Chapter 14, Problem 44P

Suppose the insulating qualities of the wall of a house come mainly from a 4.0-in. layer of brick and an R-19 layer of insulation, as shown in Fig. 14-19.What is the total rate of heat loss through such a wall, if its total area is 195 ft²and the temperature difference across it is 35 F°?

Brick Insulation

Problem 45 Two layers insulating a wall.

To determine

The rate of heat loss through a wall, if its total area is 195 ft² and temperature difference is 35⁰ C.

Answer to Problem 44P

Solution:

100 J/s.

Explanation of Solution

The temperature difference is $\Delta T={\text{35}}^0\text{C}$

Area of the wall=195 ft²

So,

$\begin{array}{l}\frac{dQ}{dt}=\frac{A\Delta T}{R}\\ \frac{dQ}{dt}=\frac{195{{\displaystyle \text{ft}}}^{\text{2}}\times {35}^{0}C}{(19+0.8){{\displaystyle \text{ft}}}^{\text{2}}\times h\times C^0/\text{Btu}}\\ \frac{dQ}{dt}=344.696\text{ Btu/h}\\ \frac{dQ}{dt}=344.696\times \frac{1055.056}{3600\text{ s}}\text{ J (As 1 Btu=1055}\text{.056 J)}\\ \frac{dQ}{dt}={{\displaystyle 10}}^2\text{ J/s}\end{array}$