This tunnel, which forms an ascending incline of a mile and a quarter
length from the terminal station in Lime-street London and N. W.
Railroad, was worked until recently by a rope and stationary engine,
to avoid fouling the air of the tunnel by the passage of locomotives;
but the increase of the traffic having necessitated the abandonment of
the rope and the substitution of locomotives for bringing the trains
up through the tunnel, it became requisite to provide some efficient
means of ventilation for clearing the tunnel speedily of the smoke and
steam after the passage of each train. A large exhausting fan has been
designed by Mr. John Ramsbottom for this purpose, which works in a
chamber situated near the middle of the length of the tunnel, and
draws the air in from the tunnel, through a cross drift; discharging
it up a tapering chimney that extends to a considerable hight above
the surface of the ground over the tunnel. The fan is about thirty
feet diameter, and is made with straight radial vanes; it revolves
on a horizontal shaft at a speed of about forty-five revolutions per
minute, within a brick casing, built concentric with the fan for the
first half of the circumference, and afterwards expanding gradually
for discharging into the base of the chimney, the air from the tunnel
being drawn in at the center of the fan at each side, and discharged
from the circumference of the fan by the revolution of the vanes.
The engine driving the fan is started by telegraph signal at each
departure of a train from the terminal station, and the fan is kept
running until the discharge from it becomes quite clear, showing that
no steam or smoke remains in the tunnel; this is usually the case in
about eight minutes after the time of the train entering the lower end
of the tunnel, the passage of the train through the tunnel occupying
about three minutes. The fan draws air in at both ends of the tunnel
simultaneously, and begins to clear the lower end immediately upon the
train entering; the clearing of the upper end commences as soon as the
train has passed out of the tunnel, and as the fan is situated nearer
the upper end of the tunnel than the lower, the clearing of both
lengths is completed almost simultaneously. The fan is so constructed
as to allow an uninterrupted passage through it, for the air, whilst
the fan is standing still; and the natural ventilation thus obtained
by means of the large chimney is found sufficient for clearing the
tunnel during the night and some portion of the day, without the fan
being worked at those times. This natural ventilation is aided by the
engine exhaust and the boiler discharging into the chimney. The fan
has now been in regular operation for three-quarters of a year, and
has been found completely successful.
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