coal trade bulletin - Clpdigital.org

If hoisting is done in a downcast shaft the
shaking screens should not be placed immediately
adjacent to the shaft, and if they are near the
shaft, vacuum dust collectors should be installed
over the screens and chutes. Otherwise, a large
quantity of dust may be drawn down the shaft.
In a certain mine in England in which
ROCK DUST
yvas used to counteract the danger of coal dust
the writer observed a thick film of coal dust on
top of the rock dust, the deposit extending for a
distance of 500 or 600 feet from the shaft. Had
it not been for the light-colored rock dust the deposit
could not have been seen. It had been collecting
for only tyvo months subsequent to the
time when the rock dust had last been laid. This
mine has since put in vacuum dust collectors over
its screens. In many of the recently built European
plants it is the practice to place the screening
plant 100 to 200 feet distant from the downcast
shaft.
It should always be borne in mind that, except
in anthracite mines, coal dust is the agency that
causes an explosion to sweep through a mine,
leaving a trail of wreckage and death. Certain
means of prevention have already been indicated,
but are reviewed beloyv. Apart from lessening
the production of dust, there are two principal
ways of fighting the coal dust danger—first, wetting
the dust by various means, such as humidifying
the air current or yv-ashing down the dust
with hose or with pump yvater cars; second,
spreading rock dust over it.
With the humidifying system the intake air
current is so saturated or supersaturated as to
carry the moisture into the mine in minute but
constant quantities every minute of the day. Details
of the system are given in Bureau of Mines
Bulletin 20,® and in many articles in current
journals.
Some operators have placed steam heating coils
at the intake entrances of their mines in order
to heat the entering air current in cold weather
to the temperature of the workings. This permits
the immediate saturation of the ventilating
current by steam jets, without serious fogging of
the air by particles of condensed moisture from
the steam jets being carried in suspension until
the air receives heat enough from the mine walls
to absorb them.
Preheating the air, if done yvith careful regulation,
has the further merit that
THE TEMPERATURE
of the roof and walls of the airway near the entrance
is, or may be, kept more nearly uniform,
summer and winter. This prevents the constant
expansion and contraction of the roof, which otherwise
takes place and probably tends to increase
roof falls.
THE COAL TRADE BULLETIN. 51
From inquiries received by the Bureau of Mines,
it is evident that there has been misapprehension
regarding the purpose of preheating; some persons
nave thought that merely heating tne air
to a summer temperature would suffice to produce
humid conditions underground. This is not the
case, as the mere heating of the air does not increase
the amount of moisture it carries. On
the other hand, tne amount carried being the
same at the higher mine temperature, the percentage
of relative humidity is decreased; hence
the necessity of introducing artificial moistening
by fine sprays of water, or, more easily, by jets
of exhaust or live steam.
Inquiries have been received as to the size of
the steam coils necessary for heating. To determine
the size it is necessary to know (1) the
volume of the ventilating current; (2) the temperature
of the mine yvorkings, and (3) the lowest
outside temperature at the mine in winter. It
does not seem necessary to take the temperature
of an extremely cold day, but the average during
a single cold wave. If the ventilating current
is 100,000 cubic feet of air per minute, if the temperature
of the mine workings is 65° F., and if
the average temperature of the coldest cold wave
is zero F., then the temperature of 100,000 cubic
feet of air must be raised 65° F. every minute.
Knowing the steam pressure available, it becomes
an ordinary steam-heating problem to determine
the size of the coils, the amount of steam, and
the coal consumption required to heat the entering
air to mine temperature.
A word of caution about humidifying seems
necessary. As already mentioned, tests at the
bureau's experimental mine show that accumulations
of dry coal dust do not take up moisture
enough to prevent their sperading an explosion.
Therefore, whenever dry dust accumulates, the
accumulations must be promptly removed as
cleanly as possible and the
AREA THOROUGHLY WET.
Evidently, in most mines, supplementary treatment
yvith hose or yvith water cars is necessary,
for, regardless of whether the air current is saturated,
the dust must be wet.
Water hose is proving a good means of washing
down the coal dust, not only in Utah mines, where
the use of such hose is compulsory, but in some
mines in other parts of the country. If the waterpipe
system goes throughout all the entries of the
mine not only does it have tbe merit of giving
fire protection, but a stream of water can be used
to sweep dust from any projection of the ribs or
from timbers.
Efficient pump cars giving a strong spray that
sweeps throughout the yvhole area of an entry
©Rice, G. S.. and others. The explosibility of conl dusl
1913, pp. SO, 16S.