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!!!

Correlations are developed from experimental data to compute Nu as a f(Re m,Pr n ) Overall Average Nusselt number All properties are evaluated at the freestream temperature, except Pr s which is evaluated at the surface temperature. Cross Flow Past A Cylinder

Cylinder in Cross Flow The empirical correlation due to Hilpert Re D Cm

Draught Systems for Steam Generators

Square Cylinder in Cross Flow D Valid for 5 X 10 3 < Re D < 10 5 D

Hexagonal Cylinder in Cross Flow D Valid for 5 X 10 3 < Re D <1.95X10 4 Valid for 1.95X10 4 < Re D < 10 5 D Valid for 5 X 10 3 < Re D < 10 5

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 S T and longitudinal pitch S L.

Inline Tube Bundle Staggered Tube Bundle Characteristic Dimension of External Flow

Definition of Parameters for Reynolds number If staggered and or

For tube bundles composed of 10 or more rows

All properties are evaluated at the film temperature.

If number of tubes are less than 10, a correction factor is applied as: And values for C 2 are from table

Power Plant Heat Exchangers

FSH Platen SHTR RHTRRHTR LTSH Economiser APH ESP ID Fan drum Furnace BCW pump Bottom ash stack screen tubes Thermal Structure of A Boiler Furnace

Thermal Balance in Convective SH. The energy absorbed by steam 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 When  T max /  T min < 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  T max /  T min  1.7.

Platen SH, U (W/m 2 K)120 – 140 Final SH, U (W/m 2 K)120 – 140 LTSH, U (W/m 2 K)60 – 80 Typical Values of U

Thermal Ratings of CHXs

FSH Platen SHTR RHTRRHTR LTSH Economiser APH ESP ID Fan drum Furnace BCW pump Bottom ash stack screen tubes Thermal Structure of A Boiler Furnace

Gas Temperatures Platen Super Heater: Inlet Temperature: C Outlet Temperature: C Final Super Heater: Inlet Temperature: C Outlet Temperature: C Reheater: Inlet Temperature: C Outlet Temperature: C Low Temperature Super Heater: Inlet Temperature: C Outlet Temperature: C Economizer: Inlet Temperature: C Outlet Temperature: C Steam Temperatures Platen Super Heater: Inlet Temperature: C Outlet Temperature: C Final Super Heater: Inlet Temperature: C Outlet Temperature: C Reheater: Inlet Temperature: C Outlet Temperature: C Low Temperature Super Heater: Inlet Temperature: C Outlet Temperature: C Economizer: Inlet Temperature: C Outlet Temperature: C

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.

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/m 2 s The waterside resistance should not exceed 5 – 8 %. Of drum pressure. Flue gas velocity : 7 – 13 m/s.

Extended Surfaces to Economizer