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Reference
The pressure loss caused by the outdoor air is as follows:
Pressure loss caused by outdoor air (Pa)
r
1.2
2
=
× V
=
2
2
r : Air weight 1.2 kg/m
{
v : Velocity (m/s)
2.2 Calculation of duct pressure loss
When selecting a model that is to be used with a duct, calculate the volumes according to Tables 3, 4, 5 and 6, and then select
the unit according to the air volume and static pressure curve.
(1) Calculation of a rectangular pipe
Table 3 Conversion table from
rectangular pipe to circular pipe
The circular pipe diameter having equal hydraulic radius
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5.2
5
4
3
2
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
5.2
Short side of
rectangular pipe
How to read Table 3
Select the unit as per each duct. In the
above example, the
520 rectangular pipe
only goes as far as 17. Thus, the long side,
short side and converted circular pipe
values are all multiplied by 100. The point
560 where the two lines cross is hence the
value where the rectangular pipe equates to
the circular pipe.
2
× (velocity)
3
(2) How to obtain the duct resistivity
Table 4 Circular duct friction loss
(steel plate duct, inner roughness ε = 0.18 mm)
0.1
1 00 0000
800 000
600 000
400 000
200 000
100 000
80 000
60 000
40 000
20 000
10 000
8 000
6 000
4 000
2 000
1 000
800
600
400
200
100
0.1
How to read Table 4
The point where the line of the circular duct
diameter (leftward slanting line) and of the
required air velocity (horizontal line)
intersect is the pressure loss per 1 m of
duct.
The value of the slanted line to the lower
right of the intersecting point is the average
velocity.
CHAPTER 4
180
160
140
120
100
80
60
40
20
2 4 6 8 10 12 14 16 18
Friction loss
0.2
0.4 0.6 0.8 1.0
2.0
4.0
6.0 8.0 10
20
0.2
0.4 0.6 0.8 1.0
2.0
4.0
6.0 8.0 10
20
Friction loss (Pa/m)
(Outline of Table 4)
Characteristics
Outdoor air (m/s)
30 40 50 60 80 100
30 40 50 60 80 100
560 mm
Average
velocity
8 m/s
Resistance
1.17Pa/m
41
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