Lecture 4 natural ventilation
- 2. Objectives 1 Discuss the different types of natural ventilation application in building design 2. Discern some techniques to reduce solar and internal gains in a building Key Words Stack Effect Positive Air pressure Inlet Openings Solar Gain Airtight Building Envelope Windward Glazing Exhaust Openings
- 3. Introduction A building is a man-made environment (a machine in Le Corbusier terms) within the bigger natural surrounding. The whole endeavor of architecture, apart from space definition, is to moderate the level of interaction with the environment. In doing so, openings are provided on the envelope of the building for ventilation and space cooling purposes. However, providing the “holes” on the “walls” is not a simple task of necessity. Rather it is a multi variable function of the immediate surrounding (wind direction, nearby elements like buildings, trees, shrubs) and the opening parameters (size, placement and orientation).
- 4. Taking into consideration all above the elements reduces energy consumption for air conditioning and mechanical cooling.
- 5. Description Natural ventilation is the use of outdoor airflow into the building to provide ventilation and space cooling. It can be achieved with operable windows when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside (stack effect) thus forcing cool outside air to be drawn into the building naturally though openings in the lower areas.
- 6. These systems use very little energy but care must be taken to ensure the occupants‘ comfort. In warm or humid months, in many climates, maintaining thermal comfort via solely natural ventilation may not be possible so conventional air conditioning systems are used as backups
- 8. Concepts Natural ventilation is a whole building design concept. the design utilizes the stack effect and wind pressure to supply outdoor air to building interiors for ventilation or space cooling purposes. The aim is to have an airtight building envelope while controlling outdoor air supply to provide the required ventilation. Features of naturally ventilated buildings include operable windows, exhaust units located high in the building with intakes located low in the building, and open building plans to facilitate air movement. Designs can incorporate lobbies, internal stairwells, vent chimneys and small fans to move vent air.
- 9. Natural ventilation reduces energy consumption for fans and mechanical cooling and in most cases gives occupants control over their office space. Further benefits include no fan noise and in some cases elimination of the mechanical cooling
- 11. Benefits provides ventilation (outdoor air) to ensure safe, healthy and comfortable conditions for building occupants without the use of fans, provides free cooling without the use of mechanical system, reduces building construction cost and operation cost, when carefully designed, reduces energy consumption for air conditioning and circulating fans and eliminates fan noise
- 12. Limitations Greater temperature swings than normal with mechanically cooled spaces will occur and have to be acceptable to the occupants, flow rate varies with the outdoor conditions, Cool air drawn into room, warm air extracted occupants must be willing to open or close vents and windows to regulate thermal comfort, not as effective during warm, humid, summer months, difficult to retrofit in buildings and fire code may restrict design options.
- 13. Application 1.The long façade of the building and majority of the openings should be oriented with respect to the prevailing summer breezes,
- 14. 2. exhaust vents or outlets should be on the leeward side as high as possible in the building,
- 15. 3. vegetation and site objects should not obstruct inlet openings,
- 16. 4. rooms should have an inlet and outlet openings located in the opposing pressure zones, e.g. windward and leeward walls, windward wall and roof 5.inlets should supply air low in the room, outlets Should Be Located Possibly Across the room and at high level,
- 17. 6. vertical distance between the inlet and exhaust openings should take advantage of the stack,
- 18. 7. all occupied spaces should have an inlet opening and outlet opening, one or both of which may be an operable window, 8. the total area of outlet openings should be operable and accessible by the occupants,
- 19. 9. inlet openings should not be obstructed by furniture and interior partitions, to ceiling height should not be less than 3m Maximize reflectivity of ground and building surfaces outside facing winter sun Provide reflective panels outside of glazing to increase winter irradiation
- 20. Some techniques to reduce solar and internal gains 1.window areas should not be excessive and be protected with exterior shading devices Window size > 0.3 A x B Window sizes for industrial buildings: glass area-1/20 of room area; window width- 1/10 (M+N-O+P)
- 21. 2. design for high thermal capacity and exposed ceilings for night cooling 3. minimize warming of walls by the sun through the use of light colored exteriors, trees and shrubs to provide shading and evaporative cooling,, grass and other ground cover to keep ground temperature low, and ponds and fountains to enhance evaporative cooling
- 22. 4. internal loads such as lighting controls, high efficiency lighting,, high efficiency mechanical equipment and pipe and duct installation should be low 5. narrow floor plan
- 23. Basic Principles •As the wind approaches the face of the building, the airflow is slowed, creating positive pressure. Separated air increases suction (negative pressure) along wall surfaces. •If windows are placed in both windward and leeward, the building would be cross ventilated and eddies will develop •Windows placed in sidewalls will enhance ventilation sue to increased suction and greater air circulation Ventilation is best for room with windows on 3 adjacent walls than 2 opposite walls
- 24. • If wind is from windowless side, there is poor ventilation • A building with one window wall negligible ventilation will occur, but architectural solutions are available • As airflow passes through a well-ventilated room it forms an air-jet • If openings are near to wall, ceiling, floor, the air stream attaches itself to the surface • It is best to offset the inlet and outlet • The window size for the inlet and outlet should be the same
- 25. Attic and crawl space ventilation 1.when there is attic space under roof, venting of attic will reduce heat build up, provide escape route for moisture and in cold climates, help prevent ice dams from forming
- 26. 2. Even when there is no attic, the venting effect can still be achieved with at least a 2.54cm air space above the insulation
- 27. 3. In some cases, the argument can be made for having no venting at all. This can be done in dry climates and building types where vapor is less of a problem or if the “wet” side of the roof is sealed against vapor migration.