PASSIVE VENTILATION in Buildings

Post 648 -Gautam Shah

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Passive ventilation sustains good quality of air in interior spaces. It works on circulation or movement of air without the use of power utilities. It relies on principals of source management and dilution, rather than any filtration. Passive ventilation is substantially based on the quality of built-form and to a smaller extent on immediate surroundings. It is a very important method of adjusting heat and moisture in Hot-arid and Hot-humid climates. The quality of air is determined by temperature, moisture content, presence and proportion of ‘other’ gases and airborne particulate matter. The quality of air has important bearing on our body. A body may endure or adopt to certain abnormal conditions for a period of time but there may occur side effects. The side effects may be realized in a different form and at a different time.

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Passive ventilation relies, as much on external or macro conditions, as the interior spatial lay. It is based on three factors:

  1. Air movements due to the differing pressures and temperatures and the buoyancy forces that result across a building and its surroundings,
  2. Location of tasks and activities that support or hinder such patterns of air movements, circumstantial and designed apertures in the building shell.
  3. Factors that define the ventilation in a building are, space-profile (section), base levels of inward-outward nodes of ventilation, nature of surroundings and neighbourhood, sill level, depth and its profile-shape, task-intensive operative plane and its volume, and constraints enforced by elements such as size and shape of external overhangs.

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Passive ventilation occurs with Two major operants.

Differential air pressures are formed by wind movements in the immediacy of the building, the pattern and size of the buildings scheme and individual components of the building. Air movement also occur as a buoyancy effect caused by the temperatures of surfaces and surroundings near the wind-ward and wind-off sides. Air pressure difference occurs, across buildings’ interiors and exteriors, across the openings and temperature of surfaces and surroundings, near the windward and wind-off sides. Entry and exit point for air, though continually shift around due to the changes in pressures.

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Temperature differential depends on the direction and inclination of sun, climate, seasons of the year, local massing of the shadows, surface materials, vegetation, water bodies, and presence of heat evolving entities. Dark surfaces and thin body objects warm up very fast and begin to radiate the heat, creating local heat related buoyancy in air.

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Ventilation requirements vary depending on whether one wants to gain or lose heat, add or subtract moisture, dilute or remove ‘other’ gases and airborne particulate matter contaminants. Ventilation needs change depending on, distance of space occupation from the inlet-outlet for air, functional use of space, types of tasks, work-schedules, crowding in the space and presence of heat evolving means (hearths, machines, etc.). One important aspect is the feeling of air movement near-over the body. It depends on several factors such as air velocity, fluctuations in air velocity, temperature of air, and personal factors like overall thermal sensation and activity level. Even for the same person, sensitivity to air movement may change from day to day.

635px-Wall_Decoration_at_Kesava_Temple_in_Somanathapura_retouchedDraughts (Draft) are very low velocity air movements. These are not always perceptible, as they do not cause any sensation of pain or pressure on skin. Draughts are more felt due to air pressure thresholds near cracks and such leakage points in small and enclosed spaces. Draughts, however, help in convective heat exchange, evaporation and dilution of pollutants in air. Draughts cause localized cooling or heating of sensitive organs of our body.

Breeze or low to medium velocity air movements generally affect only local areas. Breeze does not let airborne particulate matter to settle down. Skin sensation can be avoided by appropriate screening and deflection of the breeze. Since breeze causes effective pressure on skin, with very immediate and very perceptible change sensation.

Winds are high air velocity movements of air affecting larger regions. Winds raise particulate matter in the air, cause rapid change in level of humidity and often cause discomfort due to high pressure sensation on the skin.

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In hot and cold both types of climates people often close all the openings to reduce the air movements and thereby control the convective heat gain or loss. Turbulent air velocity is less comfortable than a Laminar air velocity. Turbulent air movement achieves a better mix of air whereas laminar helps in greater displacement of air mass. This is the reason why in hot arid climates small size openings are used to create turbulence or a viscous flow, and in hot humid climates the body level openings of horizontal nature create a laminar flow to displace the humidity.

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Passive ventilation occurs through macro and micro openings. Macro or formal openings (doors, windows, gaps etc.) are designed with a characteristic size, shape, passage section, adjunct elements on internal and external faces, and occur close to the location of need (for ventilation). Micro openings are circumstantial, and are much smaller in size like cracks, crevices, gaps or apertures. Micro openings offer a passive and consistent ways of managing comfort in enclosed and semi-open spaces. These manifest as intentional gaps, unplanned crevices and cracks of structural stresses. Both, macro and micro openings can be broadly be classified by their locations.

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  • Roof level openings are such as in the thatched and country tile roofs, loosely laid roof slates, stone sheets, or intentionally placed micro passive vents such as lattices, chutes, hoppers, etc.
  • Upper section openings in walls are such as the unpacked ends of corrugated sheets or roofing tiles, ends of purlins and truss and eyelets or oculi like holes and lites in gables.
  • Other openings manifest as doors, windows, cut-outs, chowks, in joinery, leaky fitments, ajar shutters, door bottom space, peep-holes, latticed constructions such as of woven mats or fabrics, louvered openings, crack or fissures in building elements, expansion joints, unsealed joints, etc.

Movement of air through openings encourages evaporation and increases cooling in the interior space. In dry arid climates dwellers locate their activities in the strongly directional air movement formed by small and deep-set openings. Deep-set openings also increase absorption of heat in the mass of the structure. Simple passive cooling devices such as water wetted grass mats or fountains help cooling. Outside air has lesser moisture (except during raining conditions) then indoor air, so any level of ventilation, dilutes the interior humidity level and adds to the comfort.

620px-AlfedPalmersmokestacksAir borne particulates arrive from outside sources like heavy vehicular traffic, polluting industry (mines, thermal power plants, mineral grinding plants, in the vicinity, or sand storms, and internal sources like cooking or craft fuels, and processing materials (grinding, spinning-weaving). Outside particulate can be controlled by changing the ventilation gaps to different location or elevation. Filtration screens occupy more then 60% of opening area, and much lesser due to frequent choking. Dynamic screening like water bodies or sprays can be useful, but costly and perhaps beyond the concept of a passive device. Non turbulent wind flow helps in keeping the particulate matter to lower sections.

Moisture control in interior space occurs by dilution, greater air movement and siting management of moisture generating amenities. Isolation of cooking, washing, bathing areas in dwelling is a common practice in hot-humid climates.

Quality of indoor air mainly depends on the external circumstances. The ‘feel-good aspect’ in a tropical climate (hot-arid or hot-humid) zone is not only regulated by the obvious temperature, rates of movement (pressure) and the moisture content, but also by the level of fouling of air. Some experts have claimed that air quality of a room is chiefly determined by its CO2 concentration. In tropical buildings concentration of CO2 and other gases is not a critical factor, as numerous openings and micro gaps remain substantially open. Location of cooking area is a segregated entity or an outdoor activity, and like cold climates no interior fire places.

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