Cross Flow Heat Exchangers

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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!!!
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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.

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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
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Draught Systems for Steam Generators
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Square Cylinder in Cross Flow
Valid for 5 X 103 lt ReD lt 105
Valid for 5 X 103 lt ReD lt 105
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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
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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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Characteristic Dimension of External Flow
Inline Tube Bundle Staggered Tube
Bundle
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• Definition of Parameters for Reynolds number

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

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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
And values for C2 are from table
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Power Plant Heat Exchangers
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Thermal Structure of A Boiler Furnace
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Thermal Balance in Convective SH.

• The energy absorbed by steam

• 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

• 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.

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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
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Thermal Ratings of CHXs
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Thermal Structure of A Boiler Furnace
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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

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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.

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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