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Natural ventilation is the process of supplying and removing air through an indoor space by natural means. There are two types of natural ventilation occurring in buildings: wind driven ventilation and stack ventilation. The pressures generated by buoyancy, also known as 'the stack effect', are quite low (typical values: 0.3 Pa to 3 Pa) while wind pressures are usually far greater (~1 Pa to 35 Pa). The majority of buildings employing natural ventilation rely primarily on wind driven ventilation, but stack ventilation has several benefits. The most efficient design for a natural ventilation building should implement both types of ventilation.

ProcessEdit

The static pressure of air is the pressure in a free-flowing air stream and is depicted by isobars in weather maps. Differences in static pressure arise from global and microclimate thermal phenomena and create the air flow we call wind. Dynamic pressure is the pressure exerted when the wind comes into contact with an object such as a hill or a building and it is related to the air density and the square of the wind speed. The impact of wind on a building affects the ventilation and infiltration rates through it and the associated heat losses or heat gains. Wind speed increases with height and is lower towards the ground due to frictional drag.

The impact of wind on the building form creates areas of positive pressure on the windward side of a building and negative pressure on the leeward and sides of the building. Thus building shape is crucial in creating the wind pressures that will drive air flow through its apertures. In practical terms wind pressure will vary considerably creating complex air flows and turbulence by its interaction with elements of the natural environment (trees, hills) and urban context (buildings, structures). Vernacular and traditional buildings in different climatic regions rely heavily on natural ventilation for maintaining human comfort conditions in the enclosed spaces.

DesignEdit

Typical building design relies on rules of thumb for harnessing the power of wind for the purpose of natural ventilation. Design guidelines are offered in building regulations and other related literature and include a variety of recommendations on many specific areas such as:

• Building location and orientation
• Building form and dimensions
• Window typologies and operation
• Other aperture types (doors, chimneys)
• Construction methods and detailing (infiltration)
• External elements (walls, screens)
• Urban planning conditions

Wind driven ventilation has several significant benefits:

• Greater magnitude and effectiveness
• Readily available (natural occurring force)
• Relatively economic implementation
• User friendly (when provisions for control are provided to occupants)

Some of the important limitations of wind driven ventilation:

• Unpredictableness and difficulties in harnessing due to speed and direction variations
• The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area
• May create strong draughts, discomfort.

Wind driven ventilationEdit

Wind driven ventilation or roof mounted ventilation design in buildings provides ventilation to occupants using the least amount of resources. Mechanical ventilation drawbacks include the use of equipment that is high in embodied energy and the consumption of energy during operation. By utilising the design of the building, Wind driven ventilation takes advantage of the natural passage of air without the need for high energy consuming equipment. Windcatchers are able to aid wind driven ventilation by directing air in and out of buildings.

Wind driven ventilation depends on wind behavior, on the interactions with the building envelope and on openings or other air exchange devices such as inlets or chimneys. For a simple volume with two openings, the cross wind flow rate was calculated by Aynsley et al.:[1]

Q=Uwind√((Cp1-Cp2)/(1/A12C12)+(1/A22C22) (1)

The knowledge of the urban climatology i.e. the wind around the buildings is crucial when evaluating the air quality and thermal comfort inside buildings as air and heat exchange depends on the wind pressure on facades. As we can see in the equation (1), the air exchange depends linearly on the wind speed in the urban place where the architectural project will be built. CFD (Computational Fluid Dynamics) tools and zonal modelings are usually used to calculate pressure. One of these CFD tools, called UrbaWind (UrbaWind) makes the link between this pressure and the real urban climatology. It computes with a macroscopic method the mass flow rate incoming the building for each wind characteristic (incidence and velocity magnitude), to finally give cross ventilation statistics according to the wind statistics of the considered urban location. It helps quantifying the natural cross ventilation induced by the wind flow crossing the buildings.

Stack driven ventilationEdit

(For more details, see Stack effect)

Stack effect is temperature induced. When there is a temperature difference between two adjoining volumes of air the warmer air will have lower density and be more buoyant thus will rise above the cold air creating an upward air stream. Forced stack effect in a building takes place in a traditional fire place. Passive stack ventilators are common in most bathrooms and other type of spaces without direct access to the outdoors.

In order for a building to be ventilated adequately via stack effect the inside and outside temperatures must be different so that warmer indoor air rises and escapes the building at higher apertures, while colder, denser air from the exterior enters the building through lower level openings. Stack effect increases with greater temperature difference and increased height between the higher and lower apertures. The neutral plane in a building occurs at the location between the high and low openings at which the internal pressure will be the same as the external pressure (in the absence of wind). Above the neutral plane, the air pressure will be positive and air will rise. Below the neutral plane the air pressure will be negative and external air will be drawn into the space. Stack driven ventilation has several significant benefits:

• Does not rely on wind: can take place on still, hot summer days when it is most needed.
• Natural occurring force (hot air rises)
• Stable air flow (compared to wind)
• Greater control in choosing areas of air intake
• Sustainable method

Limitations of stack driven ventilation:

• Lower magnitude compared to wind ventilation
• Relies on temperature differences (inside/outside)
• Design restrictions (height, location of apertures) and may incur extra costs (ventilator stacks, taller spaces)
• The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area

Natural ventilation in buildings relies mostly in wind pressure differences but stack effect can augment this type of ventilation and partly restore air flow rates during hot, still days. Stack ventilation can be implemented in ways that air inflow in the building does not rely solely on wind direction. In this respect it may provide improved air quality in some types of polluted environments such as cities. For example air can be drawn through the backside or courtyards of buildings avoiding the direct pollution and noise of the street facade. Wind can augment the stack effect but also reduce its effect depending on its speed, direction and the design of air inlets and outlets. Therefore prevailing winds must be taken into account when designing for stack effect ventilation.

Examples of stack effect ventilation can be seen on aluminium smelters, steel mills, and glass plants. Stack effect ventilators have undergone numerous evolutionary steps in recent years to correspond to new safety standards for protection against weather pentration, air hygiene for plant workforce and methodology of construction to reduce total installed costs of greenfield and brownfield projects.

Estimating stack effect ventilationEdit

The natural ventilation flow rate can be estimated with this equation:[2]

$Q_{S} = C_{d}\; A\; \sqrt {2\;g\;H_{d}\;\frac{T_I-T_O}{T_I}}$
English units:
 QS where: = Stack vent airflow rate, ft³/s = cross-sectional area of opening, ft² (assumes equal area for inlet and outlet) = Discharge coefficient for opening = gravitational acceleration, around 32.2 ft/s² on Earth = Height from midpoint of lower opening to neutral pressure level (NPL), ft = location/s in the building envelope with no pressure difference between inside and outside       (ASHRAE 2001, p.26.11) = Average indoor temperature between the inlet and outlet, °R = Outdoor temperature, °R
SI units:
 QS where: = Stack vent airflow rate, m³/s = cross-sectional area of opening, m² (assumes equal area for inlet and outlet) = Discharge coefficient for opening = gravitational acceleration, around 9.8 m/s² on Earth = Height from midpoint of lower opening to neutral pressure level (NPL), m = location/s in the building envelope with no pressure difference between inside and outside       (ASHRAE 2001, p.26.11) = Average indoor temperature between the inlet and outlet, K = Outdoor temperature, K

Natural ventilation of boiler rooms and industrial buildingsEdit

Due to high internal heat loads, natural ventilation of boiler rooms, warehouses, and other similar spaces is often employed. Often, conventional or overhead doors are manually opened to provide ventilation. When natural ventilation does not suffice alone, large box fans are often employed to enhance air movement.

But to provide security, and cooling-by-ventilation, some buildings have two sets of overhead doors in hot boiler and equipment rooms. The second set of doors are custom-made grilles with bird screens, similar to the security grilles used by some stores at indoor shopping malls. Some of the custom grilles have solid slats in the lowest section to reduce the amount of trash that might blow into the rooms. During hot weather the grilles help secure the opening while the solid doors are fully open. During cool and cold weather the solid doors can be partially or fully closed.