Improving airflow measurement accuracy in VAV terminal units using flow conditioners

A variable air volume (VAV) terminal unit adjusts its supply airflow rate to meet the heating or cooling load and/or the ventilation requirement of the served space. Consequently, the accuracy of the VAV airflow sensor is highly important to the VAV system operation, and an inaccuracy of the VAV airflow sensor could lead to an energy waste or insufficient ventilation. ASHRAE Research Project (RP) 1353 identified non-ideal inlet conditions, such as an elbow or kinked duct before the VAV terminal unit, as causes of observed inaccuracies of up to 45% in VAV airflow measurements. VAV airflow measurement errors are normally mitigated by on-site balancing; however, it is difficult to achieve accurate reference airflow measurements in the field because of limited straight ductwork before VAV terminal units, as well as ductwork leakage. This study explored the potential solution of using a VAV flow conditioner to regulate the velocity profile upstream of the VAV airflow sensor and increase the VAV airflow measurement accuracy. A variety of flow conditioners were evaluated with computational fluid dynamics (CFD) modeling, and a CFD-optimized prototype of a 60%-porosity K-Lab/Laws plate was fabricated and tested. For all tested inlet conditions, airflow rates, and VAV boxes, the prototype reduced the VAV airflow reading error to ±5% when it was installed immediately before the VAV box inlet, regardless of upstream duct conditions. The prototype flow conditioner had a pressure drop equivalent to that of a 2-row VAV reheat coil.