Heat Loss Transmission

Overview

Engineering reference data for Heat Loss Transmission in thermodynamics.

Key Formulas

First Law

$\Delta U = Q - W$

Energy is conserved — heat added minus work done.

Ideal Gas Law

$PV = nRT$

Relates pressure, volume, and temperature of an ideal gas.

Heat Transfer

$Q = mc\Delta T$

Sensible heat transfer.

Carnot Efficiency

$\eta = 1 - T_C/T_H$

Maximum efficiency between two temperatures.

Variables

Deeper Dive: U-value & R-value Calculations

The total thermal resistance ($R_{\text{total}}$) of a building assembly is the sum of the resistances of its individual layers and the surface film resistances. The overall U-value is the inverse of this total resistance.

For a multi-layer assembly: $R_{\text{total}} = R_{\text{si}} + \sum_{i=1}^{n} R_i + R_{\text{so}}$

Where:

• $R_{\text{si}}$ is the interior surface film resistance
• $R_{\text{so}}$ is the exterior surface film resistance
• $R_i$ is the resistance of the $i^{\text{th}}$ material layer

The U-value is then: $U = \frac{1}{R_{\text{total}}}$

The resistance of a single, homogeneous layer can be calculated from its thickness ($s$) and its thermal conductivity ($k$): $R_{\text{layer}} = \frac{s}{k}$

Example Calculation: For a concrete wall ($s = 0.25$ m, $k = 1.7$ W/m·K) with surface resistances of $R_{\text{si}} = 0.13$ m²K/W and $R_{\text{so}} = 0.04$ m²K/W:

1. Layer resistance: $R_{\text{concrete}} = \frac{0.25}{1.7} \approx 0.147$ m²K/W
2. Total resistance: $R_{\text{total}} = 0.13 + 0.147 + 0.04 = 0.317$ m²K/W
3. U-value: $U = \frac{1}{0.317} \approx 3.15$ W/m²K

Key Definitions

• U-value (U-factor): A measure of the rate of heat loss or gain through a building component. It represents the overall thermal transmittance (W/m²K or Btu/hr·ft²·°F). A lower U-value indicates better insulating performance.
• R-value: A measure of thermal resistance. It quantifies a material's ability to resist heat flow. A higher R-value indicates better insulating performance. For a single material layer, $R = s/k$.
• Inverse Relationship: U-value and R-value are inversely related for a homogeneous layer: $U = 1/R$. This relationship does not hold directly for multi-layer assemblies without considering the total R-value.

Interactive Charts

References

• Original Source