BSEE VENTILATION


Adversing: 01622 699116 Editorial: 01354 461430 W


Mike Gosling, Technical Manager, at TROX UK,


discusses the design consideraons for displacement venlaon systems and why such systems can provide a highly eecve alternave oering a number of benefits.


DISPLACEMENT VENTILATION Thinking outside the box


hen designing ventilation systems for open spaces, such as airport concourses, factory floors,


shops, or sports venues, the usual air handling strategies often fail to meet the requirements.


Typically, it is because the nature of the space throws up technical and practical challenges around the need to deliver fresh air over larger than average distances. In these instances, it can be advisable to reject traditional mixed air distribution strategies in favour of a fully-stratified air distribution approach harnessing displacement ventilation technology.


There are a number of potential benefits of a displacement ventilation approach (such as low noise, effective removal of airborne contaminants, and comparatively low energy requirements). There are, however, important design considerations that need to be addressed to ensure effective operation of the system.


Air fiow characteriscs uFigure 1.


Conventional mixed air distribution delivers hot or cold supply air at relatively high velocity from ceiling- mounted diffusers. As long as the ceiling diffusers are properly selected and positioned, this high velocity air does not result in occupant discomfort because it is delivered outside the occupied zone. See Figure 1.


When this slow moving air mass encounters a heat load, it rises and carries the heat (and pollutants) towards the ceiling, to be carried away by the extract air system.


Design consideraons


Key aspects to consider when planning a displacement ventilation installation include the following: Total cooling load – Displacement ventilation is typically recommended for cooling loads less than 60W/m² for comfort, however individual applications must be investigated to ensure comfort is achieved. Therefore, before selection, ensure the project requirements are within achievable limits. Heating requirement – Displacement ventilation requires the supply air to be cooler than the room air. So this approach is only suitable for cooling with a supply temperature range of -4K to -6K. As a general rule, heating is not recommended using displacement ventilation. Room height – The height of the room is an important factor in displacement ventilation design since there can be a considerable amount of mixing in the region below the ceiling. This arises due to the interaction between upward and downward moving buoyant air flows. Hence, it is better to have high ceilings. Generally, buoyancy-driven ventilation is less effective where ceiling heights are low, for example less than 2.5m. Adjacent zones – Displacement ventilation diffusers are ideal for spaces


furthest projection is no more than 10m. When room dimensions exceed 10m in length or width, it is best to place displacement diffusers on more than one wall. By placing diffusers on opposing walls, rooms up to 20m can be supplied from side walls. Another solution for large rooms is to place 360- degree diffusers throughout the interior space or by means of floor-mounted displacement terminals.


Correction also needs to be applied if multiple terminals are installed side by side. As with all other forms of ATD’s the throw (near zone) increases when multiple units are installed in a linear fashion. If the spacing between units (1 way discharge) is 500mm, the following factor should be applied: Extent of near zone for n units = near zone for 1 unit×n0.3


Advantages


As long as the above design considerations are addressed, displacement ventilation can provide a highly-effective alternative. It has particular advantages in sites such as auditoria, airport concourses, theatres, cinemas and conference rooms, in addition to sports halls, fitness rooms, restaurants, shops and factories. These benefits include: uImproved removal of airborne contaminants because airborne pollutants (generally lighter than air) tend to rise above the breathing zone, to the upper zone below the ceiling. This effectively increases concentrations in


‘ Displacement


venlaon requires the supply air to be cooler than the room air. So this approach is only suitable for cooling with a supply temperature range of 4K to 6K. As a general rule, heang is not recommended using


displacement venlaon.


’


uFigure 2.


By contrast, in displacement (fully- stratified) air distribution, cool supply air is delivered at reduced velocity from low level, sidewall diffusers. See Figure 2. The supply air is always cooler than the room air, so it quickly drops to the floor and moves slowly across the room.


in which occupants move through the zone (transient). However, this air movement strategy involves cool air moving along the floor in a stratified flow with a relatively constant depth (typical depth is about 200mm) with the maximum velocity in the stratified flow around l0% of this depth (approximately 20mm from the floor). So an occupant sitting at a desk in the ‘non-comfort’ zone adjacent to the displacement ventilation diffuser, for example, could therefore sense cool air at ankle level. As a result, furniture layout and zone occupation need to be determined before the unit type is selected.


Design ps


Displacement diffusers can provide coverage into a room that is up to six times the length of the ‘non-comfort’ zone (this is any area in the occupied zone where local air velocities exceed 0.2m/s at a height 200mm above the floor). We recommend a maximum of 6m for comfort and 8 –10m for industrial applications. Large rooms can be supplied from the side walls so long as the distance from the diffuser face to the


14 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2018


the unoccupied upper zone while reducing concentrations in the breathing zone. See Figure 3. uGreatly reduced energy


requirements to cool occupied spaces in mild climates, as circulation is provided by the presence/movement of occupants and other heat sources in the space, without additional building services-related input. uEnhanced opportunities for ‘freecooling’ via thermal wheels or heat exchangers when outside air temperatures are low. uReduced ventilation air requirement due to increased air distribution effectiveness. Internal heat load concentrations actually help to extend the projection of a displacement system by drawing the air across the room. uVery low diffuser noise levels, as air is supplied at low velocity. uFewer complaints, as supply of air at low velocity removes the risk of draughts. For detailed guidance on design considerations for displacement ventilation systems, request TROX Technical Leaflet L-08-1-04e.


www.troxuk.co.uk VISIT OUR WEBSITE: www.bsee.co.uk


uFigure 3.


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