The first practical application of this invention to other than liquid materials is supposed to have been in connection with the conveying of cotton in a loose form, as an improvement upon the manual shifting of large bales. This development was made about 1867, and after this date great progress was made, principally in connection with the handling of grain, wheat, malt, etc., and largely owing to the work of an American named A. K. Williams.
At the present day it is hardly possible to enumerate all the successful schemes for the pneumatic handling of materials. In addition to installations for the conveyance of materials such as those mentioned above is the pneumatic tube, for conveying papers, messages and cash in offices and between shop counter and cash desk, etc. Also, there is the suction cleaner, ranging in application from the handling of refuse and dust from saw mills and woodworking machinery to the removal of fine abrasive and poisonous particles in certain manufacturing processes. Small suction cleaners are, of course, now quite familiar domestic appliances. The sand-blasting machine is really another example of a self contained pneumatic conveyor on a small scale.
Ashes, coal, oranges, sugar, chemicals, spent oxide, iron ore, spent tanning bark, and many other materials are now actually transported, elevated or conveyed pneumatically, and it must here be acknowledged that the first really successful plant in this country was due entirely to the initiative and inventive genius of Mr. Frederic Eliot Duckham, late Chief Engineer to the Millwall Dock Co., who in 1888 commenced experiments in grain-handling by suction. In 1892 he produced a very successful floating plant for unloading ships into land silos, and this installation was the prototype of many similar plants which were placed in commission all over the world. Many improvements have been carried out and numerous patents issued for pneumatic handling apparatus, but the original scheme as designed by Mr. Duckham has never been departed from seriously.
Fundamental Principles and Components. The pneumatic conveyance of materials along pipes is most easily understood when the equipment is considered as a pump producing a high velocity stream of air in which the material to be transported is floating, and with which it is carried through the pipe system. It is necessary fully to understand this, as it is otherwise difficult to realize how it can be possible to lift solids at the rate of 100 tons or more per hour, several hundred feet up a pipe in which the vacuum does not exceed about 7 inches mercury column (say, 11 lb. per sq. in. absolute).
A modern pneumatic conveying plant of the suction type comprises: (1) An exhauster, or an air pump, of either the reciprocating or the rotary type. (2) A suction nozzle. (3) A discharger, whereby the material is extracted from the pipe line at the desired position, without “breaking” or losing the vacuum. (4) One or more appliances for filtering the air and extracting any foreign material which may have been carried over from the discharger and which would damage the cylinder walls of the exhauster if allowed to enter the plant.
It is necessary here to mention that all pneumatic conveying is not done by exhausting, but frequently by the use of pressure, that is, that the air is not sucked along the transport line but is actually blown in under pressure by fans, air compressors, or rotary blowers, according to the circumstances.
The suction system is preferable when it is required to convey materials from several outlying points to one central storage bunker or area. On the other hand, the pressure system is less expensive in first cost, when it is required to transport from one central point to numerous outlying plants in the area to be served. A combination of the suction and pressure systems is now being developed for the handling of materials in cases where neither the “suction” nor the “blowing” scheme alone can be said to be successful. This combined system is known as the induction system and is discussed in Chapter VI.
Advantages over Mechanical Conveyors. The reasons why pneumatic methods for elevating and conveying are now receiving such attention are to be found in the advantages of pneumatic over mechanical conveying. These may be summarized as follows: Economy in labour; flexibility of plant both in design and operation; elimination of dust and its harmful effects on the employees; and in many instances the recovery of dust which is valuable and which would otherwise represent a serious financial loss. Wear and tear on moving parts is reduced to a minimum. All obstacles such as buildings, roads, rivers, railways, etc., can be overcome easily since the conveyor is “only a pipe.” Should circumstances not permit of a straight single run for such appliances as bucket elevators, it is usually necessary to bag and cart the material from one position to another, but this is avoided by pneumatic elevating and conveying, because the pipe can be carried up or down, round corners, over or under roads, etc.
Hundreds of instances still exist in which loose material in barges is shovelled into bags, which are lifted by an ordinary friction hoist into a building over the quay-side. The sacks are wheeled by manual labour into the building, and emptied into hoppers or silos, the empty bags being lowered again for a further cycle of operations. This costly multiple handling may be obviated by the erection of a small pipe line which will automatically feed itself at one end, and discharge evenly and continuously into the receptacle provided. The whole of the “moving parts” may be situated in one position, which enables them to be carefully inspected, oiled and kept in repair, etc.
Cranes, hoists, telphers, and belt conveyors all have their special spheres of usefulness, but no other plant can claim all the advantages of the pneumatic system as outlined above.