Heat Transfer Coefficient Converter

Heat Transfer Coefficients in Engineering Design

The heat transfer coefficient (h or U) quantifies the efficiency of heat exchange between a fluid and a surface, or between two fluids through a separating wall. It is defined by Newton's Law of Cooling: q = h·A·ΔT, where q is heat transfer rate, A is area, and ΔT is temperature difference. The SI unit is watts per square metre per kelvin, W/(m²·K). The US Customary equivalent is BTU per hour per square foot per degree Fahrenheit, BTU/(hr·ft²·°F).

The heat transfer coefficient is one of the most important parameters in process engineering, HVAC system design, and building energy analysis. It governs the sizing of heat exchangers in power plants, petrochemical facilities, food processing, and data centres. Accurate conversion between W/(m²·K) and BTU/(hr·ft²·°F) is essential when using US equipment data sheets alongside European design codes, or when comparing ASHRAE-based HVAC specifications with UK Building Regulations Part L.

Types: Convective, Radiative, and Overall

The convective heat transfer coefficient (h) describes heat transfer between a flowing fluid and a solid surface. It depends on fluid properties, flow velocity, and surface geometry. Free convection of air over a vertical wall gives h ≈ 5–25 W/(m²·K); forced air convection gives h ≈ 25–250 W/(m²·K); water in turbulent flow gives h ≈ 300–20,000 W/(m²·K). These values directly determine whether natural convection cooling is sufficient or forced cooling is required.

The overall heat transfer coefficient (U) combines all thermal resistances in series: convection on one side, conduction through the wall, and convection on the other side. For a double-glazed window, U ≈ 1.4–2.0 W/(m²·K) is the combined resistance of indoor air film, glass panes, argon fill, and outdoor air film. UK Building Regulations Part L and ASHRAE 90.1 both specify maximum U-values for building envelope elements, using W/(m²·K) and BTU/(hr·ft²·°F) respectively.

Heat Exchangers: Shell-and-Tube and Plate

Heat exchanger design requires knowledge of the overall heat transfer coefficient U for the specific fluid pair and geometry. For a water-to-water shell-and-tube heat exchanger, U typically ranges from 800–1,800 W/(m²·K) (140–317 BTU/(hr·ft²·°F)). For a gas-to-gas heat exchanger, U is much lower: 10–40 W/(m²·K). These values determine the required heat transfer area (A = Q/(U·LMTD)), and hence the physical size and cost of the equipment.

The UK and US process engineering industries use both unit systems. TEMA (Tubular Exchanger Manufacturers Association, US) standards specify U-values in BTU/(hr·ft²·°F); European EN 13445 standards use W/(m²·K). An engineer sizing a heat exchanger for a cross-border project must convert fluently between these units. The conversion factor: 1 BTU/(hr·ft²·°F) = 5.67826 W/(m²·K).

Building Façades and Glazing

For building engineers, the overall heat transfer coefficient U is the primary metric for window and wall thermal performance. The UK Window Energy Rating (WER) system and BFRC (British Fenestration Rating Council) publish U-values in W/(m²·K). ENERGY STAR (US) and NFRC (National Fenestration Rating Council) publish U-factors in BTU/(hr·ft²·°F). A triple-glazed window with UK U-value of 0.6 W/(m²·K) corresponds to a US U-factor of 0.6 ÷ 5.678 = 0.106 BTU/(hr·ft²·°F).

Building information modelling (BIM) software such as Revit and IES VE supports both unit systems, but thermal analysis plugins (EnergyPlus for US, SAP/SBEM for UK) require consistent unit input. Architects working on international projects import manufacturer data sheets from the US (BTU/(hr·ft²·°F)) and must convert to W/(m²·K) for UK compliance calculations.

Boiling and Condensation

Phase-change heat transfer (boiling and condensation) achieves extremely high heat transfer coefficients: nucleate boiling of water on a heated surface reaches h = 5,000–100,000 W/(m²·K); film condensation of steam gives h = 4,000–25,000 W/(m²·K). These high values are exploited in steam condensers, refrigeration evaporators, and heat pipe technology. Power plant condensers operating at these coefficients can transfer enormous quantities of heat through compact surface areas, and their design relies on accurate h-value data in consistent units.