Transient sensor errors and their impact on fixed-bed regenerator (FBR) testing standards

Abstract

Fixed-bed regenerators (FBRs) are a favorable option for energy recovery in building HVAC systems due to their high sensible effectiveness. Unlike other types of energy recovery exchangers, the air temperature at the outlet of FBRs varies with time, which creates challenges when measuring the outlet temperature and effectiveness of FBRs since the actual outlet air temperature will include the transient response of the FBR and the temperature sensor. In this article, a validated numerical model of FBRs that takes into account the sensor response is used to quantify the temperature and effectiveness errors that result due to sensors’ response characteristics over a wide range of design parameters. The main contributions of this article are the practical recommendations for the temperature measurement for different types of FBRs developed for HVAC applications. The recommendations presented in this article could be implemented in future versions of the current standards (ASHRAE 84 and CSA C439-18 standards) for performance testing of air-to-air energy exchangers.

Acknowledgements

Financial support from the College of Engineering and the College of Graduate and Postdoctoral Studies of the University of Saskatchewan, National Science and Engineering Research Council (NSERC), Canada, Tempeff North America Inc., Winnipeg, Canada (Project No: 533225-18) are gratefully acknowledged.

Example A.1. Design and operating conditions. This example demonstrates the impact of temperature sensor on the system performance. Consider a space that requires 10,000 CFM of ventilation air to meet the outdoor air requirements. Heat exchanger design parameters and operation conditions, as well as the sensor time constant, are presented here.

For this condition, the actual effectiveness of the heat exchanger is 80.2%, and the recovered heat would be 13.5 kW. With temperature sensors with a time constant of 6 s, the measured effectiveness is 85.9% (from Table A.3), and hence the predicted recovered heat would be 14.5 kW. Therefore, the measured values overpredict the recovered heat by 1 kW at the described conditions. If this sensor is located in the buildings, the inaccurate measurements cause the systems to add less heat to the supply air and could deteriorate the thermal comfort within the building.

It should be noted that in an actual building, there are other thermal masses in the system that might affect the temperature profile that occupant experiences. This model, however, considers the sensor time constant and its effect on the measured temperature and effectiveness at the outlet of the exchanger only.