Air Movement and Natural Ventilation

Factors influencing

Ventialtion

VENTILATION is the process by which fresh air is introduced and used 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.

Types of ventialtion

Natural ventilation

Mechanical ventilation

Natural ventilation

Natural ventilation is the process of supplying and removing air by means of purpose-provided aperture (such as openable windows, ventilators and shafts) and the natural forces of wind and temperature-difference pressures.

types of natural ventilation

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.

types of natural ventilation

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 either supply, extract or balanced ventilation for an occupied space.

Purposes of ventilation

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).

Principles of Natural Ventilation

For air to move into and out of a building, a pressure difference between the inside and outside of the building is required.

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.

Air movement

around buildings

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

Wind flow

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 depend on:

1. wind speed and wind direction relative to the building;

2. the location and surrounding environment of the building; and

3. shape of the building.

Wind induced pressure differences

Positive pressure is created on the building sides that face the wind (windward sides) whereas suction regions are formed on the opposite sides (leeward sides) and on the side walls.

This results in negative pressure inside the building, which is sufficient to introduce large flows through the building openings.

In a general case, n airflow of air is induced on the windward side and an outflow on the leeward side. Airflow through an external opening is mainly attributed to a wind induced pressure difference across it.

Landscape and wind flow

Stack effect

When air movement is due to temperature difference between the indoor and outdoor, the flow of air is in the vertical direction and is along the path of least resistance.

The temperature difference causes density differentials, and therefore pressure differences, that drive the air to move.

How 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 replaced by colder outside air that enters the building near its base.

Factors influencing

Air movement through buildings

Factors influencing air movement through buildings

ORIENTAION SIZE OF OPENINGS POSITION OF OPENINGS CROSS VENTIALTION CONTROL OF OPENINGS

Orientation of openings

Cross ventilation

Cross-ventilation Cross-ventilation utilises

differential wind pressure. When the air outside is

cooler, windows on opposite sides of the home can be opened.

Cool air enters on the windward side and passes out on the other side, replacing warm inside air with cool outside air.

 

Size of openings

Inlet & Outlet Areas In both cross and stack

ventilation, the amount of heat removed from a building is directly proportional to the inlet and outlet areas.

The larger the inlet and outlet areas, the more air can travel through the building and the more heat can be removed.

Increasing the inlet and outlet area increases ventilation.

Control of openings

Control of openings

Air movement through buildings

Effect of window width on wind speed

Inner obstructions

Internal obstructions

Air must be free to move from inlet to outlet. The two are not very useful if there is a wall between them. In addition, the cross sectional area of the paths from inlet to outlet should be at least the area of the smaller of the inlet and outlet.

A clear path between inlet and outlet must be maintained.

Design strategies for air movement

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 wingwalls, 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 maximised in the occupied zones for bodily cooling;

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;

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, a balanced and greater ventilation can be obtained.