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

Description of the Tubes, Method of Laying, etc.

—This Chestnut Street line consists of two tubes, one for despatching carriers from and the other to the main post-office. The distance between the two stations is two thousand nine hundred and seventy-four feet, requiring five thousand nine hundred and forty-eight feet of tube. The inside diameter of the tube is six and one-eighth inches, and it was made in sections each about eleven feet long, with “bells” cast upon one end, in order to join the sections with lead and oakum, calked in the usual manner of making joints in water- and gas-pipes, with this exception, that at the bottom of the bell a counter-bore was turned to receive the finished end of the next section. By thus machining the ends of each section of tube and having them fit accurately together, male and female, a practically continuous tube was formed with no shoulders upon the interior to obstruct the smooth passage of the carriers. Joints made in this way possess another great advantage over flanged and bolted joints, in that they are slightly yielding without leaking, and so allow for expansion and contraction due to changes of temperature. Each joint takes care of the expansion and contraction of its section, which is very slight, but if all were added together would amount to a very large movement. Another advantage of the “bell” joint is that it permits slight bends to be made in the line of tube without requiring special bent sections. Where short bends had to be made, at street corners, in entering buildings, and other similar places, brass tubes were used, bent to a radius of not less than six feet, or about twelve times the diameter of the tube. (One of the brass bends may be seen in Fig. 10.) The bends were made of seamless tubing, bent to the desired form and radius in a hydraulic machine. To prevent them from being flattened in the process of bending, they were filled with resin, which was afterwards melted out. Flanges were screwed and soldered to the ends of the bent brass tubes, and they were bolted to special flanged sections of the iron tube.

The tubes were laid in the trench and supported by having the ground firmly tamped about them. Usually one tube was laid above the other, with an iron bracket between, but frequently this arrangement had to be departed from in order to avoid obstructions, such as gas- and water-pipes, sewers, man-holes, etc. The depth of the tubes below the pavement varied from two to six feet, and in one place, in order to pass under a sewer, the extreme depth of thirteen feet was reached. At the street crossings it was frequently difficult to find sufficient space to lay the tubes. At the intersection of Fifth and Chestnut Streets a six-inch water-main had to be cut and a bend put in. A seven-strand electric cable, used for telephoning and signalling, was laid upon the top of one of the tubes, protected by a strip of “vulcanized wood,” grooved to fit over the cable. The cable and protecting strip of wood were fastened to the tube by wrought-iron straps and bolts.

The tubes enter the main post-office on the Chestnut Street side, through one of the windows, and are suspended from the ceiling along the corridor in the basement for a distance of nearly two hundred feet. Fig. 8 shows the tubes thus suspended. They terminate upon the ground floor about the centre of the building, and near the cancelling machines.

Fig. 9.
DUPLEX AIR-COMPRESSOR IN THE BASEMENT OF THE MAIN POST-OFFICE.

Fig. 10.
TANKS AND TUBE IN THE BASEMENT OF THE MAIN POST-OFFICE.

Air-Compressor—Method of Circulating the Air.

—The current of air that operates the tubes is supplied by a duplex air-compressor located in the basement of the main post-office. This machine is shown in Fig. 9, and requires no detailed description, as it does not differ materially from air-compressors used for other purposes. The stroke is twenty-four inches, the diameter of the steam-cylinders ten inches, and the air-cylinders eighteen inches. The air-cylinders are double acting, with poppet-valves, and have a closed suction. The speed of the machine varies slightly, being controlled by a pressure-regulator that maintains a practically constant pressure in the tank that feeds the tube. The engines develop a little over thirty horse-power under normal conditions. The pressure of the air as it leaves the compressor is usually six or seven pounds per square inch. Compressing the air heats it to about 156° F., but this is not sufficient to require water-jackets about the air-cylinders. From the compressor the air flows to a tank, shown on the right in Fig. 10, where any oil or dirt contained in the air is deposited. The principal purpose of the tank is, however, to form a cushion to reduce the pulsations in the air caused by the periodic discharge from the cylinders of the compressors, and make the current in the tube more steady. From this tank the air flows to the sending apparatus on the ground floor of the post-office and thence through the outgoing tube to the sub-post-office. At the sub-post-office, after flowing through the receiving and sending apparatus, it enters the return tube and flows back to the main office, passing through the receiving apparatus there and then to a tank in the basement,—the left tank in Fig. 10. The air-compressor draws its supply from this tank, so that the air is used over and over again. This return tank has an opening to the atmosphere, which allows air to enter and make up for any leakage or escape at the sending and receiving apparatus, thereby maintaining the atmospheric pressure in the discharge end of the tube and in the suction of the compressor. The tank serves to catch any moisture and dirt that come out of the tube. Fig. 11 is a diagram showing the direction and course of the air-current. It will be noticed that both the out-going and return tube are operated by pressure, in distinction from exhaust. The air is forced around the circuit by the air-compressor. There is no exhausting from the return tube. The pressure of the air when it enters the tube at the main post-office is, say, seven pounds per square inch; when it arrives at the sub-post-office the pressure is about three and three-quarters pounds, and when it gets back to the main office and enters the return tank, the pressure is zero or atmospheric. Thus it will be seen that the pressure becomes less and less as the air flows along the tube. This is not the pressure that moves the carriers, but the pressure of the air in the tube, a pressure that exists when there are no carriers in the tube. It is the pressure that would be indicated if you should drill a hole into the tube and attach a gauge.