# Heat Exchangers with Cross Flow past Cylinders P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Another Common Industrial Application!!!

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Heat Exchangers with Cross Flow past Cylinders P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Another Common Industrial Application!!!

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

For Reynolds number If staggered and or

For tube bundles composed of 10 or more rows

Tube-Side Nusselt Number For turbulent flow, the following equation developed by Petukhov- Kirillov is used: Properties are evaluated at mean bulk temperature and constants are adjusted to fit experimental data. Validity range: 10 4 < Re t < 5 x 10 6 and 0.5 < Pr t < 2000 with 10% error.

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

More recent results have been obtained by Zhukauskas. All properties except Pr s are evaluated at the arithmetic mean of the fluid inlet and outlet temperatures. Values for C and m.

tρcpcp μ *10 6 ν *10 6 K [ O C][kg/m 3 ][kJ/kgK][Pas][m 2 /s]W/m.K 01.2951.04215.812.20.026 1000.951.06820.421.540.032 2000.7481.09724.532.80.038 3000.6171.12228.245.810.0448 4000.5251.15131.760.380.0506 5000.4571.18534.876.30.0565 6000.4051.21437.993.610.0623 7000.3631.23940.7112.10.0678 8000.331.26443.4131.80.0732 9000.3011.2945.9152.50.0782 10000.2751.30648.4174.30.0837 11000.2571.32350.7197.10.0891 12000.241.34532210.0946 Thermo-physical Properties of Combustion Products

Power Plant Heat Exchangers

Paths of Steam and Gas Water walls Drum Economizer

Convective Superheater (Pendant) Convective super heaters are vertical type (Pendant ) or horizontal types. The Pendant SH is always arranged in the horizontal crossover duct. Pendant SH tubes are widely spaced due to high temperature and ash is soft. Transverse pitch : S 1 /d > 4.5 Longitudinal pitch : S 2 /d > 3.5. The outside tube diameter : 32 – 51mm Tube thickness : 3 – 7mm S1S1 S2S2

Paths of Steam and Gas

Convective Superheater (Horizontal) The horizontal SH are located in the back pass. The tubes are arranged in the in-line configuration. The outer diameter of the tube is 32 – 51 mm. The tube thickness of the tube is 3 – 7 mm. The transverse pitch : S 1 /d = 2 – 3. The longitudinal pitch :S 2 /d = 1.6 – 2.5. The tubes are arranged in multiple parallel sets. The desired velocity depends on the type of SH and operating steam pressures. The outside tube diameter : 32 – 51mm Tube thickness : 3 – 7mm S1S1 S2S2

Thermal Balance in Convective SH. The energy absorbed by steam The convective heat exchange in the super heater: Overall Coefficient of Heat Transfer, U

Effectiveness Factor The ratio of the heat transfer between fouled and clean tubes.

Thermal Balance in Super Heater. The energy absorbed by steam The convective heat lost by flue gas Overall Coefficient of Heat Transfer, 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

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: 724.3 0 C Outlet Temperature: 481.3 0 C Economizer: Inlet Temperature: 481.3 0 C Outlet Temperature: 328.5 0 C Steam Temperatures 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: 540 0 C Low Temperature Super Heater: Inlet Temperature: 359 0 C Outlet Temperature: 404 0 C Economizer: Inlet Temperature: 254 0 C Outlet Temperature: 302 0 C

Real Mean Temperature Differences Three dimensionless parameters are introduced and used to compute real mean temperature difference.

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