Airflow measurements for balancing of air distribution system — tracer-gas technique as an alternative?

Abstract

Traditional instrumentations such as, vane anemometers and pitot-tubes, have been widely used for measuring airflow rates in HVAC systems to balance air distribution system. Vane anemometers and pitot-tubes are used to measure air velocities at the supply diffusers and in the ducts, respectively. The airflow rates will be calculated with the knowledge of the cross-sectional area of the ductwork. The complexity of the air distribution system has posed a great problem for airflow measurements using pitot-tubes. Air velocity in a duct is seldom uniform across any section and a traverse is usually made to determine the average velocity. The measuring process is slow and errors can incur in the airflow measurements. The simple and useful tracer-gas techniques that are commonly used for ventilation measurements in building can be a good alternative to the pitot-static traverse method. This paper describes the advantages of tracer-gas technique over the traditional instrumentations. Airflow measurements using tracer-gas technique and pitot-static traverse method were first conducted in the laboratory with a straight length of 300 mm×300 mm duct. This preliminary study is to determine the viability and accuracy of the tracer-gas technique. Results show that there is a good correlation between the airflow rates measured by using a pitot-tube and tracer-gas technique. This technique is further validated in the field with more complex air distribution system. Tracer-gas technique offers good accuracy when compared to the pitot-static traverse method. This technique can be a good alternative to the traditional pitot-static traverse method in the measurement of airflow in air distribution system during balancing. It is simple, accurate and possesses numerous advantages over the traditional instrumentations.