When deep level "tube" railways were first constructed in Lon don, it was supposed that adequate ventilation would be obtained through the lift-shafts and staircases at the stations, with the aid of the piston action of the trains which, being of nearly the same cross-section as the tunnel, would, it was supposed, drive the air in front of them out of the openings at the stations they were approaching, while drawing fresh air in behind them at the stations they had left. This expectation, however, was disappointed and it was found necessary to employ mechanical means. On the Central London railway, which runs from the Bank of England to Shepherd's Bush, a distance of 6 m., the ventilating plant installed in 1902 consists of a 30o h.p. electrically driven fan, which is placed at Shepherd's Bush and draws in fresh air from the Bank end of the line and at other inter mediate points. The fan is 5 ft.
wide and 20 ft. in diameter, and makes 145 revolutions a minute,.
its capacity being i oo,000 cu.ft. a minute. It is operated from I to 4 A.M. and, the openings at all the intermediate stations be ing closed, it draws fresh air in at the Bank station. The tunnel is thus cleared out each night, dur ing the period when trains are not operated, and the air is left in the same condition as it is outside. The fan is also worked during the day from II A.M. to 5 P.M., the intermediate doors being open. In a number of the later tube railways in Lon don—such as the Baker street and Waterloo, and the Charing Cross and Hampstead lines— electrically driven exhaust fans are provided at about half-mile intervals; these each extract 18,5oo cu.ft. of air per minute from the tunnels and discharge it from the tops of the station roofs.
The Boston system of electrically operated subways and tun nels is ventilated by electric fans capable of completely changing the air in each section about every 15 minutes. Air admitted at portals and stations is withdrawn midway between stations.
In the southerly 5 m. of the first rapid transit subway at New York, constructed between 190o and 1904, which is a four track structure of section, having the area of 65o sq.f t. and built as close as possible to the surface of the streets, ventilation by natural means through the open staircases at the stations was at first relied upon. The results were satisfactory as regards the proportions of carbonic acid gas found in the air, but when in tensely hot weather prevailed the tunnel air was sometimes 5° hot ter still, due to the conversion of electric energy into heat. Venti lation chambers were added on each side of the subway at points between stations, and the condition became much improved. These chambers are beneath the sidewalks and covered by grat ings, and they have been incorporated in the construction of all subways built later. In addition, a partition wall separates track ways for trains running in opposite directions and the piston action of the trains induces a satisfactory circulation of the air.
(192o) natural draft was relied upon for the ventilation of all such tunnels in operation, including the Blackwall and Rotherhithe tunnels, in London, which were by far the most important. The traffic in them, however, was only about ioo motor vehicles per hour for each tunnel, compared with an estimated traffic of 1,900 motor vehicles per hour in each tube of the Holland tunnel. Re search studies and exhaustive tests were made for this tunnel with the co-operation of the U.S. Bureau of Mines to determine first, the amount and composition of exhaust gases from motor vehicles; second, the physiological effects of these gases and third, the friction losses and power required to handle large quantities of air through concrete ducts.
In the Holland tunnel, the transverse system of ventilation was adopted. The fresh air is introduced continuously along each tube, from conduits provided for the purpose, and taken off at frequent intervals at opposite points. The air therefore travels a course transverse to that of the vehicles and there is a practically equal degree of purity at all points in the tubes. Because carbon monoxide is lighter than air and the exhaust motor gases are warmer than the fresh air introduced in the tunnel and therefore tend to rise, the fresh air ducts are placed beneath the roadways and the exhaust ducts above the roadway ceiling but all in the same tunnel tube. The fresh air is introduced in each roadway by continuous slots along each side above the floor, and the vitiated air is exhausted through louvers in the roof at 15 ft. intervals. The entire tunnel atmosphere is completely changed every II minutes or 4o times per hour, but so uniformly and gently that the current is hardly perceptible. This eliminates the fire hazard incident to a longitudinal circulation of draught. There are four ventilation shafts, one at each pierhead line and one about midway between the pierhead and portal on each side of the river. The fans are located in the shaft buildings and there are 84 in all, one-half blowers and one-half exhausters.
The Liberty tunnels were the first long vehicular tunnels in which artificial ventilation was used. The method adopted is the longitudinal draft system, based on Saccardo's, with modifications by C. S. Churchill. The flow of air in each tunnel is with the traffic, and is induced from a shaft near the centre of each tunnel. Each shaft is divided into two compartments, one of which is used to exhaust the air which enters at the portal from the first half of the tunnel, and the other to blow fresh air into the tunnel at the centre and force it forward to the exit portal. Suitably designed nozzles at the centre of the tunnels, at the shafts, pre vent the mixing of the air being introduced and ejected. The flow of air is continuous in each tube in opposite directions and wind pipes are provided at the exit of each tube to prevent inter ference with the ventilation by adverse winds. The ventilating plant and fans are on the hilltop near the centre of the tunnels, at the shafts. The plant was designed to provide for a double line of motor cars in each tube, spaced i oo ft., moving at 15 m. per hour and a proportion of carbon monoxide at 6 parts in 10,000 at the point of exit, the average, therefore, being 3 parts in io,000. This required the supplying of 280,000 cu.ft. of air per minute in each tube at a velocity of 6 m. per hour. (See also TRACTION, ELECTRIC; TRANSPORT; AQUEDUCTS; etc.) (R. RY.)