Pneumatic Conveying Systems. As mentioned above there are three main systems of pneumatic conveying, viz.—
(1) Conveying by air above atmospheric pressure.
(2) Conveying by air below atmospheric pressure.
(3) Conveying by air above and below atmospheric pressure, by a combination of (1) and (2), known as the induction system.
Although these are the main headings under which the subject may be considered, they must be sub-divided further as follows—
(1) Conveying above Atmospheric Pressure. (a) Low pressure systems using single-stage centrifugal fans, and suitable for conveying such materials as wool, cotton, bark, chopped straw, paper clippings, sawdust, shavings, jute, and fibrous materials of many kinds.
(b) High pressure systems using multi-stage fans and blowers of the rotary type or positive design, and suitable for conveying materials of a denser nature such as dry sand, sugar, etc., also for pneumatic despatch tubes.
(2) Air Suction Systems. (a) Large pipe systems using ordinary steel plate centrifugal fans, and applied to handling the waste products and injurious dust in many industries, such materials being sawdust and shavings from woodworking machinery; emery dust from grinding wheels; dust and lint from polishing mops; leather dust from heel and sole scouring plants; starch dust in confectionery works; colour dust in works manufacturing pigments; and bronze dust in printing works. Many other examples will no doubt occur to the reader.
(b) Small pipe systems using multi-stage centrifugal fans or rotary blowers. Under this section the most important plants are undoubtedly those for domestic vacuum cleaning. Large stationary plants for this purpose are installed in many hotels, offices, buildings, workshops, theatres, large private houses, etc.
(c) Heavy commercial systems in which the fan is replaced by a reciprocating exhauster, producing currents of air suitable for conveying such materials as coal, ashes, sugar, leather tanning bark, ores, granite chippings, wood blocks, oranges, etc., which it would be impossible, or undesirable, to pass through a fan.