Page 236 - Mechanical Engineers' Handbook (Volume 4)

7 Thermal Properties of Materials 225

Thermal conductivities vary with temperature, usually inversely for iron, steel, and some
alloys, and conversely for common refractories. At usual temperatures of use, average values
of k in Btu/(ft hr F) are in Table 5.
To expedite calculations for nonsteady conduction of heat, it is convenient to use the
factor for ‘‘thermal diffusivity,’’ defined as

k Thermal conductivity
D
dC Density Specific heat
in consistent units. Values for common furnace loads over the usual range of temperatures
for heating are:

Carbon steels, 70–1650 F 0.32
70–2300 F 0.25
Low-alloy steels, 70–2000 F 0.23
Stainless steels, 70–2000 F
300 type 0.15
400 type 0.20
Aluminum, 70–1000 F 3.00
Brass, 70/30, 70–1500 F 1.20

In calculating heat losses through furnace walls with multiple layers of materials with
different thermal conductivities, it is convenient to add thermal resistance R r/k, where r
is thickness in ft. For example,

r k r/k
9-in. firebrick 0.75 0.9 0.833
1
4 ⁄2-in. insulating firebrick 0.375 0.20 1.875
1
2 ⁄4-in. block insulation 0.208 0.15 1.387
Total R for wall materials 4.095
Overall thermal resistance will include the factor for combined radiation and convection
from the outside of the furnace wall to ambient temperature. Wall losses as a function of
wall surface temperature, for vertical surfaces in still air, are shown in Fig. 7, and are included
in the overall heat loss data for furnace walls shown in the chart in Fig. 8.

Table 5 Average Values of k (Btu/ft hr F)

Mean Temperature ( F)
100 1000 1500 2000 2500
Steel, SAE 1010 33 23 17 17
Type HH HRA 8 11 14 16
Aluminum 127 133
Copper 220 207 200
Brass, 70/30 61 70
Firebrick 0.81 0.82 0.85 0.89 0.93
Silicon carbide 11 10 9 8 6
Insulating firebrick 0.12 0.17 0.20 0.24