2016 ASHRAE Annual Conference

This paper details experimental measurements and mathematical modeling of air flow through a perforated duct with an open area of 22% and capped at the end. Measurements were conducted on ducts with uniform diameters of 12 in., 10 in., and 8 in. (0.20 m, 0.25 m and 0.30 m). All ducts were 20 ft (6.10 m) long and inlet flow rates ranged from approximately 350 to 700 cfm (165 to 330 L/s). Flow rates were measured along the length of the duct using the pitot traverse method. The static pressure was also measured. The flow through the duct was modeled assuming one-dimensional flow and a differential equation was derived using the mass, momentum and energy equations. The resulting differential equation was solved numerically and the results were compared to the experimental measurements. Good agreement was achieved when comparing the experimental and model flow rates for all test runs with a maximum difference of 14.0% and an average difference of 2.0%. Results for the static pressure showed the same trends between the experiments and the model. The pressure was largest at the capped end of the duct where the experimental measurements exceeded the model results by a maximum of 21.8%.

Low evaporator airflow is one of the common faults for rooftop units, which can be caused by dirty filter, evaporator fouling, or loose belt. Low airflow could result in frozen evaporator coil, reduced cooling capacity and indoor comfort issues. Accordingly, more fan power is consumed as longer operating time is required. With the widespread use of variable frequency drives (VFDs) on rooftop units, low evaporator airflow can be potentially detected by monitoring the fan power variation. In the paper, the principle of fan-power based detection is introduced first. Then, the detection algorithm is proposed including development of baseline and comparison of operational data with baseline. At last, the field test was conducted to verify the proposed method. The test results indicated that fan power based method can effectively detect low evaporator airflow for rooftop units.

A traditional mass and energy balance component approach was used to characterize the performance of fixed airflow series fan powered terminal units for applications in building simulation programs. The approach included developing relevant energy and mass balance equations for the components in a fan powered terminal unit – heating coil, fan/motor combination, and mixer. Fan motors that included permanent split capacitor motors controlled by silicon controlled rectifiers or electronically commutated motors were included in the model development. The paper demonstrates how to incorporate the fan/motor combination performance models for both permanent split capacitor and electronically commutated motors into the mass and energy balance approach. The fan models were developed from performance data that were provided by multiple fan powered terminal unit manufacturers. The fan/motor performance data included 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 (248.6 to 745.7 W).

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.