Heat and Mass Transfer
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

40 mm
800 mm
25 mm
160 mm

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Heat and Mass Transfer
Film coefficient is defined as the ratio of

Thermal conductivity to the equivalent thickness of the film of fluid
Thickness of film of fluid to the thermal conductivity
Thickness of film of fluid to the temperature drop through the films of fluids
Temperature drop through the films of fluids to the thickness of film of fluids

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