Objective:
Quantity measurement of ventilation air in a
simulated mine roadway using Single point, continuous traversing, and precise
traversing methods.
Introduction:
Quantity of airflow (volumetric flow rate) in a roadway is simply the product of the average air velocity and the roadway cross-sectional area. The air velocity varies over the cross-section of the roadway depending on the frictional characteristics of the roadway walls, the Reynolds number of the airflow, and the roadway cross-section geometry. For a regular roadway cross-section the maximum velocity occurs at the center & this value decreases reaching a minimum at the walls of the roadway. In order to account for this variation in velocity, special techniques are devised to compute average air velocity.
·
Single point measurement: for a uniform roadway cross-section the
velocity-measuring instrument is positioned at the center of the roadway. The
observed velocity is then multiplied with a “method factor” to obtain average
air velocity. A highly turbulent flow as encountered in underground roadways
may dictate the use of a method factor of 0.8.
·
Continuous traversing: in this case, an integrating type of quantity
measuring device such as an anemometer is traversed continuously up and down
(more convenient than side to side traversing) starting at one corner of the
roadway cross-section and reaching the diagonally opposite corner.
Subsequently, dividing the measurement obtained with the time required for
traversing produces the average velocity.
·
Precise traversing: This technique needs demarking the roadway cross-section into a
number of equal area squares or rectangles similar in shape to the roadway
cross-section. Velocity measurements are made by placing the instrument at
center of each of the smaller areas. The mean of these observations provides an
accurate measure of the average velocity of the air.
Instruments:
Anemometer, Stopwatch, Measuring Tape.
Vane
anemometer or simply anemometer is essentially a small horizontal axis
windmill consisting of a number of radial blades or vanes. The mechanical
action of air in motion rotates the vanes around the axis of the instrument.
The rotational speed of the vanes is proportional to the air velocity. The
number of revolutions correlated to the distance traversed by the air is
recorded in a set of counting dials operated through gears. Before starting a
measurement the dial reading is brought to zero with a resetting device.
Observation commences by operating a clutch system to engage the counting
dials. The instrument is held in the direction of for certain as measured with
a stopwatch, before the clutch is disengaged. The air velocity integrated over
time is noted. The pointer over the dial remains in locked position until the
instrument is reset. Anemometers used in mine cover a velocity range of 1 m/s
to 15 m/s. in order not to disturb the flow pattern in the roadways, it is
advisable to mount the anemometer on a rod. The clutch in such cases is
operated from a distance by means of a string.
Certain
anemometer models contain built in timing devices. Pressing a single button in
such anemometers serves the function of resetting, winding the (clockwork),
engaging the gears after a little time gap of 4 seconds and disengaging the
gears automatically after one minute of anemometer operation.
Procedure:
(1)
At
the quantity measuring station, determine the roadway cross-section by taking a
few representative measurements for height and with.
(2)
Mark the center of the roadway, and the
centers for the small equal areas (precise traversing stations) by stretching
wires or strings vertically and across the roadway cross-section.
(3)
Take
three observations each for single point measurement at the roadway center, and
for continuous traversing process.(excluding the handling anemometer, the remaining party should stay at a
distance away in the downwind direction ensuring least blockage of the roadway.
Uniformity in traversing and coverage of roadway cross-section is essential).
(4)
For
each precise traversing station take to velocity measurements.
Calculations:
(1)
Obtain
the mean air velocities for the single point measurement, continuous traversing
and precise traversing methods.
(2)
Taking the precise traversing method as basis
establishes a method factor for the roadway and velocity condition.
Remarks:
(1)
In
order to obtain reliable results anemometers must be frequently calibrated. The
purpose of such calibrations is to establish the parameters A and B in the
following relationship.
True air velocity = A + B
* velocity measured with Anemometer
(2)
While taking measurements misalignment of the
anemometer (yaw) could produce substantial errors in observed velocity.
(3)
The vanes are extremely delicate, finely
crafted and mounted. To ensure error free operation of the instrument, one must
never touch the interior components of the anemometer.
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