Heat Transfer
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)

α 1/T
Independent of T
α T⁴
αT

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Heat Transfer
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because

Boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system
Total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect
Heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system
Total heat transfer area of all the effects is -less than that in a single effect evaporator system

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Heat Transfer
The thermal efficiency of a reversible heat engine operating between two given thermal reservoirs is 0.4. The device is used either as a refrigerator or as a heat pump between the same reservoirs. Then the coefficient of performance as a refrigerator (COP)R and the co-efficient of performance as a heat pump (COP)HP are

(COP)R = 2.5; (COP)HP = 1.5
(COP)R = (COP)HP = 2.5
(COP)R = (COP)HP = 0.6
(COP)R = 1.5; (COP)HP = 2.5

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