Example Data Table
| Scenario | Mode | Key inputs | Computed net Q |
|---|---|---|---|
| Heated panel | Surroundings | ε=0.90, A=1.50 m², Ts=200°C, T∞=25°C | ≈ 1,545 W |
| Facing plates | Two-surface | A1=1.0 m², A2=1.0 m², ε1=0.8, ε2=0.6, F12=1.0, T1=400K, T2=300K | ≈ 518 W |
| Low emissivity shield | Surroundings | ε=0.15, A=2.00 m², Ts=150°C, T∞=25°C | ≈ 211 W |
Formula Used
- σ = 5.670374419×10⁻⁸ W·m⁻²·K⁻⁴
- Temperatures must be in Kelvin in the equation
- Q > 0 indicates net heat leaving the surface
How to Use This Calculator
- Select a mode based on your physical setup.
- Choose temperature units and preferred output units.
- Enter emissivity, areas, and temperatures carefully.
- For two surfaces, provide an appropriate view factor.
- Enable optional outputs for flux, breakdown, or sensitivity.
- Press Submit and review results above the form.
- Download CSV or PDF for documentation and sharing.
Engineering ranges you can test quickly
Thermal radiation grows with the fourth power of absolute temperature, so it becomes decisive in hot equipment. A surface at 600 K emits about 16× the blackbody power of the same area at 300 K. For a 1 m² black surface at 500 K, σT⁴ is roughly 3.5 kW/m², while at 350 K it is about 0.85 kW/m². Use the surroundings mode to estimate net heat loss from heaters, kiln doors, exhaust manifolds, or radiant panels before CFD.
How emissivity changes results
Emissivity ε scales radiative exchange for diffuse-gray surfaces and can vary by finish, oxidation, and coating. Holding temperatures and geometry constant, increasing ε from 0.20 to 0.90 increases net radiation by about 4.5×. Typical ranges used in preliminary design are: polished aluminum 0.03–0.10, stainless steel 0.10–0.30, oxidized steel 0.70–0.90, and matte paint 0.85–0.95. If ε is uncertain, run two cases to bracket best and worst performance.
Why Kelvin conversion matters
The Stefan–Boltzmann relationship requires Kelvin. A small input mistake becomes large because temperature is raised to the fourth power. For example, 400°C equals 673.15 K; using “400” as Kelvin would underpredict emission by roughly (400/673.15)⁴ ≈ 0.12, an 88% error. The calculator converts °C and °F to Kelvin internally so you can report results consistently across teams and specifications.
Two-surface exchange and view factor
When two surfaces exchange radiation, geometry is captured by the view factor F12. Closely facing, parallel plates can have F12 near 1, while small targets, large separations, or angled surfaces may be far lower. The two-surface mode uses a radiation-resistance network that combines surface resistances with the space resistance 1/(A1·F12). This is useful for estimating shield performance, enclosure heat transfer, and radiator-to-panel coupling in assemblies.
Interpreting outputs for design checks
Use net Q to size insulation, estimate cooling loads, or compare coatings. Heat flux (W/m²) helps evaluate local limits and hot-spot risk. The emitted-versus-absorbed breakdown clarifies whether the surroundings drive heating or cooling. Sensitivity (W/K) indicates how quickly Q shifts with temperature drift; multiplying sensitivity by an expected ΔT provides a fast “what-if” estimate. For mixed-mode problems, combine this result with convection and conduction in an energy balance.
FAQs
1) What does a positive net heat transfer mean?
A positive value means the hotter surface is losing heat by radiation to the surroundings or to the cooler surface. A negative value indicates net radiative heating of the surface from its environment.
2) Can I use this for very high temperatures?
Yes, as long as temperatures are entered correctly and converted to Kelvin internally. At high temperatures, validate emissivity data, surface oxidation, and whether the “diffuse-gray” assumption is reasonable for your materials.
3) What view factor should I use if I don’t know it?
Start with a conservative estimate based on geometry: near 1 for closely facing plates, lower for small targets or large gaps. For critical designs, compute F12 from standard view-factor formulas or specialized tools.
4) Why is radiation sometimes larger than convection?
Radiation scales with T⁴ and does not require a fluid boundary layer. Above a few hundred Kelvin temperature difference, radiative exchange can exceed typical convection coefficients, especially for high-emissivity surfaces and large areas.
5) Does this include conduction through shields or insulation?
No. This tool focuses on radiative heat transfer. If your system has insulation, contact resistance, or multilayer shields, you should model conduction and radiation together using a combined thermal-resistance network.
6) How accurate are the CSV and PDF downloads?
They reproduce the on-screen results and key details for reporting. The CSV is plain text for spreadsheets. The PDF is a visual capture of the results panel, so it matches what you see in your browser.