320
8 Secondary Ventilation
speed of the fan motor. The omission of such a frequency drive would entail an excess
of airflow at the early stages of excavation that might be annoying for operators.
8.11 Air Velocity Inside the Duct
For plastic or fibre ducts, the maximum recommended air velocity is usually about
20 m s
−1 . Other duct materials tend to have lower maximum air velocities: 19 m
s
−1 for metal ducts, 17 m s
−1 for inflatable ducts and 12 m s
−1 for spiral ducts (see
Table 8.4).
8.12 Air Leaks in the Duct
Air leaks are generally a consequence of defects in the air ducts. If a duct is not
sufficiently airtight, there will be a reduced airflow rate downstream of the fault.
This results in a loss of air speed, and consequently, a reduction in dynamic pressure
(Fig. 8.12) within the duct.
Fig. 8.12 Variation of the
static (P s ), dynamic (P v ) and
total (P T ) pressures along a
permeable duct
P v
P s
P T
Duct air pressure
Distance along the duct
Intuitively, it can be deduced that an increase in static pressure within the duct
would result in increased leakage. Similarly, the larger the area of the orifices through
which air can escape, the greater the leakage. In this way, loss of airflow due to leaks
can be approximated using what is termed the regulator or equivalent orifice formula
(Eq. 8.30):
Q p =
A
ρ st
P
ρ real
n
= K
n
(8.30)
where
8 Secondary Ventilation
speed of the fan motor. The omission of such a frequency drive would entail an excess
of airflow at the early stages of excavation that might be annoying for operators.
8.11 Air Velocity Inside the Duct
For plastic or fibre ducts, the maximum recommended air velocity is usually about
20 m s
−1 . Other duct materials tend to have lower maximum air velocities: 19 m
s
−1 for metal ducts, 17 m s
−1 for inflatable ducts and 12 m s
−1 for spiral ducts (see
Table 8.4).
8.12 Air Leaks in the Duct
Air leaks are generally a consequence of defects in the air ducts. If a duct is not
sufficiently airtight, there will be a reduced airflow rate downstream of the fault.
This results in a loss of air speed, and consequently, a reduction in dynamic pressure
(Fig. 8.12) within the duct.
Fig. 8.12 Variation of the
static (P s ), dynamic (P v ) and
total (P T ) pressures along a
permeable duct
P v
P s
P T
Duct air pressure
Distance along the duct
Intuitively, it can be deduced that an increase in static pressure within the duct
would result in increased leakage. Similarly, the larger the area of the orifices through
which air can escape, the greater the leakage. In this way, loss of airflow due to leaks
can be approximated using what is termed the regulator or equivalent orifice formula
(Eq. 8.30):
Q p =
A
ρ st
P
ρ real
n
= K
n
(8.30)
where