CHAPTER 3 VENTILATION

Content Introduction Important of ventilation Natural Ventilation Wind Stack effect Combination wind and stack effect Mechanical Ventilation Exhaust Supply Balanced Plenum Types of fan used for ventilation systems

Introduction: Ventilation The process of changing air in any space to provide high indoor air quality. (i.e. to control temperature, replenish oxygen, remove moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide). Allows for the exchange of air from the outside and circulation of air within building. A proportion of air within the enclosed space should be continuously withdrawn and replaced by fresh air to maintaining acceptable air quality in buildings.

Why ventilation is important? Maintaining the air purity Preservation of oxygen content – this should be maintained at approximately 21% of air volume. Removal of carbon dioxide Control of humidity- between 30-70% RH (Relative Humidity) is acceptable for human comfort. Prevention of heat concentration from machinery, lighting and people Prevention of condensation Dispersal of concentrations of bacteria Dilution and disposal of contaminants such as smoke, dust, gases and body odors. Provision of freshness – an optimum air velocity lies between 0.15 and 0.5 m/s. Ventilation/air supply must able to improved human comfort conditions. Perfect ventilation depends on: Volume of air required Temperature of air Speed of air movement Humidity Purity

Air movement Principles of air movement: Pressure difference between inside and outside building allows for the air to move from outside to inside/inside to outside. Air flow rate through the building affect by building resistance. Controlled natural ventilation and infiltration are driven by pressure difference across the building envelope.

BFC3072: BUILDING SERVICES Air movement The pressure difference is caused by: wind (or wind effect) stack effect combination of both wind and stack effects. BFC3072: BUILDING SERVICES

Natural Ventilation Process of supplying and removing air through an indoor space by natural means (without a fan or other mechanical system) Two types: 1. Wind driven (wind effect) ventilation 2. Stack ventilation Most often used through operate able windows for small spaces. Complex building using the stack effect to allows the warm air to risen up and flow to the upper opening and forcing cold air at the outside to be drawn into the building through openings in the lower area.

Figure 1 Air flow around a building WIND EFFECT When air flow is due to wind, air enters through openings in the windward walls, and leaves through openings in the leeward walls. The pressure distribution patterns due to wind in a number of cases are illustrated in Figure 1. Figure 1  Air flow around a building

Wind pressures are generally high/positive on the windward side of a building and low/negative on the leeward side. The occurrence and change of wind pressures on building surfaces depends on: wind speed and wind direction relative to the building; the location and surrounding environment of the building; shape of the building.

(b) Ventilation through wall (a) Ventilation through window (b) Ventilation through wall (c) Ventilation through roof - Opening on roof

STACK EFFECT In a completely sealed enclosure, thermal differences between the inside and outside will result in a pressure difference because warm air is less dense than cold air. Therefore when there is an opening air will flow across the roof terminals boundary to equalise the pressure.

BFC3072: BUILDING SERVICES STACK EFFECT Benefits does not rely on wind natural occurring force stable air flow greater control in choosing areas of air intake sustainable method Limitations lower magnitude compared to wind ventilation relies on temperature differences (inside/outside) Design restriction and may incurs extra cost The quality of air may be polluted. BFC3072: BUILDING SERVICES

BFC3072: BUILDING SERVICES STACK EFFECT Cold/winter season (see Figure 2a), the following stack effect occurs: indoor temperature is higher than outdoor temperature; the warmer air in building then rises up; the upward air movement produces negative indoor pressure at the bottom; positive indoor pressure is created on the top; warmer air flows out of the building near the top; and the air is replaces by colder outside air that enters the building near its base Hot/summer season (see Figure 2b), the reverse occurs when indoor temperature is lower than outdoor temperature. Figure 2  Stack effect BFC3072: BUILDING SERVICES

Combination of wind and stack effect In most buildings, natural ventilation relies on a combined wind force and stack effects.

The Stack Effect Wind Pressure Air movement in the attic may be caused by the stack effect, wind pressure or both. Balanced ventilation also uses the natural forces of wind pressure to create areas of high and low pressure areas around a house. High-pressure areas form where wind strikes the building and backs up. Low-pressure areas occur on the downwind side. Theoretically, air moves into the building through openings in high-pressure areas and exits at low-pressure areas.

BFC3072: BUILDING SERVICES Examples for attic ventilation… BFC3072: BUILDING SERVICES

Natural ventilation depends on 6 factors: Building orientation and shape – opening and air movement. External elements – house surrounding/tree. Cross ventilation – allow the air movement from one opening to one opening…window, wall…. Opening location Opening size – high and width of window, double door… Opening control – types of opening : sliding/ram window - full / half opening…

Maximize Natural Ventilation

Maximize Natural Ventilation Through Windows Your Choice?.... give your reasons BFC3072: BUILDING SERVICES

Horizontal pivot windows offer the highest ventilation capacity: Windows should be designed to admit natural ventilation. The most important design issue is whether the windows provide single-sided or cross-ventilation to interior spaces. The amount of air that flows through a window (ventilation capacity) depends on the area and vertical distribution of openings. These depend on the way the window opens. Horizontal pivot windows offer the highest ventilation capacity: For single-sided ventilation, place them as high as possible to exhaust warm air at ceiling level. Air entering through the upper opening is directed toward ceiling, making night-time cooling more effective.

Quiz 2 Discuss natural ventilation in Malay traditional house.

Explanation on natural ventilation in Vernacular Architecture: The Malay House