QC-06-053 - Heat Transfer Pathways in Underfloor Air Distribution (UFAD) Systems PDF

This paper reports on a modeling study to investigate theprimary pathways for heat to be removed from a room withunderfloor air distribution (UFAD) under cooling operation.Compared to the standard assumption of a well-mixed room aircondition, stratification produces higher temperatures at theceiling level that change the dynamics of heat transfer withina room as well as between floors of a multi-story building. Asimplified first-law model has been used to estimate andcompare the relative magnitudes of the heat being removedfrom a room through two primary pathways: (1) heat extractionvia warm return air exiting the room at ceiling level orthrough the return plenum and (2) heat entering the underfloorsupply plenum either through the slab from the floor below orthrough the raised floor panels from the room above. Surprisingly,it is shown that up to 40% of the total room cooling loadis transferred into the supply plenum and only about 60% isaccounted for by the return air extraction rate. The implicationsfor the design and operation of UFAD systems arediscussed.