QC-06-045 - Air Distribution in a Room with Ceiling-Mounted Diffusers— Comparison with Wall-Mounted Diffuser, Vertical Ventilation, and Displacement Ventilation PDF

Experiments with room air distribution that is generatedby a radial ceiling-mounted diffuser and a diffuser generatingflow with swirl are compared with the air distribution obtainedby mixing ventilation from a wall-mounted diffuser, verticalventilation, and displacement ventilation. The air distributiongenerated by a radial diffuser is partly controlled by themomentum flow from the diffusers and partly from gravityforces where the thermal load and the temperature differencebetween room air and supply air deflect the radial wall jetdown into the occupied zone. The ceiling diffuser with swirlingflow generates a flow pattern in the room that is rather uninfluencedby the thermal load. The flow is highly mixed abovethe occupied zone, and the air movement penetrates the occupiedzone close to the walls.

All systems were tested in the same room with a loadconsisting of two manikins, each sitting at a desk with two PCsand two desk lamps, producing a total heat load of 480 W.In all five cases, the design of the air distribution systemwas based on flow elements from the diffuser, a maximumvelocity assumption, and a critical vertical temperature gradientin the room. The characteristics of the air distributionsystems are addressed by analyzing the acceptable conditionsfor the supply flow rate and the temperature difference for thedifferent systems.

This paper shows that an air distribution system with ceiling-mounted air terminal units is able to generate comfortablevelocity and temperature conditions at the same and at slightlyhigher loads as can be obtained by a vertical ventilationsystem, a mixing ventilation system with wall-mounteddiffuser, and a displacement ventilation system with a lowvelocitywall-mounted diffuser.

The comparison is extended by considering both the localdiscomfort caused by draft and the percentage of dissatisfieddue to the temperature gradient when this is relevant to thesystems. The draft rating is low for the ceiling-mounted diffusersas well as for the low impulse system (textile terminals), andthe temperature gradient is also low because of the high levelof room air mixing.