Understanding Convective Heat Transfer: Coefficients, Formulas & Examples

Tool for understanding convective heat transfer, coefficients, and calculating heat transfer rates using Newton's Law of Cooling. Includes typical coefficients for various fluid flow applications.

Understanding Convective Heat Transfer

Convective heat transfer is the heat energy transferred between a surface and a moving fluid with different temperatures. This is a combination of diffusion and bulk motion of molecules.

Newton's Law of Cooling

The fundamental equation for convection heat transfer is:

q = hc × A × ΔT

Where:

  • q = heat transferred per unit time (W, Btu/hr)
  • hc = convective heat transfer coefficient (W/(m²°C), Btu/(ft² h °F))
  • A = heat transfer area of the surface (m², ft²)
  • ΔT = temperature difference between surface and bulk fluid (°C, °F)

Types of Convection

  • Forced/Assisted Convection: Fluid flow induced by external force (pump, fan, mixer)
  • Natural/Free Convection: Caused by buoyancy forces due to temperature-induced density differences
  • Boiling/Condensing: Also classified as convective heat transfer processes
Calculator

Inputs

Results

Heat Transfer Rate-- W
Heat Transfer Rate-- kW
Fluid Flow TypeMinimum hcMaximum hcUnit
Free Convection - Air/Gases/Dry Vapors0.51000W/(m²K)
Free Convection - Water/Liquids503000W/(m²K)
Forced Convection - Air/Gases/Dry Vapors101000W/(m²K)
Forced Convection - Water/Liquids5010000W/(m²K)
Forced Convection - Liquid Metals500040000W/(m²K)
Boiling Water3000100000W/(m²K)
Condensing Water Vapor5000100000W/(m²K)

Unit Conversion Factors

  • 1 W/(m²K) = 0.85984 kcal/(h m² °C) = 0.1761 Btu/(ft² h °F)
  • 1 Btu/(ft² h °F) = 5.678 W/(m² K) = 4.882 kcal/(h m² °C)
  • 1 kcal/(h m² °C) = 1.163 W/(m²K) = 0.205 Btu/(ft² h °F)

Convective Heat Transfer Coefficient for Air

For air flow, the convective heat transfer coefficient can be approximated using empirical equations (valid for velocities 2-20 m/s):

In kcal/m²h°C: hc = 10.45 - v + 10√v

In W/m²°C: hc = 1.16 (10.45 - v + 10√v)

Where v = relative speed between object surface and air (m/s)