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In older buildings, the space above the dropped ceiling was often used as a plenum space for ventilation systems, requiring only enclosed ducts that deliver fresh air into the room below, with return air entering the ceiling space through open grilles across the ceiling. That practice is now used less frequently in new buildings.
Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum; however, some drop-ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air-handling space. [1] [2]
Indoor units with directional vents mount onto walls, suspended from ceilings, or fit into the ceiling. Other indoor units mount inside the ceiling cavity so that short lengths of duct handle air from the indoor unit to vents or diffusers around the rooms.
Conventional overhead mixing systems usually locate both the supply and return air ducts at the ceiling level. Supply air is supplied at velocities higher than typically acceptable for human comfort and the air temperature may be lower, higher, or the same as desired room temperature depending on the cooling/heating load.
A chilled beam is a type of radiation/convection HVAC system designed to heat and cool large buildings through the use of water. [1] This method removes most of the zone sensible local heat gains and allows the flow rate of pre-conditioned air from the air handling unit to be reduced, lowering by 60% to 80% the ducted design airflow rate and the equipment capacity requirements.
For residential buildings, which mostly rely on infiltration for meeting their ventilation needs, a common ventilation rate measure is the air change rate (or air changes per hour): the hourly ventilation rate divided by the volume of the space (I or ACH; units of 1/h). During the winter, ACH may range from 0.50 to 0.41 in a tightly air-sealed ...