Pneumatic conveying

by Ernest George Phillips

The induction system, however, is flexible at both intake and discharge ends of the pipe. It is only necessary to lower the nozzle into the material to be removed, and to place the delivery pipe over the receptacle for the material, and to turn on the air jet. The delivery may be handled easily while working, and the material distributed where required; or suitable valves and branches may be fixed, and a number of discharge pipes used in turn to deliver into different bins or into various floors.

The source of power for the operation of the induction conveyor is the air compressor. As every operating engineer is well aware, all machinery is kept in better condition and runs more economically, when it is housed in proper environment and receives skilled attention. The compressor, in this case, need not be erected near the work, but may be placed some distance away, preferably in the power station, as the pipe line connecting the conveyor with the machine will have a very small bore, compared with the air pipe to the exhauster on a suction plant, and will also be inexpensive to erect and maintain. In the instance quoted above, where a number of suction and delivery points are required, only these small pipes need be run from a common main, and turned on and off as needed, the compressor running continuously at or near its most economical load.

Construction of Induction Plant. The induction conveyor may be said to be a compromise between the suction and blowing methods. The air jet is fixed in the conveyor pipe at a suitable angle, some distance above the intake nozzle, and a stream of air at high velocity is passed along the pipe in the direction of the discharge. This air jet is designed carefully for the duty it has to perform, and its discharge entrains the free air in the pipe, causing it to move in the required direction. A partial vacuum is created in the conveyor pipe, behind the jet, and free air rushes in at the intake, carrying the material along with it.

In order to effect the greatest economy in the operation of this plant, it is important that the power unit should be carefully chosen, and that the pipe system should be designed to give the full pressure at the jet. With modern multiple-stage compressors of the rotary or reciprocating type, working at about full load, very high efficiencies can be obtained, and the pressure pipe line should be arranged to avoid loss by friction as far as possible. The receiver should be of sufficient capacity to absorb any pulsations, and to throw down oil and moisture before the air enters the pipes. A separator of good design should also be incorporated.

In designing the conveyor pipe line, bends should be avoided when possible, by erecting the lifting pipes at an angle with the horizontal. It is not sufficiently well appreciated that bends and angles rapidly increase the frictional resistance to the flow of the conveying medium, and mean loss of power; in fact there is no doubt that the difference between success and failure in pneumatic conveying is largely a matter of design. Many substances which are otherwise quite suitable for handling in this manner are very fragile, and any friction on pipe walls or contact with metal baffles at high velocity, so reduces or pulverizes them that their value is reduced considerably. In the case of ashes from boiler furnaces, this effect is advantageous rather than otherwise, but when dealing with coal it is necessary to arrange the system so that the minimum amount of damage is done to the material. Some coals, such as Derbyshire bituminous, is not easily broken or abraded, and can be lifted very satisfactorily by the usual suction method. Welsh coal, on the other hand, is very friable, and if conveyed into the usual discharger, will emerge in a finely divided state, even though it may be fed to the intake in large pieces. For handling such materials, the induction method is most suitable, as the discharge end may be arranged so that the material is not delivered at high velocity, and does not strike any obstacle which would reduce it or break it up. It is possible to elevate potatoes and even oranges by the induction process, and it is quite within the bounds of probability that eggs may be delivered in this manner, without more than the usual percentage of breakages.

In conveying many materials, which are conveyed whilst hot, it is better if they can be kept at practically the same temperature at the delivery as when they enter the pipe. This is provided for by heating the air to a suitable temperature just before it enters the jet.

This is also an additional source of economy in operation. As is well known, air, like all other gases, increases in volume with the temperature, and if the heat lost by the air cooling after compression be replaced at the jet, considerably more power is obtained. If the compressor is situated in such a position that most of the heat of compression is delivered at the jet, there is little to be gained by reheating. In most cases, however, the air has returned to normal temperature by the time it reaches the point where it is to be used, and, if a suitable air heater is installed at this point, the volume may be increased greatly by a comparatively small expenditure.

A heater consisting of tubes through which the air passes, these tubes surrounded by water under high steam pressures, offers the most convenient and satisfactory method of heating the air. The air pipes between the heater and conveyor pipe should be lagged in order to retain the heat.

The pressure of the air may be increased by 50 per cent. by heating to the temperature of steam at 200 lbs. per sq. in. gauge pressure, while the cost will be comparatively small. Theoretically, a gain of about 40 per cent. in economy should be obtained, and the practical results should be reasonably close to this figure.

Air Receivers. It is a decided advantage in practice to install an efficient separator between the ordinary receiver of the compressor and the pipe line, as large quantities of moisture will travel over with the air, and will be condensed directly they meet some cooler surface. The ordinary receiver is supposed to fulfil this function, but it does not do so because it is, in effect, an enlargement of the pipe line, and, being filled with hot air under pressure, has no tendency to condense the moisture. The latter does not begin to cool to any extent until it reaches the small diameter pipes, with the consequence that these pipes contain quantities of oil and water which eventually reach the jet, and are blown into the material handled.