Room air stratification in combined chilled ceiling and displacement ventilation systems

Abstract

Radiant chilled ceilings with displacement ventilation (DV) represent a promising integrated system design that combines the energy efficiency of both sub-systems with the opportunity for improved ventilation performance resulting from the thermally stratified environment of DV systems. The purpose of this study was to conduct laboratory experiments for a typical U.S. interior zone office to investigate how room air stratification is affected by the ratio of cooling load removed by a chilled ceiling to the total cooling load, η, for two different chilled ceiling configurations. The experiments were carried out in a climatic chamber equipped with radiant panels installed in the suspended ceiling. In the first test configuration representative of thermally activated slab applications, 12 panels covering 73.5% of the ceiling were used. During the second series of tests, 6 panels covering 36.7% of the ceiling were used, representing a typical installation of metal radiant panels. The cooling load removed by the panels varied between 0 and 73 W/m² (0–23.1 Btu/(h ft²)) (based on radiant panel area) or between 0 and 28 W/m² (0–8.9 Btu/(h ft²)) (based on room area). The average mean water temperature of the panels varied over a more moderate range of 20°C–24°C (60°F–75.2°F) for the 12-panel tests and over a colder range of 16.5°C–22.6°C (61.7°F–72.7°F) for the 6-panel tests. The displacement ventilation airflow rate varied between 1.65 and 4.03 l/(s m²) (0.32–0.79 cfm/ft²), and the supply air temperature was kept constant at 18°C (64.4°F). The results showed that increasing η, the relative amount of the cooling load removed by the chilled ceiling, reduced the total room stratification. However, a comparison between the colder 6-panel tests and the warmer 12-panel tests indicated that average radiant surface temperature (mean chilled water temperature in panels) was a stronger predictor of stratification performance. When smaller active radiant ceiling areas are used (e.g., for a typical radiant ceiling panel layout), colder radiant surface temperatures are required to remove the same amount of cooling load (as a larger area), which cause more disruption to the room air stratification. Despite the impact that the chilled ceiling has on stratification, the results indicate that a minimum head–ankle temperature difference of 1.5°C (2.7°F) in the occupied zone (seated or standing) will be maintained for all radiant ceiling surface temperatures of 18°C (64.4°F) or higher.

Acknowledgment

The present work was supported by the California Energy Commission (CEC) Public Interest Energy Research (PIER) Buildings Program and in-kind contributions of laboratory facilities by E.H. Price, Winnipeg, Manitoba. The authors would like to thank Tom Epp for the help in the laboratory work and preparation of figures.

Stefano Schiavon, PhD, PE, Associate Member ASHRAE, is Assistant Professor. Fred Bauman, PE, Member ASHRAE, is Research Specialist. Brad Tully, PEng, Member ASHRAE, is R&D Manager. Julian Rimmer, PEng, Member ASHRAE, is Manager of New Technology