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.
