Convey velocities is Low 200 to 2000fpm.
• Convey distances high upto 10000 ft or longer
• Air mover is compressor (Screw,rotary,reciprocating)
• Operating Pressure is upto 125psig
• Air/Material ratio is less than 0.2
3. Air-activated gravity conveying is a means of moving product along a conveyor on a cushion of air.
This course outlines the distinguishing characteristics of dense and dilute phase transport. The design of dilute phase systems is dealt with in detail and the approach to design of dense phase systems is summarized.
Essentially, the two most distinct categories of pneumatic conveying can be described as either low pressure (dilute phase) or high pressure (dense phase) systems. The choice between dilute and dense phase operation is typically dependent on the material properties. In general, bulk materials with poor air retention capabilities are less suited for dense phase system and granular products with narrow particle size distribution have good air permeability and are well suited for dense phase conveying.
In addition, dense phase operation (with its relatively low velocities) is usually preferable if the conveyed material degrades easily or is highly abrasive. Dense phase conveying can handle high throughputs over long distances while requiring smaller line sizes than either dilute-phase or vacuum conveying. A drawback of the dense phase option is that dense-phase conveying is essentially a batch process, which is less convenient for most chemical process operations than is the continuous mode used by dilute phase and vacuum systems.