Operating Above Design Firing Rates. Another reason for increased heat loss
compared with design could be operating the furnace at a higher firing rate than design.
The refractory thickness is designed based on inside surface temperature, thermal
conductivity of the refractory material, ambient temperature, and wind conditions.
Many plants operate the furnaces at higher than original design throughput, gradually
pushing the firing rate above the design and causing the firebox temperature to increase.
If the refractory thickness or material is not changed correspondingly, this will lead to
higher skin temperatures and higher heat loss.