Heat and Mass Transfer
In counter flow heat exchangers

Both the fluids at exit are in their hottest state
Both the fluids at inlet are in their hottest state
One fluid is in hottest state and other in coldest state at inlet
Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state

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

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

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