Status (updated 9/4/2007): Continuing
Funding sources: CBE Industry Consortium, Research Grants
Project Objective
Develop simplified, practical design procedures and associated software tools to determine cooling airflow requirements for underfloor air distribution (UFAD) systems in interior and perimeter zones.
Significance to Industry
Design engineers often cite methods for airside design sizing as one of the most important unanswered questions regarding UFAD system design. Determining zone cooling airflows for UFAD systems to optimize energy and comfort performance requires consideration of significant differences between well mixed (i.e., overhead systems where the space air is mixed due to the action of the supply diffusers) and stratified UFAD systems. Currently, engineers attempt to apply their experience with mixed systems to the UFAD design problem without adequately considering several important differences. However, in developing a UFAD design procedure it is desirable to leverage the considerable body of knowledge of mixed system load calculations.
Research Approach
We will use a combination of test chamber data, emerging stratification theory, and detailed heat transfer analysis under stratified room conditions, along with established cooling load calculation procedures for mixed systems to derive modification factors that will allow cooling airflows to be determined for UFAD systems. The technical approach will also account for another key feature of UFAD systems, the thermal performance of the underfloor plenum and structural slab in multi-story buildings, based on ongoing full-scale testing and modeling research. In particular, the design tool will calculate the heat transfer to the supply plenum, which can amount to a substantial portion of the total room cooling load. The work will involve the development of a suitable format for associated software tools and testing of its usefulness through consultation with practicing engineers.
The work will leverage previous CBE research results and use existing standard load estimating procedures for baseline load calculations. The scope will include a limited amount of analysis with modeling tools such as the emerging UFAD version of EnergyPlus and CBE's thermal comfort model. These steps are considered critical to understanding the differences between OH and UFAD cooling loads. We will also use CBE's Thermal Comfort Model to evaluate the thermal comfort conditions in the occupied zone of modeled typical UFAD and OH spaces. This will enable "equivalent comfort conditions" to be established between well-mixed environments produced by OH systems and stratified environments produced by UFAD systems.
Status of Design Tool
A preliminary version of the design tool for interior zones was completed in early 2007 and will be published soon [Bauman et al. In press]. Additional funding has been obtained and work is ongoing to improve and refine the UFAD cooling airflow design tool. The final design tool is scheduled for completion in 2008 and will include spreadsheet-based software with users manual, a perimeter zone model, improved guidance to account for plenum air leakage, and further validation by comparison with experiment and EnergyPlus/UFAD simulations.
Publications
Bauman, F., T. Webster, and C. Benedek, 2007. Cooling Airflow Design Calculations for UFAD, ASHRAE Journal, pp. 36-44, October.
