ASHRAE Research Project 1480 Report - Food Service Technology ...

The discharge velocities were measured from the two 4-way ceiling diffusers with the airflow
rate of 1000 cfm through each diffuser. The appliances were off and the makeup air was
exhausted by the hood. The velocity readings were taken with a 4-inch rotating vane anemometer
against the ceiling at a distance of 12.0 inches from the center of each diffuser. The velocity
readings are shown in Figure 18.
485
Side 4-Way
MUA
2024
821
1662
1309
Front 4-Way
MUA
663
1578
1493
Figure 18. Velocity Profile of 4-Way Ceiling Diffusers
The velocity profile of the 4-way diffuser is dependent on the orientation of the supply duct. The
greater vertical approach to the neck of the diffuser, the more uniform the velocity distribution
from the diffuser. It was also found that the proximity of the diffuser to the filter bank had an
effect on the diffuser velocity towards the filter hood. As the exhaust rate increased, the diffuser
velocity towards the hood increased more dramatically for the diffuser closer to the filter bank,
even though the diffusers were the same distance from the edge of the hood. As a result, a fourway
diffuser closer to the filter bank (i.e., at the side of a double island hood) would have a
greater impact on the velocity at the lower edge of the hood due to its proximity to the filter
bank. Therefore, for a given air flow rate from a diffuser, the diffuser closer to the filter bank
(not necessarily the hood) would have a more dramatic impact on capture and containment of the
cooking effluent at the lower edge of the hood.
2-17
RP-1480