E-PolyLearning

61. A cable of 10 mm outside is to be laid in an atmosphere of 25 degree Celsius (h = 12.5 W/m² degree) and its surface temperature is likely to be 75 degree Celsius due to heat generated within it. How would the heat flow from the cable be affected if it is insulated with rubber having thermal conductivity k = 0.15 W/m degree?
a. 43.80 W per meter length
b. 53.80 W per meter length
c. 63.80 W per meter length
d. 73.80 W per meter length
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Answer: (b).53.80 W per meter length

62. Chose the correct one with respect to critical radius of insulation
a. There is more heat loss i.e. conductive
b. There occurs a decrease in heat flux
c. Heat loss increases with addition of insulation
d. Heat loss decreases with addition of insulation
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Answer: (c).Heat loss increases with addition of insulation

63. A heat exchanger shell of outside radius 15 cm is to be insulated with glass wool of thermal conductivity 0.0825 W/m degree. The temperature at the surface is 280 degree Celsius and it can be assumed to remain constant after the layer of insulation has been applied to the shell. The convective film coefficient between the outside surface of glass wool and the surrounding air is estimated to be 8 W/m² degree. What is the value of critical radius?
a. 9.31 mm
b. 10.31 mm
c. 11.31 mm
d. 12.31 mm
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Answer: (b).10.31 mm

64. For an object i.e. spherical the value of critical radius would be
a. 2k/3h
b. 3k/h
c. 2k/h
d. k/h
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Answer: (c).2k/h

65. Maximum value of critical radius is
a. 0.01 m
b. 0.04 m
c. 0.06 m
d. 0.0001 m
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Answer: (a).0.01 m