Q1 Figure shows a scale drawing of the temperature profile through a composite plane wall.
(i) Identify the layer with the largest thermal conductivity,
(ii) Identify the layer that provides the greatest resistance to the flow of heat.
Figure: Temperature profile through a composite plane wall
Q2A 3kW electric water heater is a cylindrical element having outside diameter 10mm. The heat transfer between the heater and the surrounding water is 980W/m2K.
Determine the length of the heater if the surface temperature must not exceed 95oC when the water is at 65oC.
The element comprises an electric resistance heater, diameter 2mm in the centre of the 10mm diameter stainless steel tube which has a wall thickness of 1mm. The gap between the resistance heater and the inside of the tube is packed with a ceramic material, Thermal conductivity 2W/mK.
Determine the temperature of the electric heater element. You may assume that the heater element is at uniform temperature and the thermal resistance between the element and the ceramic is negligible. The thermal conductivity of the stainless steel is 16W/mK.