Heat Transfer
In Fourier's law, the proportionality constant is called the

Thermal diffusivity
Stefan-Boltzman constant
Thermal conductivity
Heat transfer co-efficient

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

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Heat Transfer
Finned tube heat exchangers

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

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