4231 -- Mixing Effectiveness of AHU Combination Mixing/Filter Box with and without Filters PDF

The mixing effectiveness of an air-handling unit combination mixing/filter box was presented in a previous paper (Robinson 1998). This mixing box was equipped with a bank of 2 in. pleated throwaway filters (30% efficient) so that a separate filter section was not required. The mixing box was equipped with parallel blade dampers that directed the two airstreams toward each other. The outdoor airstream entered the top of the mixing box, and the return airstream entered the back of the mixing box. Thirty temperature sensors were used to determine how well the mixing box mixed the return and the outdoor airstreams by measuring the temperature spread at the discharge of the mixing box. The mixing tests were carried out with filters installed in the mixing box. The resulting mixing effectiveness for the mixing box ranged from a low of 0.12 (when the outdoor air damper was open 15° and the return air damper was open 75°) to a high of 0.54 (when the outdoor air damper was open 75° and the return damper was open 15°). In a second series of tests, the filters were removed from the mixing box to determine what effect they had upon the mixing effectiveness. The second series of tests showed that the mixing effectiveness ranged from a low of 0.21 (when the outdoor air damper was open 15° and the return air damper was open 75°) to a high of 0.65 (when the outdoor air damper was open 75° and the return damper was open 15°). The results from this test indicate that the placement of filters within the mixing box decreases the mixing effectiveness of an air-handling unit mixing box. Even with the filters removed from the mixing box, the mixing effectiveness was not sufficient to reduce the stratification present in the unit to acceptable levels.

Units: Dual