Local exhaust system is an effective but energy-consuming method for capturing oil mist particles in machining workshops. To reduce the flow resistance of an exhaust system for minimal fan energy consumption, the method of applying individually shape-optimized exhaust hoods, namely mass-production design, is feasible. However, the combined effect of multiple exhaust hoods in an exhaust system may not be optimal in reducing the flow resistance. This investigation thus firstly validated the shape optimization of an individual exhaust hood by a discrete adjoint method. The discrete adjoint method could adjust the shape of an exhaust hood automatically in the direction of reducing the flow resistance. The design variables were the coordinates of wall boundaries of the exhaust hood. The validation used measured data from a small-scale experiment. This study then applied the validated discrete adjoint method to conduct customized design through the shape optimization of multiple exhaust hoods simultaneously in the exhaust system. The flow resistance under customized design was compared with the method of mass-production design. The results revealed that the customized design led to different shapes of individual exhaust hoods and they were different from the shape of the individually optimized exhaust hood. The flow resistance of the exhaust system under customized design was reduced by 57%. However, only 36.5% reduction in flow resistance was achieved when the mass-production design method was employed. The customized design method was more effective in reducing flow resistance of the exhaust system.