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Heat Transfer & Thermal Glossary
Key terms and definitions for thermal engineering.6 terms to help you understand our calculators.
Thermal Conductivity
A material property indicating how well it conducts heat, denoted as k with units W/(m·K). It represents the rate of heat transfer through a unit thickness of material per unit area per degree temperature difference.
R-Value (Thermal Resistance)
A measure of thermal resistance to heat flow, calculated as R = L/k where L is thickness and k is thermal conductivity. Higher R-value means better insulation. R-values are additive for layers in series.
U-Value (Thermal Transmittance)
The overall heat transfer coefficient, calculated as U = 1/R_total. It represents the rate of heat transfer per unit area per degree temperature difference through a complete building element or assembly.
Coefficient of Thermal Expansion (CTE)
A material property indicating how much a material expands per unit length per degree of temperature change, denoted α. Units are 10⁻⁶/°C (ppm/°C). Linear expansion is calculated as ΔL = α × L₀ × ΔT.
Thermal Stress
Stress that develops in a material when thermal expansion or contraction is constrained. Calculated as σ = E × α × ΔT, where E is elastic modulus, α is CTE, and ΔT is temperature change. Can cause yielding or buckling.
Expansion Loop
A U-shaped or Z-shaped configuration in piping that absorbs thermal expansion through bending rather than developing axial stress. The loop flexes as the pipe expands, keeping stresses within acceptable limits.