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一个尺寸为 Æ 60mm ́ 3mm 的铝合金钢管,外包一层 30mm 厚的软木及一层 30mm 厚的保温材料( 85 % MgO ),管内壁温度 110 °C,保温材料外表面温度为 10 °C,已知 l 1 = 45W·m -1 ·K -1 , l 2 = 0.043W·m -1 ·K -1 , l 3 = 0.07 W·m -1 ·K -1 ( 1 )求每米损失的热量( W × m -1 ) ( 2 )若将两绝热层物互换,设互换后,管内壁温度和保温材料外表面温度不变,则每米损失的热量又是多少 W × m -1
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