Heat Loss Factors and Graphs
(Click on graphs to enlarge.)
Heat Losses from Uninsulated Surfaces
Heat Losses at 70°F Ambient
How to use the graph for more accurate calculations
Convection curve correction factors:
For losses from top surfaces or from horizontal pipes
- Multiply convection curve value by 1.29
For side surfaces and vertical pipes
- Use convection curve directly
For bottom surfaces
- Mulitply convection curve value by 0.63
Radiation Curve Correction Factors
The radiation curve shows losses from a perfect blackbody and are not dependent upon position. Commonly used block materials lose less heat by radiation than a blackbody, so correction factors are applied. These corrections are the emissivity (e) values.
Total Heat Losses =
- Radiation losses (curve value times e)
- + Convection losses (top)
- + Convection losses (sides)
- + Convection losses (bottom)
- = Conduction losses (where applicable)
Helpful Hint:
The graphs for losses from uninsulated and insulated surfaces are hard to read at low temperatures close to ambient. Here are two easy-to-use calculations that are only rule-of-thumb approximations, but they are reasonably accurate when used within the limits noted.
Rule #1:
Losses from an uninsulated surface (with an emissivity close to 1.0): (This applies only to temperatures between ambient and about 250ºF)
Rule #2:
Losses from an insulated surface: (This insulation is assumed to be 1 inch thick and have a K-value of about 0.5 Btu-in/hr-ft2-°F. Use only for surfaces less than 800°F.)
Some Material Emissivities/Metals
| Material | Specific Heat Btu/lb.-°F | Emissivity | ||
|---|---|---|---|---|
| Polished Surface | Medium Oxide | Heavy Oxide | ||
| Blackbody Aluminum Brass Copper Inocoloy®800 |
0.24 0.10 0.10 0.12 |
0.09 0.04 0.04 0.20 |
0.75 0.11 0.35 0.03 0.60 |
1.00 0.22 0.60 0.65 0.92 |
| Inconel® 600 Iron, Cast Lead, Solid Magnesium Nickel 200 |
0.11 0.12 0.03 0.23 0.11 |
0.20 - - - - |
0.60 0.80 0.28 - - |
0.92 0.85 - - - |
| Nichrome,-80-20 Solder, 50-50 Steel mild stainless 304 stainless 430 |
0.11 0.04 0.12 0.11 0.11 |
- - 0.10 0.17 0.17 |
- - - 0.75 0.57 0.57 |
- - 0.85 0.85 0.85 |
| Tin Zinc |
0.056 0.10 |
- - |
- 0.25 |
- - |
Some Material Emissivities/Non-Metals
| Material | Specific Heat Btu/lb.-°F |
Emissivity |
|---|---|---|
| Asbestos Asphalt Brickwork Carbon Glass |
0.25 0.40 0.22 0.20 0.20 |
Most Non-Metals: 90 |
| Paper Plastic Rubber Silicon Carbide Textiles Wood, Oak |
0.45 0.2-0.5 0.40 0.20-0.23 - 0.57 |
Heat Losses from Insulated, Water & Metal Surfaces
- Based upon combined natural convection and radiation losses into 70ºF environment.
- Insulation characteristics
k = 0.67 @ 200ºF
k = 0.83 @ 1000ºF. - For molded ceramic fiber products and packed or tightly packed insulation, losses will be lower than values shown.
For 2 or 3 inches Insulation:
multiply by 0.84
For 4 or 5 inches Insulation:
multiply by 0.81
For 6 inches Insulation:
multiply by 0.79
Heat Losses from Oil or Paraffin Surfaces
Wind Velocity Effects on Surfaces