Development of a Critical AirFlow Venturi for Air Sampling

Abstract

Air sampling is a key element in environmental studies and occupational exposure assessments. Airflow control and measurement is critical to the accuracy of air sampling. Commercially available flow controlling orifices were found to lack the required precision and accuracy. Most critical orifice designs require a pressure drop in excess of 47 kPa to ensure a stable flow. To achieve this pressure drop, a special high-power vacuum pump must be used. There is no existing design standard available for small critical orifices or venturi (throat diameter smaller than 12.5 mm). Many efforts have been made to experimentally develop a critical airflow control device with low pressure drop, but the procedure and the products are both labour-intensive and inaccurate. Based on the theory of fluid mechanics and experimental optimization, a series of unique critical airflow venturi have been developed to provide a constant air flow rate at low pressure drops (approximately 9 kPa), with variations of less than 1% of the full scale flow rate. These venturi can be widely used in air quality studies and other field for accurate gas flow rate control.