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Assessment and Improvements of the CBE Underfloor Air Distribution (UFAD) Cooling Load Design Tool

Abstract

The necessity of energy efficient Heating Ventilation and Air Conditioning (HVAC) technologies has been growing with the development of green buildings to reduce the impact of built environment on the US energy consumption. In this context, underfloor air distribution (UFAD) appears as an innovative air distribution method that has the potential benefits of reduced energy use, improved indoor air quality (IAQ) and thermal comfort. Currently, there are two UFAD design tools (CBE and RP-1522 tools) in the ASHRAE UFAD design guide to help HVAC designers predict thermal stratification and calculate air flow rate. The aim of this thesis is to comprehensively compare these two UFAD design tools and update the CBE UFAD Cooling Load Design Tool with new stratification models and extended capabilities. The comparison will provide HVAC designers with a reference when deciding which design tool to use.

The comparison consists of two parts. The first one compares features, including design cooling load, thermal stratification profile, air distribution models, diffuser types, supply plenum heat balance, plenum configurations and air distribution effectiveness. The second part is a numeric comparison to assess the accuracy of thermal stratification predictions of the two tools using a new database that is composed of 79 full-scale experiments and 31 CFD simulations. A new stratification model was then developed for each type of diffuser based on the combined database. The old and updated stratification models were also compared. Major functionality that has been added to the CBE tool includes a new linear bar grille in the perimeter zone, air distribution effectiveness, and the in-floor cooling unit. Design examples are provided to illustrate how users can interact and make design decisions based on the feedback from the design tool.

The results of the comparison show that both tools have practical advantages and limitations. The CBE tool has the key advantage of being able to predict the UFAD cooling load, calculate heat gain in the supply plenum, model different plenum configurations and zone types. It has the limitation of primarily being used in office buildings and not able to calculate air distribution effectiveness. The RP-1522 tool covers more buildings types (classrooms, offices, workshops, restaurants, retail shops, conference rooms and auditoriums), and is able to calculate the air distribution effectiveness. However it requires users to input the zone cooling load, supply plenum factor and the supply airflow rate of each diffuser, which is difficult to get during the design stage for UFAD system. The two design tools are both acceptably accurate prediction models for design purposes, thus validating both air distribution models, which were developed independently. The updated stratification prediction models developed from the combined database do not yield significant differences in stratification prediction compared to the old models, except in the case of VAV directional diffusers in the interior zone. The new model for interior VAV directional diffuser has been implemented in the updated CBE UFAD Cooling Load Design Tool.

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