Heat and Mass Transfer
The heat transfer by conduction through a thick sphere is given by

Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)

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Heat and Mass Transfer
A steam pipe is to be insulated by two insulating materials put over each other. For best results

Better insulation should be put over pipe and better one over it
Whether to put inferior OIL over pipe or the better one would depend on steam temperature
Inferior insulation should be put over pipe and better one over it
Both may be put in any order

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