Natural Ventilation PDF

Official (Closed), Non-Sensitive School of Architecture & the Built Environment MECHANICAL SERVICES BE6812 Ventilation Objectives : To provide students with an understanding of the needs for ventilation, the principles of...

Official (Closed), Non-Sensitive School of Architecture & the Built Environment MECHANICAL SERVICES BE6812 Ventilation Objectives : To provide students with an understanding of the needs for ventilation, the principles of natural ventilation and the effects of poor ventilation. 3.1 General Ventilation is the process of supplying fresh air to a room or some other internal space, by natural or _______________ means, in a quantity sufficient for the needs of the occupants. In any occupied space ventilation is necessary to provide oxygen and to remove contaminated air. Ventilation should be distinguished from air-conditioning and infiltration. _______________________ can be regarded as the process of treating the internal air so as to clean or purify, and to control its temperature and humidity. _____________ is the leakage of air through a building due to imperfections in the structure such as cracks on external walls, and around doors/windows. This is not a form of ventilation. 3.2 Necessity of Ventilation To provide _________________for human comfort. To provide _________in the room for the proper functioning of the human body. To keep the concentration of carbon dioxide from respiration to an acceptable level. To dilute the ____________________ and toxic gases present in the space. The contaminants may be carbon monoxide or fumes produced from combustion. To dilute odour. To remove moisture. To remove bacteria in areas where they might do harm, e.g. in a hospital. To remove excessive or ____________________. BE6812 Ventilation Page 1  Official (Closed), Non-Sensitive 3.3 Ventilation Rates ( m3/hr ) (a) Factors affecting the ventilation rate The quantity of fresh air required to provide sufficient oxygen. The type of ______________ The ____________ of occupants. The ___________ of the enclosed space. The need to reduce excess heat gain from the surrounding. The need to maintain adequate air movement within an enclosed space. (b) Recommended ventilation rates All rooms or occupied spaces which are not naturally ventilated or air-conditioned shall be provided with mechanical ventilation. Based on SS 553 Code of Practice for Air Conditioning and Mechanical Ventilation in Buildings, the quantity of outdoor air supply, through mechanical ventilation, for such rooms or space shall be based on its volume as follow :- Application Air Changes per hour Offices 6 Kitchen (commercial & industrial) 20 Restaurant, canteens 10 Shops 6 Classrooms 8 Car parks 6 Lobbies, corridors, staircases 4 Toilets 10 Note : One air change = volume of the enclosed space (m3) Table 1- Ventilation Rates in non air-conditioned buildings with no natural ventilation Example 1 An office which measures 10m x 6m x 4m (height) requires a ventilation rate of 4 air changes per hour. Calculate the volume (in m3) of fresh air that is required to be brought into this office in one hour. Solution BE6812 Ventilation Page 2  Official (Closed), Non-Sensitive Ventilation systems can be broadly classified into two types, i.e. natural ventilation and mechanical ventilation. When natural ventilation system is not able to provide the required air change rate, then mechanical ventilation system will have to be employed. 3.4 Natural Ventilation in Buildings Natural ventilation is provided by the following two mechanisms :- Air pressure difference such as that caused by the wind movement over and around a building. (This is also sometimes referred to as natural ventilation by induction.) Stack effect which is caused by the natural convection of warm air rising within a building. (a) Air Pressure Difference (or Induction) Outlet Inlet Section View Fig. 1 - Air Pressure Difference Wind causes a positive pressure to act on the windward side of the building and a negative pressure to act on the leeward side. Air moves from the positive pressure region, where the action of the force is high; towards the negative pressure region, where the action of the force is low. Openings (eg. a window) on faces of positive pressure will serve as inlets and those on faces of negative pressure as outlets for air movement. Inlets and outlets should be as large as possible in order to optimize airflow. Cross ventilation results when the inlets are directly opposite the outlets, thus encouraging continuous flow of air through the buildings. BE6812 Ventilation Page 3  Official (Closed), Non-Sensitive (b) Stack Effect Section View Fig. 2 - Stack effect Stack effect is created _____________________________________________ __________________________ of a building. ____________________________________ is especially applicable in cool climate regions. In hot humid climate regions (eg. Singapore) where outdoor temperature is about the same as the indoor temperature, the stack effect is insignificant. Stack effect is only possible in high rise building and the higher the building the greater will be the air movement inside the building. 3.5 The Effectiveness of Ventilation by Air Pressure Difference The effectiveness of induced ventilation depends on the :- average wind velocity; prevailing wind direction; interference from local obstructions; depth of the building (on plan view); size of inlet and outlet openings (eg. a window). V2 Velocity of air increases Prevailing with height, i.e. V2 > V1 Building Height wind direction Building V1 Section View Wind Velocity (m/s) Fig. 3 - Wind Velocity BE6812 Ventilation Page 4  Official (Closed), Non-Sensitive a1 Window a2 Opening Plan View Fig. 4 - Prevailing Wind Direction Low building on leeward side Low building on windward side Section View Fig. 5 - Local Obstruction BE6812 Ventilation Page 5  Official (Closed), Non-Sensitive Stagnant (or "Dead") air area Section View Fig. 6 - Deep plan Building ( Note : Where the position of the inlet and outlet are quite far apart. ) Areas of stagnant air within the building are those areas where little or no air movement occurs. Roof vent Air drawn out from "dead" area The provision of roof vents can improve ventilation in Section View Fig. 7 deep plan buildings. 3.6 Building Regulations on Natural Ventilation In Singapore, the Building Control Regulations demand a minimum openable area for openings (eg. a window) to provide for natural ventilation in buildings. Type of Space Minimum Size of Opening Any room in a building 5% of floor area Multi Storey Carparks 15% of floor area Table 2 * Source : Approved Document, Building Control Authority (2013) BE6812 Ventilation Page 6  Official (Closed), Non-Sensitive Example 2 An aluminium casement window is installed in a room which measures 4m x 3m (on plan). The overall size of the casement window is 600mm x 1200mm. Show, by calculation, whether the size of the window opening complies with the ventilation requirements of the Building Control Regulations. Solution 3.7 Advantages and Disadvantages of Natural Ventilation Advantages :- No maintenance or breakdown problems compared to mechanical ventilation. Supports “green building” design concepts and environmental sustainability. Disadvantages :- There is no means of filtering the incoming air as compared to mechanical ventilation. Degree of control and extent of ventilation is limited, i.e. areas of stagnant air may occur (eg. deep plan buildings). It is not effective when the difference in temperature between the indoor air and the outdoor air is small. BE6812 Ventilation Page 7  Official (Closed), Non-Sensitive 3.8 Effects of Poor Ventilation Causes of poor ventilation are usually related to the control of fresh air intake, under-design, poor installation, operation and maintenance of ventilation systems. Poor ventilation will result in poor indoor air quality and ___________________________ may develop if the problem gets worse. (a) Indoor air quality (IAQ) problems in buildings Most modern air-tight buildings suffer from poor indoor air quality due to poor ventilation systems. Most of the complaints in such buildings are :- __________________ fresh air intake; ___________________ air movement; _________ air distribution; Stale air being re-circulated/reused; Unpleasant odour and chemical contamination; Thermal discomfort. To counter the problem of indoor chemical pollutants, air ionisers can be used to clean and refresh the indoor air to a certain extent. Air ionisers are also known as air purifier. (b) Sick Building Syndrome (SBS) Sick building syndrome refers to a range of unrelated health symptoms appear to be linked to the time one spends in a building. The symptoms then seemed to resolve on their own after the occupant leaves the building. The symptoms of occupants suffering from SBS are sensory irritation of the eyes, nose & throat. Headaches, nausea, dizziness and breathing difficulties. _________________ Nat Vent/YHK/ BE6812 Ventilation Page 8