Heat and Mass Transfer
In counter flow heat exchangers

Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
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

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Heat and Mass Transfer
Kirchhoff's law states that

The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body
None of these
The wave length corresponding to the maximum energy is proportional to the absolute temperature

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Heat and Mass Transfer
According to Prevost theory of heat exchange

Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
It is impossible to transfer heat from low temperature source to t high temperature source
Heat transfer by radiation requires no medium
All bodies above absolute zero emit radiation

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