Heat Transfer

21. A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in _________ minutes.

1. 10
2. > 10
3. < 10
4. Either (B) or (C), depends on the mass of the body

22. A body is called grey if the monochromatic emissivity of the body is

1. Zero
2. Unity
3. Same for all wavelengths
4. Different for all wavelengths

23. A BTU/hr.ft.² °F is equal to

1. 1 kcal/hr. m²°C
2. 4.88 kcal/hr. m.2°C
3. 1 kcal/hr. m².°K
4. None of these

24. A composite flat wall of a furnace is made of two materials 'A' and 'B'. The thermal conductivity of 'A' is twice of that of material 'B', while the thickness of layer of 'A' is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material 'A' is

1. 125
2. 133
3. 150
4. 160

25. A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are k_A and k_B and the specific heat capacities are C_PA and C_PB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when

1. C_PA > C_PB
2. C_PA < C_PB
3. k_A < 0.5 kB
4. k_A >2 _KA

26. A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest?

1. Cube
2. Plate
3. Sphere
4. All will cool at the same rate

27. A dephlegmator is a

1. Total condenser
2. Vacuum evaporator
3. Partial condenser
4. Double pipe heat exchanger

28. A diathermanous substance _________ the thermal radiation completely.

1. Absorbs
2. Reflects
3. Transmits
4. None of these

29. A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because

1. Total heat transfer area of all the effects is -less than that in a single effect evaporator system
2. Total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect
3. Boiling point elevation in a single effect system is much higher than that in any effect in a multi-effect system
4. Heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system

30. A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter 'd'. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to

1. d0.8
2. d-0.2
3. d^-1
4. d^-1.8