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Displacement Has Its Place

Aug. 26, 2013
Displacement-ventilation systems introduce cool air into a space at low face velocities and let convection and stratification move warm contaminated air up to returns.

When designing a cooling system for a large open space with a high ceiling, such as an open-floor-plan office, auditorium, casino, restaurant, or theater, consider a displacement-ventilation (DV) system, in which cool supply air displaces warm room air at very low velocities. DV systems offer energy efficiency, indoor-air-quality (IAQ) benefits, and low maintenance costs compared with traditional mixed systems.

DV falls into one of the three main categories of air distribution defined by ASHRAE: fully mixed systems; partially mixed systems, in which mixing occurs at a low level and stratification occurs at a high level (such as underfloor heating systems); and fully stratified systems, in which displacement is the key component.

In a DV system, cool supply air is introduced directly into an occupied space at very low velocities. The supply air is introduced at a slightly higher temperature than traditional mixed-system supply air, and therein lies some of the energy savings. The warm, contaminated air in the room is displaced by the cooler supply air, and is pushed to returns in the ceiling (Figure 1). As a result, DV systems generally cool just the occupied space—from floor level to about 6 ft. In large spaces, such as auditoriums, it makes no sense to cool an entire 25-ft-ceiling-height room if the occupants are only using the first 6 ft of it.

FIGURE 1. Displacement ventilation model showing stratification as supply air entering the occupied space encounters a heat source and a convection plume lifts contaminants into the unoccupied zone to the return.

The typical supply-air temperature in a DV system is 63°F to 68°F. This is considerably warmer than traditional mixed-system supply air provided through diffusers and grilles. Additionally, DV systems provide air at a lower velocity than mixed systems. With an overhead diffuser or grille, face velocities can be as high as 400 to 500 fpm, whereas DV face velocities generally are 50 to 90 fpm.

ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, defines required fresh air changes; floor supply of cool air and ceiling return of warm air as found in a DV system results in a reduced requirement for ventilation air. DV systems require 17 percent less ventilation air than traditional fully mixed systems. That results in energy savings because lower-pressure systems require less fan power than higher-pressure systems, and pressure is money.

Choose Applications Carefully

The locations of displacement diffusers should be chosen carefully to avoid stationary occupants within the near zone of the diffuser. The air velocity at which occupants will begin to feel air upon them generally is 50 fpm. Even at a DV system’s low face velocity of 70 to 90 fpm, there is an area very near the diffuser faces where an occupant could feel a draft. This is described as an adjacent-zone or near-zone draft, and it limits some of the applications in which displacement diffusion can be used. The optimal customization of a DV near zone can be accomplished by using diffusers that have pattern controllers.

The applications for DV are not universal; they are best for large spaces in which occupants are not located directly adjacent to the displacement diffusers. It generally is not advisable to locate a displacement diffuser in an area where an occupant is going to be permanently stationed, such as near a desk where someone is going to be sitting all day.

DV systems are not well-suited for use in rooms with ceiling heights of less than 9 ft. Low ceiling height can make removing the stratified heat from an occupied space difficult for a DV system.

Displacement diffusers should not be used in conjunction with mixed systems. Traditional supply grilles or overhead diffusers mix the air , which defeats the purpose of  stratification.

DV is not appropriate for areas with many obstructions. Cubicles in an open-floor plan, for example, can be problematic for displacement diffusion because the supply air moves slowly throughout the occupied space until it hits a heat source or an object. A cubicle wall would stop the flow of low-velocity air to the cubicle. The problem could be solved by careful placement of diffusers and alignment of cubicles, but spaces with numerous obstructions to airflow are not ideal applications for DV.

Displacement diffusion can reduce energy usage in a building by allowing increased use of a system’s economizer mode. Because the system delivers air at a higher supply temperature than a mixed system, there is potential for increased economizer operation in which the outside temperature is between 55°F and 65°F.

The use of low-horsepower fans in displacement systems results in reduced operational expense. The decreased energy demand of DV systems provide opportunities for a building to earn Leadership in Energy and Environmental Design (LEED) credits.

Numerous Mounting Options Increase Versatility

A wide variety of displacement diffusers are available. They can be used unobtrusively in a variety of environments, which is important from an architectural perspective.

There are displacement diffusers that mount in the corners of rooms, flush in walls, or around columns, and these diffusers can be blended into an occupied space in places where they are not going to affect adjacent-occupant comfort. There is a displacement-diffuser type that mounts directly into the risers of stairs and podiums, and it addresses perfectly the requirements of a room in which the goal is to cool just the occupied space.
Another benefit of DV—and one of the reasons it is popular in schools, auditoriums, and theaters—is its quiet operation. The system pressure is low, the velocities are very low, and occupants do not hear the large volumes of air that normally are delivered with a fully mixed system.

One of the drawbacks of displacement diffusers is that they are large. Because they run at lower velocities, they must have more free area in the face. With larger displacement panels it often is desirable to have a wide variety of finishes available to blend the panel into the adjacent architectural elements around it. This often is achieved through custom powder-coat finishes or wood-grain or special finishes that can mimic stone or other materials that may be found in an architecturally aesthetic environment.

Schools Lead the Way

The largest application for displacement diffusers in North America has been schools. DV systems dovetail well with the California K-12 initiative, which the California Public School System developed to improve the IAQ, comfort levels, and energy-efficiency in public schools. A report prepared by the California Energy Commission’s Public Interest Energy Research (PIER) program found that DV systems, with their ability to clear contaminants such as carbon dioxide from indoor spaces, reduced the number of sick days and increased academic performance in schools while saving energy.1

DV system first costs are comparative to a traditional mixed system. Fewer displacement diffusers are needed compared with the ductwork, diffusers, and grilles required in traditional overhead systems, and displacement diffusers are installed at floor level, which eliminates much of the work above the ceiling.

Displacement diffusers require no more maintenance than traditional mixed-system diffusers and grilles. In fact, cleaning a DV system is arguably easier, because all the diffusers are located at occupant height, eliminating the need for ladders or scaffolding typically required to clean ceiling-mounted diffusers and grilles in a mixed-air system.

Selecting the right DV system can be difficult because these systems are relatively new to the North American market. Most engineers know how to lay out mixed systems but are less familiar with DV systems. Software can be an extremely important tool in designing and specifying displacement diffusers. Generally speaking, any manufacturer that is really serious about displacement diffusion should be able to provide state-of-the-art selection software for its systems.

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1. Eley C, and Arent J. Thermal DV: Improved IEQ and Reduced Energy Use. Prepared for the Coalition for Adequate School Housing Annual Meeting participants. Downloaded from http://www.cashnet.org/resource-center/resourcefiles/603.pdf

Mark Costello is a product manager for Titus. The company manufactures grilles, registers, diffusers, and terminal units.

DV Impresses a Tough Audience

Titus uses a displacement ventilation (DV) system at its training room at the company’s headquarters in Plano, Texas. The room, a large open space with high ceilings in a cooling climate, is well-suited to take advantage of the benefits of DV.

The company conducts training for consulting engineers, specifying engineers, and sales representatives, and many attendees comment about the comfort achieved using three small displacement diffusers in a space that would typically require eight or 10 traditional diffusers and associated ductwork. The quietness also impresses attendees: many often wonder if the system is even running, because of the diffusers’ quiet operation and low face velocities.

Titus headquarters in Plano, Texas, utilizing displacement ventilation in training room.