School districts in Southern California are looking to increase the number of their schools that have air conditioning in an attempt to improve learning environments. One technology many school districts are interested in is thermal displacement ventilation (TDV), which stratifies air by temperature so only lower levels need to be cooled.

AECOM, a global provider of professional technical- and management-support services, provided consultants to determine whether the use of TDV is feasible in Southern California schools and to provide guidelines on how the technology should be used. AECOM used computational-fluid-dynamics (CFD) software designed for building interiors to simulate the use of TDV in various classroom configurations. The consultants also developed guidelines for classroom suitability and furniture arrangement.

HOW TDV WORKS

TDV introduces cool air at about 65°F — compared with a standard mixed system's 55°F air — at a low velocity at a low level using a raised floor or terminals around a space's perimeter. Cool fresh air passes over heat sources, such as people and equipment. The heat sources warm the air, causing it to rise and accumulate moisture and contaminants, such as carbon dioxide (CO₂). This warm, moist, contaminated air collects at a high level — above the breathing zone — where it is removed from the space.

TDV reduces energy consumption because it is necessary only to cool occupied zones. Most high-level heat loads, such as from lights and heat gain through roofs and walls above 7 ft, do not have to be cooled if a 100-percent-outside-air system is used. Because a TDV system's supply-air temperature is higher than a mixed system's, free cooling is available more often. Finally, all contaminants, including CO2, collect at a high level, so the air quality of a well-designed TDV strategy is better than that of a standard mixed system with the same quantity of ventilation air.

CFD SOFTWARE DESIGNED FOR HVAC

The consultants used FloVent CFD software from Mentor Graphics' Mechanical Analysis Division (formerly Flomerics) to analyze TDV use in classrooms.

“We have a number of different CFD codes in our company,” Jim Saywell, an engineer for AECOM, said. “But I use FloVent whenever a project involves building interiors because it is designed specifically for modeling heating, ventilation, and air-conditioning applications, so it is both quicker and easier to use than general-purpose CFD codes.”

In the first part of the study, AECOM addressed whether TDV provided a practical, low-energy cooling solution for Southern California schools. For TDV to be effective, a ceiling needs to be high enough that warm, moist, contaminated air can collect without dropping into the breathing zone. FloVent was used to analyze identical classrooms with various ceiling heights to determine which schools or classrooms could best utilize TDV technology.

“It was decided that the best guidance we could give was never to implement TDV on a classroom with a ceiling lower than 8½ ft, and it was recommended that the ceiling height should be at least 9 ft,” Saywell said. “Some classrooms have sloped ceilings. In these cases, if the average height of the ceiling is above 9 ft, then this will be sufficient height for a TDV strategy to properly function.”

DIFFUSER LAYOUT

The next step was to find the most effective terminal layout, one that would have minimal impact on how teachers could make use of space. The goal was to take as little perimeter space away from the classroom as possible while still providing enough cooling and fresh air.

“The simulation results showed that providing two terminals with as narrow a width as possible, capable of supplying 600 cfm each, was sufficient to provide acceptable air distribution,” Saywell said. “Placing these terminals in opposite corners of the room is more effective than placing both in corners of the same wall. But this would mean running ductwork along two walls instead of one, possibly taking usable wall space away from the teacher. Placing terminals on opposite corners also might be impossible due to the location of a door or window. For this reason, it was considered acceptable from an air-distribution standpoint — and preferable from a space-efficiency standpoint — that both terminals be placed in two corners of the same wall.”

The next step was to find areas in which vertical temperature gradients or insufficient cooling were likely to cause thermal discomfort. A survey of several example classrooms revealed that most teachers use all available space around the perimeter of a classroom for bookshelves, boxes, and other objects. Thermal gradients were found to be significant when an object, such as a piece of furniture, was placed too close to a terminal, directing airflow toward occupants.