The use of ducts to improve the control of supply air temperature rise in UFAD systems: CFD and lab study

Cool supply air flowing through the underfloor plenum is exposed to heat gain from both the concrete slab (conducted from the warm return air on the adjacent floor below the slab) and the raised floor panels (conducted from the warmer room above). The magnitude of this heat gain can be quite high, resulting in undesirable loss of control of the supply air temperature from the plenum into the occupied space. These warmer supply air temperatures can make it more difficult to maintain comfort in the occupied space (without increasing airflow rates), particularly in perimeter zones where cooling loads reach their highest levels. How to predict plenum thermal performance is one of the key design issues facing practicing engineers – evidence from completed projects indicates that excessive temperature rise in the plenum can be a problem.