A $150$ mm steam pipe has inside diameter of $120$ mm and outside diameter of $160$ mm$.$ It is insulated at the outside with asbestos. The steam temperature is $150$C and the air temperature is $20$C$.$ The heat transfer coefficient on the steam side is $100$ W$/$m$2-$C and on the air side is $30$ W$/$m$2-$C$.$ The thermal conductivity of asbestos is $0\mathrm{.}8$ W$/$m$-$C and of steel is $42$ W$/$m$-$C$.$ What is the heat loss per unit length if the asbestos insulation is $25$ mm thick?