Page 237 - Mechanical Engineers' Handbook (Volume 4)

226 Furnaces

Figure 7 Furnace wall losses as a function of surface temperature. 1

The chart in Fig. 9 shows the thermodynamic properties of air and flue gas, over the
usual range of temperatures, for use in heat-transfer and fluid flow problems. Data for other
gases, in formula form, are available in standard references.
Linear coefficients of thermal expansion are the fractional changes in length per F
change in temperature. Coefficients in terms of 10 net values are listed below for materials
6
used in furnace construction and for the usual range of temperatures:
Carbon steel 9
Cast HRA 10.5
Aluminum 15.6
Brass 11.5
Firebrick, silicon carbide 3.4
Silica brick 3.4

Coefficients for cubical expansion of solids are about 3 linear coefficients. The cubical
coefficient for liquid water is about 185 10 .
6

8 HEAT TRANSFER
Heat may be transmitted in industrial furnaces by radiation—gas radiation from combustion
gases to furnace walls or direct to load, and solid-state radiation from walls, radiant tubes,
or electric heating elements to load—or by convection—from combustion gases to walls or
load. Heat may be generated inside the load by electrical resistance to an externally applied
voltage or by induction, with the load serving as the secondary circuit in an alternating
current transformer. Nonconducting materials may be heated by dielectric heating from a
high-frequency source.