Heat and Mass Transfer
The thermal diffusivities for solids are generally

Less than those for liquids
Less than those for gases
More or less same as for liquids and gases
More than those for liquids and gases

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Heat and Mass Transfer
In counter flow heat exchangers

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 exit are in their hottest state
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
Thermal conductivity to the equivalent thickness of the film of fluid
Temperature drop through the films of fluids to the thickness of film of fluids
Thickness of film of fluid to the thermal conductivity

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