Analysis of variable air volume unit pressure sensor factors with specific reference to measurement of low air velocity

Abstract

The purpose of this study was to identify the optimal construction of an averaging pitot tube and measure flows typical in building ventilation, considering low-velocity flows. The construction should be simple to implement in variable air volume (VAV) controllers. This paper presents mostly novel constructions, which have not been tested before, and fills the existing gap in low-velocity measurements, which is important considering the correct control with airflow reduction in the VAV system. Eleven constructions based on a circular section of the basic element were chosen. Empirical studies were conducted to determine the aerodynamic properties. The tests were conducted within a typical velocity range in ventilated systems (0.6–7.5 m/s), based on measuring the pressure difference at suitable locations on the upstream and downstream surfaces. The magnification factor (m) was obtained and compared. Sensors consisting of two parallel and slightly separated tubes from each other with upstream and sideward ports on the inner side of the sensor were characterised by a constant and highest value of the factor m (approximately 2.0). These sensors yielded approximately 25% higher values of m than the lowest values. The best sensor constructions among the tested structures were the variants with two parallel tubes with front and side ports from the inner side of the sensor. The application of narrowing and placing of the side port enabled a stable operation in the highest range of velocity.