Radiant Ceiling Cooling

uildings with radiant ceiling cooling systems, also known as “chilled beam” systems, incorporate pipes in the ceilings through which chilled water flows. The pipes lie close to the ceiling surfaces or in panels, and they cool the room via natural convection and radiation heat transfer (Figure 1). Although the technology has existed for more than 50 years, it has had problems in the past. Condensation of moisture on the cooled surfaces sometimes damaged ceiling materials (e.g., plaster) and created conditions favorable to biological growth. As noted by Mumma, current systems usually require dedicated outdoor air systems (DOAS) and tight building envelopes to manage humidity. Most commercial buildings avoid condensation on the chilled panels by using a separate system to maintain the dew point of the indoor air below the panel temperature. Ventilation makeup air is the predominant source of peak humidity load in most buildings. Consequently, humidity loads can be handled separately from the chilled ceiling by dehumidifying the makeup air before it enters the space (with enough extra humidity removal to address internal moisture sources). Mumma reports that with a good base dewpoint control, the chilled panels can manage temporary increases in local moisture loads without condensation formation. A radiant ceiling cooling system delivers sensible cooling directly to spaces, which de-couples maximum air delivery from the cooling load. Radiation and natural-convection heat transfer each account for about half of the approximately 50 Btu/ft (150 W/m) cooling capacity of passive radiant ceiling panels. At these heat transfer rates, radiant ceiling panels can meet peak sensible loads with about one-third of the ceiling area covered by cooled panels (for a cooling load of 16 Btu/h · ft [50 W/m]). Active chilled beam units that use recirculated room airflow induced by the ventilation makeup air supply can supply up to 79 Btu/h · ft (250 W/m). Each unit can be controlled separately, which simplifies zoning.