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Heat Loss Factors and Graphs

Heat Losses from Uninsulated Surfacesheat 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: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: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 vertical insulated surfacesheat losses from horizontal metal surfacescombined convection and radiation losses from water surfaces

 



Heat Losses from Oil or Paraffin Surfaces


heat losses from oil or paraffin surfaces


 

Wind Velocity Effects on Surfaces


wind velocity effects on surfaces