Ventilation displacement (DV) This is a room air distribution strategy where conditioned outdoor air is supplied at low speeds from air supply diffusers located near the floor and extracted over occupied zones, usually at sky heights -sky.
Video Displacement ventilation
System design
Special ventilation systems, such as in office space, supply cold air from the air handling unit (AHU) through a low induction air diffuser. Different types vary by app. Diffusers can be placed on the wall ("wall-mounted"), in the corner of the room ("angle-mounted"), or on the floor but not against the wall ("free standing"). Cool air accelerates because buoyancy, spreading in a thin layer above the floor, reaches a relatively high speed before rising due to heat exchange with heat sources (eg, occupants, computers, lamps). Absorb heat from a heat source, cold air becomes warmer and less dense. The difference in density between cold air and warm air creates an upward convective stream known as hot clots. Instead of working as a stand-alone system in an interior space, the displacement ventilation system can also be combined with other cooling and heating sources, such as radiant cold ceilings or pole heating.
Maps Displacement ventilation
History
Ventilation displacements were first applied in an industrial building in Scandinavia in 1978, and have been frequently used in the same application, as well as office space, throughout Scandinavia since then. In 1989, it was estimated that Resettlement ventilation comprised 50% in industrial applications and 25% in offices in the Nordic countries. Applications in the United States have not been widespread as in Scandinavia. Several studies have been undertaken to assess the practicality of these applications in the US market due to distinctive distinctive spatial and application designs in hot and humid climates, as well as research to assess the potential for indoor environmental quality and energy-saving benefits of this strategy in the US and elsewhere.
Apps
Ventilation displacements have been implemented in many famous buildings such as Suvarnabhumi International Airport in Bangkok, Thailand, the NASA Flight Flight Propulsion Flight Jet Building, and the San Francisco International Airport Terminal 2 between other applications.
General characteristics
Airflow distribution
The flow of heat and air supply from the diffusers, which determines the speed of airflow on the floor, plays an important role in the DV system. You need to set the airflow speed of the diffuser to avoid drafts.
Conditioning type
Due to the unique nature of thermal stratification, the displacement vents are usually used for cooling rather than for heating. In many cases, separate heating sources, such as radiators or pedestals, are used during the heating period.
Space requirements
Rescue vents are best suited for higher spaces (more than 3 meters [10 feet]). Standard mixing vents may be more suitable for smaller spaces where air quality is less apprehensive, such as a single occupant office, and where the height of the room is not high (for example, lower than 2.3 meters [7.5 feet]).
Benefits and Limitations
Local inconvenience: vertical and draft temperature differences
The displacement ventilation system is quieter than the conventional overhead system with better ventilation efficiency. Therefore, it can improve indoor air quality and provide the desired acoustic environment. The displacement ventilation system corresponds to the space in which high ventilation is required, such as classrooms, conference rooms, and offices.
Ventilation removal can be a cause of discomfort due to gradients and large vertical draft temperatures. According to research by Melikov and Pitchurov, cold sensations caused by vertical temperature differences and designs typically occur at the bottom of the foot/ankle/leg, while warmer sensations in the head are reported. The study also shows that concept modeling can predict the design risk with good accuracy in rooms with displacement ventilation systems.
There is a tradeoff attached to these two problems: by increasing the flow rate (and the ability to remove larger heat loads), the vertical temperature gradient can be reduced, but this may increase the risk of draft. Pairing the displacement vents with a series of cold ceilings is an attempt to address this problem. According to some studies, the displacement ventilation system can only provide acceptable comfort if the corresponding cooling load is less than about 13 Btu/h-sf or 40 W/m 2 .
Indoor air quality
One of the benefits of ventilation displacement is probably superior indoor air quality achieved with the exhausting contaminated air out of the room. Better air quality is achieved when the source of pollution is also a source of heat.
Energy consumption
Several studies have shown that displacement ventilation can save energy compared to standard mixing ventilation, depending on the type of building use, design/mass/orientation, and other factors. However, for the evaluation of energy consumption of displacement vents, numerical simulation is the main method, since annual measurements are too expensive and time-consuming. Therefore, whether removal vents can help save energy is debatable. In general, displacement ventilation draws to the core area within the building as it does not require heating. However, the perimeter zone requires high cooling energy.
Design guidelines
Different guides have been published to provide guidance on designing displacement ventilation systems, including:
- Skistad H., Mundt E., Nielsen P.V., Hagstrom K., Railo J. (2002). Ventilation displacements in Non-Industrial Places. Federation of Heating Associations and AC Europe.
- Skistad, H. (1994). Ventilation removal. Research Studies Press, John Wiley & amp; Sons, Ltd., western Sussex. UK.
- Chen, Q. and Glicksman, L. (2003). Performance Evaluation and Development of Design Guidelines for Ventilation removal. Atlanta: ASHRAE.
Among the guidelines listed above, developed by Chen and Glicksman are specifically addressed to meet US Standards. Below is a brief description of each step of their guidelines.
Ã, Ã, Step 1) Assess the application of displacement vents
Step 2) Calculate the cooling load of the summer design.
Step 3) Determine the required flow rate of supply air for summer cooling.
Step 4) Find the fresh airflow rate required for acceptable indoor air quality.
Step 5) Determine the air supply flow rate.
Ã, Ã, Step 6) Calculate the air supply flow level.
Step 7) Determine the ratio of fresh air to the supply air.
Step 8) Select the size and number of the air diffuser.
Step 9) Check the winter heating situation.
Step 10) Estimate the first cost and annual energy consumption.
List building using Displacement Ventilation (DV)
References
Source of the article : Wikipedia
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