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find the open running fans have been displaced
by the spiral casting. Disk and propeller blade
fans have
FALLEN LARGELY INTO DISUSE
where an efficient ventilator is required and a
current to be maintained against the resistance
of an airway. Blades normal to the outer circumference
and approaching tangency at the
throat circle are now extensively adopted. The
expansion of the spiral casing is made continuously
with the evase chimney of the fan drift
leading to the mine. The best designing practice
is supposed to be understood in regard to the
points before mentioned. There are other essential
features, however, equal to perfect design
that remain in dispute. Prominent among the
latter is the tapering width of the fan blade from
the throat circle towards the circumference, this
form of blade having appeared as a characteristic
feature of the Waddle and Schiele fan, its object
being to render the sectional area or area of passage
in the fan uniform from the throat to the
circumference. Another feature in dispute having
many adherents and equally as many opponents
is the special form of fan blade too common
in many types of ventilating fans, the blades
being generally curved backwards from the direc
tion of motion. The work of designing an air
motor required insight and great study on the
part of the designer. This is due to the fact that
the work of the motor is stored work. It cannot,
as many suppose, be compared with the simple
interchange of mechanical pressures that we have
in the pump. We are not able to determine the
number of expansions as in the steam engine due
to the volumetric cut-off by valves as some writers
have likened the action of the centrifugal fan to
that of a pump. We also notice as in the splitting,
or any change in the area of the conduit, a
large quantity of air is circulated by the same
power. The equivalent orifice (0) of the fan
will vary and the above factor is therefore not
constant. This limits the application of a formula
to cases of comparing the yield of a fan,
run at several different speeds, while discharging
into an airway, having a constant area, (A) or
exhausting from the same; or, we may compare
the yield at a uniform rate of speed by lengthening
the airway, or in effect by obstructing the
flow; but we find that in all cases where a change
of conditions in the flow results from any other
cause without a corresponding change in the
power, a new value for the orifice must be obtained.
For instance, the results of certain fan
tests do not fulfill anticipations, or there seems
to be disparity arising as the anxious observer
explains from certain unfavorable conditions in
the airway or in the motor itself. This may
cause all the trouble or it may have very little
THE COAL TRADE BULLETIN. ID
effect upon the results obtained, but the embarrassment
of the situation is relieved, though nothing
has been proven. Daniel Murgue explains the
lack of conformity of results obtained in thin
veins in the use of his own method by the effect of
lowering the shutter, the change of the impact of
the air as it enters the blades and the want of
the full flow between the vanes. These causes
operating might be the real cause of the disparity.
The depletion increasing with the pressure, thereby
becoming more manifest in dealing with thin
veins, than larger seams. It will now be noticed
that we reckon the efficiency of the motor as the
ratio between its effective work and its theoretical
calculated work. In this we make no
reference, as is often done, to the indicated power
of the engine driving the ventilator.
THE ENGINE HAS AN EFFICIENCY
of its own that must not be confounded with
that of the ventilator. The mechanical efficiency
of the engine increases with its speed while that
of the fan follows a different law, decreasing as
the speed increases. We sometimes find wellbuilt
and well-designed fans that manifest weakness
in the shaft, either the diameter being too
small or the distance between the bearings being
too great or a settlement of the foundation of
one of the bearings may cause trouble. We have
knowledge of 6 to 10-inch steel shafts of fans
being snapped asunder while working against a
comparatively light horse power, which may be
due to the settlement of the foundation commonly
occurring in the vicinity of a mine shaft, or again.
it may be due to failure to heat the ingots to the
proper temperature while forging the shaft.
(TO BE CONTINUED.)
Mine Accidents in New Zealand.
The report of the Mines Department of New
Zealand for the year 1903 gives the number of
deaths by accident in coal mines during the year
as 4, or 1.40 per thousand employes. The death
rate was 1.151 per thousand in gold alluvial workings
and 2.50 in gold quartz mines. The number
of coal mines employed was 2,852 and the number
of miners in all lines was 13,062. The number
of accidents showed a considerable increase over
the years 1902 and 1901. This was due to the
employment of an increased number of men, some
of them necessarily unfamiliar with mining conditions
and regulations.
It was announced at Swansea, Wales, recently
that representatives of 24 anthracite collieries had
definitely decided to open negotiations with other
owners to form an anthracite combination with a
capital of $10,000,000.