cross flow heat exchangers p m v subbarao professor mechanical engineering department i i t delhi a...
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Cross Flow Heat Exchangers
P M V SubbaraoProfessor
Mechanical Engineering Department
I I T Delhi
A Major Element for the Success of Combustion based
Power Plants!!!
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Correlations are developed from experimental data to compute Nu as a f(Rem,Prn)
Overall Average Nusselt number
6
4131
10Re1500Pr7.0
:for Valid
Pr
Pr Pr Re
D
s
mDD C
k
DhNu
•All properties are evaluated at the freestream temperature, except Prs which is evaluated at the surface temperature.
Cross Flow Past A Cylinder
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Cylinder in Cross Flow
3/1PrRemDD C
k
DhNu
The empirical correlation due to Hilpert
ReD C m
0.4 -4 0.989 0.330
4 - 40 0.911 0.385
40 -- 4000 0.683 0.466
4000 -- 40000 0.193 0.618
40000 -- 400000 0.027 0.805
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Draught Systems for Steam Generators
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Square Cylinder in Cross Flow
D
3/1588.0 PrRe246.0 DDk
DhNu
Valid for 5 X 103 < ReD < 105
D3/1675.0 PrRe102.0 DD
k
DhNu
Valid for 5 X 103 < ReD < 105
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Hexagonal Cylinder in Cross Flow
D
3/1638.0 PrRe160.0 DDk
DhNu
Valid for 5 X 103 < ReD <1.95X104
3/1782.0 PrRe0385.0 DDk
DhNu
Valid for 1.95X104 < ReD < 105
D3/1638.0 PrRe153.0 DD
k
DhNu
Valid for 5 X 103 < ReD < 105
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Convection heat transfer with banks of tubes
• Typically, one fluid moves over the tubes, while a second fluid at a different temperature passes through the tubes. (cross flow)
• The tube rows of a bank are staggered or aligned.
• The configuration is characterized by the tube diameter D, the transverse pitch ST and longitudinal pitch SL.
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Inline Tube Bundle Staggered Tube Bundle
Characteristic Dimension of External Flow
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•Definition of Parameters for Reynolds number
DV
Dmax
max,Re
VDS
SV
T
T
max V
DS
SV
D
T
)(2max
If staggered and 2
DSS T
D
or
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•For tube bundles composed of 10 or more rows
3/11 PrRe13.1 max,
mD DCNu
10
0.7r
104Re2000
:for valid
4max,
L
D
N
P
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All properties are evaluated at the film temperature.
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If number of tubes are less than 10, a correction factor is applied as:
)10(2
)10(
LL ND
ND NuCNu
And values for C2 are from table
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Power Plant Heat Exchangers
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FSH
Platen S
HT
R
RHTR
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCWpump
Bottom ash
stack
screentubes
Thermal Structure of A Boiler Furnace
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Thermal Balance in Convective SH.
• The energy absorbed by steam )( sup,sup,, inoutsteamconabs hhmQ
meanSHlosscon TUAQ
,
• The convective heat exchange in the super heater:
• Overall Coefficient of Heat Transfer, U
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Mean Temperature Difference• The average temperature difference for parallel flow and counter flow is
expressed as
min
max
minmax
log3.2TTTT
Tmean
2minmax TT
T
• When Tmax /Tmin < 1.7, the average temperature may be expressed as:
• Generally, the flow direction of the flue gas is perpendicular to the axes of tubes.
• Cross flow creates a conditions close to Tmax /Tmin 1.7.
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Platen SH, U (W/m2 K) 120 – 140
Final SH, U (W/m2 K) 120 – 140
LTSH, U (W/m2 K) 60 – 80
Typical Values of U
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Thermal Ratings of CHXs
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FSH
Platen S
HT
R
RHTR
LTSH
Economiser
APH ESP ID Fan
drum
Furnace
BCWpump
Bottom ash
stack
screentubes
Thermal Structure of A Boiler Furnace
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Gas Temperatures
• Platen Super Heater:• Inlet Temperature: 1236.4 0C• Outlet Temperature: 1077 0C• Final Super Heater:• Inlet Temperature: 1077 0C• Outlet Temperature: 962.4 0C• Reheater:• Inlet Temperature: 962.4 0C• Outlet Temperature: 724.3 0C• Low Temperature Super
Heater:• Inlet Temperature: 724.30C• Outlet Temperature: 481.3 0C• Economizer:• Inlet Temperature: 481.3 0C• Outlet Temperature: 328.5 0C
Steam Temperatures
• Platen Super Heater:• Inlet Temperature: 404 0C• Outlet Temperature: 475 0C• Final Super Heater:• Inlet Temperature: 475 0C• Outlet Temperature: 540 0C• Reheater:• Inlet Temperature: 345 0C• Outlet Temperature: 5400C• Low Temperature Super
Heater:• Inlet Temperature: 3590C• Outlet Temperature: 404 0C• Economizer:• Inlet Temperature: 254 0C• Outlet Temperature: 302 0C
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LMTD
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Two Pass Tube Bank
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Multi Pass Tube Bank
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Counter Cross & Parallel Cross
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Real Mean Temperature Differences
• Three dimensionless parameters are introduced and used to compute real mean temperature difference.
incoldinhot
incoldouthot
TT
TT
,,
,,
cold
hot
C
C
hotC
UA
pc ZZ 1
324.01136.05.0 Z
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1
1ep
1
1
ec
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CHX for Low LMTD
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Economizer
• The economizer preheats the feed water by utilizing the residual heat of the flue gas.
• It reduces the exhaust gas temperature and saves the fuel.• Modern power plants use steel-tube-type economizers.• Design Configuration: divided into several sections : 0.6 – 0.8 m
gap
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Tube Bank Arrangement
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Thermal Structure of Economizer
• Out side diameter : 25 – 38 mm.• Tube thinckness: 3 – 5 mm• Transverse spacing : 2.5 – 3.0• Longitudinal spacing : 1.5 – 2.0• The water flow velocity : 600 – 800 kg/m2 s• The waterside resistance should not exceed 5 – 8 %.
Of drum pressure.• Flue gas velocity : 7 – 13 m/s.
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Extended Surfaces to Economizer