CH-18-004 -- Modeling Variable-Airflow Parallel Fan-Powered Terminal Units with a Mass and Energy Balance Approach

In the research reported in this paper, a performancemodel capable of characterizing parallel fan-powered terminalunits (FPTUs) in the variable-airflow operation was developedby assembling its component models (a heating coil, amixer, and a variable-speed fan) in a mass and energy balanceapproach. Two configurations of parallel FPTUs with differentheating coil locations were considered. In addition, theperformance impact of the air leakage from the FPTU housingto the plenum space was captured by integrating a leakagefraction into the model development. To demonstrate the use ofthe developed model, sample calculation results with four airleakage levels are provided over heating and cooling operatingranges, along with the equations for the model implementation.Incorporating the variable-airflow operation and airleakage allows the developed model to accurately representthe performance of parallel FPTUs at realistic operatingconditions. This cannot currently be achieved using existingbuilding energy simulation tools due to the exclusion of thevariable-speed fan and air leakage in the performance modeling.