Heat and Mass Transfer
The thermal diffusivities for solids are generally

More than those for liquids and gases
More or less same as for liquids and gases
Less than those for liquids
Less than those for gases

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Heat and Mass Transfer
A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,

The better insulation must be put inside
One could place either insulation on either side
One should take into account the steam temperature before deciding as to which insulation is put where
The better insulation must be put outside

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