Modeling and parametric studies of heat transfer in a direct-fired batch reheating furnace

A mathematical system model of a direct-fired continuous reheating furnace has been developed. The furnace is modeled as several well-stirred gas zones with one-dimensional (1-D) heat conduction in the refractory walls and two-dimensional heat transfer in the load. The load travels either parallel or counterflow to the combustion gases which enter the furnace throughout its length. The convective heat-transfer rate to the load and refractory surfaces is calculated using existing correlations from the literature. Radiative heat exchange within the furnace is calculated using Hottel’s zone method by considering the radiant energy exchange between the load, the combustion gases, and the refractories. The nongray characteristics of the combustion gases are considered by using a four-gray gas model to treat the mixture as a radiatively participating medium. An extensive parametric investigation has been completed to determine the furnace design and operating characteristics that lead to optimum furnace efficiency. The parametric investigations included in this paper study the effects of the load and refractory emissivities and the height of the combustion space on the thermal performance of the furnace.