Lesson 7 - Radiation and Combined SAQ

** Calculate the overall heat transfer coefficient through a multi-layer wall. From the outside moving in, the wall contains a 100 mm brick section, a 50 mm insulation section, a 15 mm cement section, and a 5 mm plaster finish section. The film heat transfer coefficients for the inside and outside walls are, respectively, 8 and 34 Wm^-2 K^-1. The thermal conductivities of the materials are listed in the table below.

Response:

We first need to determine the equation for the overall heat transfer coefficient, U. The equation needs to have a term for each layer of resistance to heat transfer. In this scenario we have convective heat loss into the wall, conductive heat loss through each layer of the wall and convective again into the air. Therefore, the equation for U is:

It is important to keep track of the material thickness and thermal conductivity for each term. Entering the values, we have:

At this point, it would be good practice to alter the problem on your own to see the change in the overall heat transfer coefficient. Perhaps you add another layer of insulation or increase a layer thickness? What would happen to the rate of heat transfer?