Heat Transfer
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for

Cylinder
Equal for all the three
Cube
Sphere

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Heat Transfer
Finned tube heat exchangers

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

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

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