# Heat Loss Factors and Graphs

## 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

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)

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
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