AIRFLOW
Principles
The
flow of air in a mine is induced by a pressure difference between the
intake opening (head) and the exhaust opening. Pressures within a
ventilation system are in units of inches of water gage. It is noted
that 1 inch of water gage ( 1 in. of water ) is equal to 5.2 pounds per
square foot (lbs/ft^2).
Notes
- Pressure difference is caused by imposing some form of
pressure at one point or a seriew of points in the ventilation system
- Pressure difference created must be high enough to
overcome frictional resistance called "shock loss"
- The head and exhaust openings serve as air ducts
- Air circulated from a high pressure area toward a low
pressure area
- The relationship between pressure and volume is TWICE
THE VOLUME REQUIRES FOUR TIME THE PRESSURE
- Exhausted airflow is NEGATIVE and intake airflow is
POSITIVE with respect to atmospheric pressure
- The drop in pressure at the head must be equal and
opposite of the drop in pressure at the exhaust
Pressure Loss
Losses
in pressure with in a mine ventilation system are caused by some sort
of friction between the head and the exhaust of the system. Not only
can an object obstruct airflow, but the friction between the walls of
the mine shaft and the air can cause a drop in pressure as well. With
so many different ways in which airflow can be slowed down, it is all
categorized as “head loss”. Knowing how much head loss there is
with in a ventilation system is important in order to maintain air
regulations set by MSHA (Mine Safety and Health Administration).
Aside from closure of the mine and a heavy fine by MSHA if
regulations are not met, human life is at risk as well. The standard
equation used to calculated head loss is:
Hv = K*L*O*V^2
5.2*A
- "H" is the total head loss ( in. of water gage )
- "K" is the friction coefficient
- "L" is the length of the mine shaft ( ft. )
- "O" is the perimeter of the mine shaft ( ft. )
- "V" is the velocity of the airflow ( ft/min )
- "A" is the area of the mine shaft ( ft^2 )
Since the friction coefficient is a
constant, it must be obtained from a chart that was
published by the U.S. Bureau of Mines (USBM). These friction factors
were established through research in 1910 and used throughout the
mining industry ever since. Many equations used in ventilation systems
and any other engineering-related calulations can be found at the
user-friendly site: ENGINEERING
TOOLBOX.COM The only other varible not given is the velocity of the
airflow, which can only be obtained through measuring the air velocity
inside the mine with either a Paulin Altimeter or a Wallace &
Tiernan Altimeter.
