Using a Mass and Energy Balance Approach to Model the Performance of Parallel Fan-Powered Terminal Units with Fixed Airflow Fans (ST-16-026)

A mass and energy balance approach was used to characterize the performance of parallel fan powered terminal units for applications in building simulation programs. The approach included developing relevant mass and energy balance equations for each component in a parallel fan powered terminal unit – heating coil, fan/motor combination, and mixer. Only fixed airflow applications were included. Two locations of the heating coil were considered. One location, designated as the traditional configuration, was at the discharge of the unit. The second location, designated as the alternative configuration, was at the secondary air inlet. Fixed airflow parallel FPTUs use fan motors that include either permanent split capacitor motors controlled by silicon controlled rectifiers or electronically commutated motors. The paper demonstrated how to incorporate fan/motor combination performance models for both permanent split capacitor and electronically commutated motors into the mass and energy balance approach. These fan models were developed from performance data provided by multiple fan powered terminal unit manufacturers The fan/motor performance data included FPTU a fan airflow range from 250 to 3500 ft³/min (0.118 to 1.65 m³/s) and a motor size range from one-third to one hp (249 to 746 W). Leakage was included in the models. Sample runs were used to illustrate the effect of leakage in both cooling and heating operations.