Heat Flow through a Wall

The composite wall is constructed from different materials in layers. The thickness of layers are, x1, x2,..., xn. The thermal conductivity of layers are,

2,...,

n. The fluid on one side of the wall is at temperature TA, and the heat transfer coefficient from fluid to wall is

A. The temperature and heat transfer coefficient for the fluid on the other side of the wall are TB and

B. By using Fourier's law of conduction and Newton's law of cooling, it can be shown that for a steady state heat transfer:

where,

dQ/dt= The transferred heat per unit time
A= Area of the wall

U which has the same unit as heat transfer coefficient is known as the overall heat transfer coefficient.