8.18 Sizing the Auxiliary Fan
365
A empty
A T
(n) Shock losses at the upstream silencer (P sil (e) ):
These losses vary greatly depending on the precise characteristics of fan configuration. For example, silencers can be located before and after the fan. In most cases,
manufacturers will supply information about these losses which are generally in the
region of 60–100 Pa.
(o) Shock losses at the downstream silencer (P sil (s) ):
As above: 60 Pa.
(p) Total friction losses in the ducts (P f ):
The duct is made from fibreglass for which there is a generally accepted value of
K 1.2 = 0.0028 Ns
2 m
−4 .
Duct-to-face distance: 5
√
S = 5
√
30 = 27 m
This is within the normal range of 20–30 m.
P f =
K 1.2 O L Q
2
A 3
where: O is the perimeter (m) of the duct cross section, L is the duct length (m) and
Q is the average airflow rate in the duct (m
3 s
−1 ) (item k).
Considering that the elbow extends for 5 m into the main gallery, we have:
P f =
0.0028
Ns
2
m 4 · (2 · π · 0.65)m (200 + 5 − 27)m ·
36.63
m
3
s
2
π · 0.65 2 m 2
3
= 1167.93 Pa
Another option is to use the manufacturer’s nomograms.
(q) Friction losses at the joints (P u ):
This depends on the characteristics of the duct:
• layflat duct, 2 m of equivalent length can be added per joint.
• spiral duct, 2.5 m of equivalent length can be added per joint.
365
A empty
A T
(n) Shock losses at the upstream silencer (P sil (e) ):
These losses vary greatly depending on the precise characteristics of fan configuration. For example, silencers can be located before and after the fan. In most cases,
manufacturers will supply information about these losses which are generally in the
region of 60–100 Pa.
(o) Shock losses at the downstream silencer (P sil (s) ):
As above: 60 Pa.
(p) Total friction losses in the ducts (P f ):
The duct is made from fibreglass for which there is a generally accepted value of
K 1.2 = 0.0028 Ns
2 m
−4 .
Duct-to-face distance: 5
√
S = 5
√
30 = 27 m
This is within the normal range of 20–30 m.
P f =
K 1.2 O L Q
2
A 3
where: O is the perimeter (m) of the duct cross section, L is the duct length (m) and
Q is the average airflow rate in the duct (m
3 s
−1 ) (item k).
Considering that the elbow extends for 5 m into the main gallery, we have:
P f =
0.0028
Ns
2
m 4 · (2 · π · 0.65)m (200 + 5 − 27)m ·
36.63
m
3
s
2
π · 0.65 2 m 2
3
= 1167.93 Pa
Another option is to use the manufacturer’s nomograms.
(q) Friction losses at the joints (P u ):
This depends on the characteristics of the duct:
• layflat duct, 2 m of equivalent length can be added per joint.
• spiral duct, 2.5 m of equivalent length can be added per joint.