Basically, natural ventilation that can be applied to buildings can be divided into two types: wind pressure ventilation and thermal buoyancy ventilation. Both types of natural ventilation are caused by naturally formed air pressure differences, and different architectural designs are required to maximize the effectiveness of these two natural ventilation methods.
1. Wind pressure ventilation
When the wind blows toward the front of the building, a positive pressure area is generated on the windward side due to the obstruction of the building surface. After the airflow is deflected, it bypasses the sides and roof of the building, and a negative pressure area is generated on these sides and back. Wind pressure ventilation uses the pressure difference between the windward and leeward sides of the building to achieve natural ventilation of the building.
2. Thermal buoyancy ventilation
Due to the instability of natural wind, or due to the influence of surrounding tall buildings and vegetation and insufficient window area of the building, the principle of thermal buoyancy ventilation should be considered to accelerate ventilation. The principle of thermal buoyancy ventilation is that hot air rises and is discharged from the exhaust vents on the top of the building, and fresh cold air from the outside enters the room from the air inlet at the bottom of the building, thereby forming an uninterrupted air flow movement indoors. That is, ventilation is achieved by using the air density difference caused by the temperature difference between indoor and outdoor air and the height difference between the inlet and outlet.
When using thermal buoyancy ventilation design, it must be considered that the air inlet for air flow should be set at a low position in the room, and the air outlet should be set on the other side of the room facing the air inlet, and at a high position. The vertical distance between the air inlet and the air outlet should make full use of the thermal buoyancy principle, using skylights or roof vents. The thermal buoyancy ventilation method does not rely on wind power. In the windless summer, the naturally formed thermal buoyancy can still produce relatively stable air flow. In addition, since the thermal buoyancy effect does not rely on wind pressure and wind direction, there can be greater autonomy in the setting of the air inlet.
The shape of the building can create wind pressure, thereby effectively driving the air flow through the opening part of the building. Of course, there are many other considerations for building ventilation design. In the design guidelines of many building regulations, the following suggestions are usually given:
1. Orientation and location of the building: When the ventilation capacity needs to be adjusted to the maximum, a windy location should be selected for the building, and when designing the orientation of the building, the windward wall should be designed to be perpendicular to the local summer wind direction
2. Shape and size of the building: Buildings using natural ventilation cannot be designed too deep, otherwise it will be difficult to deliver fresh air to every part of the building
3. Other considerations: type and use of windows, type, shape and size of openings; construction methods and details; external elements; considerations for urban planning
In the design of natural ventilation, the design concepts of the two ventilation methods mentioned above, wind-driven and thermal buoyancy, should be fully utilized. Considerations in the design include:
1. Reduce air intake obstructions from the outside (such as plants or site obstacles) or from the inside (such as furniture and interior partitions) to increase air circulation
2. The air intake and air outlet of the room should be set in the opposite pressure area of the room, including openings on the windward wall and the leeward wall, or openings on the windward wall and the roof
3. All rooms should have an air intake and an air outlet, and at least one of the openings should be equipped with an adjustable window to control airflow. The air intake should be set at a lower position in the room; the air outlet should be set on the other side facing the air intake and at a higher position
In order to build a more reliable, less costly and more energy-efficient ventilation system, we must clearly understand the limitations of the building and use the various design strategies mentioned above to integrate them into the building design. Building limitations include the following aspects: building type, local environment, climate and building regulations. Knowing these requirements, designers can determine the size of fans, building openings and ventilation ducts.
1. Building type: the purpose of the building, the proposed building's orientation and appearance, and the size and location of the windows.
2. Local environment: the common local wind direction, air quality, and the structures around the proposed building
3. Climate: local temperature and humidity
4. Building rules: local rules, standards (such as ASHRAE standards) or guidelines. Design guidelines generally specify specific ventilation requirements. Ventilation requirements include: maximum allowable concentration of air pollutants, heat generation rate, and air renewal rate, etc.
