Heat Transfer
In the equation Q = UAΔt; Δt is

Geometric mean temperature difference
The difference of average bulk temperatures of hot and cold fluids
Arithmetic mean temperature difference
Logarithmic mean temperature difference

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Heat Transfer
Which of the following situations can be approximated to a steady state heat transfer system?

A sub-cooled refrigerant liquid at 8°C flowing at the rate of 6 Kg/minute through a copper pipe exposed to atmospheric air at 35°C
Boiling brine kept in open vessel when the bottom surface temperature of the vessel is maintained constant at 180°C
A red hot steel slab (having outside surface temperature as 1300°C) exposed to the atmosheric air at 35°C
10 kg of dry saturated steam at 8 kgf/cm² flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C

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Heat Transfer
Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the

Constant temperature equal to that of the base
Average temperature of the fin
Surrounding temperature
Temperature of the fin end

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