Heat Transfer
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that

Boundary surfaces are isothermal
Material is anisotropic
Constant temperature gradient exists
There is no internal heat generation

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Heat Transfer
Finned tube heat exchangers

use metal fins of low thermal conductivity
are used for smaller heat load
give larger area per tube
facilitate very large temperature drop through tube wall

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

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