10 VEN Duct design.ppt [režim kompatibility]
Transcript of 10 VEN Duct design.ppt [režim kompatibility]
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Ventilation10 Duct Design
Vladimír Zmrhal (room no. 814)
http://users.fs.cvut.cz/~zmrhavla/index.htmDpt. Of Environmental Engineering
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Laminar and turbulent flows
Reynolds number
laminar flow Re 2300
transitional flow 2300 < Re < 10000
fully turbulent flow Re > 10 000
air… kinematic viscosity [m2/s] = 14,5.10-6 [m2/s]
Rewd
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Flow characteristics
n …exponent f(Re)
1/
max
1n
w yw r
2
1/
max2
1
11 2
s
sS
n
sS
V w S
w wdSr
yw w ydy
r r
max
0,817sww
Laminar and turbulent flows
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Bernoulli equation (energy)
Pressures in the duct
Pressure losses
2 21 1 1 2 2 22 2s sp h g w p h g w p
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2s d
wp p p p
2 21 1 2 2 1 22 2s s t tp p w p w p p
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friction
local pressure losses
2 2
.2 2
lf
t
pp
l w wp R l Z
d
22
2t
l wp kV
d
Pressure losses
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Laminar flow
Turbulent flow
Colebrook (1939)/d – relative roughness
Smolik (1959) for = 0,15
64Re
0,125 0,11
0,0812Re d
1 / 2,512log
3,71 Re
d
Friction losses
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Turbulent flow
for smooth pipes and duct (plastic)
5000 < Re 80 0004
0,3164
Re
Friction losses
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Material (mm)
Galvanized steel 0,15
Concrete duct – smooth surface 0,5
Concrete duct – rough surface 1,0 – 3,0
Smooth brass, copper 0,015
Hose pipe 0,6 - 3
Plastic pipe 0,007
Roghness height of the conduit wall surfaces
Friction losses
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Hydraulic diameter
Rectangular ducts
4 4 22( )h
A ab abd
O a b a b
dC
1,1 0,1b
Ca
Friction losses
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Moody‘s diagram
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Local pressure losses are caused by the fluid flow through the duct fittings: which change the direction of the flow (elbows, bands, etc.)
affect the flow in the straight duct with constant cross-section(valves, stopcocks, filters etc.).
… local loss coefficient (experiments - see Idelchik 1986)
Borda loss prediction
Local pressure losses
2
2l d
wp p
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109876543210
Local pressure losses
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109876543210
0,08
1,11ab
Local pressure losses
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Duct design
Methods
velocity method
equal-friction method
static regain method
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Velocity method
Duct design procedure:
1) Find the main line
Rule no. 1: the main line is the maximum pressure loss line
(longest line, most segment line (?))
2) Air flow rate V (m3/h) in duct sections is known
3) Selection of the air velocity in the duct w
Rule no. 2: Air velocity increase towards the fan
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Air velocity w (m/s)
Main section Side section
Ventilation and low-pressure air-conditioning
recomend. max. recomend. max.
- residential buildings 3,5 - 5 6 3 5
- public buildings 5 – 7 8 3 – 4,5 6,5
- industry 6 - 9 11 4 - 5 9
High-pressure air-conditioning 8 - 12 15 - 20 8 - 10 18
Velocity method
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4) duct area A (m2) diameter d or a x b
nominal diameter dN or aN x bN
Rule no. 3: Duct sizes: 80, 100, 125, 140, 160, 180, 200, 250, 315,355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250,1400, 1600, 1800, 2000
4Vd
w
Velocity method
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5) dN real velocity wreal
6) calculation of dynamic pressure pd
7) Reynolds number friction coefficient 8) local loss coefficients 9) pressure loss of the duct section pz,i
2
4real
N
Vw
d
2
,, 2
i iz i
s i
l wp
d
Velocity method
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Rule no. 4: Balancing
10) total pressure loss is the sum of the duct sections pressurelosses
,ext z ip p
, , , ,z F z E z G z Ip p p p
Velocity method
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, , , , ,z z A z B z D z G z Ip p p p p p
1 2 3 4 5cV V V V V V
Velocity method
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Example
Example 1: Dimension the air duct system. Use the velocity method. air velocity w = 6 - 10 m/s,air density = 1,2 kg/m3, kinematic viscosity = 14,5.10-6 m2/s.
V1 = 9 000 m3/h
V2 = 1 440 m3/h
V3 = 2 160 m3/h
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Example
Example 1:
2
0,125 0,11
2 2
,
4 4
0,0812Re
Re
2 2
( )
calc N realN
realt N
N
real realf l
t i f l el
V VD D w
w D
w D
D
w wlp p
D
p p p p
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Example
Line l V V wcalc Dcalc DN wreal pd Re l pf pl pel pt
- m m3/h m3/s m/s mm mm m/s Pa - - Pa - Pa Pa Pa
0,41 19
0,96 0
0,46 0
2,04 0
TOTAL XX
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Equal-Friction Method
Duct design procedure:
1) selection of pressure loss per unit length R = 0,8 – 4 Pa/m
2) local pressure losses friction in straight duct with equivalentlength
3) duct section pressure loss
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wR
d
2 2
2 2e
e
l w wl d
d
z ep R l l
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Friction chart
Choice:
R = 1 Pa/m
Air flow rate:
1 000 m3/h
diameter D:
280 mm
Velocity :
w = 4,5 m/s
Equal-Friction Method
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Static Regain Method
for uniform air supply
constant static pressure before thebranch
Principles
cross section reduction afterbranches to change the dynamicpressure
decreasing of dynamic pressurebalances the pressure losses in theduct section
3 2 3 2z d dp p p
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Assumptions:
V = const.
b = const.
i = n, n - 1, …. 1
calculation of dimension a
1
1 11
11 i
i i ii
ia a l
d i
Static Regain Method
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Deduction:
kde 2 2 2
1 1 2 1
1 2 2 2l w w wd
2 1 2 1z d dp p p
1 21 2
1 2
,V V
w wa b a b
2 2 2
1 1 2 12 2 2 2 2 2
1 1 2 1
l V V Vd a b a b a b
Static Regain Method
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2 2 21 1 2 1
2 2 21 1 2 1
l V V Vd a a a
1 2, 2V V V V
2 22 2 2 2 2 2 21 1
2 2 2 2 2 21 1 1 2 1 1 1
1 11 1 2 i
i i i i
i il V V V l id a a a d a a a
22 2 21 11 1 1
1 1
11 1 1i i
i i i i ii i
l ia i a i a a l
d d i
Static Regain Method
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Duct systems
Shapes
rectangular
round
flexible duct
Materials
steel galvanized
aluminium
plastic PVC
textile
ALP
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Duct leakage rate
where Sv … duct surface [m2]
0,67vV m p S
Duct systems
Class Charakteristics of the leakage pathm [m3/s per m2]
A 0,027 . 10-3
B 0,009 . 10-3
C 0,003 . 10-3
D 0,001 . 10-3
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Purpose
condensation risk
heat losses/gains
Thickness of TI
indoor 45 – 60 mm
outdoor 80 – 100 mm (with sheet covering)
Thermal insulation
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Thank you for your attention