Saint Xavier: a model student for getting the most from CFD
Rather than simply adding another residence hall to their southwest Chicago campus using traditional design and building methods, Saint Xavier University seized the op-portunity to create a performance structure using the highest standards of green tech-nology available. One of the important examples of green technology used in Saint Xa-vier’s new Arthur Rubloff Hall is the use of displacement ventilation, in which ventilation air is delivered directly to heat sinks such as people.
Since this technology is relatively new, Environment Systems Design, Inc. (ESD), the designer of the mechanical systems, simulated the ventilation system with computa-tional fluid dynamics (CFD).
“The simulation evaluated the comfort of the building under a wide range of conditions and demonstrated that the new design, which uses displacement ventilation, would not only be energy efficient but also comfortable and healthy,” said James Vallort, ESD engineer. “The result is that Arthur Rubloff Hall is the first university residence hall in Illinois to obtain a Certified Gold Level rating under the LEED® green building system and provides 30% energy savings compared to traditional designs.”
NOT JUST ANOTHER UNIVERSITY RESIDENCE HALLA $9 million, 37,000 sq-ft residence hall, Arthur Rubloff Hall will allow Saint Xavier to keep up with growing demand for on-campus housing. The new building has enabled the uni-versity to welcome an additional 88 students to live on its campus. Arthur Rubloff Hall con-sists of four floors of residential student housing including apartment-style rooms, six one-bedroom units, a penthouse conference room, and green roof garden. The five-story building also features a colonnade connecting the new residence hall to another residence hall. The ground floor includes a reception area, offices, and storage.
The builder is Henry Bros, with construction and architectural services from Solomon Cord-well Buenz and Associates. ESD designed the mechanical, electrical, plumbing, and fire protection systems, was the lead in LEED certification, and also handled the commissioning of the MEP systems.
ESD started with a target of reducing energy consumption by 30% compared to ASHRAE’s 90.1 energy standard for buildings. “Achieving this level of savings required a break from conventional design methods and ESD engineers felt this could best be achieved by using displacement ventilation,” Vallort said. Conventional ventilation systems deliver air at a temperature of approximately 55°F from diffusers located in the ceiling. The air is delivered at a high velocity so that it will thoroughly mix the fresh cool air with the warm air in the room.
Displacement ventilation, on the other hand, uses diffusers at ground level to deliver air at a lower temperature and velocity. The heat generated by people and other heat sources in the room creates plumes of hot air rising to the ceiling. This in turn produces a void that is filled by the cool air coming from the displacement ventilation diffusers. The flow rising from the heat sources also tends to remove contaminants and maintain high levels of air quality.
DISPLACEMENT FINDS A HOME“Displacement ventilation provides energy savings by concentrating cooling in the areas occupied by people and allowing the other areas of the building to be warmer,” Vallort said. “The savings come from only having the ventilation system deliver air in the neigh-borhood of 65° and also from operating the diffuser fans at a much lower speed.” During the summer months, humidity levels will be unacceptably high if the air is simply cooled to 65° in a conventional air handler. This challenge was overcome on this project by using an AHU-1 dual path air handler. The dual path air handler has two separate cooling coils, one on the outdoor airstream and one on the return airstream. The outdoor air is cooled to 50° in order to provide humidity control and is then mixed with the tempered return air to reach the 65° supply air temperature.
The university liked the energy savings provided by the displacement ventilation approach but wanted to be sure it could achieve a high level of comfort and indoor environmental quality. ESD addressed these concerns by simulating the operation of the displacement ventilation system. ESD engineers used Flovent software from Mentor Graphics Mechanical Analysis Division (formerly Flomerics) to analyze the building. Flovent is designed specifically for modeling heating and cooling applications so it is both easier to use and more powerful than general purpose CFD codes.
ESD engineers used hand calculations to determine the cooling load and specify the chillers and fans. Of course, the hand calculations did not take the geometry of the building or location of heat sources into account nor were they capable of determining temperature, humidity, and airflow in specific areas of the building.
MODELING THE BUILDING WITH CFDThe CFD model provides a much more detailed and accurate simulation of the environ-mental conditions in the building. “ESD engineers created this model by defining the ge-ometry of representative rooms within the building based on the architectural drawings,” said Aliza Skolnik, ESD engineer. “The heat gain through the windows of the rooms was modeled by incorporating both radiant and convective heat sources. The mechanical sys-tems of the building were modeled including the location of the diffusers and how much air was delivered at what temperatures. The diffusers are important to the accuracy of the simulation but are difficult to model because their size is so small in relation to the building.”
ESD engineers overcame this problem by using the Diffuser SmartPart (included with Flovent) to create a compact model of the diffusers that closely match the performance of the diffusers without having to model their geometry in detail. The extract units in the ceiling, consisting of rings near the outer edges of the walls, were modeled using another Flovent SmartPart called a “fixed flow device.” Different numbers of people were included in the simulations and they were modeled as heat sources. Lights and a project were also included in the model of the conference room as heat sources. The rooms were modeled under both average and peak summer conditions.
SIMULATION VALIDATES DESIGN CONCEPT“The results of the simulation clearly show the displacement ventilation system working as intended,” Skolnik said. “The simulation results helped the building owner feel comfortable with the displacement ventilation concept, and verified the significant energy savings achieved by this method.”
When the building was completed, ESD engineers performed a variety of tests as part of the commissioning process. These included personal comfort tests and temperature measurements. “The tests showed that the actual temperatures matched the simulation predictions within +1°,” Vallort said.
“The LEED Gold designation is further confirmation of Saint Xavier’s commitment to pro-vide cutting-edge facilities for our students,” said Saint Xavier University President Judith A. Dwyer, Ph.D. “Arthur Rubloff Hall sets the new standard in Chicago and in the state for environmentally friendly construction on a university campus.” ES