Heat Losses at 70°F Ambient
(Click on graph to enlarge.)
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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)
Losses (W/in2) =  T (°F) rise above ambient
200
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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.)
Losses (W/in2) = T (°F) rise above ambient
950
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