Saturday, April 2, 2011

NATURAL AND ARTIFICIAL VENTILATION OF BUILDINGS

AR-461: BUILDING SCIENCE
By:
RAVINDAR KUMAR
Assistant Professor
Department of Architecture and Planning
NED University of Engineering and Technology
Karachi
LECTURE NO. 06
TOPIC:                         NATURAL AND ARTIFICIAL VENTILATION OF BUILDINGS

INTRODUCTION:[1]

Ventilation is the process by which fresh air is introduced and ventilated air is removed from an occupied space. The primary aim of ventilation is to preserve the qualities of air. Sometimes, ventilation may also be used to lower the temperature inside an occupied area.

NATURAL VENTILATION
Natural ventilation is the process of supplying and removing air by means of purpose-provided aperture such as open-able windows, ventilators and shafts and the natural forces of wind and temperature-difference pressures. Natural ventilation may be divided into two categories:

Controlled natural ventilation is intentional displacement of air through specified openings such as windows, doors, and ventilations by using natural forces (usually by pressures from wind and/or indoor-outdoor temperature differences). It is usually controlled to some extent by the occupant.

Infiltration is the uncontrolled random flow of air through unintentional openings driven by wind, temperature-difference pressures and/or appliance-induced pressures across the building envelope. In contrast to controlled natural ventilation, infiltration cannot be so controlled and is less desirable than other ventilation strategies, but it is a main source of ventilation in envelope-dominated buildings.

MECHANICAL VENTILATION:
Mechanical or forced ventilation is the process of supplying and removing air by means of mechanical devices, such as fans. It may be arranged to provide supply, extract or balanced ventilation for an occupied space. There are also specialized areas in which ventilation is vital, such as ventilation for industrial processes, mines, tunnels and underground developments. However, in this lecture we will focus only on natural ventilation.

DEFINITION OF NATURAL VENTILATION:[2]
It 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 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.

PURPOSES OF NATURAL VENTILATION:[3]
Maintaining human comfort and health are two key reasons for providing ventilation in buildings. To achieve these purposes, a ventilation system should be able to meet the following criteria:
  • Provide sufficient supply of air/oxygen for the physiological needs of human beings (a minimum of 0.2 l/s/person is required for breathing purpose) and/or livestock;
  • Provide sufficient supply of air/oxygen for industrial, agricultural and other processes (for example, provision of oxygen for burning and combustion processes);
  • Remove the products of respiration and bodily odour (including those from smoking) of human and/or animal occupants;
  • Remove contaminants or harmful chemicals generated by processes or from building materials;
  • Remove heat generated by people, lighting and equipment inside the occupied space;
  • Create some degree of air movement which is essential for feelings of freshness and comfort (usually a velocity of 0.1 to 0.3 m/s is required).

PROCESS OF NATURAL VENTILATION:[4]
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.
DESIGN OF NATURAL VENTILATION:[5]
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 VENTILATION:[6]
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. Wind catchers 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. 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. 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 modeling are usually used to calculate pressure. One of these CFD tools, called Urban Wind. Urban Wind 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 VENTILATION:[7]
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
  • It can take place still on 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 aluminum 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 penetration, air hygiene for plant workforce and methodology of construction to reduce total installed costs of Greenfield and Brownfield projects.

NATURAL VENTILATION OF BOILER ROOMS AND INDUSTRIAL BUILDINGS:[8]
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 sets 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.

PRINCIPLES OF NATURAL VENTILATION:[9]
For air to move into and out of a building, a pressure difference between the inside and outside of the building is required. The resistance to flow of air through the building will affect the actual air flow rate. In general, controlled natural ventilation and infiltration are driven by pressure difference across the building envelope. The pressure difference is caused by:
  • Wind or wind effect;
  • Difference in air density due to temperature difference between indoor and outdoor air stack or chimney effect; or
  • Combination of both wind and stack effects.

GUIDELINES FOR NATURAL VENTILATION:[10]
The following guidelines are important for planning and designing natural ventilation systems in buildings:
  • A natural ventilation system should be effective regardless of wind direction and there must be adequate ventilation even when the wind does not blow from the prevailing direction;
  • Inlet and outlet openings should not be obstructed by nearby objects;
  • Windows should be located in opposing pressure zones since this usually will increase ventilation rate;
  • A certain vertical distance should be kept between openings for temperature to produce stack effect;
  • Openings at the same level and near the ceiling should be avoided since much of the air flow may bypass the occupied zone;
  • Architectural elements like wing walls, parapets and overhangs may be used to promote air flow into the building;
  • Topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum exposure to breezes;
  • In hot, humid climates, air velocities should be maximized in the occupied zones for bodily cooling;
  • To admit wind air flow, the long fa├žade of the building and the door and window openings should be oriented with respect to the prevailing wind direction;
  • If possible, window openings should be accessible to and operable by occupants;
  • Vertical shafts and open staircases may be used to increase and generate stack effect;
  • openings in the vicinity of the neutral pressure level may be reduced since they are less effective for thermally induced ventilation;
  • If inlet and outlet openings are of nearly equal areas, balanced and greater ventilation can be obtained.

BARRIERS TO THE APPLICATION OF NATURAL VENTILATION:[11]
A successful application of natural ventilation strategies is only possible when there are no problems in many areas at various levels from the design stage to actual operating demands placed on the building users. These potential barriers include:

  • Barriers during building operations
    • Safety concerns
    • Noise from outdoor
    • Dust and air pollution
    • Solar shading covering the openings
    • Draught prevention
    • Knowledge of the users about how to take the best advantage of natural ventilation

  • Barriers during building design
    • Building and fire regulations
    • Need for acoustic protection
    • Difficult to predict pattern of use
    • Devices for shading, privacy & Daylighting may hamper the free flow of air
    • Problems with automatic controls in openings
    • lack of suitable, reliable design tools

  • Other barriers
    • Impact on architectural & envelope design
    • Fluctuation of the indoor conditions
    • Design a naturally ventilated building requires more work but could reduce mechanical system (design fee on a fixed percentage of system's cost)
    • Increase risk for designers
    • Lack of suitable standards

REFERENCES:
  1. http://en.wikipedia.org/wiki/Natural_ventilation (retrieved April 2, 2011)
  2. http://www.arch.hku.hk/teaching/lectures/airvent/sect02.htm (retrieved April 2, 2011)
  3. http://www.arch.hku.hk/teaching/lectures/airvent/sect03.htm (retrieved April 2, 2011)

No comments:

Post a Comment