Heat and Mass Transfer
A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100°C and leaves at 60°C. A cold fluid enters the heat exchanger at 40°C. The mean temperature difference between the two fluids is

60°C
40°C
20°C
66.7°C

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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 wave length corresponding to the maximum energy is proportional to 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

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Heat and Mass Transfer
According of Kirchhoff's law

Emissive power and absorptivity are constant for all bodies
Radiant heat is proportional to fourth power of absolute temperature
Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body
Emissive power depends on temperature

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