a form of pneumatic conveyor for moving documents and small objects through tubes by means of an airflow. It is used to send documents in communications enterprises, libraries, and banks; medical histories and medicines in hospitals; and parts, tools, and specimens (for example, hot metal) in the field laboratories of industrial enterprises. The first operating pneumatic dispatch system, with tubes 100 m long, was installed in the London telegraph office in 1853.

The main components of a pneumatic dispatch installation are the tubes, the transporting containers, the sending and receiving equipment, and the blowers. The transporting containers are cartridges or capsules that enclose the objects and are placed in the tube through the sending and receiving equipment. The pressure differential created by a blower causes the cartridges to move from the sending station to a receiving station, where they are removed.

A distinction is made between internal pneumatic dispatch systems, which operate within a building, and external systems, which connect enterprises and institutions throughout a city. The tubes in an internal system are usually made of thin-walled seamless pipe with an inside diameter of 50–120 mm. Their total length may be several hundred meters. The smallest radius of curvature in a tube is about 1 m. The tube material is brass, Duralumin, steel, or frequently, since the 1960’s, polyvinyl chloride. Tubes of rectangular cross section (for example, 10 × 70 mm) are sometimes used to move documents and objects of standard shape without packing them into capsules.

The tubes used for external pneumatic dispatch installations are 65–1,000 mm in diameter. They are usually made of steel, plastic, or asbestos cement and are laid in the ground. The distance between stations is up to several kilometers, and the total length is up to several hundred kilometers (for example, 600 km in Paris).

The cartridge is a short section of pipe with a diameter about 25 percent smaller than the inside diameter of the tube. Two (sometimes one) sealing caps made of felt or leather are mounted on the outer surface of the cartridge. The average speed of a cartridge carrying a weight of 1–2 kg is 6–20 m/sec (in some installations, up to 45 m/sec). The capacity of a pneumatic dispatch installation is as high as 2,400 cartridges per hr.

In its simplest form the sending and receiving equipment is a gap or a longitudinal cutout in the tube, which is closed manually by a movable sleeve. In single-tube reversible pneumatic dispatch installations the sending and receiving stations are hermetically sealed boxes in which the tube has a longitudinal cutout. Cartridges are removed automatically by a wedge pushed out by an electromagnet.

Air is supplied in pneumatic dispatch installations by blowers or fans that create a negative or positive pressure in the tubes. The pressure is regulated by means of baffles or throttle valves.

The stations of an installation are connected in linear, radial, or annular layouts. For low traffic volumes (up to 100 cartridges per hr), several stations may be connected by a single tube, which operates in both directions. Only one cartridge at a time can move through such a line. To increase the capacity of single-tube external pneumatic dispatch installations, sidings (bypasses) are used both in the middle portion of the line (between two stations) and at stations. With this design several cartridges can move simultaneously through the line.

The two-tube line permits independent movement of several cartridges in both directions. Two to six lines can be connected to a single junction (a distribution center, which has manual or automatic control), where the cartridges are transshipped and sorted. In the annular layout the cartridges can be forwarded between any two stations without transshipping. In a two-tube line with an annular layout, the receiving stations are equipped with switches at the points where branches leave the line. The switches are controlled by a “carrier memory,” which is a system of contacts or magnetic rings on the cartridge sleeve, or by a centralized system, such as telephone selectors.

A promising trend in the development of pneumatic dispatching is the use of large-diameter tubes (450 mm in the Federal Republic of Germany, 600 mm in France, and 1,020 mm in the USSR) and wheeled containers joined into trains of five or six, which makes it possible to transport loads weighing about 10 tons at a speed of 40–60 km/hr.