Heat Transfer Coefficient Calculator

The overall heat transfer coefficient, commonly denoted as U, relies on the principle of thermal resistance in series. Just as electrical circuits resist current flow, multi-layered walls resist heat flow. The total thermal resistance (R_total) is the sum of individual conductive resistances of each material layer plus the convective resistances at the inner and outer fluid boundaries. Mathematically, the individual conductive resistance for a layer is calculated as R_layer = t / k, where t is the layer thickness and k is the thermal conductivity. Convective boundary resistances are determined by taking the inverse of the respective film coefficients (1/h_i and 1/o_o). The overall heat transfer coefficient is simply the inverse of this total resistance: U = 1 / R_total.

Consider a simple double-layer wall exposed to indoor and outdoor air. We have an inside convective heat transfer coefficient (h_i) of 10 W/m²K and an outside coefficient (h_o) of 25 W/m²K. The wall consists of a 0.15 m thick concrete layer with a thermal conductivity (k_1) of 1.4 W/m·K, followed by a 0.05 m thick insulation layer with a thermal conductivity (k_2) of 0.04 W/m·K. First, calculate the inner and outer film resistances: R_in = 1/10 = 0.10 m²K/W and R_out = 1/25 = 0.04 m²K/W. Next, compute the conductive resistance of the concrete: R_concrete = 0.15 / 1.4 = 0.107 m²K/W. Then, compute the insulation resistance: R_insulation = 0.05 / 0.04 = 1.25 m²K/W. Summing all resistances gives R_total = 0.10 + 0.107 + 1.25 + 0.04 = 1.497 m²K/W. Finally, the overall heat transfer coefficient U = 1 / 1.497 = 0.668 W/m²K.

Always ensure your input units are consistent—converting millimeters to meters before dividing by thermal conductivity is a common requirement to avoid massive calculation errors. Pay close attention to boundary layer conditions, as fluid velocity, surface orientation, and roughness heavily influence internal and external film coefficients (h_i and h_o). When dealing with industrial piping or cylindrical vessels, remember that flat-wall formulas must be adjusted to account for changing radial surface areas.