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Studying the Performance of Active Chilled Beams for Heating and Cooling Under Varied Conditions

Studying the Performance of Active Chilled Beams for Heating and Cooling Under Varied Conditions

From ASHRAE Journal Newsletter, June 30, 2020

The orientation and boundary conditions of an active chilled beam (ACB) can affect occupants’ thermal comfort and the air distribution in a room. Therefore, it is important to select, orient and design ACBs to match the proportions and boundary conditions of a space, in addition to managing thermal loads.

In a recent Science and Technology for the Built Environment article, researchers investigate the thermal comfort, airspeed and temperature distribution and more for active chilled beams (ACB). Authors Rodrigo Mora, Ph.D., Associate Member ASHRAE; Rohit Upadhyay, Associate Member ASHRAE; Marc-Antoine Jean and Mike Koupriyanov, P. Eng, Associate Member ASHRAE, discuss the research.

1. What is the significance of this research?

The research demonstrates the effects of an active chilled beam (ACB) orientation, heating/cooling mode of operation, and simulated asymmetric open-office boundary conditions on room air distribution, room air-change effectiveness and the occupants’ thermal comfort. As such, it emphasizes the importance of properly selecting, orienting and designing ACBs, not just to offset room thermal loads, but to match the proportions and boundary conditions of the space.

2. Explain the steps of this research project. What did the process look like?

The process started with an extensive literature review on the performance gaps of ACBs. The review was followed by field experiments in three private offices in a LEED Gold office building. The field experiments revealed real-world challenges in the design and operation of ACBs. However, in the field experiments, the ACB orientation was fixed, and the field experiments did not allow testing challenging boundary conditions, either because the offices were well shaded from the sun or because the weather was mild during the experiments. Furthermore, while the field experiments were conducted in single offices, we learned that occupants in the cellular open offices with ACBs in the same building were not as happy with their thermal environment as occupants in single offices. Therefore, we decided to conduct laboratory experiments at the PRICE Research Centre North (PRCN) to test an open office, under more challenging boundary conditions, and ACB oriented in two orthogonal directions.

The laboratory experiments were conducted in a room climate simulator consisting of an office room and a climate chamber simulating the outdoor conditions. These were separated by a window. A set of experiments was designed and optimized, to test all relevant performance hypotheses from the literature and from the field experiment, using the smallest number of experiments, given that each laboratory experiment could take about half a day. With the room laid out, the ACB was selected and sized based on the room heating and cooling loads and using the manufacturer’s software. The room was fully instrumented. Experiments were conducted with the ACB oriented parallel and perpendicular to the window. Surface temperature sensors were placed in all walls, floors and ceilings. Four threes were placed close to the occupant manikins, each measuring air temperature, relative humidity, and operative temperature. The ACB supply air speed, temperature, and air pressure was measured. For the air-change effectiveness tests, CO2 was injected into the supply air stream and measured in positions of the room close to the manikins. The experiments were conducted in ACB heating and cooling operation, with asymmetric conditions simulated through the window getting cold in winter and warm in summer, and floor panels simulating the sun penetrating through the window. Since the experiments were steady state, it would take a couple of hours for the room simulator to achieve the experimentally designed conditions. Once these were reached, measurements were taken.

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

Most room air terminal devices, including ACBs, are tested in the laboratory under isothermal and symmetric boundary conditions. However, actual field conditions result in unexpected design and operational challenges that are seldomly considered by designers and often result in occupants’ discomfort and complains. We hope that the results from this research will motivate designers to sharpen their designs by taking a closer look at the open office configuration, the location of the occupants, the office exposure to solar irradiation, and even the thermal performance of the windows, in selecting, sizing, and orienting the ACBs.

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

ACBs are energy-efficient air terminal devices. However, just like with any other terminal device, ACB performance is affected by the room configuration and its exposure to the elements that will challenge achieving room thermal uniformity and effective air distribution. It is important to follow the manufacturer guidelines on ACB orientation. However, specific room conditions may challenge these orientation guidelines. The performance of ACBs is highly reliable in cooling operation. However, synergistic room air forces caused by high room solar gains may result in cold air drafts in the ankles of the occupants at the opposite side of the room. The performance of ACBs is mainly challenged in heating operation where increased risks of ACB supply air short-circuiting the room, and room thermal stratification, may result in occupants’ discomfort and reduced air quality. Our experience with the field study taught us that HVAC designers sometimes sacrifice selectivity and specificity in specifying ACBs for the room’s particular conditions, in favor of convenience and economies of scale in selecting all ACBs generically based on office types. 

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

This research brings new evidence regarding the importance of practicing integrated design at all levels. These levels include whole building, plant, distribution system, and most importantly at the room level with consideration for room orientation, solar exposure, window performance, and occupants’ location in open offices. Because ACBs are high-performance devices, their performance is often taken for granted. Our field experiments in private offices that were available to us, gave us the opportunity to talk to the occupants of the nearby open offices, which sparked our interest in testing ACB performance in the laboratory for those types of offices. In particular, we found the selection of ACBs in open offices to be far more challenging. Another aspect worth noting, which was not tested in this research, is that the type of internal cubicle partitions in those cellular offices will affect air distribution. Some partitions are sealed at the bottom. In our laboratory experiment, the partitions were only at the desk, visual, level, which allowed the air to circulate at the floor level, unlike the full floor-to-visual partitions. In summary, aside from the room thermal loads, the sizing and selection of ACBs in open offices needs to consider three factors: the envelope performance and exposure to the elements, the occupants’ location and the types of partitions and furniture in the room. Further research is needed to test the effects of different types of room partitions on ACB performance in cellular open offices. 

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

In our field ACB study, we found the differences in sensitivity and attitude of the occupants a difficult factor to deal with in design of room air terminals. While some occupants are not sensitive to minor discomfort, others are very sensitive. For example, while ASHRAE Standard 55-2017, Thermal Environmental Conditions for Human Occupancy, comfort fed by our measurements indicated that an office occupant was uncomfortably cool most of the time, when asked, the occupant was perfectly comfortable. This was not so in other offices. The laboratory experiment taught us how challenging it can be to specify ACBs for open offices with asymmetric boundary conditions. An idea that came from this research is for ACB manufacturers to improve ACB designs by enabling the differential tuning and adjusting of each side of the ACB. In this way, once installed, their airflow settings are not fixed but can be differentially adjusted to the conditions of the room.