Title: Cross Flow Heat Exchangers
1Cross 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!!!
2Cross 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.
3Cylinder 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
4Draught Systems for Steam Generators
5Square Cylinder in Cross Flow
Valid for 5 X 103 lt ReD lt 105
Valid for 5 X 103 lt ReD lt 105
6Hexagonal 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
7Convection 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.
8Characteristic 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
11All properties are evaluated at the film
temperature.
12If number of tubes are less than 10, a correction
factor is applied as
And values for C2 are from table
13Power Plant Heat Exchangers
14Thermal Structure of A Boiler Furnace
15Thermal Balance in Convective SH.
- The energy absorbed by steam
- The convective heat exchange in the super heater
- Overall Coefficient of Heat Transfer, U
16Mean 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.
17Typical 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
18Thermal Ratings of CHXs
19Thermal Structure of A Boiler Furnace
20Gas 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
21LMTD
22Two Pass Tube Bank
23Multi Pass Tube Bank
24Counter Cross Parallel Cross
25Real Mean Temperature Differences
- Three dimensionless parameters are introduced and
used to compute real mean temperature difference.
26(No Transcript)
27CHX for Low LMTD
28Economizer
- 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
29Tube Bank Arrangement
30Thermal 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.
31Extended Surfaces to Economizer