Heat Transfer Rate (\( Q_k \)):

\[ Q_k = \frac{A \cdot (T_1 - T_4)}{\frac{x_{12}}{k_{12}} + \frac{x_{23}}{k_{23}} + \frac{x_{34}}{k_{34}}} \]

Where:

• \( Q_k \) is the heat transfer rate, in watts (W).
• \( A \) is the surface area, in square meters (m²).
• \( T_1 \) is the inside wall temperature, in kelvin (°K).
• \( T_4 \) is the outside wall temperature, in kelvin (°K).
• \( x_{12} \) is the thickness between surface 1 and 2, in meters (m).
• \( x_{23} \) is the thickness between surface 2 and 3, in meters (m).
• \( x_{34} \) is the thickness between surface 3 and 4, in meters (m).
• \( k_{12} \) is the thermal conductivity between surface 1 and 2, in watts per kelvin-meter (W/°K-m).
• \( k_{23} \) is the thermal conductivity between surface 2 and 3, in watts per kelvin-meter (W/°K-m).
• \( k_{34} \) is the thermal conductivity between surface 3 and 4, in watts per kelvin-meter (W/°K-m).