Uncertainty Studies of Airflow Measurements in Non-Ideal Conditions in Variable Air Volume Air Handling Units (ST-16-C033)

Methods like the Log-Tchebycheff and Equal Area are commonly used to define the average air velocity across a traverse section. The testing, adjusting and balancing (TAB) of HVAC systems has been adopting those methods to ensure that the installed system is meeting its design capacity. The flow measurements are compromised when space constraints limit the optimal air handling system design; consequently, inadequate straight ductwork with close upstream and downstream fitting disturbances cause common measurement issues. For example, according to ASHRAE 111-1988 field airflow measurements over CAV systems made by experienced technicians commonly have an error as much as 30% when recommended vane anemometers are used while reading irregular flows. Through in-situ airflow measurements in ten air-handling-units, this paper summarizes the statistical studies of measurement uncertainties to explain why the large errors occur in field measurements even though the standard procedures are strictly followed. As a result, a more accurate and robust in-situ airflow local measurement method is introduced in this paper. The proposed method uses hot-wire as the air velocity measurement device due to its proven accuracy at speeds lower than 800 fpm (1 m/s). In order to overcome additional turbulence in the ductwork that is caused by limited duct space, a holding device was also developed for facilitating the time weighted of local airflow measurements.