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8 Secondary Ventilation
8.15 Practical Rules for Estimating Leakage
In the case where exact operating conditions are unknown, as a rule of thumb, fan
airflow rates can be estimated assuming losses of around 20–25% (Workplace Safety
North 2010). This means that the volumetric flow supplied by the fan at the free
outlet of the duct is reduced by 20–25%. This results in the loss of speed, and thus
of dynamic pressure (often termed velocity pressure and sometimes expressed as
velocity head).
Expressions of the type shown in Eq. 8.60, often appear in commercial catalogues:
Q v = Q f +
Q v L F
100
(8.60)
where
Q v : Airflow rate at the fan (“ventilator”, v), and
Q f : Airflow rate at the face.
where LF is the % leak rate per 100 m of ducting. Thus, for a loss of 2% per 100 m,
there would be a total loss of 20% at 1000 m. This expression is more accurate than
the basic rule of thumb as it accounts for the total length of the duct.
If you have a leaky duct, the mean flow (Q m ) is usually approximated by the
geometric mean of the flow rates at either end of the duct, i.e. (Eq. 8.61):
Q m =
2
Q f Q v
(8.61)
This can be approximated as shown in Eq. 8.62:
Q m = Q f + 0.5 Q
(8.62)
where ΔQ is the airflow losses.
8.16 Friction Losses in Ducts
Where precise values for the Atkinson friction factor (K 1.2 ) of ducting is not available,
the following ballpark figures can be used (de Souza 2004):
• Fibreglass: 0.0028 N s
2 m
−4 ,
• Canvas: 0.0037 N s
2 m
−4 , and
• Spiral: 0.00111 N s
2 m
−4 .
In any case, manufacturers usually supply their own nomograms. The one in
Fig. 8.16, corresponds to glass wool ducts from Flexadux.
8 Secondary Ventilation
8.15 Practical Rules for Estimating Leakage
In the case where exact operating conditions are unknown, as a rule of thumb, fan
airflow rates can be estimated assuming losses of around 20–25% (Workplace Safety
North 2010). This means that the volumetric flow supplied by the fan at the free
outlet of the duct is reduced by 20–25%. This results in the loss of speed, and thus
of dynamic pressure (often termed velocity pressure and sometimes expressed as
velocity head).
Expressions of the type shown in Eq. 8.60, often appear in commercial catalogues:
Q v = Q f +
Q v L F
100
(8.60)
where
Q v : Airflow rate at the fan (“ventilator”, v), and
Q f : Airflow rate at the face.
where LF is the % leak rate per 100 m of ducting. Thus, for a loss of 2% per 100 m,
there would be a total loss of 20% at 1000 m. This expression is more accurate than
the basic rule of thumb as it accounts for the total length of the duct.
If you have a leaky duct, the mean flow (Q m ) is usually approximated by the
geometric mean of the flow rates at either end of the duct, i.e. (Eq. 8.61):
Q m =
2
Q f Q v
(8.61)
This can be approximated as shown in Eq. 8.62:
Q m = Q f + 0.5 Q
(8.62)
where ΔQ is the airflow losses.
8.16 Friction Losses in Ducts
Where precise values for the Atkinson friction factor (K 1.2 ) of ducting is not available,
the following ballpark figures can be used (de Souza 2004):
• Fibreglass: 0.0028 N s
2 m
−4 ,
• Canvas: 0.0037 N s
2 m
−4 , and
• Spiral: 0.00111 N s
2 m
−4 .
In any case, manufacturers usually supply their own nomograms. The one in
Fig. 8.16, corresponds to glass wool ducts from Flexadux.