Heat Transfer problems
- 1. The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K= 0.04 W/m.°K and thickness 0.16m is 10 W/m2. The temperature of the inside wall is - 5°C. The outside wall temperature is
Options- A. 25°C
- B. 30°C
- C. 35°C
- D. 40°C Discuss
Correct Answer: 35°C
- 2. Hot water (0.01 m3 /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately
Options- A. 32
- B. 37
- C. 45
- D. 50 Discuss
Correct Answer: 32
- 3. In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
Options- A. increases
- B. decreases
- C. is not affected
- D. may increase or decrease; depends on the feed Discuss
Correct Answer: decreases
- 4. The number of kg vaporised per kg of steam fed to the evaporator is defined as
Options- A. capacity
- B. rate of evaporation
- C. economy
- D. rate of vaporisation Discuss
Correct Answer: economy
- 5. For an ideal black body
Options- A. absorptivity = 1
- B. reflectivity = 1
- C. emissivity = 0
- D. transmissivity = 1 Discuss
Correct Answer: absorptivity = 1
- 6. In a heat exchanger, floating head is provided to
Options- A. facilitate cleaning of the exchanger.
- B. increase the heat transfer area.
- C. relieve stresses caused by thermal expansion.
- D. increase log mean temperature gradient. Discuss
Correct Answer: relieve stresses caused by thermal expansion.
- 7. When vaporisation takes place directly at the heating surface, it is called
Options- A. film boiling
- B. nucleate boiling
- C. vapour binding
- D. none of these Discuss
Correct Answer: nucleate boiling
- 8. The steam ejector is used to
Options- A. remove condensate from the steam pipelines.
- B. create vacuum.
- C. superheat the steam.
- D. none of these. Discuss
Correct Answer: create vacuum.
- 9. In the equation Q = UA?t; ?t is
Options- A. geometric mean temperature difference.
- B. arithmetic mean temperature difference.
- C. logarithmic mean temperature difference.
- D. the difference of average bulk temperatures of hot and cold fluids. Discuss
Correct Answer: logarithmic mean temperature difference.
- 10. A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be
Options- A. greater than that for uninsulated steam pipe.
- B. less than that of the uninsulated steam pipe.
- C. equal to that of the uninsulated steam pipe.
- D. less than the steam pipe with 5 cms insulation. Discuss
Correct Answer: less than that of the uninsulated steam pipe.
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