Analogy with the Theory of Electricity

Equation (4.154) gives conduction for the one-dimensional case with constant thermal conductivity  

Following from Ohm s electrical law (theory of electricity), a heat resistance can be defined  

For conduction the heat resistance is the distance divided by the heat conductivity, R = 8/X.A, and the heat conductance is heat conductivity divided by distance, U = X.A/8. For convection and radiation the heat resistance is 1 divided by the heat transfer factor, 1/aA, and the heat conductance is the same as the heat transfer factor, U aA. A coefficient of heat flow is also used, the K value, which is the total conductance  

The following connecting rules are based on the above analogy heat resistance R in series connection 

The heat resistance between the fluids on the two sides of the pipe wall in Fig. 4.25 is 

---