Page 35 - Advanced Mine Ventilation

18 Advanced Mine Ventilation

owing to geology or ground control conditions. Such airways are treated as “airways in
series.” Similarly, often two to five airways are needed in parallel to carry a given vol-
ume of air. Both “series” and “parallel” airways are mathematically analyzed for air
flow distribution and pressure losses. Finally, an equation is provided to estimate
the horsepower needed to run a fan that can provide a prescribed ventilation air volume
at a required pressure differential.
A coal mine is basically a network of roadways that are mostly rectangular in sec-
tion. The roadways/airways have many bends, support pillars, and other obstructions
that cause shock losses. The air flow is mainly created by mechanical fans and at times
assisted by natural ventilation (to be discussed later in the book). In view of large sizes
of roadways (20 6 ft, for example) and high velocities required by law, the flow is
generally turbulent. It is also a steady-state flow. The flowing air is a real viscous fluid
that creates friction. Even though air is definitely compressible, the pressure differen-
1
tial to create the flow in mine airways is low (less than / 2 psi), and hence the air is
treated as a noncompressible fluid. It often takes a number of fans with
1000e3000 horsepower drivers to keep a typical coal mine well ventilated. A large
coal mine producing 5 to 7 million tons per year may circulate 3 to 5 million cubic
feet of air per minute at 5e15 in. of water gauge (W.G.).

2.2 Derivation of Basic Fluid Flow Equation

Solutions of mine air flow problems are derived from energy principles, the equation of
continuity, and equation of fluid resistance [1]. Resistance to flow in mine airways is
offered not only by frictional resistance but also by roof support structures in the air-
ways and bends in the airways, which create turbulence and additional dissipation of
energy.
Early experiments (by Darcy, c. 1850) on the flow of water in pipes indicated that
the pressure loss was directly proportional to the length of pipeline and the velocity

2
head, v , and inversely proportional to the diameter of the pipe, d. Mathematically,
2g
it can be expressed as:
l lv 2
h ¼ (2.1)
2gd
where h is the pressure loss over a distance; l, in feet of the fluid; v is the velocity in ft/
s; l is the length of the pipe in feet; d is the diameter of the pipe, in feet; g is the ac-
2
celeration due to gravity (32 ft/s ); l is a coefficient of proportionality, commonly
called the friction factor. It is dimensionless.

2.2.1 Determination of l in Eq. (2.1)
To calculate the head (pressure) loss in Eq. (2.1), the only thing not known is the fric-
tion factor, l.