Displacement ventilation for a better indoor climate
Since the energy crisis of the late 1970s, the building industry has made buildings more energy efficient and airtight. This allows less fresh air to enter through the skin of the building envelope, leaving us with just one source of fresh air — the air conditioning system.
Air conditioning systems have improved significantly over the past decades, with new products and solutions. However, air-distribution systems and air terminals — the devices that deliver fresh air to building inhabitants — are far behind.
Look up: See that diffuser above your head? It’s a good chance your grandpa saw the same thing. No wonder architects are trying to hide these mixing diffusers out of sight, making air-distribution system efficiency even worse.
Air distribution is all about having your portion of fresh air delivered to you and not to a return diffuser.
Displacement vs. mixingThere are two types of air-distribution systems. The traditional one is mixing ventilation, where the main purpose is to efficiently mix supply air with room air.
The contaminants in the room are diluted by supply air and then extracted through the exhaust grille (Figure 1). Displacement ventilation (Figure 2) does not mix the impurities; rather, it displaces them into the upper, uninhabited zone from where it is extracted.
With a displacement ventilation system, the supply air temperature is slightly lower than room air and supplied at low velocity directly into the occupied zone1. The impurities and heat loads in the space are carried by convective flows created by heat sources into the upper part of the room. Air in the upper zone, having a higher temperature, does not recirculate back into the occupied zone. That’s why the temperature and concentration of impurities at the exhaust outlet is higher than in the occupied zone.
The main objective is to maintain the temperature and concentration in the occupied zone at design conditions, while allowing higher temperature and concentration levels to rise above the occupied zone.
Instead of maintaining design conditions in the whole room, the displacement ventilation system works where it is needed — in the occupied zone — thus saving energy required to air condition the space.
Where to use a displacement systemDisplacement ventilation systems have been successfully used in Northern Europe over the last 20 years.
Initially used in industrial applications, designs have been successfully used for the ventilation of offices and other commercial spaces where, in addition to energy conservation, indoor air quality and comfort are important considerations.
Displacement ventilation is efficient to use in spaces with high heat loads, where contaminants are carried together with the warm convective flows.
Recent studies conducted at MIT2 show that the load can be as high as 38 Btu/sq ft (120 W/cu m). Displacement ventilation is especially effective when designing air conditioning for spaces with high ceilings (over 9 ft).
Displacement ventilation is mostly recommended in theatres, auditoriums, atriums, restaurants, commercial kitchens, gyms, stores, and factories. It is not recommended to use displacement ventilation when contaminants are heavier than air.
Since supply diffusers for displacement ventilation are typically installed in the occupied zone, the finish, quality, and diversity of models and shapes is important for seamless integration of diffusers into interiors of different spaces. Displacement diffusers can also be disguised, painted, or screened, allowing them to merge into a chosen background by the careful use of paint, cover plates, or grilles.
There are quite a few successful applications of displacement ventilation in the U.S., in gyms, schools, casinos, restaurants, shopping centers, and office buildings.
Design for U.S. climatesIt is not the intention of this article to give detailed design guidelines. However, we will try to point out the main considerations when designing displacement ventilation systems.
Two major points must be considered when designing airflow rates for displacement ventilation: the energy balance and convective flows.
First, the shift zone height (Figure 3) is specified; it is the upper border of the occupied zone. The warm air being carried by convective flows from heat sources will stratify above the shift zone. The most accurate method is to calculate the amount of supply air to be equal to the total amount of air being carried by convective flows at the shift zone height.
Since diffusers supply air directly into the occupied zone, special attention should be paid to the supply air temperature.
Supply temperature of 55Â°F (13Â°C), commonly used in mixing air-distribution systems in U.S., can be used in displacement ventilation systems only in industrial applications and some specific applications, like gymnasiums. In most cases, the temperature difference between supply air and design room air temperature should not exceed 10Â°F (5.5Â°C).
The air is supplied through the displacement diffuser at low velocity3, and its throw length depends on supply temperature (the lower the supply temperature, the longer the throw).
Thus, the further the occupants are from a diffuser, the lower supply temperature is allowed. Diffuser height and design also significantly affect throw length.
Originally designed for Northern Europe, displacement ventilation systems are 100% outside air systems. This approach could be used in some U.S. applications with high outdoor air requirements and mild cooling loads.
However, in most of the applications, air recirculation should be used. It is more efficient to take return air from the occupied zone, since air enthalpy in the zone is lower than in the upper zone of the rooms with displacement ventilation, not at the ceiling (as in a mixing system design).
As you can see, displacement air distribution requires a different design approach from mixing system design and calculation methods. A number of hvac consulting companies in this country are already familiar with displacement ventilation design methods, and have successfully implemented them.
Interest in displacement ventilation is growing as this form of air distribution addresses the need for improved indoor air quality. Several recent applications in the U.S. attest to the viability of displacement ventilation from an IAQ and energy-conservation perspective.
Halton Co. specializes in thermal displacement ventilation systems and can be contacted at 101 Industrial Drive, Scottsville, KY 42164; 502-237-5600; 502-237-5700 (fax); www.halton group.com (Web site).