Heat Transfer Coefficient Calculator

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.

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.

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.

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.

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.

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.

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).

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.