Natural Ventilation In Buildings Engineering Essay

Natural spreading In Buildings Engineering EssayProper design of energy conscious mental synthesiss requires a balance between devil thingsThe thermal performance of the building envelope and the detach selection of techniques for heating, cooling and day lightingAn acceptable quality of the interior(prenominal) climate in marchess of thermal comfort, public discussion system system intensity level or indoor gloriole quality.Over the past years in all the westbound countries and more especially Europe, in that location has been a conscious limit of the availability of energy since the oil crisis of 1973. The principal(prenominal) result of this crisis in term of the wind effort was therefore to reduce signifi piece of tailtly global energy consumption, mainly used for heating and line of business-conditioning while neglecting its impact on the comfort and health of its occupants. During these periods, new regulations were make to regulate these, which also so the bi rth of real development of building research in western countries. However, these strict regulations on energy reduction in buildings was accompanied by an increasing number of disorders, mainly due to humidity condensation and the growth of mould, which affected the health of the occupants leading to SBS and building related indisposition among the occupants, to overheating in the summer or in intermediate seasons, which affected the thermal comfort of the occupants and finally poor indoor air quality (IAQ) decompose to baseborn air-change rates.The evolution began in the 1990s and it is now clear that energy conservation cannot be disassociated from the quality of the indoor and outdoor environment. These environmental criteria have even led to major modifications in manufacturing and technology, such as the abandonment of CFCs in HVAC. These criteria also highlight the necessity of full integration of the building site characteristics and effectiveness in the design and this leads pictorially to a focus on more integration of passive concepts on heating, cooling or more generally indoor climate conditions. (1)1. Santamouris and Asimakopoulos, Passive Cooling of Buildings (1996)With these various aspects taken into account, natural ventilation system appears to be a very attractive solution to ensure both right-hand(a) indoor quality and acceptable comfort conditions in many regions. Further more, natural ventilation seems to provide an answer to many complaints from users concerning mechanical ventilation, which appears to be noisy, to build health problems, to require routine maintenance and to consume energy. In contrast, natural ventilation is preferred by the occupants since it is energy efficient (no need of mechanical system), it can be easily integrated into buildings and it provides a healthier and more comfortable environment if integrated correctly. (2)Fundamentals of Natural VentilationThe magnitude and pattern of natural air movement thr ough a building depends on the strength and caution of the natural driving forces and the resistance of the flow path. Good design involves the appropriate application of fundamental principles to the general strategies described in the following sub-sections. The driving forces for natural ventilation are wind and density difference.WindWind set ventilation is caused by differences in pressures acting across the external surface of a building. The distribution of pressure depends onthe type of terrain surrounding the building (open acres/city centre) and the presence of any obstructions (other buildings, tree belts etc) which provide opportunity for site layout and landscaping to enhance wind driven ventilationthe wind speed and its direction relative to the building, andthe shape of the building this provides the opportunity for the architectural form and detailing to enhance the potential for wind driven ventilation.2. Liddament, M. A Guide to Energy economic Ventilation. (19 96)Air will flow through a building from areas of high surface pressure to areas of low pressure as shown in figure 2.5. In general terms, building surfaces facing into the wind will experience positive pressures leeward surfaces and those parallel to the wind direction will experience negative pressures (suction) blood Whole Building Design GuideFigure 2.1 Wind pressure field around a buildingCareful orientation of a building in relation to the topography of the site can maximise the potential for wind driven ventilation. This potential can be intensify by landscaping, such as tree planting.Density differencesWarm air is less dense than colder air. If two columns of air at distinguishable temperatures are separated by a boundary, there will be a difference in pressure across that boundary due to the different pressure gradients on either side. Normally, where it is warmer interior the building than outside, the pressure difference acts inwards at lower levels of the building and outwards at high level. When possibilitys are placed in the boundary separating the two air columns, an upward air flow will be created through the building, exhausting warm air at high level and replacing it by cooler air at low level. This is known as the stack effect.These stack -driven flows rise vertically through a building, but will hit horizontal flows in spaces connected to the column of air rising. These horizontal flows replenish the warm air which is wear down from the shaft. The shaft can take a contour of forms (see 2.3.3), but Figure 2.2 below illustrates the general principles associated with stack driven flows in an atrium.Source Caroline Rock, Coventry UniversityFig. 2.2 Stack driven flows in an atriumStrategies for Natural VentilationWind and buoyancy, the driving forces for natural ventilation, may be used for different ventilation strategies wind variation-induced single sided ventilation, wind pressure-driven cross ventilation and stack ventilation.Single-s ided ventilationSingle-sided ventilation relies on opening(s) being on only one side of the ventilated enclosure. A close approximation is a cellular building with opening windows on one side and closed internal doors on the other side.Single openingWith a single ventilation opening in the room, the main driving force for natural ventilation in summer is wind turbulence. Compared with other strategies, lower ventilation rates are generated, and the ventilating air does not propagate so far into the spaceSource Dyer environmental controlsFig 2.3 Single-sided single opening ventilationDouble openingWhere ventilation openings are provided at different heights within the faade, the ventilation rate can be enhanced by stack effect. The ventilation rate will be further enhanced by any wind pressures acting on the ventilation opening. As well as enhancing the ventilation rate, double opening increases the penetration of fresh air into the space. Effective to a depth of about 2.5 times the floor to ceiling height.Source Dyer environmental controlsFig 2.4 Single-sided double opening ventilationCross ventilationCross ventilation occurs where there are ventilation openings on both sides of a space. Air flows from one side of the building to the other and leaves through for interpreter another window or door. Cross ventilation is usually wind driven. As the air moves across an occupied space, it picks up heat and pollutants. Cross ventilation is effective up to 5 times the floor to ceiling heightSource Dyer environmental controlsFig 2.5 Cross ventilationStack ventilationThis term is used to describe those ventilation strategies where the driving forces promote an outflow from the building, thereby drawing in fresh cool air via ventilation openings at low level. The strategy makes use of the difference in density between a column of warm air and the surrounding cooler air. Because air flows into the building at low level, then up to a high level exhaust point, great care has to be taken into account when determining thedifferent sizes of ventilation opening on each floor of the building. For equal ventilation rates, the ventilation openings at lower floors need to be smaller than those nearer to the top of the building.The effectiveness of natural ventilation can be enhanced by designing the stack outlet to be in a region of wind-induced negative pressure. The air may flow across the whole width of the building and be exhausted via a chimney, or it may flow from the edges to the middle to be exhausted via a central chimney and or atrium. e.g. Lanchester Library, Coventry University.Fig 2.6 Stack Ventilation strategy at Lanchester Library, CoventryFig 2.7 Air exhaust via allowance chimneys, light wells and the central atrium at Lanchester Library, CoventrySource Pat Non, Coventry UniversityDouble Skin FaadeA double faade construction consists of a normal concrete or glass wall combined with a glass structure outside the actual wall. Double skin fac ades bye several values. They can act as buffer zones between the internal and external environment, reducing heat loss in winter and heat crap in summer. In combination with ventilation of the space between the two facades, the passive thermal effects can be used to best advantage. go-ahead windows in the inner skin can draw air from the buffer zone into the building. The stack effect of thermal air currents in tall buildings stomach advantages over lower buildings.Night VentilationNight ventilation is not an additional mode of ventilation it is simply a different operational strategy. It takes advantage of the natural diurnal variations in temperature to promote cooling. Night ventilation offers many advantages like cooling the fabric of the building at night and also ventilating during slothful periods