What is the heat transfer coefficient?
The heat transfer coefficient (h) quantifies the convective heat transfer between a surface and a fluid. It represents the amount of heat transferred per unit area, per unit time, per unit temperature difference, expressed in W/m²K. A higher value means more efficient heat transfer between the fluid and the surface. See also our calculate Junction Temperature.
What is the overall heat transfer coefficient (U)?
The overall heat transfer coefficient U accounts for all thermal resistances in series — inner convection, conduction through each material layer, and outer convection. It is the reciprocal of the sum of all individual thermal resistances (per unit area). U is widely used in heat exchanger and building insulation design.
How do I calculate the heat transfer coefficient?
The overall U-value is calculated as: 1/U = 1/h₁ + Σ(Lᵢ/kᵢ) + 1/h₂, where h₁ and h₂ are the inner and outer convection coefficients, and Lᵢ/kᵢ is the conduction resistance of each material layer (thickness divided by thermal conductivity). The heat transfer rate is then Q = U × A × ΔT.
What is thermal resistance?
Thermal resistance (R) is a measure of a material's opposition to heat flow, analogous to electrical resistance in circuits. For a flat wall, conduction resistance is R = L/k (thickness divided by thermal conductivity). For convection, R = 1/h. Total resistance for layers in series is simply the sum of all individual resistances. You might also find our calculate Thermodynamic Processes Work Done (W) useful.
How do I calculate thermal resistance?
For conduction through a flat wall: R_cond = L / k, where L is thickness (m) and k is thermal conductivity (W/mK). For convection: R_conv = 1/h, where h is the convection coefficient (W/m²K). For multiple layers and convection on both sides in series, add all resistances: R_total = 1/h₁ + L₁/k₁ + L₂/k₂ + ... + 1/h₂.
What are typical values for convection coefficients?
Free (natural) convection in air typically gives h ≈ 2–25 W/m²K. Forced convection in air is about 25–250 W/m²K. Forced convection in liquids ranges from 500–20,000 W/m²K, and boiling/condensation can reach 2,500–100,000 W/m²K. The exact value depends on fluid properties, velocity, and geometry.
What units are used for the heat transfer coefficient?
The SI unit for heat transfer coefficient is W/m²K (watts per square meter per Kelvin), which is equivalent to W/m²°C. In Imperial units, it is expressed as BTU/(h·ft²·°F). 1 W/m²K ≈ 0.176 BTU/(h·ft²·°F).
Where is the overall heat transfer coefficient used?
The U-value is used extensively in building insulation design (to meet energy codes), heat exchanger sizing (shell-and-tube, plate, etc.), refrigeration system design, cooling of electronic components, and any engineering application where heat flows through multiple layers or between fluids separated by a wall.