E-PolyLearning

66. An electric cable of aluminum (k = 240 W/ m degree) is to be insulated with rubber (k = 6 W/ square meter degree). If the cable is in air (h = 6 W/square meter degree). Find the critical radius?
a. 80 mm
b. 160 mm
c. 40 mm
d. 25 mm
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Answer: (d).25 mm

67. The value of critical radius in case of cylindrical hollow object is
a. 2k/h
b. 2h/k
c. k/h
d. h/k
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Answer: (c).k/h

68. A wire of radius 3 mm and 1.25 m length is to be maintained at 60 degree Celsius by insulating it by a material of thermal conductivity 0.175 W/m K. The temperature of surrounding is 20 degree Celsius with heat transfer coefficient 8.5 W/ m² K. Find percentage increase in heat loss due to insulation?
a. 134.46 %
b. 124.23 %
c. 100.00 %
d. 12.55 %
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Answer: (a).134.46 %

69. A pipe of outside diameter₂0 mm is to be insulated with asbestos which has a mean thermal conductivity of 0.1 W/m degree. The local coefficient of convective heat to the surroundings is 5 W/square meter degree. Find the critical radius of insulation for optimum heat transfer from pipe?
a. 10 mm
b. 20 mm
c. 30 mm
d. 40 mm
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Answer: (b).20 mm

70. For insulation to be properly effective in restricting heat transmission, the pipe radius r0 will be
a. Greater than critical radius
b. Less than critical radius
c. Equal to critical radius
d. Greater than or equal to critical radius
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Answer: (d).Greater than or equal to critical radius

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