Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

Heat and Mass Transfer
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

Two

One

Three

Four

Two

One

Three

Four

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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

Both may be put in any order

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

Better insulation should be put over pipe and better one over it

Both may be put in any order

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

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Heat and Mass Transfer
The concept of overall coefficient of heat transfer is used in case of heat transfer by

Conduction and convection

Conduction

Convection

Radiation

Conduction and convection

Conduction

Convection

Radiation

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Heat and Mass Transfer
Thermal conductivity of non-metallic amorphous solids with decrease in temperature

May increase or decrease depending on temperature

Increases

Remain constant

Decreases

May increase or decrease depending on temperature

Increases

Remain constant

Decreases

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Heat and Mass Transfer
A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100°C and leaves at 60°C. A cold fluid enters the heat exchanger at 40°C. The mean temperature difference between the two fluids is

66.7°C

20°C

60°C

40°C

66.7°C

20°C

60°C

40°C

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Heat and Mass Transfer
In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.

Same

None of these

Less

Greater

Same

None of these

Less

Greater

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Heat and Mass Transfer

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

Heat and Mass Transfer The concept of overall coefficient of heat transfer is used in case of heat transfer by

Heat and Mass Transfer Thermal conductivity of non-metallic amorphous solids with decrease in temperature

Heat and Mass Transfer In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.

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

Heat and Mass Transfer
The concept of overall coefficient of heat transfer is used in case of heat transfer by

Heat and Mass Transfer
Thermal conductivity of non-metallic amorphous solids with decrease in temperature

Heat and Mass Transfer
In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.