Heat and Mass Transfer
In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by

Convection
Radiation
Both convection and conduction
Conduction

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Heat and Mass Transfer
Film coefficient is defined as Inside diameter of tube

Film coefficient × Inside diameter Thermal conductivity
Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity
Equivalent thickness of film
Thermal conductivity Equivalent thickness of film Specific heat × Viscosity

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