Positive pressure systems
Positive pressure dilute phase pneumatic conveying systems are typically employed to convey bulk materials from a single source to one or multiple destinations, over longer distances and with greater capacity than possible using vacuum systems.
These systems utilize a positive displacement blower with single or multiple downstream material entry points, each of which meters product into conveying lines by means of a rotary airlock valve that maintains the pressure differential between the ambient atmosphere and that of the conveying line. Material and air blown through the line exit at single or multiple use points where they are separated by means of a filter receiver or cyclone separator, or fed directly into process vessels that are vented to downstream dust collection devices.
Pressure systems offer higher efficiency than vacuum systems, can convey over longer distances, and can blow directly into process and storage vessels without the use of a rotary airlock valve.
Vacuum dilute phase pneumatic conveying systems are generally employed for transporting material from multiple sources such as storage vessels, process equipment, trucks and railcars, to individual or multiple destinations. Unlike positive pressure systems, vacuum systems allow easy pick-up of materials from open containers using wands, and do not impart heat to the material. Since vacuum systems offer superior leak containment, they are often specified on the basis of cleanliness, particularly when handling hazardous materials.
Negative pressure is created by a positive displacement vacuum pump located at the downstream end of the system. Material typically enters the system via upstream rotary airlock valves, hand-held pick-up wands, and non-flow-through/flood-feed pick-up adapters, and exits the system through filter receivers or cyclone separators that separate the material from the conveying air directly above process equipment, surge hoppers, storage vessels or other discharge points.
In addition, vacuum conveying is suited to direct feeding of blenders, reactors and other enclosed process vessels that are put under vacuum by a line terminating at a downstream bag house or other dust collection device, eliminating the need for individual filter receivers or cyclone separators above each discharge point.