Volume 23, number 1, february 2009

Increase the occupant’s comfort by opting for displacement ventilation

Ideal for classrooms, restaurants, theatres or supermarkets, displacement ventilation,
compared to dilution ventilation, improves air quality and can reduce energy consumption.

Definition

Displacement ventilation (DV) is defined by the use of air stratification to keep an environment free of contaminants. On the other hand, dilution ventilation, as its name indicates, dilutes contaminants with fresh air intake.

History

It is generally admitted that the first to have studied and mentioned the principle of displacement ventilation (associated more precisely with the industrial field) was W. Baturin.1 Baturin was a pioneer on the
subject in the 1940s. However, the design of such systems in that period was based on relatively unreliable rules of thumb.

During the 1970s, protection of workers in the industrial environment became a priority, translating into the use of improved ventilation systems. Displacement ventilation was one of the avenues studied.

Subsequently, in the 1980s, when indoor air quality became fashionable, the principle was improved.

From then on, a more scientific approach was necessary to meet the every more insistent demand for this type of ventilation. This led to the DGB Design Guide Book,2 containing the fundamental principles of displacement ventilation in the industrial environment.

Nowadays, the issue of energy efficiency is focusing attention on this ventilation technique, applicable to various non-Industrial environments.

Based on this new need, displacement ventilation was developed to ventilate spaces more efficiently, whether in an industrial or commercial or institutional environment.

Use

Displacement ventilation is much more common in Europe than in North America. Moreover, the European equivalent of ASHRAE, REHVA3, provides a goldmine of information on the subject.

All in all, the principle is very simple, it involves displacing contaminated air from the floor to the ceiling, where the return air vent are located.

Displacement ventilation seeks to displace surplus heat and contaminants to the ceiling in order to improve air quality in the occupied space.

In DV, relatively low air flows (<10 l/s) are induced at floor level at a temperature of about 18°C (65°F). This induced air is heated by the room’s internal ducts (office equipment, heating, people). As this air is heated, it produces a rising movement (convection plume), just as a piston would do in a cylinder, and produces a stratification zone free of contaminants and a hot air and contaminants mixing zone, with the contaminated air with the highest temperature near the ceiling.

Given that the displacement diffusers supply air to the occupied zone, special attention is required during supply to avoid an excessively high temperature gradient between an occupant’s feet and head.

Discomfort can be induced by the air current and the temperature gradient can prove critical at that time.

The ASHRAE 55 standard recommends a 3 K temperature difference between 0.1 m and 1.1 m above floor level. However, some researchers question these recommendations and mention that their research did not show any significant discomfort during stratification of 4 K/m.

On the other hand, REHVA mentions that the temperature variation according to height would be 1.5°C/m in the case of non-industrial applications for a flow of q = 20 l/s.

For more details on displacement ventilation, see the ASHRAE RP-949 standard, which deals with the subject.

ASHRAE also recommends never supplying air at a temperature lower than 18°C. However, air at 13°C can be used in industrial environments or for specific applications, such as gymnasiums.

Still according to ASHRAE, displacement ventilation, compared to dilution ventilation, improves indoor air quality and in some cases reduces energy consumption during the air conditioning period. This type of ventilation is recommended for classrooms, conference rooms, theatres, restaurants, supermarkets and spaces with a ceiling more than 3 metres high.

Air distribution under the floor

Design

The basic principle of displacement ventilation design differs from that of dilution ventilation. Most systems operating by dilution are designed on the single zone model, in which the air is mixed uniformly throughout the space and it is assumed that the return air temperature is the same as room temperature.

In the case of displacement systems, the ventilated space is divided into two zones: the low zone, which is occupied, and the high zone, which is unoccupied. The air temperature is only measured in the occupied space. In the unoccupied space, the temperature is always higher because of the stratification produced by natural convection.

Depending on the needs of the ventilated space, there are two possible options for displacement design.

The temperature-based design is used when temperature control is the main goal (e.g. schools, offices, auditoriums, sports facilities). The transition zone method is mainly used when contaminants are present, as in smoking rooms or places containing gaseous contaminants (with density less than or equal to the density of air), and are associated with the plume effect due to the heat sources.

While the ultimate goal of the temperature-based design method is simply to satisfy the temperature conditions in the occupied zone, the transition zone method seeks to stratify the contaminants above the zone where the occupants breathe by maintaining this transition zone as high as possible.

According to the reference guides, such as REHVA and ASHRAE, Under Floor Distribution (UFD) is mentioned for displacement ventilation, as mentioned above.

The advantages of such a system are better air quality, better diffusion, energy saving potential (free cooling), reduction of the level due to reduced air speed, and ease of development.

According to REHVA, for Europe, about 55 or more days can be expected where free cooling can be used because a higher air supply temperature can be permitted.

Moreover, the displacement ventilation principle is recognized by LEED certification in the chapter on Indoor Environment Quality (IEQ), Improvement of Indoor ventilation, and is worth 2 credits.

Daniel Gendron, Eng.
Technical Advisor
DATECH Group

1. Baturin W (1972) Fundamentals of Industrial Ventilation. Pergamon, NY

2. Goodfellow, H. and Tähti, E. (editors) (2001) Industrial Ventilation Design Guidebook, Academic Press, ISBN 0-12-289676-9

3. REHVA Federation of European Heating and Air Conditioning Associations.