Editor’s note: This is the first of two articles basedlargely on work being conducted under a researchgrant from the California Energy Commission, withadditional funding from the Center for the BuiltEnvironment, to develop software for calculating the energy performance of underfloor-air-distribution systems (www.energy.ca.gov/pier/buildings/projects/500-01-035-1.html). The companion article—“Design Guidelines for Underfloor Air-Supply Plenums” byFred S. Bauman, PE; Tom Webster, PE; and Hui Jin—will be published in the July issue of HPAC Engineering.

The control of room-air stratification is critical to the design and operation of successful underfloor-air-distribution (UFAD) systems, representing an oftentimes complex balancing act: Increasing stratification by reducing airflow or mixing for a given space heat load saves energy, while decreasing stratification by boosting airflow or mixing for a given space heat load improves occupant comfort.

The Center for the Built Environment (CBE) has conducted full-scale laboratory experiments to study the impact of stratification in interior and perimeter office spaces. Because the findings cover topics that are of considerable interest to the industry at this time, the following interim guidelines are offered. Each of the topics outlined in this article require more research for specific guidance to be offered. For a more complete overview of UFAD design requirements, see “Underfloor Air Distribution (UFAD) Design Guide”1 and the Underfloor Air Technology Web site (www.cbe.berkeley.edu/underfloorair).

ROOM-AIR STRATIFICATION
In this article, all systems—interior and perimeter— are assumed to operate under a variable-air-volume (VAV) control strategy.

Interior zones. Stratification is the result of complexinteraction between thermal plumes generatedby heat sources and turbulent airflow from floordiffusers. The height above the floor to which themixing provided by diffusers occurs—termed the“occupied zone” (OZ)—is limited. What’s more,the mixing process is less than perfect, as stratificationexists both in the OZ and above. The type andnumber of diffusers can have a significant impacton stratification in the OZ.

FIGURE 1. Room-air-stratification temperature profile.

Figure 1 illustrates two key temperature differences:
• Overall difference between return- and supply-air temperature (ÄTroom), which can be used, in combination with total room-airflow rate, to calcu- late the fraction of total room heat gainbeing removed by entering and leavingairflow (heat-extraction rate). Withconventional-air-distribution (CAD)(i.e., overhead) mixing systems, heat-extractionrate typically is equal to thesum of all of the load components ina space, allowing easy calculation ofrequired design cooling airflow. With UFAD systems, however, radiant heat exchange plays a role in determining the overall energy balance in a space. This, plus additional conduction heat exchange, results in a significant amount of energy being transferred from spaces to supply plenums, particularly in multistory buildings. For a given load, ÄTroom and room airflow primarily are determined by thermostat setting, supply-air temperature, and diffuser throw characteristics.

• OZ delta-T (ÄToz), which is a function of the vertical temperature gradient and plays an important role in determining comfort conditions. Comfort conditions (in stratified systems, to first order) are a function of OZ average temperature (Toz, avg [Figure 1]), ÄToz (not to exceed 5 F by American Society of Heating, Refrigerating and Air-Conditioning Engineers [ASHRAE] standards,2 although current research3 indicates that that may be conservative), and floor temperature (in addition to air velocity and relative humidity).

FIGURE 2. Room-air-stratification temperature profiles with varying number of swirl diffusers at constant load, supply temperature (65 F), and control set point (74 F at 4 ft).

Current CBE research indicates that stratification profiles are influenced by the throw characteristics of diffusers. Figure 2 shows room-stratification profiles for an interior zone with varying numbers of swirl diffusers operating at constant load and total room airflow. Adding diffusers at constant room airflow reduces the airflow per diffuser and, thus, throw height, leading to more stratification in the OZ. Throw characteristics are indicated by diffuser design ratio (DDR), the ratio of actual airflow per diffuser to recommended design flow. For swirl diffusers, to first order, the less airflow delivered by a diffuser (the lower the DDR), the lower the throw height. Research indicates that swirl diffusers have more potential to produce low throw (which, in turn, may produce greater OZ stratification) than variable-area diffusers (photos A, B, and C). Variablearea diffusers produce a relatively consistent stratification profile because of their consistent throw height over a broad range of operating conditions.