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

40°C
60°C
66.7°C
20°C

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Heat and Mass Transfer
Kirchhoff's law states that

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
The wave length corresponding to the maximum energy is proportional to the absolute temperature
The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
None of these

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