Explanation of Diagram
The illustration above shows two key differences between UFAD and conventional overhead (OH) systems. First, the overall difference between return and supply air temperatures (∆Trm), represents the total cooling load (heat extraction rate) being removed by the airflow entering and exiting the room. For OH systems, this heat extraction rate is typically equal to the sum of all load components in the space, and allows the calculation of the required design cooling airflow quantity. For UFAD systems these load components are derived in much the same way, but are manifested differently due to stratification. The calculated heat extraction rate for UFAD systems will not be equal to the sum of all cooling loads, due to additional heat removal that occurs in the form of heat transfer conducted into the underfloor plenum.
The second difference, the occupied zone delta T (∆Toz), is a function of the stratification profile and contributes to the design comfort condition. The comfort condition (in stratified systems, to first order) is a function of the occupied zone average temperature, the temperature gradient from ankle to head-height (not to exceed 5°F by ASHRAE standards, although current research indicates that this may be conservative), and the floor temperature (in addition to the normal factors of air velocity and relative humidity). Our current research indicates that the stratification profile is influenced by the throw characteristics of the diffuser, and also by the type of load and radiant exchange.