A consideration of the engineering factors that describe the functioning of the air-moving mechanism (e.g., fan) used with the hoods and connecting duct system completes the assembly of a basic exhaust system. The air-moving mechanism is usually a power-driven exhaust fan, either of the centrifugal type or, occasionally, the axial flow type. It may, however, consist of an air ejector (or injector) using a centrifugal blower or compressed air for jet air, or, if the contaminated air is hot, a natural draft chimney may serve the purpose. Roof ventilators operating on natural draft or by power-driven fans also properly belong in this discussion.