Factors affecting heat transfer and its applications

The rate of heat transfer depends on the temperature difference, the area available, the properties of the material or fluid, and the geometry of the flow path.

Factors affecting heat transfer

• Conduction: temperature difference, area normal to heat flow, thickness of the material and its thermal conductivity $k$. A larger $\Delta T$, larger $A$, larger $k$ and smaller thickness give a higher rate.
• Convection: temperature difference between surface and fluid, surface area, fluid velocity, fluid properties (density, viscosity, specific heat, conductivity), type of flow (laminar or turbulent), shape and roughness of the surface, and whether convection is natural or forced.
• Radiation: surface temperature (fourth-power effect), surface area, emissivity and absorptivity, the surface colour and finish, and the geometry and orientation of surfaces facing each other (shape factor).

Heat transfer can be increased by raising $\Delta T$, increasing area (fins) or the coefficient (higher velocity), and decreased by insulation, reflective shields or lower $k$.

Engineering applications

1. Boilers, condensers and other heat exchangers in power plants.
2. Cooling of IC engines (radiator, cooling fins on motorcycle cylinders).
3. Refrigeration and air conditioning (evaporator and condenser coils).
4. Cooling of electronic components with heat sinks, and thermal insulation of buildings, pipes and cold storages.