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Cross Flow Heat Exchangers

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Title: Cross Flow Heat Exchangers


1
Cross Flow Heat Exchangers
  • P M V Subbarao
  • Professor
  • Mechanical Engineering Department
  • I I T Delhi

A Major Element for the Success of Combustion
based Power Plants!!!
2
Cross Flow Past A Cylinder
Correlations are developed from experimental data
to compute Nu as a f(Rem,Prn) Overall Average
Nusselt number
  • All properties are evaluated at the freestream
    temperature, except Prs which is evaluated at the
    surface temperature.

3
Cylinder in Cross Flow
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
4
Draught Systems for Steam Generators
5
Square Cylinder in Cross Flow
Valid for 5 X 103 lt ReD lt 105
Valid for 5 X 103 lt ReD lt 105
6
Hexagonal Cylinder in Cross Flow
Valid for 5 X 103 lt ReD lt1.95X104
Valid for 1.95X104 lt ReD lt 105
Valid for 5 X 103 lt ReD lt 105
7
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.

8
Characteristic Dimension of External Flow
Inline Tube Bundle Staggered Tube
Bundle
9
  • Definition of Parameters for Reynolds number

or
If staggered and
10
  • For tube bundles composed of 10 or more rows

11
All properties are evaluated at the film
temperature.
12
If number of tubes are less than 10, a correction
factor is applied as
And values for C2 are from table
13
Power Plant Heat Exchangers
14
Thermal Structure of A Boiler Furnace
15
Thermal Balance in Convective SH.
  • The energy absorbed by steam
  • The convective heat exchange in the super heater
  • Overall Coefficient of Heat Transfer, U

16
Mean Temperature Difference
  • The average temperature difference for parallel
    flow and counter flow is expressed as
  • When DTmax / DTmin lt 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 DTmax /
    DTmin ?1.7.

17
Typical Values of U
Platen SH, U (W/m2 K) 120 140
Final SH, U (W/m2 K) 120 140
LTSH, U (W/m2 K) 60 80
18
Thermal Ratings of CHXs
19
Thermal Structure of A Boiler Furnace
20
Gas Temperatures
Steam 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
  • 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

21
LMTD
22
Two Pass Tube Bank
23
Multi Pass Tube Bank
24
Counter Cross Parallel Cross
25
Real Mean Temperature Differences
  • Three dimensionless parameters are introduced and
    used to compute real mean temperature difference.

26
(No Transcript)
27
CHX for Low LMTD
28
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

29
Tube Bank Arrangement
30
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.

31
Extended Surfaces to Economizer
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