Analysis and Application of Virtual-Flow Meter Technology for Chiller-Water Valve and Outside-Air Controls

Abstract

The following work focuses on using virtual flow meters (VFM) for HVAC controls and how they can replace physical sensors to reduce cost and increase efficiency. The study focuses on implementing VFM to measure outdoor air (OA) intake and the cooling coil chiller water flow rate. As HVAC controls advance with time, OA flow measurements became crucial to precisely maintain building pressurization, building codes, and healthy air quality for occupants. Moreover, Chiller water (CHW) flowrate measurement is essential for detecting inefficiencies in the cooling system and maintaining a stable supply air temperature. Consequently, newer systems utilize physical flow meters despite their high initial cost, whereas older systems do not. Outdoor air and chilled water VFM will help lower initial cost and improve HVAC controls which can increase efficiency and lower operation cost overall. Cascade control was also investigated to improve HVAC controls by integrating it with CHW-VFM. Thus, VFM will also eliminate the systematic error associated with hardware maintenance. Using an experimental air handling unit (AHU), both VFM were implemented and tested for their performance. The CHW VFM had a percentage error of less than 5% compared to a physical ultrasonic meter. Also, cascade control operation has increased performance compared to a single control operation in maintaining supply air temperature under various disturbances. Also, the suggested cascade control VFM covers a larger range of operating conditions, eliminating the need for seasonal resets. In addition, It was found that the OA ratio can be estimated when (TRA - TOA ) > 6 °F with a relative error of less than 8%. Also, a correlation between the outdoor air damper position and OA ratio can be established if mixing chamber pressure is maintained steadily.