I. Heat Transfer Without a Medium
Both conduction and convection require material contact, but radiation does not—it transfers heat via electromagnetic waves (mainly infrared) and can propagate even in a vacuum. The sun delivering heat to the Earth is radiation. In industry, every high-temperature surface continuously radiates heat outward.
II. The Fourth-Power Law: Out of Control Once Temperature Rises
The blackbody radiation heat flux is given by the Stefan-Boltzmann law:
q = ε · σ · T⁴
σ is a constant, ε is the emissivity (0~1), and T is the absolute temperature. The key is the fourth power: when a surface rises from 200°C (473 K) to 600°C (873 K), the radiation heat flux jumps to about (873/473) to the fourth power ≈ 11.6 times. This is why low- and medium-temperature equipment loses heat mainly by convection, while furnace bodies above 600°C quickly see radiation become dominant.
III. Radiation Scenarios in Industry
- Annealing furnaces, forging furnaces, glass furnaces: Radiation between furnace wall and workpiece is the main heating/loss path;
- High-temperature flue outer shells: Bare piping at 500°C radiates astonishingly, requiring insulation;
- Infrared drying: Actively uses radiation to directly heat the material surface, efficient and uniform.
IV. How to Suppress Unwanted Radiation
For radiation heat you do not want to lose, combine three measures:
- Insulation layer: Lower the outer surface temperature, and the fourth power of T drops immediately;
- Low-emissivity coating: Polished metal, aluminum foil, etc. have small ε and weak radiation;
- Radiation shield/sunshade: Multiple low-emissivity thin sheets inserted between the heat source and the cold surface, reducing net radiation layer by layer.
Conversely, to recover the radiation heat in high-temperature flue gas, an air-to-air heat exchanger must design its wall temperature to capture this part of radiation and prevent ash accumulation on the wall—ash layers both reduce conduction and create emissivity chaos, an invisible killer for high-temperature heat exchangers.