ABSTRACT
The air-moving mechanism is usually a power-driven exhaust fan, either of the centrifugal type or the axial flow type. It may consist of an air ejector 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 the discussion. Slippage is characteristic of all exhaust fans, and the amount of slippage is dependent on the pressure the fan is required to develop. There are occasionally important specific applications where an air injector has particular advantages over a fan for moving air. The airflow can be measured at the outlet by a simple bent tube that will register total pressure, about equal in this case to velocity pressure. The static pressure can be measured in the wall of the inlet duct as described for measurement of airflow at the inlet to exhaust hoods.
