Performance prediction of hydronic cooling coil with non-uniform air velocity (ASHRAE RP-1741)

Abstract

Hydronic coils are often exposed to non-uniform air velocities due to a series of factors, including tight spacing between a fan and a coil, flow area contraction and expansion, existence of obstructions, abrupt change in airflow direction, or the combination of above factors. Although studies showed non-uniform air velocities could cause performance degradation for direct expansion (DX) cooling coils, efforts in quantifying this impact on hydronic cooling coils are limited. The objective of this paper is to develop and validate a section-by-section calculation procedure for cooling performance prediction of hydronic coils with non-uniform air velocities. As the first step, air velocities at the coil face in a hydronic fan coil unit (FCU) were measured using a vane-type anemometer. The coil was then discretized into multiple sections, with each section using the measured air velocity for heat and mass transfer analysis. The cooling performance prediction with non-uniform air velocities was validated against experimental results. The comparative analysis showed that capturing the impact of non-uniform air velocities decreased the normalized root mean square error (RMSE) from 39.9% to 8.1% for latent capacity prediction and 9.1% to 2.6% for total capacity prediction compared with the results assuming uniform air velocities.