Displacement Ventilation(DV)& Air Conditioning

  DISPLACEMENT VENTILATION (DV)

What is displacement Ventilation (DV)?

The Displacement Ventilation (DV) is a means of providing cool supply air directly to the occupants in a space.The fresh air supplied near the floor at a very low velocity falls towards the floor due to gravity and spreads across the room until it comes into contact with heat sources.

Displacement Ventillation

The cool supply air slowly rises as it picks up heat from occupants and equipment. The warm, stale air rises towards the ceiling where it is exhausted from the space. This vertical airflow pattern near each occupant often referred to as a thermal plume, makes it less likely that germs will spread. The air distribution system provides for effective ventilation, since the fresh supply air is delivered directly to each occupant.The room air become contaminated when mix with supply air . Most commercial buildings in the United States use this type of overhead distribution system.

 Applications of DV:

It’s applications are in:

• Schools
• Restaurants
• Theaters
• Auditoriums
•  Atria and other open spaces with high ceilings.

UFAD (Underfloor Air Distribution):

There is similar concept but the pattern of air is somewhat different than DV. In this concept air is supplied from an under floor plenum to floor outlets at a low velocity. The air velocity with UFAD is lower than that of overhead air distribution.

Under Floor Air Distribution

Selection of HVAC Equipement:

The HVAC equipment with chilled water coil is the  best cooling source for a DV system. The control valve in a hydronic system allows for the supply of constant 63 F -65º F air. A typical direct expansion (DX) system is designed to provide colder 50º F–55º F air while the compressor is running and cycles on and off to meet space loads. When the compressor is off, the supply air temperature can rise to 75º F or higher. This lower temperature and larger temperature fluctuation would create a comfort problem with DV when the supply air comes in contact with occupants. However, larger DX systems with several compressors and temperature-reset capabilities will provide tighter supply temperature control and can be used as an alternative to a chilled water system.

Application of DV for Schools:

In field of Educational the classrooms  require significant amounts of outside air to maintain acceptable IAQ (Internal Air Quality). Since DV has a higher ventilation effectiveness. It provides a minimum outside air ventilation of 15 cfm per person and will have equivalent effect of a higher outside air rate from a mixing system (up to 20 cfm/person). Thus, DV can improve IAQ without increasing energy use

DV in Classrooms

Two wall-mounted diffusers provide for a steady supply of cool, fresh air for the typical 960-ft2 classroom.

Analysis performed under the California Energy Commission Public Interest Energy Research (PIER) IEQ Program3 showed that typical California classrooms require about 1,100 cfm of 65° F supply air at design cooling conditions. DV can also be used effectively in libraries,auditoriums, and gymnasiums.

Application of DV for Restaurants:

The air distribution from DV is especially useful in restaurants, another area where IAQ is of the highest importance. The vertical air movement towards the ceiling helps to remove contaminants and prevent the spread of germs.

Application of DV for Theater or Auditorium:

This is another open space that is well suited for DV. With stadium   eating, air can be supplied from underneath the seats directly to the occupants. DV helps to meet the demanding acoustic requirements of theaters and performing arts centers.

Displacement Ventillation (DV) in theaters

Application of Dv for Offices:

Dv is the most effective when office spaces are  high ceilings . Larger office spaces with open floor plans and partitions are excellent candidates for either underfloor air distribution or DV. Internal offices and conference rooms that requires steady cooling load can be well effected by DV. Perimeter offices or other spaces with low internal heat gains are not as well suited for DV. However, DV can still be used to provide ventilation or cooling, but such spaces may require supplemental heating near exterior windows to maintain comfort.

DV for Cool Air Supply

Industrial Spaces:

DV can also provide air quality and energy benefits for industrial spaces with open ceilings, and has been used this way in northern Europe since the 1970s. Industrial processes that generate dust, debris, and other pollutants can adversely affect workers. DV is effective when the contaminants are associated with heat sources, so they can be carried away by buoyancy forces towards the ceiling exhaust. It is not effective in biological laboratories or facilities where contaminants are heavier than air.

Limitations of Displacement Ventilation:

Despite its many benefits, there are conditions and applications that are not as well suited for DV. The primary space constraints are ceiling height and the wall space required for the diffusers. DV is not suitable for heating.

Ceiling Height:

The minimum height of ceiling for DV system is  9 ft . High ceilings are necessary to allow internal heat gains and contaminants to be effectively carried to the upper portion of the room.

Design Cooling Loads:

The studies recommend the use of supplemental cooling, such as chilled ceiling panels, for design cooling loads in excess of 8–10 Btu/h-ft2 . Recent research by ASHRAE and PIER program4 has shown that DV can provide for effective cooling and good comfort for spaces with cooling loads as high as 25 Btu/h-ft2 . For higher loads, a large diffuser area is required to supply sufficient air volumes while maintaining a low discharge velocity.

Load Calculations and System Sizing:

Rough Calculations:

As a first-order approximation, for a space with a ceiling height of 9–10 ft, the return air temperature can be assumed to be about 5°F warmer than the air temperature at the thermostat. Although this is a crude approximation, it can provide a first estimate at determining the required design airflow in cooling and the required system capacity. The use of higher ceilings (above 10 ft) that allow for greater displacement of internal loads will result in a higher return air temperature. Spaces with relatively low ceilings (9 ft or less) and high cooling loads will have a return air temperature that is closer to the room temperature.

The detail on Displacement Ventilation is not finished yet. More detail will be in next article which will be published soon.