8.18 Sizing the Auxiliary Fan
355
Pw shaft =
FTP Q v
η T
(8.64)
where
• FTP: Fan Total Pressure (Pa),
• Q v : Airflow rate generated by the fan (m
3 s
−1 ), and
• η T : Efficiency (fraction).
As a further consideration, when selecting the motor to drive the fan, it must
be borne in mind that there are fluctuations between the air density in winter and
summer. In this way, it is a common practice to over-size the fan by 20%.
In general, auxiliary fans are capable of delivering a flow rate of 14 m
3 s
−1 at a
pressure of up to 2.5 kPa over distances of up to about 760 m (ASHRAE, 2015).
If ventilation needs are greater, additional booster fans are often used. In the most
exceptional cases, the flow rates can be up to 28 m
3 s
−1 with pressures of up to 3.7 kPa.
The fan powers themselves are usually less than 75 kW, although the catalogues
include engines of up to 185 kW (de Souza 2004).
The sizing calculations for secondary ventilation fans are similar whether considering a forcing or an exhausting system, except in the case of flexible ducts. If flexible
ducts were used with an exhausting system, suctioning would cause cross section
reduction, thereby increasing the resistance. As a result, flexible type ducts cannot
be used for exhausting systems.
Exercise 8.15 In order to ventilate a cul-de-sac, a flow of 10 m
3 s
−1 is taken from
the main ventilation gallery by means of a duct of 1 m in diameter and 100 m long
which contains a right-angle elbow as shown in the figure. It is known that:
• Atkinson’s friction factor for the duct (K 1.2 ): 0.003 kg m
−3 ,
• Air density: 1.18 kg m
−3 ,
• Shock loss factors:
– At the duct entrance: 1,
– At the duct outlet: 1. This is because there is no grille, diffuser or contraction,
it is assumed that the velocity profile is uniform, and the static pressure at the
outlet is the atmospheric pressure, and
– At the elbow: 0.8.
• Mechanical efficiency of the fan: 70%,
355
Pw shaft =
FTP Q v
η T
(8.64)
where
• FTP: Fan Total Pressure (Pa),
• Q v : Airflow rate generated by the fan (m
3 s
−1 ), and
• η T : Efficiency (fraction).
As a further consideration, when selecting the motor to drive the fan, it must
be borne in mind that there are fluctuations between the air density in winter and
summer. In this way, it is a common practice to over-size the fan by 20%.
In general, auxiliary fans are capable of delivering a flow rate of 14 m
3 s
−1 at a
pressure of up to 2.5 kPa over distances of up to about 760 m (ASHRAE, 2015).
If ventilation needs are greater, additional booster fans are often used. In the most
exceptional cases, the flow rates can be up to 28 m
3 s
−1 with pressures of up to 3.7 kPa.
The fan powers themselves are usually less than 75 kW, although the catalogues
include engines of up to 185 kW (de Souza 2004).
The sizing calculations for secondary ventilation fans are similar whether considering a forcing or an exhausting system, except in the case of flexible ducts. If flexible
ducts were used with an exhausting system, suctioning would cause cross section
reduction, thereby increasing the resistance. As a result, flexible type ducts cannot
be used for exhausting systems.
Exercise 8.15 In order to ventilate a cul-de-sac, a flow of 10 m
3 s
−1 is taken from
the main ventilation gallery by means of a duct of 1 m in diameter and 100 m long
which contains a right-angle elbow as shown in the figure. It is known that:
• Atkinson’s friction factor for the duct (K 1.2 ): 0.003 kg m
−3 ,
• Air density: 1.18 kg m
−3 ,
• Shock loss factors:
– At the duct entrance: 1,
– At the duct outlet: 1. This is because there is no grille, diffuser or contraction,
it is assumed that the velocity profile is uniform, and the static pressure at the
outlet is the atmospheric pressure, and
– At the elbow: 0.8.
• Mechanical efficiency of the fan: 70%,