Steam at 200°C flows in a cast iron pipe (k = 80 W/m·K) whose inner and outer diameters are D, = 0.20 m and D2 = 0.22 m. The pipe is exposed to room air at 35°C. The heat transfer coefficients at the inner and outer surfaces of the pipe are 90 and 20 W/m·K, respectively. The pipe is to be covered with glass wool insulation (k = 0.05 W/m·K) to decrease the heat loss from the stream by 90 percent. The required thickness of the insulation is
(a) 1.2 cm
(b) 2.0 cm
(c) 2.8 cm
(d) 3.4 cm
(e) 4.0 cm
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