Ventilation

BSES 2335

Environmental Physics

Name: MUHAMAD SALMAN BIN MUHAMAD AZMI Matrix No.: BEB090020 Lecturer:

PN. NIK ELYNA MYEDA NIK MAT

Theories of Natural Ventilation

Natural ventilation can be generated by two methods: by thermal force or buoyancy effect, and by wind pressure force or wind-driven effect. Wind-driven natural ventilation is easier to achieve because it only needs a low wind speed to create adequate indoor air velocities that help people’s heat transfer by means of evaporation. The main purpose of natural ventilation as a passive cooling strategy is to achieve high indoor air velocities with the air that has appropriate temperature and relative humidity. Factors that influence these parameters can generally be divided into two parts: outdoor environment and building component. It is known that landscape elements such as trees and water bodies can reduce the air temperature while hard-surface elements such as concrete grounds raise the air temperature. According to Givoni, building components that affect natural ventilation include shape of the building, geometrical configuration, orientation of opening, window size and type, and subdivision of interior space.

Ventilation 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).

Stack Driven Ventilation

The stack effect used for high-rise natural ventilation 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. 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.

Guidelines for Natural Ventilation The important guidelines for planning and designing natural ventilation systems in buildings:

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a natural ventilation system should be effective regardless of wind direction there must be adequate ventilation even when wind does not blow from the prevailing direction;

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Inlet and outlet openings should not be obstructed.

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windows should be located in opposing pressure zones

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a certain vertical distance should be kept between openings for temperature to produce stack effect;

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openings at the same level and near the ceiling should be avoided since much of the

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to admit wind air flow, the long façade of the building and the door and window openings should be oriented with respect to the prevailing wind direction;

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if possible, window openings should be accessible to and operable by occupants;

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If inlet and outlet openings are of nearly equal areas, balanced and greater ventilation can be obtained.

Effectiveness of Natural Ventilation Effectiveness of natural ventilation depends on several factors: 1. 2. 3. 4.

Wind strength and direction. Size of openings. Air temperatures. Height of the building.

Wind Strength and Direction Wind strength and direction influence the effectiveness of natural ventilation. This is because, if the airflow is strong, the quantity of the air that will enter the building will increase. This will help the ventilation rate in the building. While for the direction, the design of the opening such as windows and doors must be appropriate with the wind direction. This is to help increase the rate of ventilation in a space or building. If the position is not appropriate with the wind direction, the ventilation may get worse.

Size of Openings Windows and doors act as the opening for some spaces or a building. It has quite a major impact on the ventilation rate for a building. This is because, the bigger the size of openings

the better the ventilation rate of a building. The airflow is easily access into the building. Other than windows and doors, the openings also include roof ventilators, skylights, vent shafts, and so forth.

Air Temperature Natural ventilation also depends on the air temperature of the inside and outside. Wind can blow air through openings in the wall on the windward side of the building, and suck air out of openings on the leeward side and the roof. Temperature differences between warm inside air and cold outside air can cause the air in the room to rise and exit at the ceiling or ridge, and enter via lower openings in the wall. During the summer, natural ventilation depends primarily upon wind forces since inside/outside temperature differences are small. Successful design of sidewall and ridge openings and proper use of control systems therefore require a good understanding particularly of wind effects.

Height of the Building Height of the building also influences effectiveness of the natural ventilation. When the building gets higher, the less effective the rate of natural ventilation. The idea of integrating natural means of ventilation in high-rise buildings has not been proven to be effective, especially in a hot-humid tropical climate. The lack of control, high external air temperature, low quality external air, the lack of "high-tech" image which can be presented to the client, and generally undesirable locations in urban centres could be the reasons why the possibility of natural ventilation has been neglected. The air that is moved through the exchange of indoor with "fresh" outdoor air can provide cooling, and can also act as a heat carrying medium. These natural phenomena are quite similar in both domestic low-rise and high-rise situations. However, in the case of high-rise buildings, there are other additional factors which may influence the external air flow pattern. Consequently, these will further modify the indoor air flow. The higher wind velocity in the upper atmosphere could be utilised as one of the possible cooling methods for a tall building. This natural phenomenon of gradient wind might be very useful, especially in a tropical climate where the mean surface wind velocity is mild.

References Lecture Notes Faculty of Architecture, University of Hong Kong (http://www.arch.hku.hk/teaching/lectures/airvent/sect03.htm) (http://en.wikipedia.org/wiki/Natural_ventilation)

Journal of Architectural/Planning Research and Studies Volume 5. Issue 1. 2007 Faculty of Architecture and Planning, Thammasat University Evaluation and Design of Natural Ventilation for Houses in Thailand Assistant Professor Chalermwat Tantasavasdi, Daranee Jareemit, Anake Suwanchaiskul, and Thitiporn Naklada