COAL - Clpdigital.org

48 THE COAL TRADE BULLETIN.
brick should be run between the inner and outer
arch to prevent collapse and to keep air space so
widely open that a current of air may freely pass
through it and keep the heat from the roof. The
importance of this arrangement is due to the
fact that where me roof stone contains water,
the crown arch is constantly buckling with the
pressure produced by steam and this causes the top
stone to break and fall. The furnace arch is
generally a semi-circle and the height from the
fire bars to the under surface of the arch is usually
one and one-fourth times the width of the
grate's surface. The dimensions of the furnace
are determined on the basis of the amount of
work it is intended to perform. The length of
the furnace bars should not exceed five feet, and
as this dimension is uniform for all furnaces, the
important dimension required for constructing
a furnace is its breadth. The area of the fire
grate's surface varies inversely as the square root
of the depth of the furnace shaft. Before the
width of a furnace can be determined the amount
of air necessary for the efficient ventilation of
the mine must be approximately known, and the
supposed water gauge must also be approximately
known. From these facts the horse-power of the
required furnace can be determined. The volume
multiplied by the pressure, divided by 33000
equals the horse power. We will now endeavor
to make clear that
THK QUANTITY VARIES
as the square root of the depth of the upcast shaft.
To compare the amount of the grate area per
horse power required, we will assume D. equals
depth of shaft. Thirty-four equals a constant
number, proven by many experiments. Now the
fire grate area per horse power in a shaft 50 feet
deep would be 4.8 square feet per horse power;
and for a shaft 600 feet deep would be 1.388
square feet per horse power, so if it was necessary
to circulate 100,000 cubic feet of air in a
shallow mine against one inch of water gauge
where the upcast shaft was 50 feet in depth, it
would require a grate area of 100,000 cubic feet,
multiplied by 5.2. Water gauge, 33,000 equals
15.76 horse power, and 15.76 multiplied by 4.8
square feet equals 75.8 feet total area; Proof:
Fifteen feet in width divided by 4.8 being the
number of square feet in a grate area per horse
power, and 5 feet being the length of the furnace.
Now to circulate 100,000 cubic feet of air in a
mine where the shaft is 600 feet deep, the width
of the grate would be 4.5 feet. From this calculation
we see the disadvantage of the furnace in
a shallow shaft, as against the advantages of a
furnace in a deeper shaft; in the problem solved
the width of the furnace would be as 1. is to 3.5
to circulate uie same quantity of air—which cer­
tainly proves that a furnace in deep workings is
false economy. Then again were we to use a
furnace in deep workings and the mine evolved
marsh gas, which is almost invariably the case,
it would then be necessary to supply the furnace
with fresh air directly from the down-cast, and
pass the return air into the up-cast by way of a
dumb drift to prevent the possibility of an explosion
by reason of the return air becoming
sufficiently charged with fire-damp to cause an
explosion, which might be true in a mine of any
great depth, at any time. Then again, the distance
from the furnace to a point in the up-cast
shaft where it would be safe for the return air to
enter the up-cast should not be less than 150 feet
and in some cases where bituminous coal is
burned safety is not secured until the junction
takes place at an elevation of 300 feet above the
furnace.
Returning again to the fan; as a mining engineer
or mine superintendent about to make a
selection of a mine fan, we feel very desirous to
secure the best fan that we can procure. To be
able to make a
PRACTICAL AND INTELLIGENT SELECTION
we must necessarily know something about the
principles involved in fan ventilation as well as
something of the design of the fan. Extract from
Mines and Minerals: We will first take up the
design of centrifugal fans for use as ventilators
and endeavor to explain some of the principles
that govern this subject, and soon find it beset
with many difficulties. What has been done in
this direction is the result of repeated trials and
many failures, perhaps due more to the conditions
under which machines of this class act, they
being so varied and often unsuspected, and often
this alone has resulted in much confusion. We
also find in calculating the efficiency of a ventilator
many influences are neglected, a few of
which we will enumerate. Chief among these
perhaps is the existence of a positive or negative
air column in connection with the circulating
current, whose influence has not been estimated.
Also temperature and pressure (barometrical) are
factors in all pneumatic calculations.
The absence of their notation has vitiated many
tabulated tests or results. No small amount of
error also arises from measuring the velocity
of an air current and reading the water gauge at
two distinctly separate points. The observations
of velocity pressure and temperature should always
be made at the same point in an airway. In
all scientific calculations air should be regarded
as a compressible fluid. The disregard of such
essential points as has just been mentioned often
results in a mass of contradictory conclusions by
investigators of known ability and repute. We