Natural Ventilation
Three basic purpose of ventilation: Supply of fresh air and dilute or remove the contaminated air. (health ventilation) minimum level of ventilation is required in all occupied dwellings for removal of odours removal of the products of combustion – gas cooking removal of the moisture vapour to prevent condensation
Convective cooling (structure cooling) cooling or warming by removing or adding heat from the structure by using air as medium and cross ventilating (when there is a favourable difference in temperature between the outside air and the inside air to cool the building, the outside air temperature must be cooler than the inside air temperature and to heat the building, the outside air temperature must be warmer than the inside air temperature)
Physiological cooling (air movement) cooling by passing air near the body surface and evaporating sweat/moisture from the skin, and increasing heat loss from the skin by forced convection.
Air will move only when it is pushed, pulled, heated up or cooled down
Different methods of ventilation: •
Natural ventilation- (a) Stack effect (b) Wind effect -
the process by which the air in a building or part of a building is removed and replaced by air from outside, by the action of natural forces of wind and temperature. In a passive design, the pushing and pulling has to be done by the prevailing wind, whilst the heating and cooling can be done by solar radiation, evaporation and/or thermal mass.
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Mechanical ventilation-
the process by which air in a building or part of a building is removed with help of mechanical equipment such as fans. In a active design, the pushing and pulling has to be done with help of use of devices that consumes energy.
Natural Ventilation, a form of convective cooling, has the potential to cool the human body directly through convection and evaporation, or indirectly by cooling the structure of the building surrounding the occupants. The cause of Natural Ventilation in the buildings is due to pressure difference between the inside and outside of the building. Figure - Natural Ventilation in a house
Natural Ventilation is due to: a) Wind-Driven Ventilation b) Stack Effect Ventilation
Wind-Driven Ventilation Fig – Wind Effect
The wind effect is due to pressure difference between the air inside and that outside the building thereby encouraging air flow from the area of higher pressure to that of lower pressure. Positive pressure exists on windward side and negative pressure on leeward side of the building Building form is fundamental to any wind-driven natural ventilation system. Anything that diverts or changes the path of the air will act to impede its flow
Stack Effect Ventilation Fig – Stack effect
The stack effect (Temperature effect or Thermal Buoyancy Ventilation) is the convection effect (vertical pressure gradient) arising from the temperature or vapour pressure difference between the air inside and outside of the building and the difference between the heights of outlets and inlet openings. Buoyancy results from differences in air density. The density of air depends on temperature and humidity. Cool air is heavier than warm air at the same humidity. Air temperature is higher inside than that outside a building The warmer air tends to rise and pass through any apertures that exist in the upper part of the building Colder air from outside enters through apertures in lower part of the building to replace it.
The rate of air change will depend on •Temperature difference between outside and inside air •Vertical height difference between the inlet and outlet opening of the enclosed volume of heated or cooled air •The size and design of the apertures. Tall room volumes will have strong stack effects while short room volumes will have little or no stack effects. For low-rise buildings or in medium to high wind conditions, the stack effect may be considered negligible in comparison to wind effect. The stack effect rarely creates enough air movement to cool the occupants directly, but it can provide enough ventilation for fresh air and health requirements. In high-rise buildings, the stack effect may cause strong air-movement through elevator shafts and stair towers, but the individual floors are usually separate from other floors so the stack effect within the floors will be small. E.g. Ventilators in the roof (outlet) and low level window in the building (inlet)
Four different ways of Naturally Ventilating of a building: 1. Single sided ventilation 2. Cross ventilation 3. Stack effect 4. Reverse stack ventilation
Single sided ventilation occurs when large ventilation openings such as doors and windows are situated on one external wall only. Exchange of air takes place by wind turbulence, by outward openings interacting with the local external airstreams and by local stacks.
Cross ventilation occurs when inflow and outflow openings in external walls have an internal flow path between them. Flow characteristics are determined by the combined effect of wind and temperature difference. Traditionally in some hot regions corridors were built between courtyards spaces for cross ventilation
Stack and reverse stack ventilation Summer afternoon with little wind – hot wind tower – stack effect – warmer air from the room to the tower replaced by cooler air from the courtyard Summer morning without wind – cooled wind tower – reverse stack effect – cool night air from outside is drawn to the tower replaced by warmer room air into the courtyard E.g. Wind catchers of the Middle East The higher the tower and the greater the temperature and pressure differences between the top and the bottom higher will be the ambient air speeds at the vent level at the head of the tower.
Air change per hour is the replacement of a quantity of in a volume within an hour. If a house has 1 air change per hour, all the air in the house will be replaced in a 1-hour period. No. of air changes required in a space depends on The requirements of fresh air supply The type of occupancy Number of the occupants and their activities The nature of any processes carried out in the space. The rate of air change is affected by •Variability of wind speed outside, Direction of wind, Air temperature and Pressure difference developed (the interplay of wind and stack effect) •The varying shapes of buildings, Design of aperture and the effectiveness of exposed openings
Factors affecting the indoor air flow: 1. Orientation 2. External features 3. Cross-ventilation 4. Position of openings 5. Size of openings 6. Controls of openings
1. Orientation The greatest pressure on the windward side of a building is generated when the elevation is at right angles to the wind direction, so greatest indoor air velocity will be achieved in this case. A wind incidence of 45 degree would reduce the pressure by 50%. 2. External features External features of the building itself can strongly influence the pressure build-up. The wind velocity gradient is made steeper by an uneven surface, such as scattered buildings, walls, fences, trees or scrub.
3. Cross-ventilation Ventilation achieved by placing openings in opposite walls of a room, and intended to provide air change and sensible air movement. Absence of an outlet opening or with a full partition there can be no effective air movement through a building. With windward opening and no outlet or leeward opening and no opening pressure is built-up indoors (buffeting). Several right- angle bends such as internal walls or furniture within a room can effectively stop low velocity airflow. Where internal partitions are unavoidable, some airflow can be ensured, if partition screens are used clear of the floor and the ceiling.
4. Position of openings If the opening at the inlet side is above the occupancy level (2 mts.), regardless of the outlet opening position, the airflow will take place near the ceiling and not in the living zone. In a two building the air flow on the ground floor may be satisfactory but on the upper floor, it may be directed against the ceiling.
5. Size of openings With largest air velocity will be obtained through a small inlet opening with a large outlet. When the inlet opening is large, the air velocity through it will be less, but the total rate of airflow will be higher. When the wind direction is not constant, or when air flow through the whole space is required, a large inlet opening will be preferable. The best arrangement is full wall openings on both sides, with adjustable sashes or closing devices, which can assist in channeling the flow in the required direction, following the change of wind.
6. Controls of openings Sashes, canopies, louvers and other elements controlling the openings that influence the indoor air flow pattern. Fly screens or mosquito nets can reduce the velocity of airflow.
General considerations: •In low-rise building blocks with gridiron layout pattern, stagnant air zones leeward from the first row will overlap the second row. A spacing of six times the building height is necessary to ensure adequate air movement for the second row. •If buildings are staggered in a checker-bond pattern, the flow field is more uniform, and stagnant air zones are eliminated. •Verandahs, particularly on three sides, do increase the flow rate to some extent. •Ventilators above the window level and close to the roof are generally more important. The location of ventilators has an effect on the functioning of ventilators. If placed in the path of wind they are very effective; they are least effective on the suction side. They should be at higher levels than the inlet opening.
•Full advantage of the wind direction should be taken by so locating inlet openings as to face the prevailing wind direction. Inlet openings are located on the windward side at a lower level and outlet openings on the leeward side at a higher level; this will increase the flow rate. Since the prevailing wind direction does change, it is advisable always to have windows in adjacent walls so that there are openings exposed to the wind direction and ventilation of the space is possible. •For stack effect to be useful, a vertical distance between openings (on facing walls) should be ensured. This is because for a motive head to be produced by the temperature difference, there must be a vertical difference between openings. However, if the openings are at the same level, irrespective of the temperature difference, no motive head is produced.
•Inlet openings should be well distributed along the length of the building and as far as possible should be equal to outlet openings in area; thus a high airflow rate is possible. Buildings, signboards, etc should not obstruct them. •Inlet openings may have an area between 20 to 30 % of the floor area. This gives a fairly high flow rate of air; areas larger than this do not increase the flow rate. •In case only one wall of a room is exposed to the outside, it is preferable to provide two small windows in the wall than to provide only one large window. •Windows located somewhat diagonally opposite each other with the windward window close to the corner will give a good flow rate.
Effect of size of inlet and outlet on internal wind speed and distribution: 1. small inlet and large outlet will result in a high max. speed but poor distribution, with large areas of the room experiencing low wind speeds 2. large inlet and small outlet will result in a lower max. speed but a better distribution of air movement over the room, with small area having low speeds 3. internal wind speed does not increase significantly when the window size is increased beyond 40% of the wall area.
Hot-arid regions: Summer To protect from the direct radiation, the windows are generally shut during daytime. To keep the indoor temp. lower than the outside temperature, min. ventilation is provided for the control of body odour and for the removal of the products of combustion Winter To protect from the cold wind, the windows and doors are shut during the night. Warm and humid regions: To provide large amounts of ventilation, buildings are oriented to face the direction of prevailing winds. Windows and doors are kept open in both windward and leeward sides to provide large amounts of ventilation.
Bodily Cooling Bodily cooling is effective during overheated periods when the temperature and humidity of the air are above the still air comfort range. Bodily cooling is especially useful in hot-humid climates where high humidity suppresses the range of daily temperature fluctuation making structural cooling difficult to achieve. When bodily cooling is desired, buildings should allow max. airflow across the occupied area and provide protection from the sun and rain. Light-weight structures which respond quickly to lower night temperature are desirable. In the extreme case the best “structure� consists of only an insulated roof-canopy to provide shade and protection from the rain and while allowing max. ventilation. In practice, careful siting and orientation, narrow elevated buildings, open plans, and use of exterior wingwalls, overhanging eaves, verandahs and large windows are prevalent elements of naturally ventilated buildings in warm-humid climates.
Structural Cooling Structural cooling, in which the building mass smoothes out the daily temperature variation, is effective in climates which large daily temperature variations (i.e. hot-arid climates). During the day, the building interior is unventilated and the high thermal capacity of the building structure serves as a heat sink for the interior gain. At night, the mass is cooled by long-wave radiation to the sky. Cooling may be enhanced by “flushing� the building with cool night air removing the stored structural heat and prechilling the mass for the next day. Night air must be cool enough to receive the stored heat (i.e. the night time outdoor air temperature must be lower than indoor air temperatures, and dip into or below the comfort zone.)
Landscape and Wind movement