364
8 Secondary Ventilation
In this system, all shock losses take place in segments of ducting that have an
equal cross section (ducts and elbow), which means that the static pressure losses
(P s ) and the total losses (P T ) coincide. In this way, downstream of the fan we can
look at the gradient of total pressure (TP) while upstream of the fan we look at the
gradient of static pressure (SP). FTP is obtained by subtracting the total pressure at
the fan outlet to the total pressure at the fan inlet (negative). It should be noted that
the fan curves published by some manufacturers include losses incurred at the inlet.
Therefore, these will not always have to be considered.
(l) Shock losses at the inlet cone (P i ):
When calculating this loss, we should bear in mind that is may already be accounted
for by the manufacturer in their published fan curve (Brake 2002). The dynamic
pressure in the inlet is estimated to be equal to that existing at the fan outlet P dv
[calculated in (j)].
24 Then, using the shock loss coefficient, X = 0.5, which is given
in the problem, we have
25 :
P i = X P vv = 0.5 · 596.86 Pa = 298.5 Pa
(m) Shock losses at the fan screen (P r ):
Assuming a porosity (FL) of 85%, as stated in the problem. This means that 85% of
the air passage cross section is not occupied by the grid wires and can be expressed
mathematically as:
F L =
A empty
A T
= 0.85
Then, the losses in the screen, using the expression given to us in the problem:
X r = 1.3 (1 − F L) +
1
F L
− 1
2
= 0.226
Then, using the dynamic pressure at the fan, the required shock loss will be:
P r = X r P vv
P r = 0.226 · 596.86 Pa = 135 Pa
24 A recalculation of the value will be carried out later, and it will be seen that this value is slightly
higher.
25 It should be noted that this is a somewhat high coefficient for a cone entry, although it is on the
safety side of fan sizing.
8 Secondary Ventilation
In this system, all shock losses take place in segments of ducting that have an
equal cross section (ducts and elbow), which means that the static pressure losses
(P s ) and the total losses (P T ) coincide. In this way, downstream of the fan we can
look at the gradient of total pressure (TP) while upstream of the fan we look at the
gradient of static pressure (SP). FTP is obtained by subtracting the total pressure at
the fan outlet to the total pressure at the fan inlet (negative). It should be noted that
the fan curves published by some manufacturers include losses incurred at the inlet.
Therefore, these will not always have to be considered.
(l) Shock losses at the inlet cone (P i ):
When calculating this loss, we should bear in mind that is may already be accounted
for by the manufacturer in their published fan curve (Brake 2002). The dynamic
pressure in the inlet is estimated to be equal to that existing at the fan outlet P dv
[calculated in (j)].
24 Then, using the shock loss coefficient, X = 0.5, which is given
in the problem, we have
25 :
P i = X P vv = 0.5 · 596.86 Pa = 298.5 Pa
(m) Shock losses at the fan screen (P r ):
Assuming a porosity (FL) of 85%, as stated in the problem. This means that 85% of
the air passage cross section is not occupied by the grid wires and can be expressed
mathematically as:
F L =
A empty
A T
= 0.85
Then, the losses in the screen, using the expression given to us in the problem:
X r = 1.3 (1 − F L) +
1
F L
− 1
2
= 0.226
Then, using the dynamic pressure at the fan, the required shock loss will be:
P r = X r P vv
P r = 0.226 · 596.86 Pa = 135 Pa
24 A recalculation of the value will be carried out later, and it will be seen that this value is slightly
higher.
25 It should be noted that this is a somewhat high coefficient for a cone entry, although it is on the
safety side of fan sizing.