Heat Transfer
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.

10 minutes
20 minutes
40 minutes
5 minutes

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Heat Transfer
In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m².s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m²/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is

5000
2000
4000
6000

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