PNEUMATIC
CONVEYING
Conveying by mechanical means has existed for many centuries, and is one of the earliest forms of man’s ingenuity towards labour saving as we know it to-day. The pneumatic conveyance of materials from one position to another, either horizontally or vertically, is the most recent form of automatic handling of solid substances.
Genesis and Applications of Pneumatic Conveying. The need of water for human consumption and for irrigation purposes caused the ancient inventor to carry out nearly all his experiments with that substance, and the first instance we have of anything approaching pneumatic conveying is the well known injector in which steam is passed through one pipe, placed at right angles to a second pipe, at such a velocity as to reduce the pressure in the second pipe to below the atmospheric pressure. The excess of atmospheric pressure over the reduced pressure in the second pipe then drives up the latter the material (in this case water) in which the end of the pipe is submerged. This invention is about a century old, if it emanated from the Marquis Mammonry d’Eclet, in 1818, as is usually believed.
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.
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.
(3) Induction System. A compressed air injector is used to produce a partial vacuum on the suction or inlet end of the pipe system, and to produce a pressure above atmospheric at the delivery end, thus avoiding the necessity for a discharger. This system, which is suitable for use with heavy sticky materials—such as hot sugar, saturated sand, finely ground heavy ores, spent oxide as used in gas works, etc.—is now being developed so as to make possible pneumatic conveyance of all manner of materials which are unsuitable for passing through the discharger, or air lock, of a suction system.
Factors influencing Design. In the strict sense of the term, “pneumatic conveying” really implies the conveyance of a quantity of material from one point to another, using air as the conveying agent. As will be seen from the above, the subject also embraces the removal of comparatively light dust produced in many industrial processes. Although the same name is implied in both cases, the methods to be adopted vary, and each case must be considered on its own merits.
With regard to a conveying plant proper, the points for consideration in the initial stages are as follows—
(1) The nature of the material to be handled and the quantity required in a given time.
(2) The size of the largest and smallest pieces of materials and the density of the material.
(3) The distance over which the material is to be conveyed.
(4) The difference in level between the point at which the material is fed into the system, and the point where it is delivered.
(5) The method and regularity of feeding the material into the system.
(6) The means to be adopted for separating the material from the conveying air at the desired point of delivery.
There is a very important distinction between plants in which the material passes through the fan or blower and those in which the conveying is carried out entirely under suction.
The latter system has many advantages, but it carries with it the necessity of providing automatic delivery of the material without seriously impairing the suction. In order to accomplish this, some form of “air lock” is required, and a necessary feature of this is a device with close-fitting surfaces or more or less air-tight valves. The simpler plan is undoubtedly to pass the material through the fan, as there is then no question of breaking the suction in order to get the material out of the system. It is common practice to convey for long distances such materials as wood chippings and sawdust, cotton, jute, wool, esparto grass, paper chippings and many other materials. In all cases, however, the designer is confronted with the problem of separating the material from the air, and in many instances to do this satisfactorily is more difficult than the actual conveying; especially is this so with certain sticky materials or materials which will readily “pack” or build up when entering any mechanical discharger at the high velocity necessary in the suction system.