A virtual supply airflow rate meter for rooftop air-conditioning units

A proper amount of supply airflow is critical in all kinds of air-based HVAC systems to maintain desired control effectiveness, energy efficiency and indoor air quality (IAQ). Although knowledge of supply airflow rate (SCFM) is certainly very important, measuring and monitoring SCFM in rooftop air-conditioning units (RTUs) by using the conventional SCFM metering devices are very costly and more than often problematic. This paper proposes a low-cost but accurate virtual SCFM meter to solve the dilemma for RTUs. The SCFM values are indirectly derived from a first-principle model in combination with accurate measurements of low-cost virtual or virtually calibrated temperature sensors. Modeling, uncertainty analysis and experimental evaluation through a wide range of laboratory testing for both cooling- and heating-based approaches are performed respectively in the development. The study reveals that the heating-based method surpasses the other in terms of its simplicity, accuracy (uncertainty is ±6.9% vs ±13.8%) and reliability and is chosen to be the virtual SCFM meter in RTUs. This cost-effective application is promising with a number of merits, such as easy to implement, economical for use, and generic in RTUs with the same constructed gas furnaces. For applications, it could be applied as a permanently installed monitoring tool to indicate the SCFM and/or to automatically detect and diagnose improper quantity of SCFM for RTUs.