Thermal Displacement :
For better indoor air climate
Dr. Andrey Livchak,
Why Bother With It?
Whether we like it or not, we spend most of our time indoors and this situation is unlikely to change in the foreseeable future. It is difficult to estimate the effect the indoor air climate has on our health, productivity, and overall enjoyment of life. Since the energy crisis of the late 1970's, the building industry has made our buildings more energy efficient and air tight. This allows less fresh air entering through the skin of the building envelope, leaving us with the 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 that precious fresh air to building inhabitants, are far behind the progress. Look above, do you 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 and Mixing Ventilation, what is the difference?
There are two types of air distribution systems. The traditional type is mixing ventilation. It's 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 grill (Fig.1). Displacement ventilation (Fig. 2) does not mix the impurities, 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 occupied zone . 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 re-circulate back into the occupied zone. That is why the temperature and concentration of impurities at the exhaust outlet is higher than in the occupied zone.
A typical distribution of temperature and concentration of impurities in a room with displacement ventilation is shown in Fig. 3. 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, displacement ventilation system works where it is needed - in the occupied zone, thus saving energy required to air condition the space.
Figure 3 - Temperature and concentration graphs in a
typical room with displacement ventilation
Where to use Displacement Ventilation?
Displacement ventilation systems have been successfully used in Northern Europe during the last twenty 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 MIT show that the load can be as high as 38 Btu/ft2 (120 W/m2). Displacement ventilation is especially effective when designing air conditioning for spaces with high ceilings, over 9 ft. Displacement ventilation is mostly recommended in theaters, 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, their finish, quality and diversity of models and shapes is important for seamless integration of diffusers into interiors of different spaces (Fig. 4).
Displacement diffusers can also be disguised, painted or screened allowing them to merge into chosen background by the careful use of paint, cover plates or grilles (see photos below). There are quite a few successful applications of displacement ventilation in the USA - in gyms, schools, casinos, restaurants, shopping centers and office buildings.
Displacement Ventilation - design for US climate
It is not the intention of this short article to give detailed design guidelines. However, we will try to point out the main considerations when designing displacement ventilation systems.
There are two major points to be considered when designing airflow rates for displacement ventilation: the energy balance and convective flows. First, the shift zone height (see Fig. 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 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 are supplying air directly into the occupied zone, special attention should be paid to the supply air temperature. 55°F (13°C) supply temperature, commonly used in mixing air distribution systems in USA, can be used in displacement ventilation systems only in industrial applications and some specific applications like gymnasiums. In most of the cases, the temperature difference between supply air and design room air temperature should not exceed 10°F (5.5°C). The air is supplied through the displacement diffuser with low velocity and its throw length depends on supply temperature - the lower the supply temperature the longer the throw (Fig. 8). Thus the further the occupants from a diffuser, the lower supply temperature is allowed. Diffuser height and design also significantly effects its throw length.
Originally designed for Northern Europe, displacement ventilation systems were 100% outside air systems. This approach could be utilized in some of US applications with high requirements for the amount of outdoor air and mild cooling loads, such as schools, theaters and other similar applications. However, in most of the applications air re-circulation should be used. It is more efficient to take return air from the occupied zone since air enthalpy in the occupied zone is lower than in the upper zone of the rooms with displacement ventilation, not at the ceiling as in mixing system design.
As you can see displacement air distribution design requires a different 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 critical need for improved indoor air quality. Several recent applications in the USA attest to the viability of displacement ventilation from an IAQ and energy conservation perspective.