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Evaluating the Performance of Coupled Displacement Ventilation, Passive Chilled Beam Systems

ASHRAE Research Project 1666

Evaluating the Performance of Coupled Displacement Ventilation, Passive Chilled Beam Systems

From ASHRAE Journal Newsletter, September 8, 2020

Research shows that displacement ventilation systems deliver a high level of indoor air quality. However, these systems’ ability to remove heat is limited. Passive chilled beam systems have a better capacity to remove heat, so a solution could be to combine the two systems to provide both good indoor air quality and heat removal capabilities.

In ASHRAE Research Project 1666, Experimental Evaluation of the Thermal and Ventilation Performance of Stratified Air Distribution Systems Coupled with Passive Beams, researchers evaluated and tested the ventilation and thermal performance of a combined DV-PCB system. Purdue University researchers, Qingyan Chen, Ph.D., Member ASHRAE, and Zhu Shi, Student Member ASHRAE, discuss the research.

1. What is the significance of this research?

Displacement ventilation (DV) creates high indoor air quality since it directly supplies fresh air to an occupied zone. However, because its supply air temperature cannot be too low, it has limited capability in removing a high cooling load. Meanwhile, a passive chilled beam (PCB) system may be able to remove a large cooling load. This study coupled these two systems and investigated its thermal and ventilation performance with the goal of developing a practical design guide for the coupled DV-PCB system.

2. Why is it important to explore this topic now?

Research shows that when the cooling load is larger than 40 W/m2 (12.68 Btu/h · ft2), a DV system could create a large vertical temperature gradient, which leads to thermal discomfort. Since people on average spend more than 90% of their time indoors, such thermal discomfort may have negative impact on occupants' productivity and even health. With the increasing use of electrical equipment in buildings nowadays, it is particularly crucial that this limitation of DV system be addressed.

The results from this study show that PCB was very effective in reducing the temperature gradient by cooling the indoor air through the induced downward cold jet. With a good design, this coupled system can maintain good thermal comfort and satisfactory indoor air quality in high cooling load scenarios. However, the system can easily turn the airflow in an indoor space to a mixing condition so that the benefits of displacement ventilation would disappear.

3. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?

PCBs can effectively reduce the temperature gradient created by a DV system and hence remedies the limitation of DV in removing a high cooling load. But this investigation also showed draft exists beneath PCB (percentage dissatisfied, or PD, of >15%) because of the induced downward cold jet. In the design of an indoor space, it is important to avoid placing chairs or desks under PCBs.  Meanwhile, PCBs could bring part of the airborne contaminants near the ceiling downwards to an occupied zone to form a mixing ventilation. Hence, it is critical that the design parameters are well specified so the contaminant stratification can still be maintained. This research provides a guide on how to design the coupled DV-PCB systems that could meet thermal and ventilation requirements.

4. How can this research further the industry's knowledge on this topic?

The five-step design guide developed from this research provides designers with a procedure to correctly design a coupled DV-PCB system. The system designed with this guide could meet thermal and ventilation requirements set by ASHRAE standards. Moreover, to make the design process more convenient, this study also created a user-friendly design interface to automate and visualize the design process. Once a designer enters required inputs, this design interface will do the calculation automatically and display the recommended design parameters.

5. Were there any surprises or unforeseen challenges for you when preparing this research?

One challenge was the complexity of the research with a large number of parameters in the coupled system. These parameters included total cooling load, percentage of load removed by PCBs, airflow rate, geometric parameters, etc. These factors increased the difficulty in both the experimental measurement and computational fluid dynamics simulation parts of the investigation.

What came as a surprise was that the combined system could easily turn airflow into a mixing ventilation. This would make the applications of the system limited, in addition to the well-known problem of possible condensation on the cooling coils of PCB units.