Cross Flow Heat Exchangers P M V Subbarao

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!!!

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.

Cylinder in Cross Flow The empirical correlation due to Hilpert Re. D 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 Valid for 5 X 103 < Re. D < 105

Hexagonal Cylinder in Cross Flow Valid for 5 X 103 < Re. D <1. 95 X 104 D Valid for 1. 95 X 104 < Re. D < 105 D Valid for 5 X 103 < Re. D < 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.

Characteristic Dimension of External Flow Inline Tube Bundle Staggered Tube Bundle

• Definition of Parameters for Reynolds number or If staggered and

• 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

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

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 DTmax / DTmin < 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.

Typical Values of U 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

Thermal Ratings of CHXs

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

Steam Temperatures Gas Temperatures • • • • Platen Super Heater: Inlet Temperature: 1236. 4 0 C Outlet Temperature: 1077 0 C Final Super Heater: Inlet Temperature: 1077 0 C Outlet Temperature: 962. 4 0 C Reheater: Inlet Temperature: 962. 4 0 C Outlet Temperature: 724. 3 0 C Low Temperature Super Heater: Inlet Temperature: 724. 30 C Outlet Temperature: 481. 3 0 C Economizer: Inlet Temperature: 481. 3 0 C Outlet Temperature: 328. 5 0 C • • • • Platen Super Heater: Inlet Temperature: 404 0 C Outlet Temperature: 475 0 C Final Super Heater: Inlet Temperature: 475 0 C Outlet Temperature: 540 0 C Reheater: Inlet Temperature: 345 0 C Outlet Temperature: 5400 C Low Temperature Super Heater: Inlet Temperature: 3590 C Outlet Temperature: 404 0 C Economizer: Inlet Temperature: 254 0 C Outlet Temperature: 302 0 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