A small reheating furnace wall consists of 200 mm of firebrick. The inner surface of the wall is at a temperature of 320 ℃ and the outside temperature is 25 ℃ Calculate the rate at which the heat is transferred, by conduction, through a unit area of the wall. The thermal conductivity of the firebrick used can be taken as 1.4 W m-1K-1. If the outside surface area of the furnace is 80 m2 estimate the heat losses through the furnace wall per hour. A pipe carrying superheated steam at 300 °C has an outside diameter of 150 mm and is lagged with two layers of insulating material. The first layer (adjacent to the outer pipe wall) is 20 mm thick and has a thermal conductivity of 0.049 W m-1 K-1. The second layer (covering the first layer) is 25 mm thick, has a thermal conductivity of 0.063 W m-1 K-1 and an outside temperature of 25 °C. Estimate the rate of heat loss per metre length of pipe (assume the thermal resistance of the pipe wall is negligible). The Grashof and Reynolds numbers appear in most correlations of experimental data for convective heat transfer. Explain, in a maximum of 150 words, the mechanisms of natural and forced convection with particular reference to the above nondimensional groups.