Cycle 2 - Ventilation And Daylighting Notes PDF

CYCLE 2 – VENTILATION AND DAYLIGHTING 2.1 DEFINITION OF NATURAL VENTILATION 2.3.3 PRINCIPLE 3 The term natural ventilation is used to indicate the intentional airflow through windows, doors or other openings designed for the purpose, obtained without the use of fans or any...

CYCLE 2 – VENTILATION AND DAYLIGHTING 2.1 DEFINITION OF NATURAL VENTILATION 2.3.3 PRINCIPLE 3 The term natural ventilation is used to indicate the intentional airflow through windows, doors or other openings designed for the purpose, obtained without the use of fans or any other mechanical means. 2.2 CAUSES FOR NATURAL VENTILATION It is created by Pressure differences caused by the wind and/or BERNOULLI EFFECT Temperature differences between the inside Air, as all fluids, is subject to the Bernoulli effect, and the outside because of which there is a reduction of pressure 2.3 BASIC PRINCIPLES OF VENTILATION when speed increases; this effect is exploited in Natural ventilation is driven by some basic the wing of an aeroplane, whose shape is such principles, which we recall below. that it forces air passing above it to follow a longer path, resulting in greater speed than that 2.3.1 PRINCIPLE 1 of the air flowing beneath it; the pressure at the Air always moves from a high-pressure zone to a top is then lower than at the bottom and there is low pressure zone. a push from the bottom upwards: the lift; 2.3.2 PRINCIPLE 2 2.3.4 PRINCIPLE 4 LAMINAR FLOW An air flow is called laminar when the speed is low VENTURI EFFECT and all the fluid streamlines move parallel. When an air stream is forced through a smaller section, there is an increase in speed and a decrease in the pressure in correspondence to the narrowing. This effect is called as Venturi Effect. 2.3.5 PRINCIPLE 5 TURBULENT FLOW When the speed increases or there is a pronounced change of direction, the motion becomes turbulent, and fluid streamlines cease to move in parallel, giving rise to significant changes in direction and to eddies. PRESSURE DISTRIBUTION AROUND A BUILDING Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|25 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING In combination of the factors described above, Moving air acts as a heat carrying medium to when the wind hits a building, it creates areas of heat or cool the interior of a building. low pressure along the sides parallel to its Occurs when there is a temperature difference direction and on the leeward side and areas of between the outside and inside air. high pressure on the windward side. To cool the building, the outside air temperature must be cooler than the inside air 2.3.6 PRINCIPLE 6 temperature and conversely warmer if the building is to be heated. In warm climates, when the indoor temperature is higher than the outdoor air temperature convective cooling can occur. C) Physiological cooling Ventilation cools the body in two ways Evaporation of moisture from skin Increasing heat loss from skin by convection Air movement must be sufficiently fast to be felt at body level. If it is not felt it will not provide STACK EFFECT effective cooling. Cooling effect is not due to the coolness of the When the cool air from outside enters a room or air but due to the velocity of air which building it gets warmer than the outdoor air due increases the evaporative and convective loss to internal heat gain. The density of warm air In very low humidities (below 30%) this cooling being lower, causes it rise and escape through effect is not great, as there is unrestricted the opening on top. The pressure inside is also evaporation even with very light air movement. lower than the outside. This triggers air movement In high humidities (above 85%) the cooling from low pressure to high pressure area. This is effect is restricted by the high moisture content called stack effect. preventing evaporation, but greater velocities (above 1.5 to 2 m/s) will have some effect. 2.4 FUNCTIONS OF VENTILATION It is most significant in medium humidities (35 to 60%). Supply of fresh air Convective cooling 2.5 APPLICABILITY OF WIND ROSE Physiological cooling DIAGRAM A) Supply of fresh air A wind rose is a graphic tool used by Replacing the used air by fresh external air for two meteorologists to give information about wind reasons speed and direction of a particular location. Presented in a circular format, the modern Removing the odours and removal of carbon wind rose shows the frequency of winds dioxide and blowing from different directions over a Supply of oxygen for inhaling specified period. The length of each "spoke" shows the A minimum quantity of fresh air needs to supplied frequency that the wind blows from a for this purpose in all occupied spaces. particular direction per unit time. The direction of the longest spoke shows the Quantity of fresh air supply is governed by wind direction with the greatest frequency. Type of occupancy A wind rose plot may contain additional Number of occupants information, in that each spoke is broken down Activity of occupants and into color-coded bands that show wind speed, Nature of any processes carried out in the temperature and humidity of the wind. space Wind roses typically use 16 cardinal directions, such as north (N), NNE, NE, etc., although they Expressed in terms of m3/h person, or number of may be subdivided into as many as 32 air changes per hour. directions. In terms of angle measurement in B) Convective cooling Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|26 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING degrees, North corresponds to 0°/360°, East to been diverted, therefore it will take some time 90°, South to 180° and West to 270°. to return to the ground surface after the obstacle, to occupy all the available 'cross- section'. Thus a stagnant mass of air is also formed on the leeward side, but this is at a reduced pressure. In fact, this is not quite stagnant: a vortex is formed, the movement is light and variable and it is often referred to as 'wind shadow’. AIR FLOW AROUND A BUILDING Consequently, vortexes are formed wherever the laminar flow is separated from the surfaces 2.6 AIR MOVEMENT AROUND BUILDINGS of solid bodies. On the windward side such vortexes are at an increased pressure and on When moving air strikes an obstacle such as a the leeward side at a reduced pressure. If the building, this will slow down the air flow, but the building has an opening facing a high-pressure air flow will exert a pressure on the obstructing zone and another facing a low pressure zone, surface. air movement will be generated through the This slowing down process effects a roughly building. wedge-shaped mass of air on the windward side of the building, which in turn diverts the rest of the air flow upwards in elevation and sideways in plan. Separation layer is formed between the stagnant air and the building on the one hand and the laminar air flow on the other hand. The laminar air flow itself may be accelerated at the obstacle, as the area available for the flow is narrowed down by the obstacle, as it were. At the separation layer, due to friction, the upper surface of the stagnant air is moved forward, thus turbulence or vortex is developed. AIR FLOW AROUND SINGLE OBSTACLE Due to its momentum, the laminar air flow tends to maintain a straight path after it has Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|27 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING 2.7 STACK EFFECT As the air gets warmer it becomes less dense and so more buoyant. This means that warm air tends to rise. This effect can be used to naturally ventilate buildings. Cooler outside air is drawn into buildings at a lower level, it is warmed by sources of heat within the building (such as people, equipment, heating and solar gain), and then rises through the building to vent out AIR FLOW – GRID IRON LAYOUT at a higher level. If buildings are placed in rows in a grid-iron This process can take place without pattern, stagnant air zones leeward from the mechanical assistance, simply by introducing first row will overlap the second row. openings at the bottom and the top of buildings. It is known as the stack A spacing of six times the building height is effect or stack ventilation. necessary to ensure adequate air movement for the second row. Temperature difference between the outside and inside air creates a pressure difference which can be used to achieve ventilation through stack effect. Rate of ventilation is calculated by the formula V = 0.117 X (A / h) * dt dt - Difference in temperature between outside air and inside air h - Difference in height between inlet and outlet AIR FLOW – CHECKERBOARD LAYOUT A - Area of inlet In a similar setting, if the buildings are Rate of ventilation depends on staggered in a checker-board pattern, the Difference in temperature between outside air flow field is much more uniform, stagnant air and inside air zones are almost eliminated. Difference in height between inlet and outlet Area of inlet Generally speaking, buildings in which cross ventilation is important should be separated by a distance of 7 times the building height to assure adequate airflow if they are directly behind one another; far less if staggered. In dense urban areas this rule cannot be followed. The reduction of cross ventilation deriving from a high-density layout can be overcome by providing ventilating ducts or shafts for deeper rooms. The stack pressure can be calculated from the equation: Ps = 0.042 × h × ΔT Ps = stack pressure in N/m2 h = height of stack in m ΔT temperature difference in degC Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|28 19ARB211T Climate Responsive Architecture  CYCLE 2 – VENTILATION AND DAYLIGHTING the constant is N/m3 degC 2.7.2 APPLICATION OF STACK EFFECT – Courtyards 2.7.1 APPLICATION OF STACK EFFECT – Solar Due to the incident solar radiation in the Chimney courtyard, the air in the courtyard becomes warmer and rises up. Hot air rises out of the chimney To replace it, cool air from the ground level drawing in more flows through the openings of the room, thus cool air producing the air flow. During the night the process is reversed. Painted black so that sun’s radiation heats As the warm roof surface gets cooled by up the cool air in Cool air chimney causing it to drawn convection and radiation, a stage is reached rise Heated up air drawn to into the when its surface temperature equals the dry house bulb temperature of the ambient air. the bottom of chimney through windows If the roof surfaces are sloped towards an internal courtyard, the cooled air sinks into the To enhance the flow rate in stack effect, an court and enters the living space through the effective solution is to increase the low-level openings and once it gains heat from temperature difference between inside and the spaces, escapes through higher level outside, using the solar chimney, exploiting openings in the spaces. solar energy to heat the rising air flow. Courtyards act as heat sinks during the day Solar chimneys are generally tall, and radiate the heat back to the ambient at wide structures constructed, facing the sun, night. with a dark colored, matt surface designed to absorb solar radiation. This concept can very well be applied in a hot dry climate and warm humid climate. As the chimney becomes hot, it heats the air inside it. The hot air rises in the chimney and is It is necessary to ensure that the courtyard gets vented out of the top, and in doing so it draws adequate radiation to produce a draft more air in at the bottom of the chimney. This through the interior. can be used to drive natural However, care should be taken that the ventilation in buildings. courtyard does not receive intense solar During cold weather the vents can be shut, radiation, which would lead to conduction allowing the chimney to trap warm air. and radiation heat gains into the building. It may use materials with high solar absorption The size of the courtyards should be such that such as metal / surfaces painted black to the mid-morning and the hot afternoon sun are maximize the temperatures achieved within avoided. the chimney. In addition, they can be provided with ponds Solar chimneys are particularly effective in and fountains for evaporative cooling. climates that are humid and hot. They are most efficient when they are tall and wide. This maximizes the surface area that can absorb solar radiation and maximizes the surface area in contact with the air inside the chimney. Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|29 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING Air in courtyard gets heated up and rises Cool air from outside entering the spaces Cool air from Courtyard outside entering surfaces getting heated up due to sun’s Roof surfaces lose heat to the cooler night sky Cooler air enters Cooler air Ground surfaces lose the spaces enters the heat to the cooler night Cooler air sinks into the Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|30 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING 2.7.4 APPLICATION OF STACK EFFECT – Wind In the presence of wind, the cool night air tower enters the tower and forces itself down into the structure. Though it is warmed slightly during There are cases in which it is difficult to provide the process, sufficient cooling can be adequate ventilation even if the location is achieved due to forced circulation. Again, fairly windy. cooling due to nocturnal radiation adds to this Such situations occur in low-rise, high-density process. settlements, where it is difficult to get good Working : Day wind access, because upwind buildings block breezes, or The hot ambient air coming in contact with the cool upper part of the tower gets cooled. When conflict between the best orientation for shade and wind forces sun protection to be It becomes cold and dense and sinks through favoured, or the tower and into the living spaces, replacing the hot air. When the shape of the plot does not allow the building to be oriented to take advantage of In the presence of wind, the air is cooled more the prevailing wind direction. effectively and flows faster down the tower and into the living area. In some countries, a traditional solution to this kind of problem is the wind catcher: a tower It must be noted that the temperature of the capable of capturing winds above the tower soon reaches that of the ambient air building, bringing in fresh air from outside. and hence, in the absence of wind, the downward flow ceases, the tower then begins A prerequisite for using a wind catcher is that to act like a chimney. the site should experience winds with a fairly good consistent speed. The operation of the tower depends greatly on the ambient fluctuations like the wind velocity, A wind tower operates in various ways air temperature changes, etc. according to the time of day and the presence or absence of wind. Wind tower is generally used in hot and dry climates for cooling purposes. Works based on stack effect. The difference in temperature between the interior and exterior parts of a building creates different pressures and different density. This in turn creates a draft, pulling air either upwards or downwards through the tower. Working : Night The tower walls and the internal walls of the air- flow passages absorb heat during the day and release it at night, warming the cool night air in the tower. Warm air moves up creating an upward draft and is exhausted through the openings. WIND TOWER : Working (Night) The pressure difference thus created pulls the cool night air through the doors and windows into the building. In the absence of wind, the tower acts as a chimney. The nocturnal radiation through the roof and the external walls brings about further cooling. Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|31 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING to daytime by exploiting the principle of evaporative cooling. It may be noted that wind towers may need to be shut off when cooling is not required, and hence, such provisions may be included in the design. 2.8 FACTORS AFFECTING AIR FLOW THROUGH BUILDINGS Size of openings Position of openings Controls of openings Orientation Cross-ventilation External features WIND TOWER : Working (Day) 2.8.1 Size of openings To improve the efficiency of its operation, Small inlet and large outlet will result in a high evaporative cooling may be introduced. The maximum speed but poor distribution of air air flowing down the tower is first sensibly within the space with large areas of the room cooled, and then further cooled evaporatively. experiencing low wind speeds. This can be achieved by providing a shower/spray or dripping of water at top of tower, or a fountain at the bottom. The reduction in the temperature of air can be as much as 10 – 15o C in arid climates. The relative humidity in hot and dry regions is low and more humidity is needed for wind towers for sinking the air down the wind tower. SMALL INLET AND LARGE OUTLET Large inlet and small outlet will result in a low maximum speed but better distribution of air within the space with only a small area of the room experiencing low wind speeds. WIND TOWER : WITH EVAPORATIVE COOLING DISADVANTAGES OF WIND TOWER Control of the volumetric flow rate is almost zero, unless adjustable dampers are used. Due consideration must also be given to prevent the entry of dust, birds and insects. Wind catchers, in hot-arid climates, should be used only for night ventilation but, if water is available, their effectiveness can be extended LARGE INLET AND SMALL OUTLET Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|32 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING 8.2 Position of openings To be effective, the air movement must be directed at the body surface. In building terms this means that air movement must be ensured through the space mostly used by the occupants: through the 'living zone' (up to 2 m high). POSITION (Horizontal) OF OPENINGS High inlet and high outlet do not produce good air movement at body level When inlet and outlet openings are aligned, cross ventilation is activated by wind. If the openings are aligned in the direction of the wind, the air flow passes right through the Low inlet and high outlet produce low level space giving rise to modest induced air wind pattern movements. If the wind blows obliquely, however, the ventilation involves a wider zone and more air movement is induced Low inlet and low outlet produce a good air movement when it is required for cooling. Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|33 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING Air flow at ceiling height produced by a high inlet is hardly affected by an outlet at low level The best conditions are created when the outlet opening is higher and wider than the inlet (the ideal is to have them of equal area). 2.8.3 Controls of openings Sashes can divert the air flow upwards. Only a casement or reversible pivot sash will channel it downwards into the living zone Canopies can eliminate the effect of pressure 2.8.4 Orientation build-up above the window, thus the pressure Figure below shows the outline of air flow at 90° below the window will direct the air flow and at 45°, to a building square in plan. upwards. In the second case a greater velocity is A gap left between the building face and the created along the windward faces, therefore canopy would ensure a downward pressure, wind shadow will be much broader, negative thus a flow directed into the living zone pressure (the suction effect) will be increased. Louvers used for protection against direct solar An increased indoor air flow will result. gains significantly affect the average indoor air If often happens, that the optimum solar speed and the airstream pattern. orientation and the optimum orientation for The position of blades in a slightly upward wind do not coincide. position would still channel the flow into the In equatorial regions a north–south orientation living zone (up to 20° upwards from the would be preferable for sun exclusion but most Horizontal) often the wind is predominantly easterly. Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|34 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING The usefulness of the above findings is obvious breezes a pathway through the structure. This is for such a situation – it may resolve the called cross-ventilation. contradictory requirements. In the absence of an outlet opening or with a full partition there can be no effective air movement through a building even in a case of strong winds. With a windward opening and no outlet, a pressure similar to that in front of the building will be built up indoors, which can make conditions even worse, increasing discomfort. It is generally best not to place openings exactly across from each other in a space. ORIENTATION - Effect of direction on the width of While this does give effective ventilation, it can wind shadow cause some parts of the room to be well- cooled and ventilated while other parts are The greatest pressure on the windward side of not. a building is generated when the elevation is at right angles to the wind direction, so it seems Placing openings across from, but not directly to be obvious that the greatest indoor air opposite, each other causes the room's air to velocity will be achieved in this case. mix, better distributing the cooling and fresh air. A wind incidence of 45° would reduce the Also, cross ventilation can be increased by pressure by 50% but would increase the having larger openings on the leeward faces average indoor air velocity and would provide of the building that the windward faces and a better distribution of indoor air movement. placing inlets at higher pressure zones and outlets at lower pressure zones. Air flow loses much of its kinetic energy each time it is diverted around or over an obstacle. Several right-angle bends, such as internal walls or furniture within a room can effectively stop a low velocity air flow. Where internal partitions are unavoidable, some air flow can be ensured if partition ORIENTATION - Effect of wind direction and inlet screens are used, clear of the floor and the opening size on air velocity distribution ceiling. Arrow indicates the direction of wind and the numbers indicate the air velocity 2.8.5 Cross ventilation When placing openings, inlets and outlets are placed to optimize the path air follows through the building. Windows or vents placed on opposite sides of the building give natural Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|35 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING In most cases rooms have only one wall facing outside and a single opening. In such cases, air movement is quite poor (if the window is on the windward side, the available wind velocity is about 10% of the outdoor velocity at points up to a distance one sixth of the room width; beyond this, the velocity decreases rapidly and hardly any air movement is produced in the leeward portion of the room). In such cases, split the single opening into two, positioning the parts as far apart as possible; If the wall is to windward, a further improvement is obtained by constructing a vertical fin (wing wall). 2.8.6 External features If the room has openings on adjacent walls, wing walls can significantly increase the effectiveness of natural ventilation. VENTILATION WITH OPENINGS ON ADJACENT VENTILATION, WITH WING WALLS, IN ROOMS WALLS, AND WING WALLS WITH ONLY ONE WALL FACING OUTSIDE AND TWO OPENINGS Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|36 19ARB211T Climate Responsive Architecture CYCLE 2 – VENTILATION AND DAYLIGHTING Prepared by : G. Sudha, Associate Professor, SAID, SRMIST Page|37 19ARB211T Climate Responsive Architecture