352
8 Secondary Ventilation
into account the effects of accessories when testing their fans, mainly because the
end-user may require any number of different configurations.
In view of the above, when sizing an auxiliary fan, it is usual to add in losses from
the following sources:
• Friction along the duct.
• Shock losses at the:
– Fan inlet,
– Grid,
– Silencer,
– Joints,
– Elbows, and
– Duct outlet.
Figure 8.18, is a diagram showing pressure losses in a booster fan. In this arrangement, a pressure measurement made just at the beginning of the inlet cone will give
the smallest value of dynamic pressure (except for the outlet). At this same point, the
static pressure (negative) will have the same absolute value as the dynamic pressure,
to give a total pressure of zero. Static pressure and total pressure will both increase
in terms of their absolute values until they reach the fan. As air traverses the fan, total
pressure increases to a maximum value equal to the FTP (Fan Total Pressure). From
that point on, total pressure and static pressure start to decrease due to the different
losses, although there will be occasional gains in static pressure (static regain) at
certain points along the duct, for example, where duct cross section increases.
At the system outlet, the fluid energy is all kinetic (i.e. due to air velocity), thus, the
total and the dynamic pressures coincide. In the case of Fig. 8.18, there is a diffuser
attached to the outlet, and the speed of air entering the diffuser will be higher than the
speed of air leaving the diffuser. The kinetic energy of the air as it exits the system
is sometimes referred to as the exit loss, which may generate some confusion.
The total exit loss for the system is calculated as an exit loss coefficient, X, multiplied by the dynamic pressure. This coefficient is dependent largely on the Reynolds
number and the shape of the outlet. The shock loss factor may be greater than, less
than, or equal to 1 depending on the Reynolds number and the configuration of the
outlet (ASHRAE 2013; Carrier Corporation 1965, pp. 2–41). Values of X o = 1 correspond to uniform speed profiles, larger than 1 (up to 3.67), to exponential, sinusoidal,
asymmetric and parabolic profiles (ASHRAE 2013). In most circumstances under
discussion here, exit losses are equal to one velocity head (i.e. X = 1).
Some authors (e.g. Brake 2002), believe that the problem can be addressed by
considering it as a case where the conduit cross section expands abruptly as the flow
enters the gallery. In our particular case, it is not exactly a loss of the system, as part
of this energy will return through the gallery doing useful work, but a loss of the
ventilation duct in which we focus our attention.
The Schauenburg Company provides the following rule of thumb for calculating
the total static losses for flexible ducts:
8 Secondary Ventilation
into account the effects of accessories when testing their fans, mainly because the
end-user may require any number of different configurations.
In view of the above, when sizing an auxiliary fan, it is usual to add in losses from
the following sources:
• Friction along the duct.
• Shock losses at the:
– Fan inlet,
– Grid,
– Silencer,
– Joints,
– Elbows, and
– Duct outlet.
Figure 8.18, is a diagram showing pressure losses in a booster fan. In this arrangement, a pressure measurement made just at the beginning of the inlet cone will give
the smallest value of dynamic pressure (except for the outlet). At this same point, the
static pressure (negative) will have the same absolute value as the dynamic pressure,
to give a total pressure of zero. Static pressure and total pressure will both increase
in terms of their absolute values until they reach the fan. As air traverses the fan, total
pressure increases to a maximum value equal to the FTP (Fan Total Pressure). From
that point on, total pressure and static pressure start to decrease due to the different
losses, although there will be occasional gains in static pressure (static regain) at
certain points along the duct, for example, where duct cross section increases.
At the system outlet, the fluid energy is all kinetic (i.e. due to air velocity), thus, the
total and the dynamic pressures coincide. In the case of Fig. 8.18, there is a diffuser
attached to the outlet, and the speed of air entering the diffuser will be higher than the
speed of air leaving the diffuser. The kinetic energy of the air as it exits the system
is sometimes referred to as the exit loss, which may generate some confusion.
The total exit loss for the system is calculated as an exit loss coefficient, X, multiplied by the dynamic pressure. This coefficient is dependent largely on the Reynolds
number and the shape of the outlet. The shock loss factor may be greater than, less
than, or equal to 1 depending on the Reynolds number and the configuration of the
outlet (ASHRAE 2013; Carrier Corporation 1965, pp. 2–41). Values of X o = 1 correspond to uniform speed profiles, larger than 1 (up to 3.67), to exponential, sinusoidal,
asymmetric and parabolic profiles (ASHRAE 2013). In most circumstances under
discussion here, exit losses are equal to one velocity head (i.e. X = 1).
Some authors (e.g. Brake 2002), believe that the problem can be addressed by
considering it as a case where the conduit cross section expands abruptly as the flow
enters the gallery. In our particular case, it is not exactly a loss of the system, as part
of this energy will return through the gallery doing useful work, but a loss of the
ventilation duct in which we focus our attention.
The Schauenburg Company provides the following rule of thumb for calculating
the total static losses for flexible ducts: