Modeling Variable-Airflow Parallel Fan-Powered Terminal Units with a Mass and Energy Balance Approach

In the research reported in this paper, a performance model capable of characterizing parallel fan-powered terminal units (FPTUs) in the variable-airflow operation was developed by assembling its component models (a heating coil, a mixer, and a variable-speed fan) in a mass and energy balance approach. Two configurations of parallel FPTUs with different heating coil locations were considered. In addition, the performance impact of the air leakage from the FPTU housing to the plenum space was captured by integrating a leakage fraction into the model development. To demonstrate the use of the developed model, sample calculation results with four air leakage levels are provided over heating and cooling operating ranges, along with the equations for the model implementation. Incorporating the variable-airflow operation and air leakage allows the developed model to accurately represent the performance of parallel FPTUs at realistic operating conditions. This cannot currently be achieved using existing building energy simulation tools due to the exclusion of the variable-speed fan and air leakage in the performance modeling.