The electrostatic precipitator (ESP) is a dry dedusting system used for BOF primary gas cleaning. For efficient operation of the ESP a balanced off-gas flow over the entire cross section is important. Typical questions that can be answered by CFD concern the transient gas flow distribution inside the ESP. As the gas flow is affected by both the batch-type BOF operation and the routing of the off-gas duct, perforated flow rectifier plates are installed in the inlet region of the ESP to ensure a proper gas exchange. Furthermore, explosive gas mixtures of O2 and CO induced by the BOF batch process have to be avoided; CFD is used to predict the maximum allowable mixture resulting from varying gas properties (density, viscosity). Another approach is to calculate the maximum explosion pressure which may occur. Based on these data, the safety devices (number, size, and location of pressure vent valves) of the overall gas cleaning system are designed.