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Q7. A steel pipe has a bore of 150 mm, a wall thickness of 10 mm and carries dry saturated steam at 12 bars.

It is covered with a 40 mm thick layer of moulded insulation which in turn is covered with a 60 mm layer of felt with an outer surface temperature of 25℃.

A change in regulations requires the felt to be replaced by a polystyrene material which cannot be used above 95℃. The heat transfer rate and outer surface temperature are required to remain unchanged.

Calculate EACH of the following:

(a) The rate of heat loss per unit length of pipe; (7)

(b) The temperature at the moulded insulation/felt interface; (2)

(c) The thickness of the polystyrene if the moulded layer is undisturbed. (7)

Note ; inner heat transfer coefficient = 550  W⁄(m²K)

thermal conductivity of steel may be ignored

thermal conductivity of the moulded insulation = 0.07  W⁄(m K)

thermal conductivity of the felt = 0.1  W⁄(m K)

thermal conductivity of the polystyrene = 0.09  W⁄(m K.)