Over the years, both furnace capacity and radiant heat intensity have increased. Most
modern furnaces operate with firebox temperatures in excess of 2200 °F and could have fuel-firing rates in excess of 400 MBtu/h (400 million Btu/h) in each furnace. Since
firebox temperature has increased over the years, it is even more important to minimize
heat loss from the firebox with sufficient insulation to reduce the external skin
temperature. A combination of high temperature brick and ceramic fiber insulation
with 13 in. thickness, designed for 2600 °F, is not uncommon in modern furnaces. The
choice of modern refractory is discussed in detail by Thorpe [3]. Firebox floors are
typically lined with 2600 °F castable insulation material. Typically, the bottom 10–12 ft
of the firebox is lined with brick due to the possibility of erosion from burners and the
possibility of refractory damage due to proximity to the entry door. The middle and upper
portion of the firebox, radiant roof, and transition into convection section are lined with
ceramic fiber, as shown in Figure 9.5.