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Q7.A 15 mm thick steel furnace cover is in the form a hemisphere and has an internal diameter of 1.65 m.

The internal surf ace is coated with a refractory material 125 mm thick which has an inner surf ace temperature of 800°C.

The external surface is coated in a layer of magnesia insulation 60 mm thick which has an outer surface temperature of 40°C.

The magnesia is to be covered in an additional layer of insulation to reduce the heat flow by 50%.

The inner and outer surface temperatures are to remain at 800°C and 40°C respectively.

Calculate EACH of the following:

(a) the heat flow through the original three layers; (6)

(b) the thickness of the additional insulating layer; (6)

(c) the change in temperature at the steel/magnesia interface. (4)

Note: thermal conductivity refractory= 0.3 W/m K

thermal conductivity steel = 50 W/m K

thermal conductivity magnesia= 0.05 W/m K

thermal conductivity insulation=0.03 W/m K