Pneumatic conveying

Numerous means have been devised to cause the necessary current of air to flow along the conveyor pipe, but the ideal method is probably yet to seek. Probably the most satisfactory and economical system, until recently, was the positive pump exhausting a vacuum chamber; the latter receiving the material, and discharging it into the receptacle provided for that purpose.

The difficulties arising in practice, however, incited the inventive genius of engineers responsible for the operation of these plants, and a number of attempts were made to induce an air current by other means.

Ejector Systems. Steam ejectors were fitted to the closed tank provided for the reception of the material, thus converting the tank into a vacuum chamber, and eliminating the discharger. In other cases injectors, also operated by steam, were placed at intervals along the conveyor pipe, usually at such convenient points as 90° bends, and the slight vacuum created by the condensation of the steam and also by the velocity of the jet, induced an air current which swept the material along with it into the receiver chamber.

Although both these methods are in practical use, their applications are strictly limited to materials which do not suffer by contact with heat and moisture; the methods are therefore used principally for conveying ashes and soot from boiler furnaces and flues. Ashes formed by the combustion of coal contain large amounts of abrasive matter, and it is very important that all this matter should be extracted from the air, before entering the exhauster of the suction system. The steam jet cuts out the exhauster entirely, but absorbs an excessive amount of power in the form of steam. It has the advantage, however, of quenching the ashes on their way to the settling tank. For flue dust, however, the steam jet is unsuitable, as the condensed vapour causes the material to cake in the pipes, and the latter rapidly become choked, involving considerable delay and trouble in cleaning out. The ejector system is used for this material, the tank being of the closed type, and the necessary vacuum being created by a steam ejector fixed in a branch at the side near the top, the dust striking a baffle and falling by gravity to the bottom of the tank. A special air-tight gate or valve is opened to empty the tank.

Air Induction. The cardinal feature of the induction system is the ease with which materials may be handled which cannot be conveyed by the suction method.

Sand, sugar, salt, soda ash, and many other substances of a granular nature, which are very troublesome when conveyed by the suction method, may be dealt with economically by the induction system and, although the latter is only in its experimental stages at the moment of writing, it is possible that it may displace all other systems in the near future.

The induction system differs from the suction system in that the air flow along the pipe is induced by a jet of air, at very high velocity, fixed at any convenient distance from the intake nozzle of the conveyor pipe, and the material conveyed is discharged either from an open end into an open container, or by some form of cyclone. The closed discharger or container, with its baffles and rotary valves or air locks, is eliminated, and the substance to be handled has a free and unrestricted flow throughout the length of the pipe.

Advantages of the Induction System. The advantages of the induction conveyor may be summarized as follows: (1) Low first cost, the power unit being the only expensive item. (2) Low maintenance cost, there being no moving parts and little wear. (3) Low labour cost, practically no attendance being required. (4) Flexibility and ease of handling. (5) High efficiency of power unit and reliability of system. (6) Ability to handle materials which are easily damaged.

In the case of a suction plant handling grain or coal, the intake end of the conveyor is fairly flexible, and the nozzle may be operated over a fairly large radius, say, all over the floor of a vessel’s hold. The discharge end, however, is fixed, unless a cumbersome and expensive gantry is provided to permit of the discharge apparatus being moved about. Even should the discharger be mounted on rails, the area over which it can operate is limited by the rails on which it runs. With a large plant, this would mean that a number of dischargers would be required to lift from a ship into a warehouse or store, from the latter into bunkers or silos, or perhaps into trucks or waggons. Each discharger would require a separate exhauster and a separate intake, and valuable space would be occupied by the plant and expense incurred for machinery which would not be in use for a considerable part of the time.

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.

Where compressors of the rotary or turbine type are installed, there will be only water in expansion, but it is good practice to remove this, even though the air be re-heated, because the moisture will recondense in the conveyor pipe, and tend to choke the latter when small grained substances are being conveyed.

Types of Compressors. Reference has already been made to the power unit, and it is hardly within the scope of this work to describe in detail the various machines available. As, however, the economy of air conveying depends in a large measure on the cost of power, it is evident that the compressor should be of the most suitable type for the duty to be performed.

For small installations, single-stage reciprocating machines, driven directly by steam engines or by electric motors are, no doubt, the most suitable. In the case of large plants, using the air continuously in a number of air jets, where the load factor is high, it is certainly more important to install a two or three-stage compressor, owing to the greater economy of working. The larger capital expenditure will be compensated by the considerable saving of energy. As compared with single-stage compression to 100 lbs. gauge pressure, a saving of 20 per cent. can be effected by three-stage working, and with a constant load of from 75 per cent. to 100 per cent. of full load, a turbine or electrically driven rotary multiple-stage compressor is decidedly the best type to adopt.

In plants where exhaust steam can be used to advantage, as in large generating stations, a steam turbo-compressor, multiple-stage, exhausting to a feed water heater will show great economy, and the operating costs of a large plant of this type are very low compared with any other form of conveyor. This will be obvious when it is pointed out that maintenance costs on the conveyor are confined to renewals of bends and junctions in the pipe lines, and of flexible hose. There are no discharge valves or air locks to be kept vacuum tight, no filter strainers or sleeves to renew, and the power unit is not subjected to undue wear through extraneous matter entering the cylinders and scoring the walls or wearing the valves.

Compared with other forms of mechanical conveying, the pneumatic induction system is very low in maintenance costs, while the serious charges incurred in employing human labour are reduced to a minimum.