An improved virtual calibration of a supply air temperature sensor in rooftop air conditioning units

Abstract

Accurate supply air temperature (SAT) measurements are vital for improving the energy management of packaged rooftop air-conditioning units (RTUs) through better sequencing control and real-time automated fault detection and diagnosis (FDD). However, the accuracy and reliability of single measured manufacturer-installed supply air temperature (MSAT) are greatly compromised in the heating mode due to severe temperature stratification and high thermal radiation. There exists unacceptable erratic measurement errors and the traditional calibration method can hardly overcome the defect. An easy-to-use and very cost-effective nontraditional calibration method was proposed previously to calibrate virtually an MSAT sensor in RTUs. In order to overcome the deficiencies of the virtual calibration (VCal) method in fault tolerance and fault diagnostics, experiments with wider combination and coverage are investigated in this study. It is found that an improved virtual calibration (IVCal) method can be obtained by correlating the offset errors with available system information (the outside air damper status[OADst]) and low-cost temperature measurements (the MSAT and outside air temperature [OAT]). Further experimental evaluations demonstrate that the IVCal method could predict the true value of SAT with greater accuracy (at a relative error of only ±1.4°F [0.8°C]) with excellent fault tolerance and significantly enhanced performance of a virtual supply airflow rate meter. Additionally, the IVCal method also inherits the good characteristics of the VCal method, such as its high cost-effectiveness, ease of use, and applicability to all RTUs with similar constructed gas furnaces.