Heat Transfer Coefficient Converter

Watt/Square Meter/Kelvin (W/(m²·K))

The SI unit of heat transfer coefficient. It represents the heat transfer rate per unit area per unit temperature difference. h = Q / (A × ΔT).

Common uses: International standards, scientific research, engineering calculations, HVAC design.

Watt/Square Meter/°C (W/(m²·°C))

Heat transfer coefficient using Celsius units. Since Kelvin and Celsius have the same interval size, W/(m²·°C) = W/(m²·K). Commonly used in regional standards.

Note: Equivalent to W/(m²·K) in magnitude.

Kilocalorie/Hour/Square Meter/°C (kcal/h/m²/°C)

Heat transfer coefficient using kilocalories and metric units. 1 kcal/(h·m²·°C) ≈ 1.163 W/(m²·K). Used in some regions and older engineering standards.

Common uses: Regional standards, historical engineering data, European applications.

BTU/Hour/Square Foot/°F (Btu/h/ft²/°F)

Heat transfer coefficient in British Thermal Units. 1 Btu/(h·ft²·°F) ≈ 5.678 W/(m²·K). Widely used in US engineering.

Common uses: US HVAC design, building thermal analysis, American engineering standards.

Convective Heat Transfer Equation

Heat transfer by convection is calculated using Newton's Law of Cooling: Q = h × A × ΔT

• Q: Heat transfer rate (Watts)
• h: Heat transfer coefficient (W/(m²·K))
• A: Surface area (m²)
• ΔT: Temperature difference (K or °C)

Typical Heat Transfer Coefficient Values

• Natural convection (air): 5-25 W/(m²·K)
• Forced convection (air): 10-100 W/(m²·K)
• Natural convection (water): 50-1000 W/(m²·K)
• Forced convection (water): 100-10,000 W/(m²·K)
• Boiling water: 3,000-100,000 W/(m²·K)
• Condensing steam: 5,000-100,000 W/(m²·K)
• Heat exchanger tube side: 100-5,000 W/(m²·K)
• Inside building surfaces: 7-10 W/(m²·K)

Factors Affecting Heat Transfer Coefficient

The heat transfer coefficient depends on many parameters:

• Fluid properties: Density, viscosity, thermal conductivity, specific heat
• Flow conditions: Velocity, flow regime (laminar/turbulent), flow pattern
• Surface characteristics: Roughness, orientation, material
• Temperature: Properties vary with temperature
• Phase change: Boiling or condensation increase h significantly

Dimensionless Analysis: Nusselt Number

The Nusselt number (Nu) is a dimensionless form of the heat transfer coefficient used in correlations: Nu = h × L / k

• h: Heat transfer coefficient
• L: Characteristic length dimension
• k: Fluid thermal conductivity

Use: Empirical correlations relate Nu to Reynolds and Prandtl numbers.

Overall Heat Transfer Coefficient

When multiple resistances are in series (e.g., heat exchanger): 1/U = 1/h₁ + R_wall + 1/h₂

• U: Overall heat transfer coefficient
• h₁, h₂: Individual heat transfer coefficients
• R_wall: Thermal resistance of wall

Note: Fouling and scale formation reduce overall heat transfer performance.

Common Applications

Heat transfer coefficients are critical in:

• HVAC Systems: Air-side and water-side design
• Heat Exchangers: Sizing and effectiveness calculations
• Cooling Systems: Electronics, engines, reactors
• Building Design: Window and envelope analysis
• Process Equipment: Reactors, boilers, condensers
• Power Generation: Turbine cooling, condenser design