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Q7. A steel pipe has an internal diameter of 100 mm and a wall thickness of 8 mm, it carries wet steam at a pressure of 7 bar and is covered with two layers of insulation, each 10 mm thick. The outer layer of insulation becomes contaminated and is removed.

The surrounding air temperature remains constant at 20℃

Calculate EACH of the following:

(a)The heat loss per meter length of pipe when covered in two layers of insulation (6)

(b)The percentage increase in heat loss when the outer layer is removed; (5)

(c)The percentage increase in outer surface temperature. (5)

Note: The heat transfer coefficient of the inner surface may be ignored

The thermal conductivity of steel = 52  W⁄(m K)

the thermal conductivity of the inner insulation=0.045  W⁄(m K)

the thermal conductivity of the outer insulation=0.13  W⁄(m K)

the heat transfer coefficient of the outer surface=15  W⁄(m²  K).