Study of heat exchanger inlet air velocity distribution for ducted tests in a psychrometric chamber

This paper presents ongoing work on ASHRAE RP-1785. RP-1785 has the global objective of providing accurate refrigerant charge and oil retention data for residential coils, collected in a controlled experiment. The final coil test matrix of the study includes several representative residential indoor and outdoor coils to be tested at various refrigerant and airside inlet conditions with a fully-dry coil. The coil performance, charge, and oil retention is strongly influenced by the airflow; therefore uniform airflow distribution to the coils is critical for providing repeatable data. The physical location of the airside ductwork is space constrained, thus the final design was given careful consideration, including a CFD analysis. The final airside duct design resulted in uniformity of airside velocity and temperature within ASHRAE Standard 33 requirements. The measured maximum air velocity entering the coil exceeds the lowest velocity by 9%, and the individual entering air temperature varies by within 0.09 K (0.17 R). Using the final duct design and coil a secondary study was performed to ensure a fully-dry coil for all tests to be performed. The effects of relative humidity on inlet air velocity were investigated: 28.7% relative humidity at 26.67 °C (80 °F) drybulb temperature is required for 2.54 m/s (500ft/min) air velocity across a 3-ton coil's slab.