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have the square root of the water gauge in inches
multiplied by gravity, divided by the weight of a
body of air whose equal bulk of water weighs one
pound multiplied by 12, there being 12 inches in
one foot, the water gauge being read in inches.
This product multiplied by 60 being the manometric
efticiency and that being reckoned at 60^ a value
probably not exceeded by the best types of fans:
13.5 w g x 32.2 gravity
.00125 x 12 x 60?
equals 219.8 feet perimeter speed per second.
The next consideration will be as to whether we
shall select a fan of small diameter to run at high
speed by belt or a fan of larger diameter at lower
speed, direct coupled. From observations we note
the universal practice in Western Pennsylvania
favors the larger diameter, while in Germany we
believe, the opposite is the case. Suppose that
for our fan in question to circulate 200,000 cubic
feet per minute we fix a maximum speed of 200
revolutions per minute. We then have 219.8 feet
per second tip speed, multiplied by 60 equals
13188 feet per minute, divided by 200 equals 65.94
feet circumference. This gives us a fan 21 feet
in diameter; knowing the diameter and area of
end of fan and number of revolutions, the next
step is to find the width. To do this the volumetric
efficiency of the fan must be known in order
to secure a correct solution to our final. In our
several steps in calculations with the Capell and
Robinson fans, 100 per cent, volumetric efficiency
may perhaps be very closely approached although
I understand that from actual test 140 per cent.
volumetric efficiency has been attained with the
Capell whue the other types of fans within
the scope of their gauges will give from 40 per
cent, to 70 per cent, volumetric efficiency. In this
case we find the high gauge necessary and we will
assume a Capell fan is used and its volumetric
efficiency is 75 per cent., then taking our factors,
200,000 cubic feet of air per minute, being required
at 200 revolutions, the speed at which it is to be
obtained, a fan 21 feet in diameter has been found
to do the work required, 21 x .7854 equals 436.36
square feet area of fan, and the width of fan will
equal two hundred thousand cubic feet divided by
the product of two hundred revolutions, multiplied
by the area of the fans, which equals three and
eighty-five (3.85) hundredths, width of fan.
And for a fan of double port of entry, the width
should be two hundred thousand, divided by two,
which will equal one hundred thousand, and width
of fan would equal in this case the square root of
100,000 multiplied by .0343, or 10.84, divided by 2,
equals 5.42 feet, width of fan.
Our observation in Western Pennsylvania is
that the fans have been built much wider than the
THE COAL TRADE BULLETIN. 49
theoretical calculation has shown they should be.
This may be due to large airways and improved
methods of ventilation, etc., which would offer less
resistance to the passage of the air through the
mine, lower the water gauge, and thence the speed
of the ventilator; then the greater breadth would
keep up the volume, or the speed remaining the
same would very much increase the volume.
Centrifugal fans are used for blowing and exhausting.
Exhausting fans are most generally
used, as this system will allow the traveling and
haulage ways to be in the intake air-ways, although
advocates of the blowing fan are numerous.
So far as mechanical efficiency is concerned,
EXHAUST FANS ANU BLOWING FANS
are practically equal. The general principles of
each are the same except in reverse order. The
exhaust fan draws the air from the mine under
atmospheric pressure, and discharges it into the
outer atmosphere, while the blowing fan draws
from the outer atmosphere and discharges into the
mine above atmospheric pressure, ventilating pressure
in either case being equal to the difference of
pressure produced by the fan's action. The advantage
claimed for the exhaust fan, is. should a sudden
stoppage occur, we have a rise of mine pressure
instead of a fall, and the gases are driven
back into the mine workings for awhile. If the
fan is a blower and such a stoppage should occur
it is followed at once by a fall in the ventilating
pressure of the mine and the gases expand more
freely into the passage ways at the very moment
when their presence is most dangerous. This
point should be very carefully considered in the
ventilation of deep workings. In shallow workings
the blower fan has a decided advantage, especially
if there is a large area of abandoned workings
that have a vent or opening to the surface.
Every crevice becomes a discharge opening by
which the mine gases find their way to the surface
instead of drawing them into the workings as
would be the case with an exhaust fan.
Any change in the atmospheric pressure affects
the expansion of mine gases to a less extent in the
blowing system than in the exhaust system. The
area of the throat of the fan must be equal to the
area of the curve surface of an imaginary cylinder
whose diameter is equal to that of the port or
ports of entry.
The area of the point of discharge in an ideal
fan should not be less than eighty-one hundredths
of the area of the port of entry. It is, however,
true that a few fans will work satisfactorily where
this port is so large, but such fans cannot give best
results. When the area of the discharge port is
too much restricted the excessive pressure required
to blow out the air is much greater than it should
be. On the other hand, if the port of discharge is