372 moleswobth’s pocket-book
Ventilation of Tunnels.
(Morrison, ‘ Min. Inst. Civ. Eng.,’ vol. xliv.)
H = Head in feet of pressure of air of same density as the
flowing air.
D = Diameter of tunnel in feet.
L = Length of pipe or passage in feet.
P = Perimeter of cross-secliou in feet.
A = Area of pipe or passage in feet.
V =x Velocity in thousands of feet per minute.
K = Coefficient of friction — -03.
„ KV!PL KV24L
H —------T----;-----r—> f°r circular section.
A 1) ’
On a portion of the Metropolitan Railway, mile long,
With 30 trains per day,, the velocity of the air should be 4Ü0
feet per minute. In a tunnel 7 miles long, with 16 trains per
day, a current of 410 feet per minute would be needed.
When long tunnels without shafts have to be ventilated, a
current of air should be passed through a fan placed near one
end of the tunnel and the end closed with doors.
For a given amount of traffic the power required to ventilate
varies as the fourth power of the length. Fur purposes of
ventilation, a double line is better than two separate single-
line tunnels.
For a given length of line there Is a limit In the number of
trains, beyond which ventilation becomes impossible. This
limit cannot be defined, but for a tunnel 22 miles in length it
cannot exceed 20 trains per day.
Proportions of Air, Gaseous Products, and Steam in a
Tunnel after the passage of 1 Tbain. (D. K. Clark.)
Per foot run. Per mile.
cub. ft. 11)3. cub. ft. lbs.
Air Gaseous products.. Steam 473 1 1-5 36 •08 '056 2,497,440 5280 »840 190,080 420 297