Ventilation and dust removal systems in industrial buildings handle particulate-laden exhaust flows. Centrifugal fans are the primary gas-moving equipment in these systems, and straight plate blades are widely used in them. These blades resist particulate deposition and erosive wear. Their aerodynamics are governed by the inlet clearance between the stationary inlet nozzle and the rotating impeller. Design and simulation routinely idealize it as clearance-free. A 12-blade fan of this type was studied numerically. Four clearance conditions were quantified over the full flow range. This idealization overestimated the design point total pressure efficiency by 1%. Overlapped clearance dominated the aerodynamic performance. A clearance of 20 mm approached the original model. A clearance of 30 mm, in contrast, reduced the efficiency to 50% at high flow. The axial clearance itself lowered the total pressure efficiency. Its size, however, barely mattered. The three axial clearance models remained within 77–78% at the design flow rate. A circumferentially non-uniform clearance lowered the design point efficiency to 79% and the high-flow efficiency to 32%. Eddy viscosity accumulation and passage blockage show that leakage-induced backflow sets these limits. These results provide a basis for controlling inlet clearance tolerances during the design and assembly of the straight plate blade fans.