Heat Transfer: Conduction, Convection and Radiation
Heat moves in three ways: through solids by conduction, with moving fluids by convection, and across space by radiation. A boiler uses all three — radiation in the furnace, convection in the tube banks, conduction through every metal wall — and each has its own rule.
You'll learn
- Describe conduction, convection and radiation
- Write the basic equation for each
- Know where each dominates in a boiler
- See why a thin layer of scale matters so much
Before this: Units and temperature scales, Laws of thermodynamics and entropy
Three ways heat moves
- Conduction: heat passing through a solid, molecule to molecule — through a tube wall, a layer of scale, insulation.
- Convection: heat carried by a moving fluid — flue gas sweeping over tubes, water circulating inside them, air over a pipe.
- Radiation: heat travelling as infrared light, needing no contact — from the flame and hot gas to the furnace walls, from a hot pipe to its surroundings.
In a boiler furnace, radiation from the flame does most of the work. In the tube banks further on, where the gas is cooler, convection takes over. Through every tube wall, heat is conducted from the gas side to the water side.
The three equations
| Symbol | Meaning | Unit |
|---|---|---|
| Thermal conductivity of the material | W/(m·K) | |
| Thickness | m | |
| Heat flow per unit area | W/m² |
| Symbol | Meaning | Unit |
|---|---|---|
| Heat transfer coefficient, set by the fluid and how fast it moves | W/(m²·K) |
| Symbol | Meaning | Unit |
|---|---|---|
| Emissivity of the surface, 0 to 1 | — | |
| 5.67 × 10⁻⁸ | W/(m²·K⁴) | |
| Absolute temperatures | K |
Radiation grows with the fourth power of absolute temperature. A perfect radiator at 1,000 °C emits 149 kW per square meter; at 500 °C, 20. That steep rise is why the hottest part of a furnace does so much of the boiler’s work.
Resistances in series, and why scale matters
Heat passing from flue gas to water crosses several layers in turn: the gas-side film, any soot, the steel wall, any scale, and the water-side film. Each resists the flow, and their resistances add, like resistors in series. The largest resistance controls the total.
- Steel tube wall 6 mm thick, conductivity about 50 W/(m·K). Scale 1 mm thick, conductivity about 1 W/(m·K) (typical approximate values).
- Resistance of each layer is thickness ÷ conductivity:
Convection coefficients vary over a huge range: condensing steam and boiling water transfer heat very readily, liquids moderately, gases poorly. That is why the gas side, not the water side, usually limits a boiler’s heat transfer, and why gas-side surfaces are extended with fins in economizers and air heaters.
Key takeaways
- Conduction through solids, convection by moving fluids, radiation across space.
- q = (k/t)ΔT; q = hΔT; q = εσ(T₁⁴ − T₂⁴) with T in kelvin.
- Radiation dominates the furnace; convection the tube banks.
- Resistances add in series; a thin layer of scale or soot can dominate.