Page 250 - Mechanical Engineers' Handbook (Volume 4)

8 Heat Transfer 239

Figure 21 Overall heat-transfer coefficients for gas and solid radiation, as function of gas emissivity
and wall-to-load area ratio, for uniform gas or wall temperature, compared to blackbody radiation. 1

Thermal conduction in some solid materials is a combination of heat flow through the
material, radiation across internal space resulting from porosity, and convection within in-
dividual pores or through the thickness of porous layers.
Conductivities of refractory and insulating materials tend to increase with temperature,
because of porosity effects. Values for most metals decrease with temperature, partly because
of reduced density. Conductivity coefficients for some materials used in furnace construction
or heated in furnaces are listed in Table 5.
A familiar problem in steady-state conduction is the calculation of heat losses through
furnace walls made up of multiple layers of materials of different thermal conductivities. A
convenient method of finding overall conductance is to find the thermal resistance (r/k
thickness/conductivity in consistent units) and add the total for all layers. Because conduc-
tivities vary with temperature, mean temperatures for each layer can be estimated from a