Pneumatic Dispatch

Dispatching and Receiving Apparatus.

On house tube systems, where only low pressures and vacuum are required, simple forms of terminals consisting of cast-iron bodies with flap-doors are used, the doors being opened to insert a carrier, and through which carriers are automatically ejected, the door closing behind them. Dispatching funnels are also used at the open end of tubes.

On the street tubes, however, a more complicated type of apparatus, called a double slide switch, is used at the central office. This consists of two vertical sections of tube secured into top and bottom plates and provided with a handle by which it is rocked between two fixed horizontal plates forming a frame, three holes being provided in each plate, the air supply pipe and dispatch tube being connected to the centre holes and funnels for inserting or removing the carriers to the other holes. The carrier on arriving is received in one of the vertical sections and on the rocker being moved to its second position the carrier drops out through the funnel provided. The second tubular section is now ready to receive a carrier, which is discharged when the rocker is returned to its first position. The process is reversed for dis patching purposes, and where the tube is used for two-way working one position is reserved for sending and one for receiving. The supply of air, and whether under pressure or vacuum for send ing or receiving purposes, is controlled by a three-way valve mounted under the switch.

A form of flap terminal, the door being restrained by a spring in view of the higher pressures used, is installed at the outlying offices in place of the receiver originally used and the carrier is automatically ejected.

On tubes where the carrier reaches a high velocity, a by-pass is provided near the terminal by means of which the air pressure behind the carrier is released after it has passed this point so as to reduce the velocity of egress. This terminal is also used for the dispatch of carriers to the central office, the supply of air to the tube being changed from vacuum to pressure at the latter point in response to a bell signal. When used for dispatch ing the by-pass is cut off by a remote control valve. Carriers may be diverted from one tube to another by means of a moving section of tube directly or remotely controlled.

A

further development is the provision of an automatic rotat ing pneumatic tube switch by which carriers can be transferred between street and house tubes for both sending and receiving purposes, the terminals used being of the flap type.

Circuit System.

Another system of working is the circuit system, in which stations are arranged in circular or loop lines round which the carriers travel in one direction only, both pres sure and vacuum being used. This system is in use in Paris and other Continental cities. In Messrs. Siemens' system a continu ous flow of air is maintained in the tube, the carriers being dis patched or removed through a form of rocking switch so designed that the movement of carriers in other parts of the system is not interfered with. More usually carriers, or trains of carriers, are dispatched at intervals, the air supply being cut off when not re quired. Long tubes may be sectionalized, means being provided

at the various stations by which air is only supplied to the working section.

In America, under the Batcheller system, tubes 8in. in diam eter are used for postal purposes in New York, Brooklyn, Chi cago, Philadelphia, Boston and St. Louis; tubes 6in. and loin. in diameter are also used. These tubes are essentially cast-iron pipes, carefully bored and equipped with suitable apparatus for introducing and receiving the carriers. The carriers for the 8in. tubes consist of cylindrical steel shells about Tin. in diameter and 2 iin. long (inside dimensions) with suitable end covers. They are fitted with lubricated soft bearing or packing rings. The work ing capacity of these tubes may be taken as 81b. per container and six containers per minute, or 2,88o1b. of postal matter per hour in each direction, the average speed being about 25 to 3om. per hour.

The dispatching apparatus is similar to that in the Siemens' system, consisting of two sections of tube supported in a rocking frame so arranged that either section may be brought into line with the main tube, in which a current of air is constantly flow ing. One of these sections maintains the continuity of the tube while the other section is loaded. The switch is then swung over by means of a pneumatically operated piston to insert the carrier in the tube. A by-pass is provided to maintain the flow of air in the tube during the operation. As soon as the carrier leaves the switch it trips a lever and the switch automatically returns to the loading position. A time lock is used to prevent carriers being dispatched too frequently. The carrier is received by an air cushion formed by closing the end of the tube with a sluice gate, a by-pass being provided before this point to allow the air to flow away.

In addition to their use for postal and telegraphic purposes pneumatic dispatch tubes are used for internal communication in offices, hotels, etc., also in shops for the transport of money and bills between the counters and the cashiers' desk. Pneumatic tubes are also used for the unloading and transport of grain and other commodities in bulk between warehouses and ships, trains, etc.

BIsmoGRAPHY.—The system as developed for use in the British post office by Messrs. Culley and Sabine is described in a paper in Min. Proc. Inst. Civil Eng., vol. xliii. The same volume contains a description of the Paris system and of experiments thereon by M. Bontemps, and also a discussion of the theory of pneumatic transmis sion by Prof. W. C. Unwin. Reference should also be made to a paper by C. Siemens (Min. Proc. Inst. Civ. Eng. vol. xxxiii.), describing the Siemens' circuit system ; to Les Telegraphes, by M. A. L. Ternant 0880 ; to the report to Congress of the American joint commission on pneumatic tube mail service, March 1919 ; General Post Office engineering department's Technical Instruction No. X (Pneumatic Tubes) ; a short paper by J. McGregor on the automatic system in the post office (P.O. Elec. Eng. Journal, vol. xix., part and to Kempe's Engineer's Year Book. (J. McG.)