Heat and Mass Transfer
In counter flow heat exchangers

Both the fluids at inlet are in their hottest state
Both the fluids at exit 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 the ratio of

Thickness of film of fluid to the temperature drop through the films of fluids
Thickness of film of fluid to the thermal conductivity
Temperature drop through the films of fluids to the thickness of film of fluids
Thermal conductivity to the equivalent thickness of the film of fluid

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