Pneumatic conveying

by Ernest George Phillips

FOOTNOTES:

[1] Inst. Mechanical Engineers Journal, March, 1920, p. 291.

CHAPTER VIII
MISCELLANEOUS APPLICATIONS OF PNEUMATIC CONVEYING

Pneumatic Despatch Tubes. The ordinary pneumatic conveyor picks up material at one point and unloads it at another and continues this course consistently, whereas the “pneumatic despatch tube” is a conveyor of small articles enclosed in a special cartridge which is built to fit the tube and which travels to and fro as required, carrying a variety of articles, or if necessary, the same articles, backwards and forwards between the same two stations or a series of fixed stations.

The despatch tube thus constitutes an effective “mechanical messenger.” One or more tubes are run between the points to be connected, with a despatch and receiving terminal at each end, or if necessary, a single line to operate in both directions can be designed. The tubes vary from 1½ ins. to 4 ins. diameter, and they are also made of oval sections up to 4 ins. × 7 ins.; rectangular tubes have been installed in special installations, chiefly in telephone exchanges for convenience in dealing with certain cards there employed.

Tubes. The tubes are of lead and are usually encased for protection against mechanical damage, and the erection is carried out with great care so as to preserve the smooth interior. Joints occur at intervals of 28 ft. or less, and are “wiped” with an ordinary plumber’s joint over an internal mandril which is heated previous to insertion in the tube. Air-tight joints and smooth interiors are absolutely essential to a successful installation.

Carriers. The carriers or cartridges in which the material to be transmitted is placed are made of gutta-percha covered at the ends with felt. One end of the container is closed and the other end is left open, but a “skirt” of felt surrounds the open end, and, as this is the “trailing” end and the air pressure is behind it, the air forces open the “skirt,” making a tight fit and preventing leakage of air past the carrier. The nose of the carrier is usually fitted with a felt “buffer” which also assists in making an air-tight fit. A carrier for a 2½ in. tube is 6¾ ins. long and weighs empty about 3 ozs. Fig. 26 shows a large carrier.

Methods of Working. Pneumatic tubes are worked either by air above atmospheric pressure or by reducing the pressure below atmospheric. In the pressure system the usual pressure is about 10 lbs. per sq. in. above atmospheric pressure, whilst in the suction system the vacuum employed is equivalent to about 6½ lbs. per sq. in.

Fig. 26.—Typical Lamson Intercommunication Carrier.

Also, the method of working may be either “continuous” or “intermittent”; in the first system the air, either above or below atmospheric pressure, is circulating continuously and the cartridge or carrier is inserted into a stream of air already in circulation, whilst in the “intermittent” system the power, either pressure or suction, is admitted to the conveyor tube only after the carrier has been inserted, and it is again cut off when the carrier reaches the end of its journey.

To a great extent the success of a pneumatic tube system is the speed at which it can transmit the message sent by this means. In the “continuous” system, working above atmospheric pressure, the speed is not so great as in the “intermittent” scheme, because the pressure in the tube is the same in front of and behind the carrier, which has to displace the air in front of it. In the “intermittent” system the pressure is turned on after the carrier is in place, and the advancing carrier has only to move the air at atmospheric pressure. On the other hand, if suction is employed, the “intermittent” system is slower than the “continuous” system because the air has to be exhausted to a certain point before the carrier begins to travel. It is true that it will begin to move as soon as the difference in pressure amounts to a few ounces, but there is a distinct “time lag” compared with inserting the cartridge into a tube continuously exhausted when it starts off at practically full pressure and speed immediately.

The difference in time is stated by Kemp to be 3 per cent. longer with “continuous” pressure, compared with “intermittent” pressure at 6 lbs. per sq. in.; the difference increasing to 6 per cent. when the pressure is raised to 14 lbs. per sq. in. The average working speed of these tubes is from 25 to 30 miles per hour.

Power Required for Operation. It is difficult to determine the actual amount of power necessary to carry a cartridge through a tube. Kemp’s Engineer’s Year Book states that, working at the standard pressure of 10 lbs. per sq. in., the power required is theoretically 3·35 h.p. for a 2½ in. tube, 1 mile long, but actual experience suggests that at least 50 per cent. should be added to allow for losses from various causes, making the actual power, say, 5 h.p. per 2½ in. tube per mile.

Pressure receivers or tanks are inserted between the pump and the travelling tube to compensate for the impulses due to the irregularity of the pumps and also to act as reservoirs furnishing additional power during periods of abnormal working.

The vacuum system takes less power (for a definite time of transmission) than is required by the pressure method of working, but local conditions always influence results considerably, and it is inadvisable to give any definite figure as to the power required, without actual knowledge of the system and conditions involved.

The air compressors are usually driven electrically, but they can, of course, be operated by any other prime mover such as oil, gas, or steam engines. It is economical to combine the pressure and suction systems by arranging the air compressor to draw air from the vacuum receiver into the compressor cylinders whence it is returned to the pressure line.

Automatic valves keep the pressures in the pressure and vacuum sides of the system within pre-determined limits. “Make up” air is admitted by the automatic opening of an atmospheric valve when the pressure side of the system is low and the vacuum side high, so that the pump is deprived of sufficient air to operate the system efficiently. Should the conditions become reversed, that is, a low vacuum and a high pressure, then the pump is working against a high back pressure, and this is reduced by the opening of an atmospheric relief valve which remains open until the vacuum is restored to normal pressure. This system is preferable to and more economical than the use of two separate pumping and exhausting machines.

Elaborate and valuable tables of horse-power required by compressors and of “transit times” for distances up to 4,500 yds. with 1½ in., 2¼ in., and 3 in. tubes are given in Kemp’s Engineer’s Year Book.

The Lamson Tube Co., Ltd., have brought what was originally invented as a means of conveying persons to a practical business accessory, capable of saving a great amount of time by despatching sketches, papers, small articles, money, etc., here, there, and everywhere at the rate of 30 miles an hour.

The utility of these plants has long been recognised by banking establishments, the General Post Office, large stores, factories, newspaper publishing offices, etc. (see Figs. [27] and [28]).

In addition to the conveyance of messages and papers, they are frequently installed to convey money and bills from the numerous departments of a large store to the cashiers, thus saving time and effecting economy in labour and floor space. One cashier can attend to from 10 to 15 stations, or in small establishments all the stations can be centralized around the book-keeper.

The installation of a power-driven plant is not essential, providing that the service required is not too great. A foot power pneumatic service is available and it is in use in many business establishments. In this system the methods of transportation are similar to those in a power plant, but the tubes are brought to a special cabinet 15 ins. square by 2 ft. 6 ins. high, in which is mounted a foot-operated pump of patented design without bellows or cords. The pump is operated as and when the service is required, and there is no loss of any description when the apparatus is not in use.

Pneumatic Tubes for Heavy Articles. It is interesting to recall, especially in view of the proposed use of pneumatically-propelled parcel-conveying trains by the G.P.O. in London, the proposal made by Mr. Medhurst, in 1810, when it was suggested that a carriage somewhat similar to the modern railway carriage should be moved through a tunnel by pneumatic means. So long ago as 1667, Denin Papin read before the Royal Society a paper entitled “A Double Pneumatic Pump,” and definite mention of despatch tubes was made in this paper.

Fig. 27.—Tube Central in Wholesale Drug House, Distributing Orders to all Departments.

Fig. 28.—Lamson Distributing Station in well-known Publishing House.

In 1840 a pneumatic railway was actually built and worked between London and Croydon, and in view of its success was followed by others between Dalkey and Kingstown and between Exeter and Plymouth. From this it will be seen that transportation by pneumatic means is not modern in its application, and was originally intended for very large tubes and weights, but modern development has been toward small tubes and light weights.

The Vacuum Cleaner. The pneumatic transporting of material in the form of dust has been brought to a very high state of perfection during recent years and an enormous number of plants is now in use, ranging from the hand-propelled machine to very large stationary equipments.

Certain hand-propelled machines have been constructed in such a way that the fan is directly operated by gearing from the running wheels, and after a few moments a very considerable speed is attained and the suction of the fan is used for lifting the dust from the surface over which the apparatus is travelling.

Numerous designs of more powerful machines actuated by hand bellows have been placed on the market and these possess the advantage that they are independent of the use of power; but it is not altogether easy to operate a machine by one hand and to manipulate the nozzle with the other.

Electrically driven machines of almost numberless designs are available. These usually employ a high speed fan of the single-stage type, but a piston pump is embodied in some designs.

In the removal of dust the same principle applies as in the conveying of heavier materials, i.e. it is not so imperative to obtain a high vacuum as it is to have a large volume of air moving at high velocity, hence the multi-stage turbine machine has distinct advantages as regards weight of material moved and economy of power.

The multi-stage exhauster consists of turbine wheels mounted on a single shaft, the air being drawn into the first wheel, from this to the second wheel and so on right through the machine, each wheel increasing the suction on the intake end according to the total number of wheels or stages. This style of machine is procurable in either the stationary or portable type, and in both it is made in various sizes, the portable machines ranging from ¹/₁₂ h.p. up to ½ h.p. for domestic purposes, and from 1½ to 3 h.p. on trucks for cleaning electrical machinery, railway carriages, etc. Figs. 29 and 30 illustrate typical stationary and portable plants respectively.

Fig. 29.—Stationary Turbo-Exhauster with Dust Separator.

Fig. 30.—Portable Turbo-Exhauster Driven by 1½ h.p. d.c. Motor.

It is not generally recognized what enormous amounts of dust and dirt may be extracted by these machines. From one London hotel a ½ h.p. cleaner removed 166 lbs. of dust from the carpets of the public rooms only. On a cleaning test in a first class dining car on one of the English railways 25 lbs. of dust was removed from 38 sq. yds. of carpet. A rug in front of a lift in a London stores yielded 91 lbs. 1 oz. of dirt to a small machine.

The stationary plants are usually installed in the basements of large office buildings, theatres, hotels, clubs, etc., and the whole building is piped suitably, wall plugs or connectors being fitted to which the staff make connection by flexible hose as and when required. The free end of the flexible hose is fitted with one or other of a series of special nozzles, the latter being adapted to the varying requirements of everything in the room from floor to ceiling.

With the portable hand sets or even with the larger truck type, the design is complete as a working unit; the equipment is used as manufactured and there is little or no chance for the user to endanger the working efficiency of the plant. In permanent plants, however, as installed in hotels, etc., it is necessary that all points previously mentioned regarding pipe lines, valves, junctions, bends, etc., should be considered and acted upon.

The pipe lines should be too large rather than restricted in any way, the suction flexible should be kept as short as possible, and if necessary extra connections should be allowed rather than require flexibles too long for use without “kinking.”

Fig. 31.—Suction Cleaning for Railway Carriage Cushions.

Fig. 32.—Sturtevant Equipment for Office Cleaning.

Fig. 31 illustrates a stationary suction cleaning plant applied to cleaning railway carriage cushions, and Fig. 32 shows a similar installation in use in an office building.

Cleaning by Air Blast. By transferring the hose from the suction side to the discharge, a suction cleaner may be used to blow dust from machinery of all kinds and from places that are high up and cannot be cleaned economically by suction. For cleaning electric generators and motors by blast, these machines have many advantages, and on account of the large volume of air handled they are much to be preferred to the small-volume high pressure jet of the ordinary air compressor often used for this purpose. With the portable turbo-blower there is no danger of damage to the insulation through high pressure, or through the carrying of moisture and oil into the windings with the air jet.

Pumping by Compressed Air. Although, generally speaking, the raising of water by compressed air is not an economical method, it is frequently adopted in mining and tunnelling where the use of steam or electricity is objectionable. In these cases, cost of operation is a minor factor, and it may be interesting to give a few particulars of this form of pneumatic conveying.

The simplest form of compressed air pump consists of a closed chamber or tank immersed in the water, to be raised or fixed at such a level that the water will flow into the tank. An air pipe is connected to the top of the chamber, and the rising main is carried inside the tank to the bottom. On opening the air valve, pressure is exerted on the surface of the water in the tank, and the water is expelled through the lift pipe or rising main. On closing the air valve, water again fills up the tank, and the process is repeated.

A decided improvement on this pump is the return air pump, which consists of two closed chambers connected through valves with the rising main. The compressed air pipe passes through a two-way valve, either into one tank or the other, this valve being positively operated. The method of working is similar to that of the single acting pump, considering each chamber separately, but one tank is filling while the other is being emptied.

The air expelled from the filling tank, instead of being discharged to atmosphere, and part of its expansive power lost, is carried back through the pipe, which would be the air intake pipe when discharging, through a port in the two-way valve, and into the compressor intake pipe. The air leaving the filling tank is naturally above atmospheric pressure, and assists the piston on entering the compressor, thus reducing the power absorbed in driving the latter.

Air-lift Pumping. The air-lift pump is a common means of conveying by pneumatic means and should not be confused with the above methods of raising water by compressed air.

In the air-lift method of pumping air under pressure is admitted at the foot of a pipe already submerged in the well. The air does not merely bubble through the water, as might be supposed, but passes up the pipe as a mixture of air and water. The introduction of the air into the rising column of water makes the latter as a whole less dense than the water around the tube, and therefore we have a difference in head between the internal and external columns of water which will carry the internal column considerably higher than the external column.

As the lifting force depends upon the “head” of water outside the rising main, it follows that the maximum height to which the water can be raised depends upon the depth to which the air pipe and rising main are submerged below the standing level of the water in the bore-hole. In other words, the greater the lift, the greater the depth to which the air pipe must be carried before releasing the air into the rising main.

Experience shows that the water pipe should be submerged 18 ins. for every 1 ft. lift above the water level in the bore-hole, and allowance must be made for the “depression” of the water level in the bore-hole, which will probably take place when pumping is in progress. This depression will vary according to the water bearing capacity of the strata, in which the hole has been bored, hence it is necessary to go carefully into the conditions before boring the hole. If available, data should be studied concerning the standing water level, and the pumping depression in other bore-holes in the immediate neighbourhood. Also tests should be made before the boring machinery is removed because, although the initial depth of bore-hole may be satisfactory on the basis of standing level calculations, it may be found that when pumping the depression is so great that the bore-hole has to be carried to a greater depth.

The air is supplied at a pressure suitable for the conditions, and can be carried down a separate tube and connected to the rising main at the correct depth (Fig. [33]), or, as is often done, one pipe may be lowered and the rising main supported centrally inside the casing tube, the annular space between the two being used as the air pipe (see Fig. [34]).

The amount of free air required is from 0·6 to 1·0 cu. ft. per gallon of water raised per min., provided that all the details have been studied carefully and the design of the plant worked out accordingly.

If the air pipe is too small the air will bubble slowly through the water, while if it is too large it will blow out with great force, spraying and losing the water: the ratio between the cross-sectional area of the air and water pipes is about 1½ to 4.

Advantages of air-lift pumping are that a greater amount of water can be obtained from a hole of given size than by ordinary pumping; and that one compressing plant can deal with several wells instead of needing a separate pump to each well.

Fig. 33.—Air Pipe Outside Riser.

Fig. 34.—Air between Casing and Riser.

Air-lift Pumping

The disadvantages are, that the mechanical efficiency is low; that a considerable amount of air is entrained in the water, and aerated water is very unsuitable for boiler feed purposes; and that means must be provided to allow air to escape by passing the discharge from the pump over a weir or similar contrivance. It is necessary to have some reliable form of oil trap between the compressor and the well to prevent contamination of the water by oil carried over by the air from the cylinders of the compressor; this is difficult, because the oil is not only “atomized” but is actually vaporized while in the compressor cylinders and as a gas it is difficult to reclaim. The air must be kept as low in temperature as possible, and it is usually passed through a cooler before being delivered down the well.

At times, air-lifts are installed for conveying other liquids to a height, and when these can be treated at a high temperature it is advisable, as the efficiency is then much improved. Even under these conditions it is advisable to cool the air to the lowest feasible temperature, before using it as a lifting medium.

When starting up, the column of water in the rising main has to be moved as a solid column, and consequently a higher pressure of air is required at starting than when the column has been set in motion, as the water and air then pass up in alternative “pellets.”

In chemical works and allied industries this pneumatic method is frequently used for pumping acids, and other corrosive liquids from one place to another. Compressed air is a very handy medium for this class of work as ordinary mechanical methods are ruled out, due to the impossibility of introducing corrosive liquids into the pumps and syphons unless great expense is incurred by the use of acid-proof materials.

The air-lift is also very advantageous for pumping water which contains a large amount of sand or similar gritty material which would cut and score the walls of an ordinary piston pump. Air-lift pumping is frequently used, therefore, on new bore-holes until the sand, etc., has been eliminated, after which the final pump can be installed without fear of damage.

The question of submergence will frequently make it impossible to use air-lift without boring many feet deeper than would otherwise be necessary, but when the water bearing strata is low this form of pumping is frequently very convenient.

Miscellaneous Applications of Pneumatic Conveying. Several other interesting applications of pneumatic conveying may be enumerated but, being somewhat outside the primary scope of this book, they will not be discussed in detail. The main object of the author is to raise interest in the handling of solid materials in a manner practically unknown to the general reader.

The housing problem has developed the pneumatic handling of cement in a liquid form, and houses are now being built of reinforced cement in the following manner. An expanded steel frame is supported between concrete or brick piers, and on wood sheeting where necessary, and liquid cement is blown on to the metal in the form of a liquid spray: the first coat dries quickly and leaves a certain amount of cement covering the framework. Then follows another coat, and again another and so on, until the whole of the framework has been covered to an appreciable thickness. The result is a thin wall or slab of cement reinforced with the steel and of great combined strength. Slightly domed roofs constructed in this manner have proved very strong and durable.

The Aerograph is an instrument working on the same principle for the application of paint, and it is used a great deal in the art world, in the manufacture of Christmas cards, in panel painting, and in interior decoration generally. Excellent “tones” and shades are obtained by the simple method of varying the thickness of the colour or the number of coats applied. It is usual to convey the surplus colour and fumes away from the operator by means of a stream of air through a special hood placed at the back of the work, thus maintaining clean pure air for the operator.

A similar machine of more crude design is used for whitewashing walls of stables, cattle pens, etc. All these plants comprise an air compressor, either power or hand operated, from which the air is led to a special injector which draws up through a second pipe a certain amount of the material to be sprayed. The paint or other material is then atomized and impelled with considerable force on to the surface to be covered.

The sand blast is another application of pneumatic conveying in which the medium conveyed is sand, which has well-known cutting and erosive effects when it impinges on a surface at high velocity. This plant is used for decorating glassware, obscuring sheet glass, and also for cleaning stone buildings by the actual removal of the face of the previously discoloured stone.

The pneumatic conveyance of energy is exemplified by rock drills, riveting machines, coal-cutters and innumerable other portable tools. Energy is expended in compressing air which is transmitted through pipes and made to yield its stored energy by driving the air motors of the tools or other apparatus in question.

Conclusion. Enough has been said to show that pneumatic conveying has made great progress, and that the possibilities of this method of dealing with the moving of solid materials are much greater than has been generally recognized.

Almost anything that will enter a pipe up to about 9 ins. diameter can be conveyed in this way, either by “blowing” or “suction” or by the “induction” method.

Weight and size is an advantage rather than otherwise, and bricks can be dealt with more successfully than flour. The writer’s experience, in the results of actual working with pneumatic conveying, indicates that no problem should be considered too difficult to be tackled by this method, and that even the most unlikely materials can be conveyed successfully by pneumatic means.