In a previous study of an actual reheat furnace with a capacity of 45 ton/h, it was found that a staggering 20% of the input energy was unaccounted for in the heat balance. It was hypothesized that this missing energy was lost to the environment as heat losses. In this work, the main losses are identified and quantified. First, the heat transfer through the wall of the furnace was determined. For this, an extensive measurement campaign was performed. Based on the measured wall temperatures and emissivity values, the heat transfer from the walls for the operating conditions at the time of the measurements was estimated. The heat rejected through the walls amounts to approximately one fifth of the total heat loss. Secondly, when the furnace door is opened, a relatively large flow rate of hot gas leaves the furnace, and a net heat loss occurs due to the radiative heat exchange between the furnace interior and the environment. As the aforementioned heat losses are very difficult to measure, a simplified theoretical model was made based on physical principles. The corresponding results indicate that the opening of the furnace accounts for a large part of the remaining heat loss
