scope:

Experiments with room air distribution that is generated by a radial ceiling-mounted diffuser and a diffuser generating flow with swirl are compared with the air distribution obtained by mixing ventilation from a wall-mounted diffuser, vertical ventilation, and displacement ventilation. The air distribution generated by a radial diffuser is partly controlled by the momentum flow from the diffusers and partly from gravity forces where the thermal load and the temperature difference between room air and supply air deflect the radial wall jet down into the occupied zone. The ceiling diffuser with swirling flow generates a flow pattern in the room that is rater uninfluenced by the thermal load. The flow is highly mixed above the occupied zone, and the air movement penetrates the occupied zone close to the walls.

All systems were tested in the same room with a load consisting of two manikins, each sitting at a desk with two PCs and two desk lamps, producing a total heat load of 480 W.

In all five cases, the design of the air distribution system was based on flow elements from the diffuser, a maximum velocity assumption, and a critical vertical temperature gradient in the room. The characteristics of the air distribution systems are addressed by analyzing the acceptable conditions for the supply flow rate and the temperature difference for the different systems.

This paper shows that an air distribution system with ceiling-mounted air terminal units is able to generate comfortable velocity and temperature conditions at the same and at slightly higher loads as can be obtained by a vertical ventilation system, a mixing ventilation system with wall-mounted diffuser, and a displacement ventilation system with a low-velocity wall-mounted diffuser.

The comparison is extended by considering both the local discomfort caused y draft and the percentage of dissatisfied due to the temperature gradient when this is relevant to the systems. The draft rating is low for the ceiling-mounted diffusers as well as for the low impulse system (textile terminals), and the temperature gradient is also low because of the high level of room air mixing.