BIZOMAT MAKEOS · Materials
Radiant Heat Loss
How much heat does this hot surface radiate to its surroundings?
Net radiant heat exchange between a surface and large surroundings from area, emissivity and temperatures (Stefan–Boltzmann law), with the equivalent radiation coefficient.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Surface area | m² | Radiating surface area. |
| Emissivity | Emissivity of the surface (oxidised steel about 0.8, painted 0.9, polished aluminium about 0.05). | |
| Surface temperature | °C | Surface temperature. |
| Surroundings temperature | °C | Temperature of the surrounding walls or space (large surroundings). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Surface area
- What it is
- The area of the hot surface that radiates to the surroundings.
- Units and format
- Enter a number in m²: square metres. The form accepts 0.000001 m² to 10000000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Radiant heat rises in direct proportion to area.
- Get it right
- Include only the surface that sees the surroundings, not faces hidden against other hot parts.
What this means: Emissivity
- What it is
- The emissivity of the surface: how well it radiates compared with a perfect black body, from zero to one.
- Units and format
- Enter a plain number with no unit. The form accepts 0.01 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Radiant heat rises in direct proportion to emissivity.
- Get it right
- It depends strongly on surface condition: polished, oxidised and painted surfaces differ a lot. Take it from surface property data.
What this means: Surface temperature
- What it is
- The temperature of the radiating surface itself, measured with a contact probe or a correctly set infrared thermometer.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -200 °C to 3000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 300 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Radiant heat rises steeply with surface temperature, as it depends on absolute temperature to the fourth power.
- Get it right
- Enter degrees Celsius; the tool converts to kelvin.
What this means: Surroundings temperature
- What it is
- The temperature of the large surroundings the surface radiates to, such as the walls of the room.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -200 °C to 3000 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the temperature of the surrounding surfaces, which may differ from the air temperature.
Method and formulas
- Q = ε σ A (Ts⁴ − Ta⁴), with σ = 5.670 × 10⁻⁸ W/m²K⁴ and temperatures in kelvin.
- Equivalent coefficient h_r = Q ÷ (A (Ts − Ta)).
- Example: 2 m² at 300 °C with ε 0.8 radiates about 9.0736 kW.
Limitations
- Grey surface facing large surroundings; view factors for nearby surfaces are not included.
- Convection adds to the total loss.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Estimate heat loss from furnace walls, ovens and hot parts
- Compare surface finishes (emissivity)
- Add radiation to convection estimates
Who it is for
- Process engineers
- Energy managers
- Maintenance engineers
What you get
- Net radiant heat
- Heat flux
- Equivalent radiation coefficient
Interpreting the result
- Low-emissivity cladding greatly reduces radiant losses from hot equipment.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 371–378. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Radiation and multilayer insulation
In Cryogenic Design by the Numbers
What it covers: Radiation from a warm vessel onto a cold mass, effective emissivity, the radiant heat tool for cold surfaces, multilayer insulation by CERN’s formula, and a quick test without it.
Method explained here · BIZOMAT original book
Heat leak and boil-off
In Cryogenic Liquids by the Numbers
What it covers: Turn a vessel's normal evaporation rate into watts, calculate a foam bath's heat leak with a mean conductivity, bound the open-lid loss by radiation, and find a bath left open between jobs.
Method explained here · BIZOMAT original book
Heat Transfer by the Numbers
What it covers: Calculate heat flows in plant: energy and power, conduction through walls and pipe insulation, convection, radiation, economic insulation, heat exchangers and heating and cooling times.
Worked example · BIZOMAT original book
Where the heat goes: available heat and the furnace load
What it covers: Follow a gas furnace's fuel into the load, the walls and the flue, calculate available heat from flue temperature and excess air, and plan a load's fuel, time and cost.
Worked example · BIZOMAT original book
Cold surfaces and the night sky
In Condensation Control by the Numbers
What it covers: Why surfaces fall below air temperature, how a clear night sky cools an enclosure roof by radiation, how much water then forms, and why a heater alone may not stop it.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.