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CHAPTER 4
Characteristics
The figure obtained from Table 4 must then be corrected for duct type at various velocities. This can be done using Table 5
below.
Duct inner surface
Very rough surface
Rough
Very smooth
An alternative, more detailed method for determining the pressure loss in duct work is as shown using the following formula:
Circular pipe section pressure loss
R
ρ
2
∆p = λ ·
· v
(Pa)
·
2
d
ρ
2
∆p = C ·
· v
(Pa)
2
2
= 0.6 C · v
42
Table 5 Friction coefficient compensation table
Example
Concrete finish
Mortar finish
Drawn steel pipe Vinyl pipe
λ : Friction resistance coefficient (smooth pipe 0.025)
C : Local loss coefficient (refer to Table 6)
d : Duct diameter (m)
R
: Duct length (m)
ρ : Air weight (1.2 kg/m
v : Wind velocity (m/s)
Average velocity (m/sec)
5
10
1.7
1.8
1.3
1.35
0.92
0.85
2
)
15
20
1.85
1.9
1.35
1.37
0.82
0.8
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