Calculator Input
The page stays single-column, while the fields use a responsive three, two, and one column layout.
Plotly Graph
The graph shows how net radiation heat transfer changes as the hot surface temperature rises while the cold surface stays fixed.
Example Data Table
| Case | Model | Hot Temp | Cold Temp | Area | ε₁ | ε₂ | F₁₂ | Heat Rate (W) | Heat Flux (W/m²) |
|---|---|---|---|---|---|---|---|---|---|
| Furnace Wall → Room Panel | Two Gray Surfaces with View Factor | 700 C | 80 C | 1.8 m2 | 0.92 | 0.75 | 0.95 | 60,166.13 | 33,425.63 |
| Heater Plate → Facing Plate | Two Gray Parallel Plates | 450 C | 120 C | 2.2 m2 | 0.88 | 0.66 | 1.00 | 18,852.30 | 8,569.23 |
| Hot Pipe Jacket → Large Surroundings | Gray Surface to Large Black Surroundings | 300 C | 30 C | 12 ft2 | 0.79 | 1.00 | 1.00 | 4,967.41 | 4,455.73 |
| Blackbody Emitter → Target Surface | Two Black Surfaces | 900 K | 200 K | 3500 cm2 | 1.00 | 1.00 | 0.72 | 9,352.38 | 26,721.07 |
Formula Used
q = σ × A × F₁₂ × εeff × (Th4 − Tc4)
εeff = 1
εeff = ε₁
εeff = 1 / [(1 / ε₁) + (1 / ε₂) − 1]
Where:
- q = net radiation heat transfer rate in watts
- σ = Stefan–Boltzmann constant
- A = effective exchanging area in square meters
- F₁₂ = view factor between the two surfaces
- εeff = effective emissivity for the selected model
- Th and Tc = absolute temperatures in kelvin
This calculator focuses only on radiative exchange. It does not add conduction or convection losses.
How to Use This Calculator
- Enter names for the hot and cold surfaces.
- Select the radiation model that best matches your geometry.
- Choose the temperature unit and enter both temperatures.
- Enter the exchanging area and pick the correct area unit.
- Provide emissivity values for both surfaces.
- Enter the view factor, unless you use the parallel plate model.
- Click the calculate button to show the result above the form.
- Review the graph, metrics, and download the result as CSV or PDF.
FAQs
1) What does this calculator estimate?
It estimates net radiative heat transfer between two surfaces using temperature, area, emissivity, and view factor. It also reports heat flux, daily energy equivalent, and an effective radiation coefficient.
2) Why are temperatures converted to kelvin?
Radiation equations use absolute temperature. The Stefan–Boltzmann law depends on T⁴, so the calculation must use kelvin even when the input is entered in Celsius or Fahrenheit.
3) What is emissivity?
Emissivity measures how effectively a real surface emits thermal radiation compared with an ideal blackbody. It ranges from near zero to one, and higher values generally increase radiative heat exchange.
4) When should I use the gray surface model?
Use it when both surfaces are real engineering materials rather than perfect blackbodies. This is the most practical option for painted metals, ceramics, coatings, insulation jackets, and many thermal enclosure problems.
5) What is the view factor?
The view factor represents the fraction of radiation leaving one surface that directly reaches the other. It depends on geometry, spacing, orientation, and shielding between surfaces.
6) Why can the result become negative?
A negative value means the entered “cold” surface is actually hotter after unit conversion. The sign only indicates direction. The magnitude still represents the size of the radiative exchange.
7) Why does heat transfer rise sharply with temperature?
Radiative exchange follows a fourth-power temperature relation. Even modest increases in absolute temperature can produce much larger heat transfer changes than linear heat transfer mechanisms.
8) Does this include convection and conduction?
No. This page isolates radiation only. If your real system also loses heat by air motion, contact resistance, or structural conduction, you should calculate those modes separately and combine the results.