28
Table 2: Pipe diameter and resistance loss in unit length
Note: Parameters in the preceding table may vary based on the design manual. For details, see the «HVAC
Design Manual». For hot water system, water speed can be a little bit smaller.
■ Calculation of local resistance loss
In the water system, water pressure drop may also be caused by some components in abnormal shape,
such as bends, tee joints, valves, filters and others, which is known as local resistance loss. It should also be
considered for hydraulic calculation. Local resistance ΔP (m) should be calculated as below:
ΔP=ξρν
2
/20000 Formula (14)
ρ— water density, kg/m3
ν— water flow velocity, m/s
ξ— local resistance coefficient, see Table 5.
Therefore, the total local resistance P
j
of the entire water system should be the sum of local resistance loss
of various components, namely, P
j
=ΣΔP.
In this way, total resistance loss of the pipeline section can be worked out and the pump head previous
selected may also be verified on this basis. Besides, pipe diameter can then be selected in more scientific
and reasonable way to achieve overall economy and rationality.
Diameter of the
steel tube (mm)
Closed water system
Open water system
Water flow (m
3
/h)
kPa/100m
Water flow (m
3
/h)
kPa/100m
15
0 - 0.5
0 - 60
--
--
20
0.5 - 1.0
10 - 60
--
--
25
1.0 - 2.0
10 - 60
0 - 1.3
0 - 43
32
2.0 - 4.0
10 - 60
1.3 - 2.0
11 - 40
40
4.0 - 6.0
10 - 60
2.0 - 4.0
10 - 40
50
6.0 -11.0
10 - 60
4.0 - 8.0
--
65
11.0 -18.0
10 - 60
8.0 -14.0
--
80
18 - 32
10 - 60
14 - 22
--
100
32 - 65
10 - 60
22 - 45
--
125
65 - 115
10 - 60
45 - 82
10 - 40
Form of Local
resistance
ξ
Form of Local
resistance
ξ
Water tank
2
Four-way joint for
direct flow
2
Sudden
expansion
1
Four-way joint for
by-pass flow
3
Sudden
shrinking
0.5
Four-way joint for
converging flow
4
Gradual
expansion
0.6
ξ at various pipe diameters
Gradual
shrinking
0.3
DN15
DN20
DN25
DN32
DN40
Above
DN50
Ω pipe
expansion joint
0.2
Straight bar stop
valve
16.0
10.0
9.0
9.0
8.0
7.0
Direct flow tee
joint
1
Sway rod stop valve
3.0
3.0
3.0
2.5
2.5
2.0
By-pass flow
tee joint
1.5
Rotational flow
valve
4.0
2.0
2.0
2.0
Converging
flow tee joint
3
Gate valve
1.5
0.5
0.5
0.5
0.5
0.5
Check valve
2
90° bend
2.0
2.0
1.5
1.5
1.0
1.0
