COAL - Clpdigital.org

(CoNTINl ED FROM JlI.Y 1).
METHODS OF MINE VENTILATION; MINE
FANS, FURNACES, STEAM JETS AND
THEIR RELATIVE ADVANTAGES CON­
SIDERED.*
The arrangement of the cut-off is of considerable
importance. There are many engineers who
believe that a V-shaped shutter, sometimes called
the Walker shutter, is an indispensable adjunct
to avoid vibrating in the fan. We have heard
fans that pounded and vibrated so as to shake
the ground, and we have known this defect remedied
by introducing a shutter which was not
always a V-shaped shutter. This trouble may
be due to one of three causes: First, a lack of
proportionment in the fan. Second, a too rapid
and too large expansion of the spiral casing be
ginning at the point of cutoff. Third, abutting
the current against a flat surface, etc.
The velocity of intake at the central opening
or eye of the fan should not exceed 1500 feet per
minute. From this you will see that we favor
double intake fans. The velocity of the periphery
flow or the velocity of air in the spiral conduit
should be a uniform velocity when the casing is
properly expanded. This velocity measured at
the point of cut-off should at least equal the velocity
of the blade tips; or, in other words, the
sectional area of the spiral conduit should be such
that air will at least travel with the fan and not
fall behind it.
In endeavoring to point out some of the principles
that govern the proportions of fans used
to ventilate mines, and by working an example
or two, we hope to give some help to mining
men who are from time to time called to perform
the somewhat bewildering task of selecting
a fan for some particular mine. The formulas
that appear in text books or mining journals have
for their object the simplifying and mak'ng plain
the underlying principles of the fan, yet they are
so complex and complicated in character that the
ordinary mining student cannot grasp them.
Quoting Mr. William Clifford's views: Several of
our mining text books very clearly deHne the
law that resistance of the flow of air in a mine
increases as the square of the quantity or volume,
and this will be shown, as we proceed, to be
A LEADING FACTOR
in fan calculations. One of the simplest formula
and most easily to be understood and one which
has been unanimously accepted by mining engineers
tollows:
U indicates speed of tips of vane in feet per
second.
G indicates 32 2-10, the velocity in feet that a
falling body has acquired at the end of the first
second.
*By I. G. ltoby, of Uniontown, Pa,
THE COAL TRADE BULLETIN. 47
.00125 equals the weight of a body of air whose
bulk of water weighs 1 -800th, the relative weight
of air and water commonly assumed in text books.
12 inches equals a foot; the water gauge being
read in inches, 62% Ios. being the weight of one
cubic foot of water; this divided by 12 equals the
weight of a square foot of water 1 inch deep, or
the weight of 144 inches of water.
The water gauge that would be produced by the
above equation of formula is not that which we
may expect to see in a fan drift, but is that oshich
would be produced if the fan and its surrounding
conditions were complete and in perfect adjustment.
The percentage utilized, which we
note reading the water gauge producing this equation
will give the manometric efficiency of our fan.
This efficiency varies in two ways; one depending
upon the mine, and one depending on the construction
of the fan.
Here I must extend my acknowledgment to Mr.
William Clifford for some very important information
and suggestions, which will follow
First: "We will note that the mine will not
allow the column the fan displaces to pass it, the
depression produced by the velocity of the tips of
the blades."
Second: "The fan may produce a false water
gauge higher than the true statistical gauge. High
manometric efficienty by no means of the mine
proves a good fan."
"Three properties in fans present themselves for
consideration, and, in my judgment, they are noted
in the order of their importance.
"(A) Volumetric efficiency, or what is often
termed in text books and mining literature generally
body output: this is the proportion or excess
of the displacement of the fan considered as a revolving
cylinder shown by the volume produced
by a fan.
"(B) The mechanical efficiency is the proportion
of the power applied to drive a fan that is
utilized in producing air and in overcoming resistance
in the mine and drifts, though we must
add in the resistance of the fan itself. (This notation
does not apply to the Capell fan).
"(C) The manometric efficiency is the proportion
of the force of the velocity of the tips of the
vanes considered as a fallen body utilized in producing
actual depression. In ascertaining volumetric
efficiency we take the whole cylindrical
contents, including the shaft and solid members
of the fan wheel and compare the displacements of
the cylinder through one revolution, or preferably
through one minute, with the column of air passed
during the period or time, with the displacement
as a divisor and the column as a dividend.
"It is suggested that in making calculations
the observer or experimenter should be particu-