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!!!
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
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
Draught Systems for Steam Generators
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
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
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.
Inline Tube Bundle Staggered Tube Bundle
Characteristic Dimension of External Flow
•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
•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
All properties are evaluated at the film temperature.
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
Power Plant Heat Exchangers
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
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
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.
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
Thermal Ratings of CHXs
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
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
LMTD
Two Pass Tube Bank
Multi Pass Tube Bank
Counter Cross & Parallel Cross
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
1
1ep
1
1
ec
CHX for Low LMTD
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
Tube Bank Arrangement
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.
Extended Surfaces to Economizer