The Pneumatic Despatch Tube System of the Batcheller Pneumatic Tube Co. / Also, Facts and General Information Relating to Pneumatic Despatch Tubes

by Birney C. Batcheller

If a larger than eight-inch tube is to be used for mail service, it should be not less than thirty-six inches. Carriers larger than eight inches cannot be handled: they are too heavy. They are also too heavy to slide through the tube, hence, must be mounted upon wheels. It is not practical to make a carrier on wheels less than eighteen or twenty-four inches, and the carrier must be at least twenty-four inches to contain a large mail-pouch. Now, if we are going to despatch mail-pouches through a pneumatic tube we must send more than one in a carrier, otherwise the service will be too slow. Such large carriers could not be despatched oftener than once or at most twice in a minute. Suppose we were to transport the mail from a railway station to a main post-office. A train arrives with, say, sixty pouches. If only one pouch could be put into a carrier and the carriers could be despatched at half-minute intervals, it would take thirty minutes to despatch all the pouches. Now, suppose we make the tube thirty-six inches. The carriers will be eight feet long and will contain from twelve to fifteen pouches. Five carriers would contain the entire train-load of mail, and they could be despatched in four or five minutes.

Fig. 22.
CROSS-SECTION OF A 36-INCH TUBE.

Fig. 23.
CARRIER FOR A 36-INCH TUBE.

System of Very Large Tubes.

—The cross-section of a thirty-six-inch tube is shown in Fig. 22. It is built flat on the bottom and sides, with an arched top. The floor is of concrete containing creosoted ties; the side walls and top are of brick, plastered with cement upon the interior. The two tubes may be built one above the other or side by side, depending upon the condition of the streets, but one common separating wall will serve for both. The carriers, one of which is shown in Fig. 23, run on two rails laid close to the sides of the tube. At curves a guard-rail is placed upon the side wall, making it impossible for a carrier to leave the track. The carriers are made of hard wood with an iron frame, and are as light as consistent with the service required of them. They are open on top. Their outside dimensions are thirty-four inches by thirty-four inches by eight feet. The sending and receiving apparatus for these very large tubes have to be specially designed for each particular station, so no attempt will be made here to describe them. The air-pressure required depends upon the length of the line. If it were not more than six or eight ounces a fan would be used to maintain the air-current, but for pressures above this, up to a pound or two per square inch, some form of positive blower would be used.

At the stations considerable floor space or “yard room” would be required for side tracks, switches, etc. Usually the basement of a building would have to be utilized for the termination of such a tube. There are but few places in our large cities where the streets are so free from pipes, sewers, conduits, etc., that it would be practicable to build a thirty-six-inch pneumatic tube. When the service can be rendered by an eight-inch tube, the cost of installation favors its adoption. Steep grades cannot be ascended by these very large tubes, while the eight-inch tubes can be placed vertically. We do not say that there is no use for eighteen- and twenty-four-inch tubes, but the demand for them would be in special cases and we will not discuss them here. For ordinary mail and parcel service we recommend the use of six- and eight-inch tubes. An eight-inch carrier is twenty-four inches long, about seven inches inside diameter, and will contain five hundred ordinary letters. It weighs about thirteen pounds empty, and one can be despatched every six to ten seconds. We estimate that eighty per cent. of all the parcels delivered from a large retail department store could be wrapped up to go into these carriers. The minimum radius of curvature of an eight-inch tube is eight feet.

General Arrangement of Apparatus in the Stations. Two-Station, Two-Compressor Line.

—We will now proceed to a description of our system in detail. Figs. 24, 25, and 26 are diagrams showing how the tubes, air-compressor, tanks, sending and receiving apparatus are connected together at the stations. These diagrams are drawn to represent an eight-inch tube, but essentially the same arrangement would be used for smaller tubes.