Effect of Inlet Duct Design on Fan Performance of Indoor Air Handling Units with Pull-Through Fan Configuration (ASHRAE RP-1743)

During testing indoor air handling units, ASHRAE Standard 37 requires installing an inlet duct that might increase the length of the testing setup to exceed the capability of some testing facilities. ASHRAE Research Project 1743 (ASHRAE RP1743) addresses this issue by evaluating the effect of inlet duct design on fan performance of indoor air handling units. The findings of the study will allow identifying alternative, shorter, inlet duct configurations maintaining the fan performance as of that obtained with ASHRAE Standard 37 inlet duct. The test matrix of the project includes various indoor units differing in motor type, such as Constant Torque Motor (CTM) and Electronically Commutated Motor (ECM), and fan configurations, such as pullthrough and pushthrough configuration. This paper focuses on the findings of testing two pullthroughfanconfiguration units, one of which with a CTM fan and the other with an ECM fan. In addition, other effects, such as atmospheric pressure and hysteresis in approaching external static pressure (ESP), were investigated to study their impacts on the repeatability of equipment testing. The results of the CTMfan unit showed that the proposed alternative inlet duct configurations can maintain the fan performance within the suggested tolerances, which are 5% in power and 2.5% in air volumetric flowrate relative to the ASHRAE Standard 37 inlet duct. This allows a reduction in the inlet duct stacklength from 38% to 74% compared to ASHRAE Standard 37 inlet duct configuration. Noticeable performance hysteresis was observed at low air flowrate and ESP, while atmospheric pressure had no effect on fan performance. For the ECMfan unit, the fan performance was maintained during testing the proposed alternative inlet duct configurations within the suggested tolerance, with a noticeable decrease in power during testing the reduced alternative inlet duct configuration. The reduction in the stack length as compared to the standard inlet duct configuration was 40%, 72%, 74% for the inlet box, 4inch inlet duct, and reduced alternative configuration, respectively. In contrast to the results of the CTMfan unit, the hysteresis effect on the ECMfan unit was found to be insignificant, while atmospheric pressure effect was significant. As the atmospheric pressure increased, air volumetric flowrate and fan’s power decreased