Tubes. The tubes are of lead and are usually encased for protection against mechanical damage, and the erection is carried out with great care so as to preserve the smooth interior. Joints occur at intervals of 28 ft. or less, and are “wiped” with an ordinary plumber’s joint over an internal mandril which is heated previous to insertion in the tube. Air-tight joints and smooth interiors are absolutely essential to a successful installation.
Carriers. The carriers or cartridges in which the material to be transmitted is placed are made of gutta-percha covered at the ends with felt. One end of the container is closed and the other end is left open, but a “skirt” of felt surrounds the open end, and, as this is the “trailing” end and the air pressure is behind it, the air forces open the “skirt,” making a tight fit and preventing leakage of air past the carrier. The nose of the carrier is usually fitted with a felt “buffer” which also assists in making an air-tight fit. A carrier for a 2½ in. tube is 6¾ ins. long and weighs empty about 3 ozs. Fig. 26 shows a large carrier.
Methods of Working. Pneumatic tubes are worked either by air above atmospheric pressure or by reducing the pressure below atmospheric. In the pressure system the usual pressure is about 10 lbs. per sq. in. above atmospheric pressure, whilst in the suction system the vacuum employed is equivalent to about 6½ lbs. per sq. in.
Fig. 26.—Typical Lamson Intercommunication Carrier.
Also, the method of working may be either “continuous” or “intermittent”; in the first system the air, either above or below atmospheric pressure, is circulating continuously and the cartridge or carrier is inserted into a stream of air already in circulation, whilst in the “intermittent” system the power, either pressure or suction, is admitted to the conveyor tube only after the carrier has been inserted, and it is again cut off when the carrier reaches the end of its journey.
To a great extent the success of a pneumatic tube system is the speed at which it can transmit the message sent by this means. In the “continuous” system, working above atmospheric pressure, the speed is not so great as in the “intermittent” scheme, because the pressure in the tube is the same in front of and behind the carrier, which has to displace the air in front of it. In the “intermittent” system the pressure is turned on after the carrier is in place, and the advancing carrier has only to move the air at atmospheric pressure. On the other hand, if suction is employed, the “intermittent” system is slower than the “continuous” system because the air has to be exhausted to a certain point before the carrier begins to travel. It is true that it will begin to move as soon as the difference in pressure amounts to a few ounces, but there is a distinct “time lag” compared with inserting the cartridge into a tube continuously exhausted when it starts off at practically full pressure and speed immediately.
The difference in time is stated by Kemp to be 3 per cent. longer with “continuous” pressure, compared with “intermittent” pressure at 6 lbs. per sq. in.; the difference increasing to 6 per cent. when the pressure is raised to 14 lbs. per sq. in. The average working speed of these tubes is from 25 to 30 miles per hour.
Power Required for Operation. It is difficult to determine the actual amount of power necessary to carry a cartridge through a tube. Kemp’s Engineer’s Year Book states that, working at the standard pressure of 10 lbs. per sq. in., the power required is theoretically 3·35 h.p. for a 2½ in. tube, 1 mile long, but actual experience suggests that at least 50 per cent. should be added to allow for losses from various causes, making the actual power, say, 5 h.p. per 2½ in. tube per mile.
Pressure receivers or tanks are inserted between the pump and the travelling tube to compensate for the impulses due to the irregularity of the pumps and also to act as reservoirs furnishing additional power during periods of abnormal working.